# When the asteroid hit the earth 65 mya did the earth's gravity pull change? By how much?

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When the asteroid hit the earth 65 million years ago, did the earth's gravity pull change? By how much?

The gravity of Earth did not change substantially.

The strength of gravity on Earth is about 9.8, but varies between 9.76 and 9.83 ms-2(due to Earth not being a perfect sphere)

This can be approximated from the mass of the Earth using $$g=GM_e/r_e^2$$

Where $M_e = 5.97219 imes 10^{24}kg$ and $r_e= 6371000m$.

The mass of the asteroid was about $10^{16}kg$ That changes the mass from $5.97219 imes 10^{24}$ to $5.97219001 imes 10^{24}$. It changes the gravity from 9.8 to 9.800000001

In other words. The change in gravity was utterly insignificant. You probably get a greater change in gravity by walking downstairs.

It is possible the gravity changed locally. Depending on what the asteroid was made of, and the details of what happened in the collision, you might end up with a lump of denser asteroidal material buried in the Earth's crust under the impact crater. That could produce a measurable (although not noticeable to a human) increase in gravity locally.

What is a "Sun Synchronous" orbit?
(b) Why are satellites launched from near the equator?

(1) Why don't its particles separate by weight?
(2) What accelerates the solar wind?

If you have a relevant question of your own, you can send it to
stargaze ["at" symbol] phy6.org

### 114. Why not use a heat shield going up?

The other day a colleague asked me a question that I could not answer and which keeps intriguing me. "We know that (the shield of) a spacecraft that re-enters the earth's atmosphere heats up spectacularly because it is hit and slowed down by air-molecules. Is there a similar problem when it is going the other way, at take off? It is going through the same amount of air and acceleration looks quite similar to deceleration, only the other way around." The only answer that I could think of is that a good part of the acceleration might take place at higher altitudes where there is less air, and braking is done only by using the atmosphere.

The reason you suggested yourself is pretty much what happens.

Going up, it is the rocket engine which provides acceleration and energy. Because air resistance robs energy and is undesirable, the rocket deliberately rises vertically, to go though the denser atmosphere as quickly as possible. The vehicle gets most of its velocity, and almost all of the kinetic energy, at high altitudes where air density is too low to make a great difference. With the space shuttle "Columbia," even that might not have been enough: by the time it reached twice the velocity of sound, the atmosphere around it was still dense enough to rip a piece of foam insulation off its fuel tank, and it hit the orbiter with great force, breaking the heat shield.

On coming down (with spacecraft which we want to come down undamaged), the atmosphere is the brake absorbing the energy. We need that air resistance, and the heating is a result of absorbed energy! The big concern that energy should not be absorbed too fast, otherwise the heat gets too intense and may melt the heat shield and everything else. That is why the space shuttle comes down at a low angle, trying to stay as long as possible in a layer with the right density: If the shuttle comes in too high, not enough energy is lost, if too low, too much. The density is also important in supporting the shuttle, which--like a kite--needs part of the resistance to help it from coming down too fast.

### 115. When and where can rainbows be seen?

This may be a very unusual request but I need to know the sun's angular elevation at noon , winter solstice, at 42 degrees N latitude. Not necessary but curious same for summer solstice.

Believe it or not this all has to do with rainbows. I had an argument with someone whom I told that I have never observed a rainbow except in the morning or evening. The Sun has to be low in the east or west, and the bow then appears on the opposite side of the sky. Never observed one in the northern sky.

Could you help me out by answering these questions?

Actually, the problem you ask about is fully discussed on my page

On the winter solstice, the noontime Sun will be 42 + 23.5 = 65.5 degrees from the zenith or 24.5 degrees from the horizon
On the summer solstice, the noontime Sun will be 42 - 23.5 = 18.5.5 degrees from the zenith or 71.5 degrees from the horizon

### 116. The unusual rotation of the planet Venus

I am a university student who was asked to research some common legends to determine if they are true. One of the "facts" which I am supposed to verify is:

" Venus is the only planet that rotates clockwise. "

Is the above statement true? And if so, why? Why would Venus rotate in the opposite direction of all the other planets?

"Why" is harder--I am not sure that anyone knows for sure. The sense in which all planets in our solar system (and the Sun) rotate is presumably the same as that of the cloud from which the solar system condensed. When that happened, presumably many fragments collided to create each planet, and some average behavior of those collisions produced the rotation. One might even speculate that Mercury and Venus, the ones closest to the Sun once had large outer gas envelopes, and the way these evaporated in the Sun's heat may have contributed to the loss of rotation. That, though, is just a guess.

### 117. Why not use nuclear power for spaceflight?

Why not use nuclear energy to power spaceflight? After all, few pounds of plutonium contain as much energy as thousands of tons of rocket fuel!

Nice idea. However, to fly in space takes rocket thrust, not just energy.

By Newton's laws, the forward momentum given to any rocket is always equal to the backward momentum given to the jet fired backwards. That momentum, in its turn, depends on two factors--how much mass is expelled by the jet, how many tons per second, and the speed with which it is expelled. Nuclear energy can supply the speed, but something must provide the expelled mass.

You might think next that given some source of mass (say, a tank filled with water), plentiful nuclear energy would make it possible to eject it much faster. But how? Rocket engines work by converting heat into directed motion, in a very efficient way, but they already run about as hot as available materials can stand. Nuclear energy could provide more heat, but no rocket engine could stand it.

Early in the space age a serious effort existed to build a nuclear rocket, getting its thrust by heating hydrogen with nuclear fission. A jet of hydrogen, coming from a rocket engine at a certain temperature, is much faster than a jet of burned rocket fuel, coming from a rocket engine at the same temperature. The reason is linked to the fact that hydrogen molecules are much lighter than those of any burned fuel.

However, the rate at which rocket engines used in spaceflight supply energy is enormous--e.g. the shuttle's engines burn a ton of fuel or more each second. The stresses are enormous, and the risk of nuclear material and waste products of fission getting into the atmosphere was too great, and so the project ended.

A visionary proposal of the 1950s proposed a "rocket" cabin with a strong flat plate on the bottom (oil would be sprayed on it for protection), and a trapdoor through which small nuclear bombs could be dropped, detonating some distance away and pushing the craft forward. On paper, it seemed feasible, but an actual nuclear test was deemed hazardous, sure to release contamination. The nuclear test-ban treaty of 1963 ended all efforts in this direction.

### 118. "Doesn't heat rise?"

I was helping my wife in her 3rd grade class yesterday and one of the students came up with an interesting question. We were talking about how it gets cooler as you increase your altitude (specifically in the mountains). I was guessing that the temperature drops about 3 degrees for every 1000 ft that you climb.

I was really caught off guard with a question from the "peanut gallery." It was - " Doesn't heat rise? " I said that that is correct, and conversely cold would tend to remain near the ground. He further questioned - "If heat rises then why wouldn't it get hotter as you increase your altitude? I had no explanation. Can you help?

-----------------------------
It is true--the atmosphere is hot at its bottom and cool higher up (at least for the first 10 miles or so). It is true even though, as we know, hot air tends to rise! It all happens because THE BOTTOM OF THE ATMOSPHERE IS WHERE AIR RECEIVES ITS HEAT.

That heat arrives when sunlight hits the ground and warms it up. Think of what would happen if no way existed for removing it! The ground would get hotter and hotter--oceans, lakes and rivers would boil away, life would become impossible. Actually, we see rather little temperature change near the ground--just day-to-night fluctuations, and slow changes with weather and seasons. Such observations suggest that on the average, heat is removed just as fast as it is received.

Where can it go? Only one place--outer space, the sky above! We know that anything that is warm shines in some sort of light ("radiates"). A lightbulb filament is hot enough to shine in visible light, but a hot teapot (say) also shines (radiates). Our eye cannot see such "infra-red" light, produced (at a much lower rate) by moderately warm objects, but a hand held close to the pot will sense the radiation, as heat streaming out. (Rattlesnakes have special sensors to detect infra-red (IR) light, helping them find warm blooded prey.)

So at first sight, this appears to be a simple situation. The Sun shines on the ground and gives it heat, and the ground returns that heat to space as invisible infra-red radiation.

But this simple process is made complicated by the so-called " greenhouse effect ." Air is relatively transparent, but some gases in it absorb and re-emit infra red very efficiently--water vapor, methane and increasingly, carbon dioxide, so called "greenhouse gases." Put a few drops of India ink in a glass of water, and it darkens appreciably with these gases, similarly, a little bit of those gases ABSORBS A LOT. By absorbing and re-emitting IR, they make the IR light bounce around, rather than letting it head straight to space.

That makes it difficult for heat to escape, and keep the ground warm something similar happens in a gardener's greenhouse, enclosed by glass panes, which let sunlight in but absorb IR. Such random bouncing-around continues until the IR (some of it, anyway) reaches a layer so high that not enough air and water vapor are left to send it back, and then the radiation escapes to space. That layer is known as the "tropopause" and it is typically 8 miles up.

This "greenhouse effect" would keep the air warm near the ground, even if our cars and power plants did not emit carbon dioxide (although those emissions make the effect more pronounced). The air then does a second thing to help getting rid of its heat: IT RISES.

Air cools as it rises, because air pressure around it is lower, and an expanding gas cools (that is how air-conditioners work). But because this process is happening everywhere and all the time, the SURROUNDING air is already cooler than the air near the ground. As long at air is warmer than its surroundings, it keeps rising. If a chunk of air started out extra-warm, it may well STILL BE warmer than the air around it, even higher up, so it keeps rising. Ideally, it rises until it arrives near the tropopause, where it can get rid of its heat. After that, being cooler, it sinks down again and is replaced by more heated air from below.

(Why does air pressure get lower as one rises? Because air near the ground is compressed by the weight of all the other air above it. If you rise about 5 kilometers--3 miles and a little bit more--half the air is below you, only half of it is above and contributes to the compression, and so, the pressure there is only half of what it was near the ground. Go up 5 more kilometers and the pressure is about half as much again--a quarter of what it is near the ground. That is where jetliners fly, and the low pressure is the reason their cabins are sealed and pressurized--also why climbers on Mt. Everest carry oxygen bottles).

This process of cooling, rising and finally radiating heat away is very important. THAT IS THE REASON WE HAVE WEATHER! The whole weather process is driven by the heat of the Sun, and by the collection of processes by which the Earth returns heat to space.

The above is very, very simplified, especially since it ignores humidity. Actual air also contains water vapor--water which was evaporated by the Sun and dissolved in the atmosphere, just as sugar dissolves in a cup of coffee. Since the Sun has provided heat energy for the evaporation, water vapor acts a bit like extra heat given to the air when the water is removed as rain, air gets that heat back and is warmed, which is what drives thunderstorms. More about all these in

### 119. Have any changes been observed on the Moon?

I happened to wonder if anyone has looked at the moon in the last 100 years or so and noticed a crater that 'wasn't there yesterday'. How many new craters have been observed and how big are they? That could kind of say things about safety HERE! We do have frequent meteorites, after all. I have even seen one myself

I once had an office on the same floor as a lady scientist, Winnifred Cameron, who very much wanted to find such changes. She used a special viewing device looking at two pictures of a region on the Moon, taken under similar conditions but at different times, flipping from one to the other and looking to see if anything changed. I don't think anything ever did. She was particularly interested in observations of a Russian named Kozyrev, who claimed to see glows.

### 120. Why isn't our atmosphere flung off by the Earth's rotation?

I have wondered for years how the earth keeps our atmosphere. The equator moves at almost 1000 MPH and the atmosphere is fluid. The fact that there isn't any wind (to speak of, at least resulting from the earth's rotation) says that the attraction of gravity is stronger than the centrifugal force trying to throw it off. Do we know if the amount of atmosphere is increasing, decreasing or remaining the same? It just seems to me that there should be a lot of turbulence in the atmosphere/space boundary region, although the 'emptiness' of space probably can't provide any drag on the atmosphere.

Concerning the atmosphere. the centrifugal force on the Earth's equator is just a fraction of 1% of gravity it makes the Earth slightly oval, but nothing falls off. The effect was found in the 1600s, when pendulum clocks accurate in Europe slowed down near the equator. Jupiter is bigger and rotates faster--so its equatorial flattening is larger, large enough to be evident in photos through the telescope.

### 121. Can kinetic energy be reconverted to work?

and have a question. Is kinetic energy available to do work later?

1. You zoom on your bicycle down a valley and gain kinetic energy. That energy can help you rise again on the upslope on the other side. Rising against gravity is doing work.

If however your car is of the new "hybrid" type with electric motors on the wheels (like the Toyota "Prius" or the Honda hybrid), by braking you connect the motors to the car's batteries. The motors act as generators and charge the batteries, turning your kinetic energy into chemical energy of the battery, which can be reused.

If however you link the shuttle to a conducting tether, as was done once (with some problems), the motion of the tether across the Earth's magnetic field lines creates a voltage, which may be tapped, e.g. for charging batteries. See http://www.phy6.org/Education/wtether.html.

The solution was a large flywheel, connected to the generator providing the current. When the current decreased, the generator acted as a motor (a bit like that of the hybrid car) and spun up a flywheel weighing a few tons. The next cycle, the flywheel provided most of the energy for generating the magnet's current, slowing down again only a little extra power was needed to make good friction losses. Thus the energy bounced back and forth between magnetic and kinetic.

Someone pointed out to me that the way the flywheel rotated was carefully chosen. In case the flywheel's bearings somehow gave way, its rotation was such that it would roll through the wall of the building and out into the field--not in the opposite direction, which would have brought it into the crowded accelerator hall.

### 122. Does any location get the same number of sunshine hours per year?

Is the period of light and dark the same for every place on earth over the course of a year? Example: Do Nome, AK and Rome, Italy get the same number of hours of light and dark per year (ignoring intensity differences).

Big question in our family recently!!
Thank You !!

A good question--and congratulations for having a family with such wide-ranging interests! Intuition seems to say "yes", but the challenge is to demonstrate it without any calculations. Below is one try.

Certain approximations are necessary. You ignore the actual size of the Sun's disk--but count as "light" times when the center of the Sun is above the horizon and as "dark" times when it is below the horizon. Also we ignore the ellipticity of the Earth's orbit, which causes the speed of the Earth's motion around the Sun to vary, but assume that it moves at constant speed. (About that variation, see Skepl2A.htm .

With those assumptions the Sun, too, seems to move with constant speed around the ecliptic over the course of the year. For the purpose of deriving average sunshine over the year, we can therefore replace it by an "average Sun" spread out evenly around the ecliptic. The question then becomes does this "average Sun" give equal hours of light and darkness to any point one Earth.

Assume first that the Earth DOES NOT rotate, so that your position--Rome, Nome or Pennsylvania--does not move. Your view from that location is then determined by your horizon, which in turn is determined by the plane tangential to the surface of Earth wherever you stand. You can call this "the plane of the horizon." Anything above that plane--exactly half the celestial sphere--is visible, anything below it is not. (It may be useful for you at this point to fetch paper and pen.)

The plane of the horizon cuts the plane of the ecliptic along SOME line of intersection, passing through wherever YOU are standing. In the plane of the ecliptic, the "average Sun" is a circle around your position, and any line through its center (=your place) is a diameter. This includes the above line of intersection. That line therefore cuts the "average Sun" into two equal halves--one above the horizon (which you see), the other below the horizon (which you don't see).

Over the year the Sun occupies equally every little bit of the circle. So half the time you see it, half you don't.

### 123. Speed of toy car rolling off an inclined ramp

This is my first time doing this. I am eleven years old and I have a science project that I need some help on. My dad built me a ramp for model cars. I want to prove that the speed of a car is determined by the weight of a car. I think that the lighter the car the faster it will go down the ramp. How do I prove this? Is there a formula? Please help me.

You are up against something very fundamental, something I hope you will remember in high school, when you study physics.

Every object has a WEIGHT, the force by which gravity pulls it down. A big stone has more weight than a small one. Weight is one way of measuring the amount of material in the stone, or its "mass." A big stone has much more mass than a small one.

However, if you drop them together, you will find that they fall equally fast!! This is, because each object also resists motion, and the resistance (called "inertia") is ALSO proportional to mass. That is why on a horizontal surface it takes much more force to get a bowling ball rolling than a tennis ball. The motion is horizontal, gravity is not too much involved, but the bowling ball has more mass and therefore much more resistance to being set in motion.

Say the big stone has 10 times the mass of the small one. It also has 10 times the inertia, and that inertia does not allow it to move any faster, even when the Earth pulls it down 10 times as strongly.

Model cars on a ramp (I suppose they are moved by gravity, like soap-box racers) obey the same rules. A heavy car is pulled more strongly, but also has more inertia, so the two should roll at the same speed. Try it! Put two toy cars--big and small--together on a slanting board, and let go. All other things being equal, they should move together.

### 124. Acceleration due to gravity

I am a high school physics student. My class was given a bonus assignment for the internet and I have yet to find an answer. I was wondering if you could help. I know that the acceleration due to gravity on the earth is

9.8 m/sec^2, but the class was asked to find a website that listed values of acceleration due to gravity at different locations on the Earth including acceleration due to gravity at our high school, Clarion-Limestone High School in Strattanville, PA. My question is:

What are values of acceleration due to gravity at different locations on Earth and what is the value closest to my school?

If you could help me out because I am really interested in finding out the answer, I would be greatly appreciative.

Have you just tried to ask Google or Yahoo for links concerned with "Acceleration of Free Fall"? I did so and got many leads

I am not really supposed to do your work--but look at http://www.haverford.edu/educ/knight-booklet/accelarator.htm Actually, the interesting questions are not "what is the number" but "why does gravity vary from place to place?" and "how do we know?"

It varies because the Earth rotates, adding a centrifugal force to the forces felt locally. That does two things: it makes the Earth bulge at the equator, so that points there are more distant from the center of Earth. And it adds there a force opposing gravity, so that the acceleration is smaller. Newton proposed that around 1690.

The process was experimentally studied by comparing the time kept by pendulum clocks ("grandfather clocks") at different locations. The period of the pendulum depends on gravity, and a clock which keeps correct time in Pennsylvania will probably run slow at the equator.

### Response

Thank you for the additional information on gravity. I actually did look at Google acceleration due to gravity but I guess I didn't look fully enough.

## Is gravity faster than light?

I've been wondering this for a while. I have read some science fiction stories where it is, and allows for instant communication over intra-system distances.

I did a quick google search to see if it had been resolved, and only got more confusion. From what I can see, nobody has done a real test due to technological limitations. Newtonian mechanics says gravity is instant, but general relativity says not so much.

So, is there information out there I missed? It seems like an interesting question.

According to general relativity, gravity travels at c, not faster.
speed of gravity

If one does a simplistic mathematical simulation of, say, the Earth's orbit around the Sun where the speed of gravity is c, it is true that the results to not correspond to reality (the Earth sling-shots off into wild black yonder) . However, a closer examination of general relativity reveal other factors that, when taken into account, do lead to results that reflect reality.

I've wondered this myself. If you put 2 masses a distance apart into an otherwise empty universe, when do they start moving towards one another?

Since we haven't yet found a way to turn gravity on and off, there isn't really any way to test this.

GR says gravity travels at the speed of light (that the speeds are equal is implicit in the theory). So, if the Sun were to disappear, the Earth would continue to orbit for another 8 minutes.

There has been at least one recent direct test of the speed of gravity, based upon the light from a quasar as Jupiter passed in front of it. The results indicate the speed of gravity is within

20% of the speed of light the speeds are consistent given the precision of the experiment. One news account can be found here.

Other direct tests include gravitational wave detectors, which have not yet detected a signal (but are only now reaching sensitivities that may allow a signal to be detected).

There is an experiment in place to determine if the predictions of GR are in fact true - LIGO.

Basically, it's a gravity interferometer. There's a detector in LA, and one in WA. The idea is that they will be able to detect gravity waves from massive emitters (say, two neutron starts rapidly rotating around a common gravity point), and then determine the speed.

Currently, according to GR, the theory is that gravity waves will travel at the speed of light. So, if your large planet is destroyed by, say, a Death Star, the moons will not leave the area until the effects of this mass being gone reach them. For our moon, it would be about 3 seconds.

Google LIGO for specifics on the best bet AFAIK of detecting gravity waves anytime soon. I think it's Large Interferometry Gravity Observatory, but I could be REALLY wrong. -- View image here: https://cdn.arstechnica.net/forum/smilies/biggrin.gif --

EDIT: The L is for "laser," but in my defense, it IS rather large. -- View image here: http://episteme.arstechnica.com/groupee_common/emoticons/icon_wink.gif --

Assuming, of course, that the planet is turned into energy and not merely reduced to its constituent atoms.

Assuming, of course, that the planet is turned into energy and not merely reduced to its constituent atoms.

Eh? Unless the constituent atoms of the planet somehow held themselves together after the planet was blown up, the moon would drift away from it's current orbit even if the planet's mass wasn't turned to energy

According to general relativity, gravity 'travels' at the speed of light.

As far as LIGO goes, it's not so much a method of testing GR. Everybody believes GR, and specific predictions of GR have been verified to extremely high accuracy. Studying gravitational waves, (I did a research internship at the Washington LIGO site last summer) is more about have another source of information about astrophysical events. It would be a big deal to detect a gravitational wave as it's extremely hard and no one has accomplished it yet, but most people generally think that at least one event will be detected in the next few years. Gravitational wave astronomy, while a long way off from becoming a major tool, would allow us to get information that we can't get from radio astronomy, such as things that go on in the interior of stellar bodies.

And being able to detect the wave/particles involved would help close up some giant holes in particle physics, right? Half the time that's all the news talks about.

You're thinking of gravitons, the postulated massless particle which is the carrier of the gravitational force. LIGO is not setting out to detect gravitons, but gravity waves, which is predicted by general relativity. Gravitons do not exist within the framework of general relativity. Gravitons are also essentially impossible to detect individually.

Well, photons are what makes up electromagnetic waves, and these are detectable individually as well as in waves. So I'm just extrapolating, without claim to know much about the state of experimentation here.

Edit: that nonsense about a detector the size of Jupiter is fascinating. So these things (if they exist) interact with all matter in the universe, but cannot be detected?

I guess photons are easy because we can tweak one emitter and receiver at a time: the atom.

Photons are easy because the EM coupling constant is large compared to gravity.

If gravity is a wave, space-time is like a very, very stiff spring -- the "spring constants" of ordinary matter are fantastically small by comparison. You can think of this much like an index of refraction difference. In that case, trying to detect the interaction of gravity waves with matter as trying to detect EM waves by looking at the scatter off of a vacuum/air interface, only the index contrast is still 3-4 orders of magnitude less. So, we are lucky that it looks like we can detect gravity waves, which are coherent excitations of many gravitons, but direct detection of a single graviton looks essentially impossible.

I think your calculation is a bit off.

Moon distance: 238,863 mi
Speed of light: 186,282 mi/s

/actually, I just wanted to post in the new forum! I didn't notice it until today.

This strays a bit from the topic at hand, but I can't resist.

Said constituent atoms would, for the most part, fall back together. Using the Earth as an example, anything that didn't get ejected at roughly Mach 23 (IIRC) would be sub-escape velocity, and would collapse back towards the center of gravity. For any reasonable weapon, some miniscule percentage of the planet's mass would be converted into energy (perhaps this explains the ring of light that all massive explosions apparently produce?), and the gravity of the system would be reduced by that amount ( c being notably higher than escape velocity under 1 g). Some mass would possibly be blown out of the system with sufficient velocity to escape the gravity well (of the planet, at least, though quite possibly not of the primary). But most of the mass would recollapse into almost as massive an object as was there initially, since it's got nothing better to do.

Obviously, one can theorize that enough energy is deposited into the planet to accelerate every bit of it up to escape velocity, in which case you'd most likely end up with a new (sparse) asteroid belt. But it seems wildly unlikely that you'd design a weapon to do that. After all, you can effectively destroy the planet just by depositing enough energy to shatter it - even when it recollapses into a spherical body of pretty much the same mass as the original planet, it's not like it's going to be habitable.

And even that would be sufficient overkill to make any political/military statement you'd care to, since that would already be vastly more than necessary just to kill everyone on the planet.

I think your calculation is a bit off.

Moon distance: 238,863 mi
Speed of light: 186,282 mi/s

/actually, I just wanted to post in the new forum! I didn't notice it until today.

Well, uh, I meant round trip? -- View image here: http://episteme.arstechnica.com/groupee_common/emoticons/icon_smile.gif --

If I was gravity, I would have stopped along the way for a few seconds to take some pictures. The view up there would be amazing. I'm just saying.

20% of the speed of light the speeds are consistent given the precision of the experiment. One news account can be found here.

I've read the news account, and I'm still slightly baffled by how this experiment worked.

Can someone explain it in more detail? Since it's the only(?) concrete test of the speed of gravity yet performed, it's quite important. Of course, it will be overshadowed by LIGO - when that's working .

There's an explanation on the linked MSNBC article, but I don't fully get it:

Jupiter moves across the field and distorts the apparent position of a distant quasar. Fine - Jupiter's gravitational field bends the radio waves from the quasar. And they measure the distortion from multiple points far apart on the Earth's surface, using the VLBA.

Then the article says (and I freely admit to losing the plot at this point):

"If the speed of gravitational propagation were infinite, the apparent position of the quasar should have moved in a perfect circle due to the bending of the radio waves, Kopeikin said. Instead, it inscribed an offset ellipse, shaped roughly as would be expected if the speed of gravity and the speed of light were equal."

## Q: What would it be like if another planet just barely missed colliding with the Earth?

Physicist: There’s a long history of big things in the solar system slamming into each other. Recently (the last 4.5 billion years or so) there haven’t been a lot of planetary collisions, but there are still lots of “minor” collisions like the Chicxulub asteroid 65 million years ago that caused that whole kerfuffle (65,000,000 years is practically this morning compared to the age of the solar system), or comet Shoemaker Levy 9 which uglied up Jupiter back in 1994.

Jupiter after a run-in with Shoemaker Levy 9. Each of those black clouds on the lower right is caused by the impact of a different chunk of the same comet, and each is bigger than Earth.

So while planets slamming or nearly slamming into each other isn’t a serious concern today, it was at one time. Of course, in solar systems where this is still a serious concern, there’s unlikely to be anything alive to do the concerning.

For the sake of this post, let’s say there’s another planet, “Htrae”, that is the same size and approximate composition of Earth (but possibly populated entirely with evil goatee-having doppelgangers with reversed names).

A direct impact, or even a glancing impact, is more or less what you might expect: you start with two planets and end with lots of hot dust. We’re used to impacts that dent or punch through the crust of the Earth, but really big impacts treat both planets like water droplets. Rather than crushing together like lumps of clay, Earth and Htrae would “splash” off of each other. A direct impact of two like-masses tends to destroy them both. A glancing, well-off-center, impact will “stir” both planets, leaving no none of the original surface on either. A glancing impact like this is the best modern theory of the origin of the Moon.

If Htrae were to fall out of the sky, it would probably hit the Earth with a speed that’s on the same scale as Earth’s escape velocity: 11 km/s (Probably more). The time between when Htrae appears to be about the same size as the Sun or Moon, to when it physically hits the surface, would be a couple of weeks (give or take a lot). The time between hitting the top of the atmosphere and hitting the bottom would be a few seconds. If you were around, you would see Htrae spanning from one horizon to the other. A few moments before impact the collective atmospheres of both planets would glow brightly as they are suddenly compressed. It’s more likely that in those last few seconds/moments you would be vaporized from a distance by the heat and light released by the impact, and less likely that you would be crushed. People on the far side of Earth wouldn’t fare much better. They’d get very little warning, and would have to suddenly deal with the ground, and everything on it, suddenly being given a kick from below big enough to go flying into space.

Generally speaking, being slapped by the ground so hard that you find yourself in deep space a few minutes later is seriously fatal.

A near miss is a lot less flashy, but you really wouldn’t want to be around for that either. When you’re between two equal masses, you’re pulled equally by both. You may be standing on the surface of Earth, but most of it is still a long way away (about 4,000 miles on average). So if Htrae’s surface was within spitting distance, then you’d be about 4,000 miles from most of it as well. Nothing on the surface of Earth has any special “Earth-gravity-solidarity”, so if you were “lucky” enough to be standing right under Htrae as it passed overhead, you’d find yourself in nearly zero gravity.

Earth and Htrae have an extremely near miss. Which way does the gravity between them point?

Of course, there’s nothing special about stuff that’s on the surface either. The surface itself would also start floating around, and the local atmosphere would certainly take the opportunity to wander off. On a large scale this is described by the planets being well within each others’ Roche limit, which means that they literally just kinda fall apart. It’s not just that the region between the planets is in free fall, it’s that halfway around the worlds gravity will suddenly be pointing sideways quite a bit. So, what does a land-slide the size of a planet look like? From a distance it’s likely to be amazing, but you’re gonna want that distance to be pretty big.

Even a near miss, with the planets never quite coming into contact, does a colossal amount of damage. There would be a cloud of debris between and orbiting around both planets (or rather around both “roiling molten masses”) as well as long streamers of what used to be ocean, crust, and mantle extending between them as they move apart. This has never been seen on a planetary scale, since all the things doing the impacting these days barely have their own gravity. The highest vertical leap on a comet would be infinity (if anyone were to try).

But the news gets worse. Unless both planets have a good reason to be really screaming past each other (maybe they were counter-orbiting or Htrae fell inward from the outer solar system or something), a near miss is usually just a preamble for a direct impact. All of the damage and scrambling that Earth and Htrae did do each other took energy. That energy is taken mostly from the kinetic energy, so after a near miss the average speed of the two planets would be less than it was before. And that means that the planets often can’t escape from each other (at least not forever). In fact, this is why Shoemaker Levy 9 impacted Jupiter a dozen times instead of all at once. Before impacting, the comet had passed within Jupiter’s Roche limit (probably several times), been pulled into a streamer of rocks, and slowed down.

What is a "Sun Synchronous" orbit?
(b) Why are satellites launched from near the equator?

(1) Why don't its particles separate by weight?
(2) What accelerates the solar wind?

Science Fair Project on the Size of the Earth

If you have a relevant question of your own, you can send it to audavstern("at" symbol)erols.com

### 114. Why not use a heat shield going up?

The other day a colleague asked me a question that I could not answer and which keeps intriguing me. "We know that (the shield of) a spacecraft that re-enters the earth's atmosphere heats up spectacularly because it is hit and slowed down by air-molecules. Is there a similar problem when it is going the other way, at take off? It is going through the same amount of air and acceleration looks quite similar to deceleration, only the other way around." The only answer that I could think of is that a good part of the acceleration might take place at higher altitudes where there is less air, and braking is done only by using the atmosphere.

Going up, it is the rocket engine which provides acceleration and energy. Because air resistance robs energy and is undesirable, the rocket deliberately rises vertically, to go though the denser atmosphere as quickly as possible. The vehicle gets most of its velocity, and almost all of the kinetic energy, at high altitudes where air density is too low to make a great difference. With the space shuttle "Columbia," even that might not have been enough: by the time it reached twice the velocity of sound, the atmosphere around it was still dense enough to rip a piece of foam insulation off its fuel tank, and it hit the orbiter with great force, breaking the heat shield.

On coming down (with spacecraft which we want to come down undamaged), the atmosphere is the brake absorbing the energy. We need that air resistance, and the heating is a result of absorbed energy! The big concern that energy should not be absorbed too fast, otherwise the heat gets too intense and may melt the heat shield and everything else. That is why the space shuttle comes down at a low angle, trying to stay as long as possible in a layer with the right density: If the shuttle comes in too high, not enough energy is lost, if too low, too much. The density is also important in supporting the shuttle, which--like a kite--needs part of the resistance to help it from coming down too fast.

### 115. When and where can rainbows be seen?

This may be a very unusual request but I need to know the sun's angular elevation at noon , winter solstice, at 42 degrees N latitude. Not necessary but curious same for summer solstice.

Believe it or not this all has to do with rainbows. I had an argument with someone whom I told that I have never observed a rainbow except in the morning or evening. The Sun has to be low in the east or west, and the bow then appears on the opposite side of the sky. Never observed one in the northern sky.

Could you help me out by answering these questions?

On the winter solstice, the noontime Sun will be 42 + 23.5 = 65.5 degrees from the zenith or 24.5 degrees from the horizon
On the summer solstice, the noontime Sun will be 42 - 23.5 = 18.5.5 degrees from the zenith or 71.5 degrees from the horizon

### 116. The unusual rotation of the planet Venus

I am a university student who was asked to research some common legends to determine if they are true. One of the "facts" which I am supposed to verify is:

" Venus is the only planet that rotates clockwise. "

Is the above statement true? And if so, why? Why would Venus rotate in the opposite direction of all the other planets?

"Venus' rotation is somewhat unusual in that it is both very slow (243 Earth days per Venus day, slightly longer than Venus' year) and retrograde. In addition, the periods of Venus' rotation and of its orbit are synchronized such that it always presents the same face toward Earth when the two planets are at their closest approach. Whether this is a resonance effect or merely a coincidence is not known."

"Why" is harder--I am not sure that anyone knows for sure. The sense in which all planets in our solar system (and the Sun) rotate is presumably the same as that of the cloud from which the solar system condensed. When that happened, presumably many fragments collided to create each planet, and some average behavior of those collisions produced the rotation. One might even speculate that Mercury and Venus, the ones closest to the Sun once had large outer gas envelopes, and the way these evaporated in the Sun's heat may have contributed to the loss of rotation. That, though, is just a guess.

### 117. Why not use nuclear power for spaceflight?

Why not use nuclear energy to power spaceflight? After all, few pounds of plutonium contain as much energy as thousands of tons of rocket fuel!

By Newton's laws, the forward momentum given to any rocket is always equal to the backward momentum given to the jet fired backwards. That momentum, in its turn, depends on two factors--how much mass is expelled by the jet, how many tons per second, and the speed with which it is expelled. Nuclear energy can supply the speed, but something must provide the expelled mass.

You might think next that given some source of mass (say, a tank filled with water), plentiful nuclear energy would make it possible to eject it much faster. But how? Rocket engines work by converting heat into directed motion, in a very efficient way, but they already run about as hot as available materials can stand. Nuclear energy could provide more heat, but no rocket engine could stand it.

Early in the space age a serious effort existed to build a nuclear rocket, getting its thrust by heating hydrogen with nuclear fission. A jet of hydrogen, coming from a rocket engine at a certain temperature, is much faster than a jet of burned rocket fuel, coming from a rocket engine at the same temperature. The reason is linked to the fact that hydrogen molecules are much lighter than those of any burned fuel.

However, the rate at which rocket engines used in spaceflight supply energy is enormous--e.g. the shuttle's engines burn a ton of fuel or more each second. The stresses are enormous, and the risk of nuclear material and waste products of fission getting into the atmosphere was too great, and so the project ended.

A visionary proposal of the 1950s proposed a "rocket" cabin with a strong flat plate on the bottom (oil would be sprayed on it for protection), and a trapdoor through which small nuclear bombs could be dropped, detonating some distance away and pushing the craft forward. On paper, it seemed feasible, but an actual nuclear test was deemed hazardous, sure to release contamination. The nuclear test-ban treaty of 1963 ended all efforts in this direction.
-------------------------

### 118. "Doesn't heat rise?"

I was helping my wife in her 3rd grade class yesterday and one of the students came up with an interesting question. We were talking about how it gets cooler as you increase your altitude (specifically in the mountains). I was guessing that the temperature drops about 3 degrees for every 1000 ft that you climb.

I was really caught off guard with a question from the "peanut gallery." It was - " Doesn't heat rise? " I said that that is correct, and conversely cold would tend to remain near the ground. He further questioned - "If heat rises then why wouldn't it get hotter as you increase your altitude? I had no explanation. Can you help?

That heat arrives when sunlight hits the ground and warms it up. Think of what would happen if no way existed for removing it! The ground would get hotter and hotter--oceans, lakes and rivers would boil away, life would become impossible. Actually, we see rather little temperature change near the ground--just day-to-night fluctuations, and slow changes with weather and seasons. Such observations suggest that on the average, heat is removed just as fast as it is received.

Where can it go? Only one place--outer space, the sky above! We know that anything that is warm shines in some sort of light ("radiates"). A lightbulb filament is hot enough to shine in visible light, but a hot teapot (say) also shines (radiates). Our eye cannot see such "infra-red" light, produced (at a much lower rate) by moderately warm objects, but a hand held close to the pot will sense the radiation, as heat streaming out. (Rattlesnakes have special sensors to detect infra-red (IR) light, helping them find warm blooded prey.)

So at first sight, this appears to be a simple situation. The Sun shines on the ground and gives it heat, and the ground returns that heat to space as invisible infra-red radiation.

But this simple process is made complicated by the so-called " greenhouse effect ." Air is relatively transparent, but some gases in it absorb and re-emit infra red very efficiently--water vapor, methane and increasingly, carbon dioxide, so called "greenhouse gases." Put a few drops of India ink in a glass of water, and it darkens appreciably with these gases, similarly, a little bit of those gases ABSORBS A LOT. By absorbing and re-emitting IR, they make the IR light bounce around, rather than letting it head straight to space.

That makes it difficult for heat to escape, and keep the ground warm something similar happens in a gardener's greenhouse, enclosed by glass panes, which let sunlight in but absorb IR. Such random bouncing-around continues until the IR (some of it, anyway) reaches a layer so high that not enough air and water vapor are left to send it back, and then the radiation escapes to space. That layer is known as the "tropopause" and it is typically 8 miles up.

This "greenhouse effect" would keep the air warm near the ground, even if our cars and power plants did not emit carbon dioxide (although those emissions make the effect more pronounced). The air then does a second thing to help getting rid of its heat: IT RISES.

Air cools as it rises, because air pressure around it is lower, and an expanding gas cools (that is how air-conditioners work). But because this process is happening everywhere and all the time, the SURROUNDING air is already cooler than the air near the ground. As long at air is warmer than its surroundings, it keeps rising. If a chunk of air started out extra-warm, it may well STILL BE warmer than the air around it, even higher up, so it keeps rising. Ideally, it rises until it arrives near the tropopause, where it can get rid of its heat. After that, being cooler, it sinks down again and is replaced by more heated air from below.

(Why does air pressure get lower as one rises? Because air near the ground is compressed by the weight of all the other air above it. If you rise about 5 kilometers--3 miles and a little bit more--half the air is below you, only half of it is above and contributes to the compression, and so, the pressure there is only half of what it was near the ground. Go up 5 more kilometers and the pressure is about half as much again--a quarter of what it is near the ground. That is where jetliners fly, and the low pressure is the reason their cabins are sealed and pressurized--also why climbers on Mt. Everest carry oxygen bottles).

This process of cooling, rising and finally radiating heat away is very important. THAT IS THE REASON WE HAVE WEATHER! The whole weather process is driven by the heat of the Sun, and by the collection of processes by which the Earth returns heat to space.

The above is very, very simplified, especially since it ignores humidity. Actual air also contains water vapor--water which was evaporated by the Sun and dissolved in the atmosphere, just as sugar dissolves in a cup of coffee. Since the Sun has provided heat energy for the evaporation, water vapor acts a bit like extra heat given to the air when the water is removed as rain, air gets that heat back and is warmed, which is what drives thunderstorms. More about all these in

### 119. Have any changes been observed on the Moon?

I happened to wonder if anyone has looked at the moon in the last 100 years or so and noticed a crater that 'wasn't there yesterday'. How many new craters have been observed and how big are they? That could kind of say things about safety HERE! We do have frequent meteorites, after all. I have even seen one myself

I once had an office on the same floor as a lady scientist, Winnifred Cameron, who very much wanted to find such changes. She used a special viewing device looking at two pictures of a region on the Moon, taken under similar conditions but at different times, flipping from one to the other and looking to see if anything changed. I don't think anything ever did. She was particularly interested in observations of a Russian named Kozyrev, who claimed to see glows.

### 120. Why isn't our atmosphere flung off by the Earth's rotation?

I have wondered for years how the earth keeps our atmosphere. The equator moves at almost 1000 MPH and the atmosphere is fluid. The fact that there isn't any wind (to speak of, at least resulting from the earth's rotation) says that the attraction of gravity is stronger than the centrifugal force trying to throw it off. Do we know if the amount of atmosphere is increasing, decreasing or remaining the same? It just seems to me that there should be a lot of turbulence in the atmosphere/space boundary region, although the 'emptiness' of space probably can't provide any drag on the atmosphere.

### 121. Can kinetic energy be reconverted to work?

and have a question. Is kinetic energy available to do work later?

You zoom on your bicycle down a valley and gain kinetic energy. That energy can help you rise again on the upslope on the other side. Rising against gravity is doing work.

If however your car is of the new "hybrid" type with electric motors on the wheels (like the Toyota "Prius" or the Honda hybrid), by braking you connect the motors to the car's batteries. The motors act as generators and charge the batteries, turning your kinetic energy into chemical energy of the battery, which can be reused.

If however you link the shuttle to a conducting tether, as was done once (with some problems), the motion of the tether across the Earth's magnetic field lines creates a voltage, which may be tapped, e.g. for charging batteries. See http://www.phy6.org/Education/wtether.html.

The solution was a large flywheel, connected to the generator providing the current. When the current decreased, the generator acted as a motor (a bit like that of the hybrid car) and spun up a flywheel weighing a few tons. The next cycle, the flywheel provided most of the energy for generating the magnet's current, slowing down again only a little extra power was needed to make good friction losses. Thus the energy bounced back and forth between magnetic and kinetic.

Someone pointed out to me that the way the flywheel rotated was carefully chosen. In case the flywheel's bearings somehow gave way, its rotation was such that it would roll through the wall of the building and out into the field--not in the opposite direction, which would have brought it into the crowded accelerator hall.

### 122. Does any location get the same number of sunshine hours per year?

Is the period of light and dark the same for every place on earth over the course of a year? Example: Do Nome, AK and Rome, Italy get the same number of hours of light and dark per year (ignoring intensity differences).

Big question in our family recently!!
Thank You !!

Certain approximations are necessary. You ignore the actual size of the Sun's disk--but count as "light" times when the center of the Sun is above the horizon and as "dark" times when it is below the horizon. Also we ignore the ellipticity of the Earth's orbit, which causes the speed of the Earth's motion around the Sun to vary, but assume that it moves at constant speed. (About that variation, see Skepl2A.htm .

With those assumptions the Sun, too, seems to move with constant speed around the ecliptic over the course of the year. For the purpose of deriving average sunshine over the year, we can therefore replace it by an "average Sun" spread out evenly around the ecliptic. The question then becomes does this "average Sun" give equal hours of light and darkness to any point one Earth.

Assume first that the Earth DOES NOT rotate, so that your position--Rome, Nome or Pennsylvania--does not move. Your view from that location is then determined by your horizon, which in turn is determined by the plane tangential to the surface of Earth wherever you stand. You can call this "the plane of the horizon." Anything above that plane--exactly half the celestial sphere--is visible, anything below it is not. (It may be useful for you at this point to fetch paper and pen.)

The plane of the horizon cuts the plane of the ecliptic along SOME line of intersection, passing through wherever YOU are standing. In the plane of the ecliptic, the "average Sun" is a circle around your position, and any line through its center (=your place) is a diameter. This includes the above line of intersection. That line therefore cuts the "average Sun" into two equal halves--one above the horizon (which you see), the other below the horizon (which you don't see).

Over the year the Sun occupies equally every little bit of the circle. So half the time you see it, half you don't.

### 123. Speed of toy car rolling off an inclined ramp

This is my first time doing this. I am eleven years old and I have a science project that I need some help on. My dad built me a ramp for model cars. I want to prove that the speed of a car is determined by the weight of a car. I think that the lighter the car the faster it will go down the ramp. How do I prove this? Is there a formula? Please help me.

Every object has a WEIGHT, the force by which gravity pulls it down. A big stone has more weight than a small one. Weight is one way of measuring the amount of material in the stone, or its "mass." A big stone has much more mass than a small one.

However, if you drop them together, you will find that they fall equally fast!! This is, because each object also resists motion, and the resistance (called "inertia") is ALSO proportional to mass. That is why on a horizontal surface it takes much more force to get a bowling ball rolling than a tennis ball. The motion is horizontal, gravity is not too much involved, but the bowling ball has more mass and therefore much more resistance to being set in motion.

Say the big stone has 10 times the mass of the small one. It also has 10 times the inertia, and that inertia does not allow it to move any faster, even when the Earth pulls it down 10 times as strongly.

Model cars on a ramp (I suppose they are moved by gravity, like soap-box racers) obey the same rules. A heavy car is pulled more strongly, but also has more inertia, so the two should roll at the same speed. Try it! Put two toy cars--big and small--together on a slanting board, and let go. All other things being equal, they should move together.

### 124. Acceleration due to gravity

I am a high school physics student. My class was given a bonus assignment for the internet and I have yet to find an answer. I was wondering if you could help. I know that the acceleration due to gravity on the earth is

9.8 m/sec^2, but the class was asked to find a website that listed values of acceleration due to gravity at different locations on the Earth including acceleration due to gravity at our high school, Clarion-Limestone High School in Strattanville, PA. My question is:

What are values of acceleration due to gravity at different locations on Earth and what is the value closest to my school?

If you could help me out because I am really interested in finding out the answer, I would be greatly appreciative.

I am not really supposed to do your work--but look at http://www.haverford.edu/educ/knight-booklet/accelarator.htm Actually, the interesting questions are not "what is the number" but "why does gravity vary from place to place?" and "how do we know?"

It varies because the Earth rotates, adding a centrifugal force to the forces felt locally. That does two things: it makes the Earth bulge at the equator, so that points there are more distant from the center of Earth. And it adds there a force opposing gravity, so that the acceleration is smaller. Newton proposed that around 1690.

The process was experimentally studied by comparing the time kept by pendulum clocks ("grandfather clocks") at different locations. The period of the pendulum depends on gravity, and a clock which keeps correct time in Pennsylvania will probably run slow at the equator.

## Disclaimer: The following material is being kept online for archival purposes.

Listed below are questions submitted by users of "From Stargazers to Starships" and the answers given to them. This is just a selection-- of the many questions that arrive, only a few are listed. The ones included below are either of the sort that keeps coming up again and again, or else the answers make a special point, often going into details which might interest many users.

### 346. Harry Paul Sprain's perpetual Motion Device

Thank you for returning my call. and for our conversation this afternoon [in which I told you about the device].

An early prototype can be viewed on YouTube by clicking on one of the following links:
or

To provide you some of my background and experience, my C.V. is attached. I look forward to talking with you again.

My advice is, stay away from this device. Do not invest good money in it.

All it uses is electromagnetic forces, and electromagnetism is well known to obey the conservation of energy (I vaguely recall something known as Poynting's theorem which expresses this mathematically).

I looked at the video, and also looked at web pages about Harry Paul Sprain's gadget. The rotating arm in your video has an electric connection, and the argument is that yes, a magnet is indeed being pulsed once each rotation, but the pulse provides less energy than is extracted. That I find hard to believe--especially since no explanation is given WHY it should happen, contrary to physics expectations.

On the mantelpiece above our fireplace in the living room is a gadget bought in California. It has a 3-armed flywheel with weights at the ends of the arms, and its axle rests between a pair of parallel supports, whose top follows a shallow curve. You would expect the wheel to rest at the bottom of the curve, but in fact, when set up, it rolls back and forth. as if something has endowed it with perpetual motion. That "something" is a battery hidden inside the base, connected to an electromagnet below the lowest part of the curve. That magnet is usually turned off. Some clever electronics, however, sense when the wheel comes close to the bottom (the weights contain iron) and then give the magnet a short burst of current, for a quick pull on the wheel, making good the energy loss to friction and helping it rise to its previous height.

Sprain's gadget seems to be something similar, and you may discuss this with physicists at the university.

### 347. Can the plasma that fills space help spaceflight?

With space filled with plasma (charged particles from suns) is it feasible to create electromagnetic space propulsion systems ?

Would these be similar to the solar sails already in use by several space agencies ? Or could they be similar to the electromagnetic propulsion systems created for water vessels (I think back in the early 90's) ?

I cannot guess all possible applications, but my first guess would be, it is probably not feasible . To create a force forward, something else needs to be pushed back , or at least something moving needs to be stopped.

A solar sail stops sunlight, or reflects it back, which doubles the force (and actually it's best to deflect it by 90 degrees, to increase the orbital velocity around the Sun). The solar wind also can be stopped--I don't know how its pressure compares, I suspect it's less than that of sunlight.

But to make use of the surrounding plasma, you need some electric circuit which pushes the plasma back. It is hard to channel an electric current when all space around you conducts electricity! It would take a lot of energy, too. Ion rockets indeed push back plasma, but the plasma is generated in the spacecraft itself, where its motion can be controlled before it is pushed away. The energy then comes from solar cells, and the acceleration is very gradual. See
http://www.phy6.org/stargaze/Sionrock.htm

### 348. Spiral arms of our galaxy

". the Sun has been in and out of the spiral arms often in the twenty times it has gone around the Milky Way at 200km/sec. "

(Sender's location: Lat: S 33.940 Lon: E 18.766 )

That was however before the impact of dark matter on galaxy rotation was appreciated:
Nowadays many astronomers believe that galaxies are drawn together by blobs of invisible "dark matter" of yet-unknown nature, and that new factor might perhaps indicate a completely different cause to spiral arms.

Stay around for a few years, or decades. Maybe we'll know than.

### 349. What powers a glider?

We are having a discussion of what powers a glider.

I keep insisting that while gravitational potential energy controls the motion, gravity isn't its source of power or energy, while a few others insist it must be powered by gravity.
Who is right?

Gravity of course pulls the glider down, but as it begins falling, it acquires some speed, and that speed creates lift on its wings, which slows down the downward motion. As explained in
http://www.phy6.org/stargaze/Sflight.htm
both lift and air resistance (" drag ") are approximately proportional to the velocity squared, which means they should be proportional to each other .

When the two forces are in equilibrium with gravity, the glider slowly descends--say, 1 foot for every 19 feet moving forward. The lift of the wing balances 19/20 of the weight of the glider, so only 1/20 of the gravitational potential energy is released each second, and is used up to overcome drag. In fact the motion is very much like sliding down a 1:19 slope.

Soaring birds and experienced pilots know that air often rises faster than that --air rising from a sun-heated parking lot, or from a ridge hit by a steady breeze. By maneuvering again and again into the rising stream they can actually gain altitude. The source of energy is then heat --of sunlight on asphalt, or of the distant weather system which created the wind. See also
http://www.soaringmuseum.org

### 350. UFOs

Is the Government of the United States, or any of its agents, contractors, assignees, etc. in possession of any empirical evidence of any kind, which would serve to substantiate/prove the existence of intelligent extra-terrestrial life forms?

Notwithstanding my BA in English Literature, I identify far more with the scientific community than with the groupie/fringe community of self-proclaimed UFOlogy experts. That's why I don't believe that I've been abducted, or that an alien object is implanted under my skin somewhere. even though I've been experiencing this awful pain just above my left wrist!

I cannot speak for the US government or anyone else in the country. But as scientist I can only say that if such evidence existed, it would create tremendous interest and could not be kept secret. In fact, there would be no reason to conceal it, on the contrary--more people are probably worried that we may be alone in the universe than that we are not , and there have existed scientific searches for such life--maybe some still continue.

I don't know what you and your companion saw--especially at night, lots of things can fool the eye and the mind. But the fact is, no reliable sign of alien intelligence has ever been identified. There used to be a big USAF project, and you may read about it in
http://en.wikipedia.org/wiki/J._Allen_Hynek
and in links from there. As far as I know, while life (intelligent or not) in the distant universe is still an interesting target, our government has given up on finding any alien UFOs here .

### 351. Maximum speed for propeller-driven airplane?

1. If so what happens to the "vehicle" being driven? Does it just stay at that speed or does the inefficiency now a cause a reduction in speed?
2. In this situation (if it exists where the prop can do no more) does the now flailing prop develop a heat problem?
3. Do engine makers and prop makers create governing processes to preclude this occurrence and what happens if a prop designed for 400 MPH is driven at 550 MPH (or equivalent rpms)?

There definitely exists a maximum speed for propeller-driven airplanes, now quoted as 528.33 mph and held by a converted WW-2 fighter:
http://en.wikipedia.org/wiki/F8F_Bearcat (see at the end there)
As explained on my web site (stargaze/Sflight.htm), as airplane speed increases, the angle of the blade relative to the airstream becomes increasingly inefficient, and while blade tips may go supersonic (very noisy!) it still does not help. Whether the pitch is variable is not important--a variable pitch helps the airplane get to its top speed, but it is the final pitch setting that counts.

The prop does not flail, and certainly does not overheat (air flowing by cools it!), but the thrust it gives levels off. In any airplane, thrust levels off--if the designed cruising speed is 400 mph, the pilot can probably fly a little faster, but will burn a lot of fuel and perhaps overstress the airframe.

The earlier record, about 100 mph less, was set by a German Luftwaffe pilot in 1939, and he was quoted saying "I could have gone faster, but the propeller would not let me." Marine props have a different problem--cavitation, bubbles which limit operation.

### 352. The speed at which gravity spreads

Enjoyed your website, though a latecomer. We know the EM wave travels at the speed of light. But how about gravity? If this is a valid question to ask--how fast does a "gravity field" travel?

For example, if the Sun were to disappear suddenly , will we fly away in darkness (as would happen if the gravity spread faster than the light, or instantly)? I happened recently to read something about a "torsion field" that, according to the writer, travels at the 10 9 c. I am not sure that is at all true science. but it started me thinking as to whether there is such a thing as the speed of gravity.

General relativity (another name for Einstein's extension of Newton's theory of gravitation) is not my field, but I am pretty sure that gravity (like electromagnetic and other fields) spreads at the velocity of light . The Sun will not suddenly disappear (mass-energy is conserved). However, some cataclysmic event (perhaps a big supernova collapse or the merging of black holes) may occur and generate gravitational waves , and these apparently travel at the speed of light. It is a tiny effect, but sensitive instruments are already in place to detect such waves, if their source is close enough. As of today, none was confirmed.

But consider a simpler example (maybe too simple--general relativity may have something to say here too). Earth orbits the Sun in a near-circular path--let us simplify the picture and assume it is an exact circle . The Earth is attracted by the Sun's gravity, which provides the force to maintain the curvature of the orbital circle, or to balance the centrifugal force, whichever way you want to put it.

However, what IS THE EXACT DIRECTION of that attraction? If gravity spreads with the speed of light, its pull is parallel to sunlight--directed to there we SEE the Sun and thus perpendicular to the orbit.

However, that same sunlight took 8 minutes to reach us, and meanwhile Earth advanced a fraction of a degree in its orbit. The "real" position of the Sun in the sky should therefore be a small distance AWAY from where we see it, along the ecliptic and towards Zodiac constellations where the Sun had been earlier in the year. If gravity spreads INSTANTLY, its pull should be towards that "real" position.

It is ALMOST the same direction, but not quite: the force will have a small vector component (see http://www.phy6.org/stargaze/Svector.htm) OPPOSING the motion of the Earth, and diminishing its energy, causing it to spiral towards the Sun (faster and faster, by the way). The fact you and I exist suggests this does NOT happen, and that gravity does not spread instantly. In fact, any velocity other than that of light would cause the Earth's motion to lose or gain energy.

### 353. Layers of the Earth

Dear Joe
Most of our information about layers of the Earth comes from the study of earthquake waves (other sources also exist, especially from the chemistry of minerals). The outer layer on which you stand IS the crust , below it is the rocky mantle (with inner and outer mantle), then halfway to the center begins the core , much denser, liquid and probably mostly iron. Inside this is a solid inner core--believed to have solidified from the liquid, because the iron (at its high pressure, from so much weight on top) is not too far from its melting point.

You can read about it on a web page, part of a new set on the solar system
http://www.phy6.org/stargaze/Searth.htm

The crust however is far from even, and in particular, it (luckily!) contains large lighter slabs , the continents , floating on top of denser rock which forms the sea bottom. The slabs of the crust slowly shift position ("plate tectonics"). If you want to pursue the evidence about this, you need to read sections on magnetism in "The Great Magnet, the Earth"
http://www.phy6.org/earthmag/demagint.htm
especially #15 and maybe also #12 and #13, and perhaps others.

Now a request . By the end of 10th grade, students should have covered a lot of that, especially if they had a course on Earth science. If it was not covered, please show this letter to your teacher, perhaps these web sites can help shape future curriculum. Also, tell your friends in school--they can learn a lot from this web material, even on their own.

### 354. Why doesn't the sky fall on us?

Hi, there
What is "the sky", anyway? All there is above you is just air , held by gravity and compressed by its own weight.

Where the jet planes fly, most of the air is below them , so there is less weight compressing the air and it is only about a quarter as dense (which is why airliner cabins are sealed and maintain pressure). Still higher there is even less air, and only a trace remains at, say, 60 miles (about 100 kilometers). We are like fish at the bottom of an ocean of air, except that water has a top surface where it ends, while air decreases gradually.

Above that, there is nothing--just empty space , with Sun, Moon, planets and distant stars, which are really far-away suns. A few atoms and electrons fly around there, but otherwise, NOTHING.

So what is "the sky"? In the daytime you see it blue , because air scatters sunlight, especially the blue part of sunlight. Instead of the blue coming directly from the Sun, some of it comes from all directions (and with less blue, the Sun looks more yellow than it would look, say, from a spacecraft above the air). But there is nothing there except a layer of air.

On a clear dark night you can really see how empty "the sky" is. Air is transparent, so you see the stars in all their glory, and there is hardly anything between you and them.

### 355. Imagine a non-rotating Earth

Dear Dr. Stern,
My name is Luke and I was wondering if there is any way you could stop Earth's rotation and create a perpetual desert? What I mean, is it theoretically possible to have one half of the earth perpetually in darkness and one half perpetually in light from the Sun. If this were possible, would this create a desert on one side of the world and freeze the other side?

I've been wondering this for quite awhile and when I found your website, "From Stargazers to Starships" , I thought I'd send you an email and ask you. Oh and when I say "stop," I don't mean stop abruptly, I mean slowly decelerate until it stops rotating. I hope you will answer my question even if it sounds bizarre or absurd, because its been bugging me for awhile.

Dear Luke
The Earth's rotation can only be changed by a force coming from outside it. Tides raised by the Sun and Moon do that, but it is an extremely slow process, probably taking billions of years.

It would still be interesting to simulate such a state on a weather-modeling computer , and I would not be surprised if someone had already done so. I gather you mean here an Earth undergoing one rotation per year and so presenting the same face to the Sun at all times. If ALL rotation stopped, sunlight would slowly circle the Earth.

The problem is complicated by the atmosphere . If Earth's atmosphere were rarefied , all water on the sunward side would evaporate and condense again as an ice-cap on the dark side, while the day side would indeed be a hot desert. On the boundary between the two air would circulate vigorously, transporting heat from day to night.

If the Earth's atmosphere were very dense , we would have something like Venus, whose temperature is more or less uniform (and hot) even though it rotates very slowly. Heat there is evenly distributed by winds, which have much more mass than those on Earth, and by the "greenhouse effect" in which molecules in the atmosphere transport heat by absorbing it and then re-radiating it in a random new direction. Which of these situations would resemble a non-rotating Earth I cannot even guess.

### 357. US Flag on the Moon

There is no air on the Moon, therefore no wind can blow, and a normal flag would hang limply down (the pressure of the solar wind is w-a-a-a-y too weak).

Unless, of course, it had a sleeve sewn into its top and a rigid cross-bar threading it and connected to the flagpole. Indeed, if you look at pictures of Apollo flags, you will notice how straight their tops are, like the tops of a coffee-shop curtains, and for the same reason.

Clarification I understand how they got the flag to look like it was blowing in the wind, but that's not exactly what I was asking. I wanted to know how they got the flag to stay implanted in the moon. .
Reply I guess you can stick a flagstaff into the the dry "regolith" (=ground-up debris) covering the Moon, just as you would stick one into beach sand. No wind will blow it down!

### 358. Rope stretched across a long lake.

Say the lake in Norway is 120 km long. One may ask, "If we have two points at the same elevation but 120 km apart, and draw a straight line between them, how deep is the deepest part of the line?"

The deepest point is in the middle, 60 km from either end. Suppose its depth there is D, while R is the radius of the Earth. You draw your diagram and use the theorem of Pythagoras :

R 2 = 60 2 + (R – D 2 )
= 60 2 + R 2 – (2R – D)D
giving approximately
2RD = 60 2 = 3600
D

3600/2R = 3600/12740

However, a ROPE through the lake will never be straight! Its weight will pull it down in the middle, probably all the way to the bottom, even if the lake is deep.

I am sure you have seen electric trains whose power is taken from an overhead cable. The electric cable must be absolutely level, otherwise the frame connecting to it jumps up and down as the train moves. One cannot stretch the cable so tight that it will be absolutely straight, so (at least in this country) there are always TWO cables, one above the other.

The top cable is steel (probably steel that does not change much in heat and cold) and it curves from the weight it supports. The bottom cable is copper to carry the electricity, and it hangs from the steel by short wires. Those wires get a little shorter in the middle between the towers that hold the steel cable, and are so adjusted that the second cable is straight, even if its supports are not.

### 359. About studying electronics in the USA

(Message from India)
I am really very satisfied with your answers to my queries. I am also very much satisfied with the outstanding contents of your site.
I have already appeared for my 12th [grade] science exams and they were good and now I want to apply for further studies at California university.

I am going to stay there with my aunt and so I will not be paying my accommodation fees and hope to pay my university fees through part time jobs. I want to study electronics and communications there. Is the future for this field going to be good? Please also suggest me some good and reasonable universities for this course. I could not contact you for the last year as I was busy with my 12th grade studies.
Last time you advised me to improve my English. Is it ok now, or do I need to still improve it?

I am not in a good position to advise you on a university in California--it depends among other things on what part of California you will be in. Please realize, first, that university can be expensive, California and other states have special fees for students from that state, but all others pay more, and the best universities are private and expensive. In addition, your visa will probably state that you are not to work. Make sure all legal issues are taken care of.

Finally, consider a good university in India, too. It used to be that American universities were automatically considered better, but for many years that is no longer true. There exist some very good universities in your country--in fact, the best ones (I understand) have competitive entrance exams and you need succeed well in those.

Personally, I am about 4000 km from California, and it is hard to recommend. However, you have the internet, and you can collect information from it and find out about locations, about what various universities consider their strong subjects, about fees and entrance requirements, legal requirements for visas and dates for registration. You really should have started your search 1-2 years ago!

Some web sites:
http://www.calstate.edu/
http://www.calstate.edu/search_find/campus.shtml
http://www.collegebound.net/content/

Most important--ask professionals in India for advice, both those who studied in the USA and those who have studied in India.

### 360. Publish Stargazers as a book?

At one time I tried to publish in print the smallest of my collections ("The Great Magnet, the Earth") and found the effort time-consuming and discouraging. In any case, the future of books probably lies with the computer . Whether future "books" will be "printed to order" as some books are now, or read by compact devices like "Kindle," I do not know. Whatever replaces them should look like a book--not be videos or flashy illustrated stories (too much eye candy distracts), but very much like paper books used now, which is what I try to produce. Perhaps they will allow space to take notes and comments--no need to write them on the pages of a paper book.

As it is, my efforts started in 1995, and the collections are still much in use (about a million pages each month), while most books go out of print much faster. The computer allows extra features, too--internal and external links, optional extensions, glossaries, time-lines, math supplements, links to hundreds of questions-and-answers like this one, translations and greatly expanded material.

All this is very easily downloaded to a CD as "zip-archives," compressed collections which after downloading are split up by the computer into the same files as the ones you find on the web. The central link is
Then go to
http://www.phy6.org/stargaze.zip
http://www.phy6.org/earthmag.zip
http://www.phy6.org/Education.zip

You can make a disk for each of your 6th graders (and also download the archives onto the home computer for off-line use). Let them explore on their own!

### 361. Charging of Earth by lightning: + or – ?

The Earth as a whole (including atmosphere) has zero electric charge . Any charge it acquires--for instance, from positive cosmic ray ions which hit it all the time--is quickly neutralized, by ions and electrons ejected from the top of the atmosphere (positive "polar wind" ions or negative photoelectrons), or else by charges drawn from surrounding space. In either case, the excess electric charge is what creates the attracting or repelling force.

However, there exists a charge separation in the atmosphere itself --high layers have extra positive charge and those near the ground have negative charge, which also determines the rate of voltage change near the ground. The separation is due to the process responsible for lightning, and is therefore greatest in thunderstorms and near them. Under quiet condition this voltage also spreads quite far horizontally, although far from its source it cannot drive any appreciable current, because air is such a good insulator. That makes such voltages hard to measure! The mechanism itself is described in
http://www.phy6.org/stargaze/Svandgrf.htm

### 362. If no stars were seen--could Earth's orbital motion be discovered?

(abbreviated correspondence)
First of all my congratulations for your excellent web pages: they are a reference point for everyone of us who wants to understand with few logical passages what is behind a physical phenomenon.

My question : we are able to demonstrate that the Earth rotates because there is the Coriolis force that accounts for the swirling water or the deviation to east of falling bodies.

Is there any equivalent proof by which we could tell the Earth orbits the Sun, without considering the seasons and the position of the stars? The only non inertial force that I can see here is the centrifugal force, but since it is perfectly balanced by the gravity force that pulls the Earth to the Sun I wonder how we can devise an experiment to prove that the Earth is orbiting around our star.

Your question reminds me of a very early science-fiction story by Isaac Asimov, " Nightfall ." It is set in a civilization like that of early Mesopotamia, except that the planet is in constant sunlight by two separate suns. When by a rare chance both suns are eclipsed and the stars (a very bright cluster) are first revealed, mass hysteria descends.

Of course, motion with two centers of attraction would not be stable enough. But suppose Earth were like Venus covered by dense clouds limiting our vision. Could we tell it orbited the Sun--short of sending a rocket above the clouds?

In a non-inertial frame, a non-moving compact object is in apparent equilibrium--only its own motion reveals any non-inertial character of its surroundings, by the Coriolis force. The human body is not sensitive enough to sense tiny forces, so it may fail to detect any difference even if a moderate amount of motion exists. A blindfolded astronaut in a space station will just feel "weightless," and a human on Earth won't sense any motion either. If you looked at the example of a draining bathroom sink (below the picture of the hurricane) in
http://www.phy6.org/stargaze/Srotfram.htm
you will realize that on the scale of the human body, the effect is negligibly small, even for the rotation of the Earth.

However, sensitive instruments are much better in telling if a system is inertial or not. A freely suspended spinning gyroscope points in a fixed direction, so if one is available, that is like having a fixed star in view. On a space station its axis (if in the orbital plane) should rotate 360 degrees every orbit, about every 90 minutes. On Earth it should move around a cone of directions in about 24 hours, as if it pointed at some star.

Suppose Earth was permanently under cloud cover, allowing one to see variations of light between day and night, but no more, and therefore guess at the direction of the Sun. The axis of a freely suspended gyroscope pointing at the Sun today should rotate relative to it in a single year. Since the year is quite long, one must be sure the suspension is free enough to isolate the axis from motions of its surroundings, and to eliminate other possible sources of such rotation. That can make a challenging technical problem (and possibly, a cute science fiction story!).

### 363. Local Solar Time in Iceland

I am trying to derive the actual solar hour of a place . Some websites state that you need to calculate magnetic declination of the place first. Is that true?
In my case, the location is Iceland , relatively close to the North Pole--65° north and 16-21° west. Iceland is 16-21° from the Greenwich longitudes, but its time zone is that of Greenwich, which seems wrong. Its magnetic declination is very high, so I need to know if that should be taken into account when finding the local solar time. Can you assist me with your knowledge on astronomy?
Is actual solar time only measured by longitude, so that to get the solar time, for each degree of longitude difference from 0° +/– one must add 4 minutes to Greenwich time (or subtract from it)?

I found this calculator here and wanted to ask you if it is accurate and if magnetic declination would be needed to find actual solar time:
http://www.jgiesen.de/SiderealTimeClock/index.html

Solar time depends on the location of the Sun in the sky and has nothing to do with magnetism of the Earth. As indicated on the web site you gave, solar noon is when the Sun "passes the meridian"--when it is exactly south, and is highest above the horizon (for that day).

It only depends on longitude , since it really measures the rotation of Earth around its axis and indeed shifts by 4 minue for each one-degree shift of longitude. Your latitude determines where the Sun actually is in the sky (it could even be night with the Sun below the horizon!). If the Sun is above the horizon, a sundial (http://www.phy6.org/stargaze/Sundial.htm) should indicate solar time, except for a small correction "the equation of time" included on your web site, which equals the difference (some minutes) between actual "solar noon" and "mean solar noon." Mean solar noon assumes all solar days are equal to 24 hours: actually, small differences exist, because of facts like the Earth's elliptical orbit.

Noon by the clock is averaged over large strips of latitude, typically 15 degrees wide, which is the angle the Earth rotates in one hour. When traveling across the sea of the land you therefore need to adjust your clock only when you pass from one "time" zone" to another, usually by one hour. See http://www.phy6.org/stargaze/Slatlong.htm.

About Iceland. I do not know. The web site you cited gives the local time zone for Reykjavik an hour behind Greenwich, as it should be for locations 16 to 21 degrees west of Greenwich. You wrote that actually Greenwich time is used , but maybe that is done out of convenience, people actually use the west European time zone to fit with local time in Scandinavia, Britain and other European neighbors. After all, time zones can adjusted for the convenience of users: all China has the same time zone, even though the country covers several zones, and the time zone of India is 5.5 hours ahead of Greenwich, since a standard time zone boundary would cut the country in half.

### 364. Is orbital motion same as free fall?

Your question has more to do with use of language than with science. A swimmer exerts a force on the surrounding material, which the Moon etc. do not so "swimming" is not appropriate. "Floating" was originally applied to light objects on top of a fluid, like a cork on water and in this sense, astronauts in space do not float.

People however also use this word for a state of being suspended without having to apply any force, just as the weight of the cork is balanced by the pressure of the water. In this sense, astronauts FEEL as if they were floating.

"Free fall" suggests motion subject to no force except gravity, and in that sense, the Moon is in free fall around the Earth. However, free fall is not always straight down. That is only true for objects starting with no velocity of their own. If you give an object some velocity (say, throw a ball) its path is curved, and with enough velocity--like that of an artificial satellite, or the Moon--the path is curved enough to wrap around the Earth.

### 365. Build a straight tower in warped space?

I was taken to one of your pages from a Google search to answer "if we build a straight structure, will it circle the globe?"
My question has to do with the curvature of spacetime, the shape of the earth, and the notion of "straightness." I'm submitting it here because I'm not sure where else I could possibly get an answer. I looked up "Spacetime" in "Wikipedia" and found the statement "In general relativity, it is assumed that spacetime is curved by the presence of matter (energy)" That seems a tough pill to swallow, after being raised on Newton's explanation of gravity being just an attractive force.

If we were able to build an ideal structure of unlimited length that was perfectly straight when measured with some theoretical ideal device, would it wrap around the earth and join with itself, or would it extend out into space like a tangent to the earth's surface (ruling out the need to support its mass)?

I am largely ignorant of general relativity, and therefore some of the things below may be wrong. But let me try.
I suspect that you indeed put your finger on the core question , when you wondered about the definition of " straight ." Euclid defined a straight line as the shortest distance D between two points, and distance D could be defined by the theorem of Pythagoras

When you deal with cosmic distances , however, time creeps into the definition too. You look at a galaxy a billion light years away, but what you actually see is where it was a billion years ago. You have no way of measuring the path to it instantly. This and other things led Einstein to modify the prescription for distance ("metric tensor" or "metric" for short) to involve time as well, and make "space-time" separation rather than just space distance the foundation of physics. Actually, time is a different kind of dimension--a "time-like" dimension--since all its applications involve the square root of (𔂿), and that requires using a wider definition of numbers. It. That was the special theory of relativity .

General relativity concluded that the presence of mass warped the metric (which is here a property of actual space--not just a mathematical artifact), causing (for instance) the path of light to curve and such effects as gravitational lensing. The strange arcs seen by telescopes suggest that even though light reaches us by the shortest route, that route is not the same as it would be in the absence of gravity. It is indeed the path a laser beam would take, and is as straight as anything can make it. However, you need extremely intense gravity to make a noticeable difference in the metric: the effect of the Sun's pull during a total eclipse in 1919 was barely noticeable. You need come close to a black hole to make space-time curve significantly, even bend a ray path onto itself.

Let me stop here. You may ask someone more closely familiar with general relativity for a clearer view, but I am not sure it can be done without math. The late John Wheeler wrote "Spacetime Physics" which gives the nitty-gritty, but I'm not sure you are ready for it.

## Talk:Gravity/Archive 6

I visited this page specifically to remind myself of the formula for the gravitational attraction of two bodies. To my mind, that's a fundamental formula in any discussion about gravity. —Preceding unsigned comment added by 91.153.156.36 (talk) 14:53, 7 November 2007 (UTC)

I agree, I didn't find any reference to the page gravitational_constant anywhere in this article, even though the earth's gravitational constant is mentionted without a second thought. --KWifler —Preceding unsigned comment added by 24.22.176.33 (talk) 07:19, 30 May 2008 (UTC)

My friend says that Gravity is only an unproven theory and that Centripetal Force is what keeps everything on the Earth. LOL! --Can Not 01:19, 26 September 2007 (UTC)

The article is now 47 kilobytes. The maximum is 32 (see: article size). I don't know who keeps putting the merge requests up, but this article needs to be chopped up. Rules are Rules. 32 kilobytes is the maximum. If I end up doing it, it's going to be sloppy. I would hope someone who has worked on this article would do it soon. I would suggest breaking this up into at least five pages that way some of us who have interest in certain areas can contribute here or there as we please.--Sadi Carnot 04:45, 9 March 2006 (UTC)

I agree. As a casual reader of science topics and Wikipedia editor, split this baby up. As indicated, splitting off the Newtonian/math section seems appropriate and fairly simple. I'd also suggest keeping quantum mechanics and quantum particle physics out of this altogether, with only brief linking statements about the deeper implications of gravity and attempts at a unified theory. Go ahead and do it Sadi, be bold. DavidH 07:22, 10 March 2006 (UTC)

How about using a /Gravity_page to partition out this idea? --Ancheta Wis 10:41, 10 March 2006 (UTC)

7 Problems with Newton's theory 7.1 Theoretical concerns 7.2 Disagreement with observation 7.3 Newton's reservations 8 Einstein's theory of gravitation 8.1 Experimental tests 9 Comparison with electromagnetic force 10 Gravity and quantum mechanics 11 Alternative theories 11.1 Recent alternative theories 11.2 Historical alternative theories 12 Notes 13 See also 14 References 15 External links

Thanks for the support and ideas, keep them coming (however we can't have 30 headers). Last week I broke up the (over-the-limit) love article into four pages, and that seemed to go fairly well:

If I break up this one, I will likely outline it similar to that of Britannica’s (CD-ROM) article on gravitation. They do a redirect from gravity to gravitation this is the way it is done in most physics textbooks as well. Britannica has over 200 year’s experience on this matter. Similarly, from the force article, as we all know:

There are four known fundamental forces in nature.

acting between subatomic particles between electric charges arising from radioactive decay between masses

Hence, when people look up “gravitation”, “gravity, “force of gravitation”, “gravitational force”, etc., for the sake of organization (without getting into some esoteric debate), we would redirect all to one page then on this main page, i.e. Gravitation, being short in length, give the following overview of gravitation (with branch-off points throughout):

• Recent theories
• References

This is rough outline (maybe with one more section?). We would then put the rest in the See also section, thus keeping the main page very simple. In this manner, it will have room to grow. If we have any grave objections we can always revert. Again, if someone else wants to do the break up then by all means do so. Are we all relatively O.K with this?--Sadi Carnot 17:50, 11 March 2006 (UTC)

Wouldn't it be useful to separate Gravity from Gravity theory? The former would include the documented evidence, and the latter would include the speculative thought. This is how many similar topics are discussed. (See: theory#List of famous theories). KSchutte 19:14, 12 March 2006 (UTC)

That would be nice in principle however, what happens is that people stumble upon the lesser of the two articles, assume it to be an untapped editing prospect, then start adding duplicate content, e.g. there were two separate history sections growing on the former separate “gravity” and “gravitation” articles. Read the arguments on the archive pages as well. --Sadi Carnot 15:44, 14 March 2006 (UTC)

Completing merge and break-up of "joint" articles per Talk:Gravity discussion, gravitation talk page archives, and gravity talk page archives.--Sadi Carnot 05:56, 14 March 2006 (UTC)

Gravity could be completely wrong nobody truly knows what keeps us on the earth gravity is just one version of it Cahan123 (talk) 08:36, 22 May 2008 (UTC)

Gravity is now a redirect to Gravitation, in accordance with the above discussions (and per archives). The main article has now been reduced from 18 pages to 5. In doing this, seven new pages have been created as branched off from the main article, and the total set of eight pages (verses one) is now very workable. I apologize if the breakup was emotional for anyone. Almost nothing was deleted, most was simply redistributed and reorganized.--Sadi Carnot 09:59, 14 March 2006 (UTC)

Congratulations and thank you. --Ancheta Wis 12:07, 14 March 2006 (UTC) Very well done, although I was taken by surprise by this. (I have not been active on the gravity page recently, but have kept an active watch here to maintain the integrity of this page.) It is nice to see that my firm stand against redirecting this topic to "gravity" has finally paid off. This is very much the kind of thing that I hope to have this page become. --EMS | Talk 16:39, 14 March 2006 (UTC)

The current text is "American (German-born) physicist Albert Einstein". My object is that Einstein was a German physicist at time general relativity was created, and IMO should be listed as such. That he later emigrated to America is immaterial to his status at the time general relativity was developed. The other option is not to deal with the nationalities of well-known people like Einstein and Newton at all. (Note that this issue also related to the page Einstein's theory of gravitation. --EMS | Talk 16:52, 14 March 2006 (UTC)

But I thought he renounced his German citizenship. OK even if he was granted German citizenship when he was professor in Germany, he lost that when he lost his job. What about simply omitting the attribution? -- or German / Swiss / American ? --Ancheta Wis 17:14, 14 March 2006 (UTC) I agree with you both however, as a general of thumb (in science writing) most attach a name to a nationality + type of scientist whereby we assume, no matter how famous, that the reader has no prior knowledge of the person. In this manner, the learning process is improved: people will attach an unknown name, such as “Albert Einstein” with an occupation, as “physicist”, to a national origin, as “German-born American”. This, by the way, is how Merriam-Webster describes Einstein in their one paragraph summary.--Sadi Carnot 17:23, 14 March 2006 (UTC) In 1915, Einstein's renouncing his German citizenship was still

20 years in the future. The issue is one of do we describe Einstein as he was when he created the theory, or as he was at his death? A case can be made for both, or for the alternative of not mentioning nationality at all in order to avoid this can of worms. At this point, my temptation is to leave it alone unless a consensus appears to change it, partially out of respect to Sadi's argument above and partially out of respect for the amount of thought and work that Sadi is putting into these articles. --EMS | Talk 18:12, 14 March 2006 (UTC) EMS, Actually he first renounced it in 1896. But SC, I concur with your work. --Ancheta Wis 18:26, 14 March 2006 (UTC) LOL! After looking at the link, it is now my conclusion that the correct text is "Swiss-American (born German) . ". --EMS | Talk 20:54, 14 March 2006 (UTC)

Sounds accurate, how about: "the German-born, Swiss-American physicist Albert Einstein "? This seems cogent. Feel free to change it.--Sadi Carnot 21:23, 14 March 2006 (UTC)

Sheesh, I can't believe that no-one suggested the simplest and wisest solution: Albert Einstein rather than the born-in-Ulm-schooled-in-Berne-worked-in-Vienna-later-lived-in-Berlin-and-still-later-fled-to-Princeton-Swiss-German-American-and-by-his-own-account-devoutly-trans-national mostly-theoretical-but-sometimes-experimental physicist-and-philosopher-and-sometime-inventor-and-as-some-would-say-theologian-which-as-some-would-say-makes-him-an-applied-mathematician-and-engineer-but-certainly-not-a-monk Albert Einstein.---CH 07:55, 21 April 2006 (UTC)

Good lord. Just change it to "then-German scientist Albert Einstein" and be done with it. 209.214.230.142 20:54, 23 June 2006 (UTC)

Interesting news going around today (2006-03-25) about some tiny aparrent changes in gravity from rotating rings of semiconductors. http://xxx.lanl.gov/abs/gr-qc/0207123.pdf Waaay over my head! Needs a physicist to explain where this fits in. If it does. —Preceding unsigned comment added by 83.104.55.73 (talk • contribs) (demon.uk in merrie old England)

Thanks for the heads-up Anon, interesting article.--Sadi Carnot 01:50, 28 March 2006 (UTC)

The article failed to reach good article status because:

• The applications section is poorly focused and the link to Lynton & Lynmouth Cliff does not work. There are obviously a number of applications of gravitation and the section would benefit from being rewritten.
• commented out the L&LC link. Ancheta Wis 01:28, 11 April 2006 (UTC)

challenged Albert Einstein's theory of relativity, announcing he was working on a Dynamic theory of gravity (which began between 1892 and 1894) and argued that a "field of force" was a better concept and focused on media with electromagnetic energy that fill all of space. : In 1967 Andrei Sakharov proposed something similar, if not essentially identical. His theory has been adopted and promoted by Messrs. Haisch, Rueda and Puthoff who, among other things, explain that gravitational and inertial mass are identical and that high speed rotation can reduce (relative) mass. Combining these notions with those of Thomas Townsend Brown, it is relatively easy to conceive how field propulsion vehicles such as "flying saucers" could be engineered given a suitable source of power.

These claims are profoundly inaccurate. Tesla's announcement provides insufficient details to compare it with any published theory. Sakharov's proposal does have some coherent motivation and is still taken seriously despite (according to the mainstream view) lack of progress along these lines Tesla's murky ideas have been entirely forgotten. The work of Haisch, Rueda, and Puthoff is fringe physics at best. Finally, Thomas Townsend Brown can only be regarded as a crank.

I have eliminated these particular cranky-POV-pushing edits, but there may well be others in this article. May the reader beware.---CH 21:57, 13 April 2006 (UTC)

I noticed some other errors, e.g. in general relativity gravitational radiation (which under the rules of quantum mechanics must be composed of gravitons) is created only in situations where the curvature of spacetime is oscillating contains at least one error. This seems to have involved trying to redress the ill effects of some massive merger/redirection war, so the cranky stuff quoted above may have been written by someone else and pasted into this article by Sadi during a rather massive reconstruction, which is why I say above the editions may have been a mistake. Be this as it may, the fact that they went unnoticed from 13 March 2006 to 13 April 2006 must be troubling to those of us concerned to improve and maintain the reliability of WP as an encylopedia.---CH 22:34, 13 April 2006 (UTC)

Hillman, why don't you do some research before pointing fingers. Every single wikipedia edit is stored in easily-accessible memory. Thus, if you would have done your investigation properly, you would have found that User:Gbleem put these questionables into the pre-merger Gravity article on Nov 2, 2005, see: Edit. Myself, I spent one day only (March 14, 2006) doing a breakup of an over-bulging Gravity + Gravitation set of articles. I would hope, in the future, that you do not go around affixing my name to any other questionable contributions that were made years ago by other people. Thank-you.--Sadi Carnot 02:39, 19 April 2006 (UTC)

Sadi, thanks for clearing this up. But please assume good faith. I think it is clear from my comment above that I did do some research, in fact I probably tried harder than most editors would have done to figure out what had happened. I even tried to clearly explain that I suspected I had not uncovered the whole story. Frankly, instead of yelling at me, I think you should give me credit for finding the edit I cited and for figuring out that it seemed to have something to do with some kind of merger or name change. I even said that I realized that you might not have been the original author! All you needed to say here was, indeed, during a merger from Gravity, which was then a separate article, I pasted in material written by Gbleem I now wish I had read that material more carefully.

The real problem is that in cases like this, where material written by user U is merged from article A into article B by user V, it can be quite difficult to figure out what happened even by a careful perusal of the history file of B, unless you somehow guess that the needed information might be contained in the history file of A! In this case, the history file of B didn't tell me what I needed to know, namely that material was being merged by V from A and that I should look in the history file of A to find that the original author of the inaccurate claim was U. Obviously, it would be very useful to have a who-wrote-this? tool which is smart enought to unravel this kind of thing even if (as sometimes happens) article A no longer exists or has been reduced to a redirect!

Anyway, thanks for your work in the merger. I do think this episode shows why anyone carrying out such a merger should try to fact check anything which appears suspicious, since any attempt to improve the WP is rather defeated if one perpetuates such a wild claim. But I do realize that this might not always be possible. I think this episode highlights some serious problems with current wikisoftware, which can all too easily lead to this kind of misunderstanding. I hope that some developer will see this and attempt to develop the kind of tool described in the previous paragraph.---CH 02:10, 21 April 2006 (UTC)

As an aside, Tesla also denounced Einstein's methods, stating that "Einstein is trying to replace science with mathematics." This is an apparent reference to the fact that his theories were considered largely unprovable at the time of their publication. 209.214.230.142 21:25, 23 June 2006 (UTC)

Gravity is not a pulling force, it is a PUSHING force, it is the VIRTUAL PARTICLES surrounding a planet, PUSHING down the objects. Matter is mostly space but some objects have farther between the smallest particles, and therefore they are lighter - there is less resistance, less particles to be hit by the virtual particles. Say a piece of lead for example, is very heavy simply because it is dense!. This "media with electromagnetic energy that fill all of space" EXISTS and is proven, it just happens to have the name VIRTUAL PARTICLES. For more in depth explanation gohere and press ctrl+F to bring up the search bar and paste "This is what gravity" into the search field and hit enter alternatively scroll down to 12/05/06 12:11 AM (Aaron Murakami). /Minoya 16:07, 14 December 2006 (UTC)

In the history section there are strange claims about ancient Indians being "the first to recognize gravity as a force of attraction". Namely, Brahmagupta said in 628 'it is in the nature of the earth to attract bodies'. Is it really something profound and ground-shattering? Does it really differ from what other ancient men thought long before him? In the article as it stands now we are supposed to believe that Newton was "building on these foundations". Are you serious? Did Newton know anything about Brahmagupta? I can see a lot of articles reflect now Hindutva propaganda in its crudest form. Am I the only person to resent it?

No. Unfortunately this topic attracts all sorts of crank edits, and is difficult to maintain without engaging in prolonged controversy. Since last man standing wins, and reasonable people tire more quickly . linas 13:02, 18 April 2006 (UTC) I don't disagree with the often-cited WP goal of correcting a eurocentric bias which is evident in some (most?) of the English language literature, but of course we should discourage anyone from trying to replace eurocentric bias with indocentric bias! In this case, I think that simply deleting any claim to the effect that Indians were the first to recognize gravity as a force of attraction should suffice. Assuming the quotation credited to Brahmagupta is undisputed, I think that is notable, but readers should be allowed make up their own minds what if anything to infer from this quotation. ---CH 02:57, 21 April 2006 (UTC) history of gravity and indian sages is innapropriate The basis for the scientific argument and its demonstration is being watered down far too much by ambigous claims of the sages in india etc. There are far too many religious and historical propostion that point to all sorts of basis for bona fide scientific work. This looks like an attempt to discredit Newton as the discoverer of gravity. Unless you can make scientific predictions from the writings of sages on critical scientific issues of today as evidence of their validity, i can bring all sorts of arguments of for example the origins of the theory of electrons on the african zulu caim of nature of 'small things' please avoid nationalistic, racial, religious pride from interfering with valid accepted truths on merit for discovery. --- —Preceding unsigned comment added by 196.200.29.165 (talk • contribs)

Okay, one can tell this is coming from someone uneducated. I'm not sure if this is the right place for any inquiry such as this, but it doesn't seem as if putting it here would harm. Now, to the point: Let us say that there is a bubble of empty space near the center of earth, offset from the center by a certain bit. Now, let us put a man in there, or any object for that sake. How will gravity work on him? And what if we placed him in the actual center of earth, disregarding all realism. Some might see this as a paradox that means that at some point (assuming mass is evenly distributed throughout the globe), a particle at a certain depth will face less gravity. As such, it seems as if pressure on the particle will also be releaved, at least to an extent. —Preceding unsigned comment added by 213.161.189.107 (talk • contribs)

Your intuitions are correct. An object at the exact center of the Earth would feel no net gravitational force and would float at the center. The attraction to mass in any direction is exactly counterbalanced by matter in the opposite direction. If slighty offset from center, it would experience some force to move toward the center. It is true that inside the earth the gravitational force toward the center is less than it is at the surface, since now some the mass is on the other side of the body. A good place on Wikipedia to ask questions like this is at Wikipedia:Reference desk/Science . Hope that helps. --GangofOne 03:04, 21 April 2006 (UTC)

A mass in free space in the gravitational center of the earth would nevertheless be in an orbit and subject to orbital forces of the sun and the moon and the planets.WFPMWFPM (talk) 04:25, 24 June 2008 (UTC)

and because of how gravity warps spacetime he'd experience less time passing than us according to our frame of reference. --some anonymous guy 07:20 1 september 2006

Why doesn't the planets spiral inwards toward the sun or away from it? Why is it rotating perfectly around the sun? Wouldn't it spiral in or out because of the imperfect balance between the mass and speed of the planets versus the gravitational pull of the sun? —Preceding unsigned comment added by 71.141.178.226 (talk • contribs) (San Francisco, CA this IP is registered to Southwestern Bell)

> There is a volume of solar wind to factor in also. ---User:Seb-Gibbs —Preceding comment was added at 16:19, 21 December 2007 (UTC)

This would be best directed toward Wikipedia:Reference desk/Science, where you will probably be told that in Newtonian theory, a central force law scaling like r -2 admits stable closed orbits (they are ellipses), whereas other exponents would not. This is a standard topic in courses on theoretical physics, for example Landau and Lifschitz, Mechanics. ---CH 04:55, 21 April 2006 (UTC)

Is "john davenport" in the Einstein section vandalism? I'm confused.

The header called "realted" should be named something more descriptive. "Related" means that the things under them are related? that the things under it are related to gravity? or the things above them? Its very vague. Fresheneesz 05:05, 10 May 2006 (UTC)

Changed to Specifics. Is this O.K.?--Sadi Carnot 13:04, 10 May 2006 (UTC) Its better, i'm still fuzzy on what exactly that header means about what under it. Fresheneesz 07:49, 11 May 2006 (UTC)

Let's begin the discussion per the protocol. --Ancheta Wis 05:26, 11 July 2006 (UTC)

Let me get this straight: we want to make stable an article that didn't even get up to "Good" status? If we're stabilizing versions in order to avoid article degradation, why would we start with article that isn't particularly stellar? And even once it is good (or hey, even featured), why not simply link to the version that was "certified good/featured"? Why not keep the Wikiwiki version the main version and make the "certified" versions easily accessible from that page? JDoorj a m Talk 21:34, 11 July 2006 (UTC) Forget I said it. The protocol does most of the things you're asking for. A good idea, just not one I'd run across before here. S B Harris 02:32, 12 July 2006 (UTC)

Just exactly how many pages have you slapped this on? It's a brand new proposal, that has all kinds of discussion still surroudning it. Please be calm abot it? -Splash - tk 20:15, 12 July 2006 (UTC)

What is this theory of gravitation of Aryabhata based on heliocentric solar system? To the best of my knowledge Aryabhata did not formulate any theory of gravity worth its name.

I propose changing the beginning of the section from "There have been numerous theories of gravitation since the time of the Greek philosopher Aristotle in the 4th century BC. He believed that there was no effect without a cause, . " to the following: "Since the time of the Greek philosopher Aristotle in the 4th century BC, there have been numerous attempts to understand and explain what we now know as the force of gravitation. Although these attempted explanations can not be called "scientific" in the modern sense of the word, they are nonetheless precursors of a scientific attitude towards natural phenomena. Aristotle believed that there was no effect without a cause, &c &c &c. " Also, in accordance with the comment direrctly preceding this one, mention of Aryabhata will be excised. Please discuss, especially why these changes should NOT be made. Hi There 12:11, 3 August 2006 (UTC)

My only request is that that ". explain what we now know as the force of gravitation" shotenned to ". explain gravitation". Please realize that in the most modern gravitation theory, general relativity, gravitation is not due to a force. I also support the removal of the mention of Aryabhata. I have tolerated the mentioj of Aryabhata, but have never been 100% sure that it belonged here (nor 100% sure that it did not). It is nice to see a consensus finally apprearing on this issue. --EMS | Talk 00:57, 4 August 2006 (UTC) Okay, I edited it and utilized your suggestion, although I made a few other slight changes from the text that I originally proposed let me know what you think. Hi There 10:15, 4 August 2006 (UTC) This looks good. Thanks much. --EMS | Talk 20:36, 5 August 2006 (UTC)

The beginning of the article states, "Gravitation is one of the four fundamental interactions in nature, the other three being the electromagnetic force, the weak nuclear force, and the strong nuclear force." I read that the electromagnetic and weak forces were aspects of the same process. I suppose it's now commonly called the electroweak force? I know that it's not an everyday occurance that they merge into the same force since it takes a lot of energy. I'm sure there's people who know way more than I here but I figured I'd chime in just in case. feel free to delete this if it's total nonsense. --some anonymous guy 07:27, 1 September 2006 (EST)

Ya, my history book says that they are the same but it requires a lot of mathematics to understand it. 68.155.149.13 18:00, 23 February 2007 (UTC) The electroweak interaction is part of the attempt to create the theory of everything used to combine the four forces in the standard model. The two forces diverged when the universe was approximately 1 second old and at a temperature of about 10^9K. It is appropriate to describe the four forces as seperate for the current universe (temperature about 3K) MDoggNoGFresh 16:32, 30 May 2007 (UTC)

• The following analysis addresses the phenomenon of gravitation (a negative electrostatic phenomenom) and it's integrated in the concepts of the Physics of Creation:

P.S.: A comment related to the measurement of the proton-electron mass-ratio, which was reported some 10 years after Dr. Harold Aspden had presented the theoretical value derived from aether theory:

"The value that they [Aspden and Eagles] calculate is remarkably close to our experimentally measured value (i.e. within two standard deviations)This is even more curious when one notes that they published this result several years before direct precision measurements of this ratio had begun." R. S. Van Dyck, Jr., F. L. Moore, D. L. Farnham and P. B. Schwinberg in Int. J. Mass Spectrometry and Ion Processes, 66, p. 327, 1985.

Can we assume that in the formula, r is in meter, m in kilogram and F in Newton? If yes, I'll just add this precision. JeDi 11:51, 7 September 2006 (UTC)

It is in any units you like, provided that r has units of length, m of mass, and F of force (or, equivalently mass*distance/time^2). G has units of exactly what it has to—length^3/(mass*time^2)—and of course its numerical value depends on the system of units used. But the point is, since the units balance on both sides, this equation (like all fundamental equations in physics) is not dependant on a particular system of units. -- SCZenz 20:24, 7 September 2006 (UTC)

Shouldn't it say something about gravitation being a theory in the first paragraph? Maya Levy 13:16, 11 October 2006 (UTC)

I don't understand what you're getting at, but I think that the first paragraph is pretty bad. I'm going to rewrite it. --Smack (talk) 16:35, 15 October 2006 (UTC) The first line reads: "Gravitation is a phenomenon through which all objects attract each other". The sentence is false since object don't attract each other. We know that since 1915. It was an old prejudice born out of efforts to explain Newtonian math, which even Newton himself considered sick. I don't think we should start a wikipedia article with an old and sick prejudice. The gravitational force is an inertial force that looked to most of 18th, and 19th century physicists as "attraction". It could be explained at the beginning of the article why it is an inertial force and why it looks as attraction if one doesn't know where to look. To bad that someone replaced my old stuff with the old (according to Einstein) and sick (according to Newton) prejudice instead of just showing how gravitational force is determined according to Einstein. Jim 11:22, 21 November 2006 (UTC)

I heard about a hungarioan who said intresting things.

"[. ] A motivation of the author to an experiment checking the UFF hypothesis in a range of pro mille, in a simultaneous free fall from 110 m fall height in vacuum with different materials, found mainly on the three “irregular” observations:
1) The observed values of G are widely scattered. For instance considering the measurements after 1995 only, the deviation of G is 0.7%. → The quantity G = G m g / mi does not appear as a constant in measurements.
2) A recalculation of Kepler’s third law by Szász with all the nine planets has discovered a composition dependency up to 0.15%. → The motions of planets are composition dependent and violate the UFF.
3.) The relative mass defects of isotopes MD A Δ offer a dependency from the mass number A up to 0.78%, Audi and Wapsta, Ref. [11]. → In microgravity, there is a loss of mi A and the change of MD A Δ depends on the number of nucleons. The Newtonian would be G = G M g / M i mg /m i ≈ G (1+ MD Δ (M)+ MD Δ (m)) Prior to the description of the experiment performed never before, the theoretical background, which is extensively [. ]" "[. ]
Simultaneous Fall Experiment with Different Materials from 110 m Height

For an experimental verification of the difference between the inertial and the gravitational mass, the author has used only solid chemical elements Li/Be/B/C/ Al/Fe/Pb and has performed a simultaneous fall experiment in the 110 m high vacuum tube at the drop tower of ZARM, University of Bremen. The weights of the test bodies were between

7 g. The purities were better than 98.8% in all cases. The test bodies were freely placed at the middle of the safety glass cylinder. On the back plane of the experimental equipment, a cm scale was fixed with 0.0 cm at 130 Measurement of UFF Violation with Li/C/ Pb Compared to Al start, and with red marks for the fall distance prognoses according to Eq. (5). The relative movement of the test bodies was recorded with a standard CCD video camera. The camera was placed in front of the middle glass cylinder through a mirror arrangement in a distance (from the front of objective to the cm scale on the back ground) of

60 cm directed to the height of 15 cm. The experimental equipment was fixed in the drop capsule falling freely in vacuum. The time resolution 0.04 s is to be calculated from 25 frames/s. From 256x256 pixels, the space resolution is in order of 1 mm for the quickest relative motion of Li. The time of fall was mirrored in by film exposure in 40 ms units. The time of fall with approximately free fall conditions and the relative fall distances in each time step can be read immediately from single pictures of film. The following sequence of four pictures shows the relative movement of the seven test bodies at fall times of 1.23 s, 2.43 s, 3.63 s and at 4.68 s, the end of the 110 m fall. [. ]"
Any comment? (Nemethpeter 18:54, 26 October 2006 (UTC))

To verify these results one would need more data. E.g. how good the vacuume was, the shape of the falling bodies, method of releasing them, etc. It looks like the experiment (or the reporting) has been done by amateurs while professionals have done them with meassurements of many orders of magnitude more accurate and didn't find any diff between Newtonian equations and the real world. So your results didn't look like worth analizing, however if you don't mind, get the missing data and calculate what is the expected standard deviation. It'll be a nice exercise and you will learn something at the same time. Jim 14:28, 21 November 2006 (UTC) I think this discussion was wrapped up nicely on my talk page. The gist of it was: 1)not enough sources 2)all results quoted were the accepted value to within experimental uncertainty. --MOBle 17:41, 21 November 2006 (UTC)

Somewhere in the article, it says that Einstein was German-born. Recently, someone added that he was Jewish after that clause. It was quickly reverted. I think it should stay, for two reasons:

1. The sentence said that Einstein was German-born. That fact that he was of Jewish descent had very serious consequences for him, even before the rise to power of the Nazis.
2. He identified himself as a Jew — a nonreligious Jew, but a Jew nonetheless. He left his personal papers (and other things, I think) to the Hebrew University of Jerusalem, and generally seemed to be proud of his heritage. I don't think we should play revisionism.

It might make sense to remove both "German-born" and "Jew", but I think that if one stays, both should stay, because they are so essentially intertwined. --MOBle 12:59, 13 November 2006 (UTC)

I would support the removal of both. The detail impresses me as being inappropriate to an overview article such as this one. If nothing else, this article is about gravitation instead of Einstein, although if there is a guideline that says that the first reference to a physicist should include their country of origin and religous affiliation I would yield to that. It also seems to me that the details of who Einstein was are covered more than adequately in the Albert Einstein article. --EMS | Talk 19:39, 13 November 2006 (UTC) Done. --Ancheta Wis 00:32, 14 November 2006 (UTC)

In the whole article as well as the linked to it an article on general relativity there is no Einsteinian equation for gravitational force nor any instruction how to calculate it. So what a high school student is going to think about Wikipedia if he is looking for explanation of gravitational force? That Wikipedians think that Newtonian stuff is good enough for him? Maybe this is what most of them think but is it right thinking?

Furtheremore there is the following item counted as achievement of general relativty:

Edwin Hubble didn't "confirm the expansion", he only proposed a law that redshift of galaxies changes linearly with their distance form us. Which was wrong anyway since Einsteinian physics predicts exponential change which simulates accelerating expansion. Anyway, since this item is wrong I propose to replace it with

• The 'anomalus' acceleration of space probes Pioneer 10 and Pioneer 11 has been predicted by Einsteinian gravitation within less than half standard deviation (this is still an original research but it might be not anymore probably earlier than Wikipedians reach a consensus on what changes should be made in Gravitation). Jim 11:51, 21 November 2006 (UTC)
1. In general relativity, gravitation is not due to a force. Therefore there is no equation for gravitational force in general relativity. Gravitation is of course not due to a force but it does not make general relativity (GR) unable to calculate a gravitational force that acts eg. on a guy standing on the Earth. If you mean that GR is not able to calculate this force you simply don't know basics of GR and you are not someone who can instruct people on what is right and what's wrong in gravitation. The way to calculate gravitational force in GR is this: take invariant energy of a guy (from any textbook on GR, for the mass of the guy of course) differentiate it with respect to guy's proper distance from let's say surface of the Earth (standard for calculating "forces") reverse the sign and your result is the gravitational force with which the guy acts on the Earth. Then you will also see why it is only an inertial force and not some mysterious attraction as 19 century thinkers call it. Pretty simple derivation and I don't see it anywhere in whole gravitation page as if it never existed and I remember well placing it there. GR would be a very poor theory if it didn't allow to calculate in a simple way, that a high school student can understand, a gravitational force. The calculation is in fact so simple that is shown mostly in popular texts (like mine) since professionals don't bother with high school calculus. Which does not mean that high school calculus is useless and can't be used in a standard way to calculate forces. Jim 11:49, 25 February 2007 (UTC)
2. While there is gravitational accelaration due to spacetimecurvature, what describes it is a tensor expression known as the Einstein field equations (EFE). This equation can be written as R μ ν − ( 1 / 2 ) g μ ν = 8 π T μ ν -(1/2)g_=8pi T_> or abbreviated to G = T , but those are most uninformative to people who lack an understanding of tensor calculus.
3. The instructions for calculating gravitational interactions starting from the EFE are hideously complex and do not belong here. See general relativity resources for textbooks and web sites that can help with this. One differentiation of energy of a particle is "hideously complex"? Jim 11:49, 25 February 2007 (UTC)
4. Solutions of the EFE such as the Schwarzschild solution could be used instead of the EFE. There is still a need to use tensor calculus with them however, and links to articles on the relevant solutions are already in this article in any case.
5. Hubble's observations did confirm that the universe is expanding as predicted by the Robertson-Walker metric. Kindly note that accelerating expansion of the universe is a result of modern observations! It can be described using an EFE solution, but is not a unique prediction of general relativity. (Note that the enabling EFE solution requires the universe to be 70% dark energy, 26% dark matter, and 4% normal matter. So the acceleration is being used to predict to composition of the universe and not the other way around.)
6. General relativity does not account for the Pioneer anomaly. The difference between the Newtonian and GR preditions is just way too small.

Let's get back to the question that was thrown out of the article section, If I'm in a geosynchronous orbit around the earth and I see two objects dropped near the north and the south poles and I see them falling towards each other, am I supposed to attribute that phenomena to the curvature of space? I can see your moving towards minimum energy concept but I dont see what the three dimensional space continuim has to do with it.WFPMWFPM (talk) 17:13, 16 June 2008 (UTC)

A question of mine that might be interesting for the article: when two bodies impact each other and join they become one with a single gravitational field, but what if they are just touching? What if we have a 50 km asteroid and a 100 m asteroid is made to touch down gently on the surface? Does just touching the larger one increase its gravitational field or does it require something else? What is the exact borderline for when objects become one in gravitational terms?

Mithridates 15:59, 11 December 2006 (UTC)

Semantically, that is a question for astrononmers. In terms of the gravitational field, the farther you are from the object (objects?), the more appropriate it becomes to treat them as one object of the combined mass instead of two objects. Thus holds true even in the objects are in orbit about each other. (For example, to model the trajectory of a spacecraft in Earth orbit you would consider the Moon as a seperate object, but to model the orbit of Pluto it is fine to treat the Earth/Moom system as a single mass unless your calculation were are extremely high precision.) --EMS | Talk 16:41, 11 December 2006 (UTC) Good question —The preceding unsigned comment was added by Makewater (talk • contribs) 19:45, 16 December 2006 (UTC). This is a question of philosophy rather than science I think. There is only one gravitational field (or if you prefer to talk about relativity, only one curvature tensor) for the entire universe at each point, it describes the amount of force exerted by gravity (degree of spacetime curvature, which causes objects to move) at that point. The gravitational field due to two objects is just the superposition of the fields due to each alone as they collide, that field looks more and more like that of one larger object. When it "officially" becomes two objects rather than one really just depends on how you want to look at things in a given problem. -- SCZenz 23:06, 16 December 2006 (UTC)

I don't think it's a verifiable fact that Galileo dropped anything from the Tower of Pisa. As far as I understand most historians consider that an urban legend, seeing that Galileo would generally be very specific in his observations and therefore it would be a very unprofessional experiment. Any thoughts?

These needed to be added to the article, or made so they stand out and are understandable to the user, because even i am left w/ these questions:

1 Can Gravitation be proved? 2 If so, is there a way to prove it other than mathematically? 3 If gravitation is real, what is it? Matter? If not, is it actually in existence? (dont bring in philisophical/religious stuff in, i.e. "spirits exist, so gravity can") 4 Why the gravitational theory is a theory and not a fact/law. toaster 02:05, 6 February 2007 (UTC) I can tell you that gravitation is an observed fact. After all, the planets go around the Sun and we are constantly pulled downward towards the center of the Earth. In essense "gravitation" is the name of an observed phenomenon. As for "proving" gravitation: You don't do it mathmatically to begin with. Instead it is the observation that matters. A combination of math and observation comes in when one seeks to verify that a given theory of gravitation is correct. In fact, this also brings up to your last question, since any explanation of gravitation is just a model and not the reality. Even if a given theory is exactly right, there is no way from first principles to prove that gravitation must exist and have exactly certain properties. Given that, any theory of gravitation must be just a theory due to its carrying with itself its own postulates. --EMS | Talk 20:12, 7 February 2007 (UTC) Point four is best addressed by reading theory theories are like the existence of God - they can never be proven. Unlike God, they can be disproven. And as far as I know, they still don't know what causes gravity, there's just the whole 'bending of spacetime' thing. Gravity is still very poorly understood as a phenomenon, though it's effects are well known. WLU 23:59, 7 February 2007 (UTC) Ok, but than what can we say gravitation is, but not just the simple attraction between two objects? Heck, what other observable phenomenon out there does not have matter?toaster 22:26, 8 February 2007 (UTC) I'm not sure I understand the question, but WP:OR, we can't speculate. We can't put anything beyond what's documented in secondary sources. Gravity will not be solved on wikipedia. WLU 00:54, 9 February 2007 (UTC) Why not? If the knowledge about Gravity, a simple direct inverse-square of distance force, seems to have been already unveiled (an electrostatic force):

Whenever one tries to replace statements of Newton's gravitation with corresponding statements of Einstein's gravitation the editors reverse the changes. Often without any good reason beyond a vague impression that it might be original research. As it happens to me a lot and they don't even answer a question what they consider original research. While I'm only trying to popularize Einstein's theory and everything I write Einstein already knew. However not editors of Wikipedia. Which is rather sad because it's almost a century of ignorance.

And this reversing is probably not because they want to prevent Wikipedia users from learning gravitation. Most likely the reason is that they don't understand a bit about gravitation and think that Newton's theory about it, which they remember from school, is science. They think that Einstein's gravitation should be close enough to Newton's and ignore the fact that while it is (almost) the same math it describes a completely different physics. So whatever is not close enough to Newtonian physics should be deleted. Even something like derivation of a gravitational force because it can't be derived with Newton's theory. They don't know how to derive it so they suspect it of being never derived even by Einstein (otherwise how it could be close enough?). They must assume that Einstein discovered theory of gravitation without addressing the issue of gravitational force. It might sound funny but this is really the case with most editors. Which is telling us that some of them must be intellectually challenged yet they keep editing Wikipedia and that's why Wikipedia is so much retarded in the area of gravitation. And that's why there is a problem with editors which better be solved. Jim 22:20, 24 February 2007 (UTC)

Has anyone seen JimJast's questions to all the board candidates? Can you believe this guy? Either he is a complete idiot, or he is speaking a language other than English (a bizarre language known only to Physics PHD students), or both. Just accept that gravity is the natural force that attracts massive objects to each other, as any kindergarten student can prove. This is not the place for writing your PHD thesis. This is an encyclopedia for ordinary people who speak English. Gravity is a remarkably simple concept. The total unreadability of all the scientific articles in this Encyclopedia is entirely the fault of scientists such as JimJast who are unable or unwilling to speak normal English or to recognise that the average editor knows a fair bit about the subject. The next person who says a line is not a line because it isn't infinately long should be made to swallow a dictionary. Carl Kenner 13:10, 5 July 2007 (UTC)

I have taken a look at it. Overall it is nothing but the same self-serving bellyaching that I have seen in other contexts. For now, I will place Jim Jast in the "a little knowledge is dangerous" category. As for the candidates, one has made a blanket statement that shows no research into this matter, another that I looked at called on Jim to work with us, and the overall majority seem to have enough sense (or lack of time) to not respond at all. Let's just say that I have a hard time faulting those who don't respond. As for the science writing here: The problem is that more editors are needed who are interested in this stuff, and who appreciate Wikipedia for what it is. Much here is written by experts, and unfortunately reflects that. This creates an ironic situation whereby the the process if improving Wikipedia means that many articles must be "degraded". An example of that ironically is Jum's edit: I actually have some sympathy for it's intent (being a general relativity person myself). However, Wikpedia is for everyone and not just Ph. D.s. That is why I agree with you about keeping the definition of gravitation simple, but do note that I for one refuse to allow the word "force" to appear in the first paragraph due to Einstein's work. --EMS | Talk 04:40, 6 July 2007 (UTC)

A couple of days ago, User:JimJast changed the term gravitational attraction from a redirect to "Gravitation" into an article of its own. I don't know whether such a separate article is required, or maybe the things should be included in the existing articles on gravitation, but the main problem is that User:JimJast dedicated that article to a philosophy of his own (starting the definition with the words "gravitational attraction is a myth", and later saying "There are no gravitational forces between particles that are free to move as e.g. between the stars and the planets of the universe"). I tried to make the article more balanced, and discussed the issues with User:JimJast on the Talk page, and presented references (including a textbook) showing that there are well-established alternative ways to look at the subject, and his general statements are incorrect. Nevertheless, he reverted most of my edits, and put his wrong ideas back. Please join the discussion. Thanks, Yevgeny Kats 21:09, 13 March 2007 (UTC)

This statement is not self-evident: "In flat spacetimes such as those of classical mechanics and special relativity, there is no way that inertial observers can accelerate with respect to each other, as free-falling bodies can do as they are each accelerated towards the center of a massive object."

Given Observer A and Observer B falling toward Mass C, the the statement implies there is no way for A and B to accelerate toward eachother. To me this says there is "no way" A and B can have a gravitational force between them (albiet small). Why not? What is the proof of this assertion? Dw31415 17:21, 23 March 2007 (UTC)

If a force is being exerted on them, they are not inertial observers. So in classical mechanics, that statement is moot. However, in general relativity, it is very, very important: In GR, there is no force that causes gravitation. So another mechanism (in this case spacetime curvature) takes the place of gravity and acts as the source of the gravitational field. --EMS | Talk 18:00, 23 March 2007 (UTC) I don't think that paragraph in the article makes sense at all. First of all, the sentence before the one cited above says: "inertial motion occurs when objects are in free-fall instead of when they are at rest with respect to a massive object such as the Earth (as is the case in classical mechanics)". It's incorrect to say that in classical mechanics a reference frame is inertial when it's attached to a heavy body. If the heavy body is accelerating, the frame won't be inertial. If the heavy body is rotating, the frame won't be inertial. Furthermore, a heavy body isn't a part of the story at all. An observer can be inertial even if he is very light and not attached to anything. As for the sentence cited by Dw31415, that sentence is wrong too. Since bodies may be free falling with a different acceleration in different places (e.g., on opposite sides of the Earth), they will have an acceleration relative to each other. Furthermore, I don't know if that new definition of inertial motion that is used in that paragraph is very standard. At least a definition should be given for what is meant by that. Yevgeny Kats 19:11, 23 March 2007 (UTC) That sentence is of my devising. It is quite technical yet more-or-less says what I want it to say. You are correct that rotation and orbital motion keep the Earth from being a inertial frame is classical mechanics. Perhaps we can hash out how to correct that. The part about inertia in GR is unforutnately quite technical, but is also concise. I am open to suggestions on how to improve that also (or even that section). It may be that this is not the best place to make the point that I am trying to make, although I personally prefer to make it here. --EMS | Talk 20:08, 23 March 2007 (UTC) EMS, I propose that the sentence be recast into subjunctive mood. --Ancheta Wis 10:06, 24 March 2007 (UTC) For example "If a photon were . then . "

Thanks for taking a look at my concern. I think this introductory paragraph would do well to continue the thread of how this theory explains the Mecury orbit does not match what would be predicted by Newton's equations. After all that's the way one theory replaces another, right (the new one is consistant with observations that the old one cannot explain). Dw31415 04:28, 24 March 2007 (UTC)

If you are suggesting what I think you are suggesting, then I strongly advise against it. "[H]ow this theory explains the Mecury orbit . " requires that use of a level of detail that I have largely avoided even in the general relativity article itself. In this article, I want to stick with the issues of "what", and leave the issues of "how" to other articles. (Maybe an orbits in general relativity article could go into the details, but that is going to be another project IMO.) --EMS | Talk 05:08, 24 March 2007 (UTC) Dw31415, please, let's not use this page to segue to another topic. Please include a link to the next topic, so that this page stays focussed. See for example the featured article Laplace-Runge-Lenz vector --Ancheta Wis 09:58, 24 March 2007 (UTC)

I'm a Mechanical Engineer by training, so I spent four happy years with Newton and we worked well together. For the last 15 years, I've tried to find a coherent explaination of how Einstein and Relativity added a more accurate explanation of what type of phenomenum (sp?). One of my biggest frustrations is that the experts always go to some highly abstracted example (like you're falling in an elevator at the speed of light. ). I think the opening paragraph is in that same vein. I think the opening paragraph should focus on real phenomenum first. Wouldn't Newton's equations properly account for the forces in the first example (two small masses and a big one)? Thanks again (And Ancheta, since I'm trying to get us to revise the transition between two theory's of gravity, this seems like the right place.) Dw31415 15:04, 24 March 2007 (UTC)

Hi Everybody. Recently the page "gravitational attraction" where I expained the nature of gravitational attraction got redirected by vote 9:1 to "gravitation" that unfortunately contains inaccuracies in its desription of nature. So, since it seems that we have no much choice but to explain the gravitational attraction in "gravitation", we need to clean up the page to reflect what about gravitation is published in reliable sources.

For that reason whatever is contradicting the contemporary knowledge of gravitation in this page has to be changed to whatever reliable sources say about real gravitation rather than what some editors put for whatever reasons into this page. The first thing that I propose to change is the opening sentence "Gravitation is a phenomenon through which all objects attract each other" since this is neither true nor reliable published source says so. It turns out that in our universe (as proposed by Einstein and we have to stick to it for already explained reasons) no object attracts any other. The alleged attraction is an over three centuries old prejudice, believed in also by many physicists, apparently for wrong reasons, since they couldn't come up yet, after so many years of trying, with consistent theory of quantum gravity. It should be finally explained why the behaviour of objects looks as they were attracted to each other. Between other things to allow those physicists to understand gravity better.

Before that though I propose to change this opening sentence back to what was there which read something like "Gravitation is a phenomenon through which objects tend to get closer to massive objects in their vicinity". After reaching an agreement on this first change we explain why they look as if they were "attracted" to thoses massive objects, as I did it in my redirected page. For now I'd like to have your agreement to this first change (which I expect knowing your opinions about Einstein might need some debate) and if the consensus is reached we might go on with this first and then with further improvements until we fix all the problems. So please express your opinions about this project. Jim 23:53, 25 March 2007 (UTC)

I just read the "General relativity" and "Introduction to general relativity" pages. IMHO, the problem with "Gravitation" page (correct me if I'm wrong) is that no one writing about it understands the simple Einsteinian physics behind it. The editors are trying to push into those pages all they know about the math forgetting that it has no meaning for those for whom encyclopedias are written. Consequently the gravitation remains a mess, a mystery to the readers, to the editors, and even to many non familiar with GR physicists. In short a bad popularization.

In particular, I didn't find in those pages an explanation of so called "gravitational attractive force", why it looks like "attraction", why it is really a pseudo force, how this pseudo force it is generated by the curvatures of spacetime (the thing that I put into "Gravitational attraction" page and it was removed from Wikipedia by 9:1 vote on a theory that it is my private POV (and possibly Landau's private POV, not acceptable to Wikipedia editors). Where this information on physics of gravitation is located in those pages? Jim 19:25, 28 March 2007 (UTC)

Are you reading the same Wikipedia as the rest of us? I foudn the following line in the first section of "Introduction to general relativity" . the gravitational field we feel at the surface of the earth is really a fictitious force like those of other non-inertial frames of reference. Furthermore, this was preceeded by a number of examples helping to make the point that you want made. It seems to me that what is "missing" is your prefered POV on this issue instead of the point itself. --EMS | Talk 20:59, 28 March 2007 (UTC) I was reading the same Wikipedia but I don't think it explains physics (forces, energies, and stuff like that) that high school student can understand. I treated seriously your call for making this page less mathematical and I'm now waiting for your appreciation of my job. However it might need some beautifying since not all high school students might understand what we do and certain things might need to be explained to them more clearly and with simpler language than just a language of regular calculus. But I think that as for an introduction it is a proper way of doing things. Because they can go to "GR" page ind got stuck there for the next 15 years until they understand everything. Even why the universe isn't really expanding in Einsteinian physics and why it is all just another illusion, like many other things in science. Jim 22:11, 28 March 2007 (UTC)

The passage below was labeled as 'pseudoscience' by a 'state-of-the-art' expert, physics graduate student of the oldest U.S. university, expressing the common materialist-positivist-reducionist approach of the 20th century academic thought applied to the long-standing but avoided work, into the ancient subtle aether, by a former student of the second-oldest university in the English-speaking world:

• Deleted from section "Gravity and quantum mechanics" > 01:46, 31 March 2007 Yevgeny Kats (Talk | contribs) (Undid revision 119178843 by 213.58.99.45 (talk) (pseudoscience)):
• Removed afterwards, for coherence reason, from section 'Newton's theory of Gravitation' > 01:55, 31 March 2007 213.58.99.45 (Talk) (→Newton's theory of gravitation - If you, expert, say so, then the same applies to this passage supported by the reference work of the same author ():

Let us see what the true Science in the future, be it in 50 or in 500 years from now, will have to say regarding this auto-proclaimed 'scientific-knowledge' age of our human history. As a great Mystic in the U.S. once wrote "The man who realizes his ignorance has taken the first step toward knowledge." Cheers.

The article claims that in scientific terminology the term "gravity" refers to the Newtonian force, while other theories (such as general relativity) should use the term "gravitation". I don't think it's true. For example, a search in Google Scholar gives 276 hits for the expression "Einsteinian gravity", and only 83 hits for "Einsteinian gravitation", which is the opposite trend from what the article claims.

We can also search for "Newtonian gravity" 1320 hits.

As you see, in both cases the word "gravity" is used about 3.3 times more than "gravitation", without any difference whether they refer to Newtonian or Einsteinian theory. Therefore, I suggest removing that section, and just mentioning both terms in the first sentence of the article as synonyms. Yevgeny Kats 01:14, 1 April 2007 (UTC)

I disagree with those who would want to diminish the distinctions between gravity and gravitation. Please consider the following in current thinking, when light from a distant star is “bent” (in Euclidean space) as it goes past a massive object, it is in fact going along a “straight” line (in curved space-time) known as a geodesic (which minimizes the “distance” that it travels). According to the general theory of relativity, to say that the light was “attracted” toward the massive object is incorrect. If the light had gone straight in Euclidean space or in any way not followed the geodesic as it passed this massive object, then we could say that it had been attracted and accelerated toward something. Similarly, for the article to say "“Gravitation" is the attractive influence that all objects exert on each other, while "gravity" specifically refers to a force which all massive objects (objects with mass) are theorized to exert on each other to cause gravitation.” rips out and throws away the heart and the central premise of the general theory of relativity which as a theory of gravitation states that there is no force of gravity and that there is no attraction (or acceleration) of bodies due to this force. AikBkj 15:43, 8 November 2007 (UTC)

Jagged 85 (talk · contribs) edited the start of the "Early history" section to read:

Since ancient times, there have been many attempts to understand and explain gravity. From the 8th century BCE, philosophers in ancient India may have understood that a form of gravitation held the solar system together. <ref>Dick Teresi (2002), Lost Discoveries: The Ancient Roots of Modern Science - from the Babylonians to the Maya, Simon & Schuster, New York, 0-684-83718-8:

"Two hundred years before Pythagoras, philosophers in northern India had understood that gravitation held the solar system together, and that therefore the sun, the most massive object, had to be at its centre."

I find this to be an amazing assertion, especially the indication that it was known that the Sun is much more massive than the Earth. Most ancient cultures thought that the Sun was small as compared to the Earth. Even with a heliocentric theory (and the resultant realization that the Sun must be bigger in order to function as the central mass), I see no evidence of a gravitational theory here, but instead a blanket statement that an influence exists. In any case, more and more scolarly documentation is needed of this business. --EMS | Talk 03:10, 12 May 2007 (UTC)

I've added another quote from an ancient Indian philosopher around that time who tried to explain gravity. I really don't see what's so amazing about this though. It's really not that hard to come up with a gravitational theory of some kind. I wouldn't be surprised if the Egyptians and Babylonians made earlier attempts aswell. As for the heliocentric theory, that's a different issue altogether and doesn't really need to be discussed here, but it just happens to be part of the quote. Jagged 85 05:57, 15 May 2007 (UTC)

Hi, I stubled upon this stray article: Gravity (fundamental forces) will someone please merge this article into this one. Thanks: --Sadi Carnot 16:43, 12 May 2007 (UTC)

I find it odd that you are calliong this article "stray" as you created it! In any case, I have merged in the one thing that I thought was worthwhile, and will now initiate an AfD on "prod" it. --EMS | Talk 17:13, 12 May 2007 (UTC)

As a supposed "fundy", I will furious to read this in my email inbox. According to a recent artilce online there is strong evidence that while scientists cannot explain why gravity works, the Evangelical Center for Faith-Based Reasoning has concluded that there is a "higher power" controlling what we understand as gravity. I am curious as to why Intelligent Falling (ie God wants things to fall) is not given proper discussion in Wikipedia. Website: (http://www.theonion.com/content/node/39512) Respectfully, Grandadd 03:27, 23 May 2007 (UTC)grandadd

Nice reference. The Onion is a well-known humor rag founded in Madison, Wisconsin, currently with editorial offices in Manhattan. Some of their work includes Our Dumb Century, with appropriate tongue-in-cheek commentary in the weekly paper about this encyclopedia. --Ancheta Wis 03:49, 23 May 2007 (UTC)

According to this article, "Newton’s law of gravitation simplifies to F = ma". While the F in in the law of gravitation can obviously be usefully equated with the F in F = ma, I do not see how the former equation can be said to "simplify" to the latter. Can anyone clarify what this statement is supposed to mean? Matt 00:32, 10 June 2007 (UTC)

At one time the article said 'simplifies to F = mg', where m and g are scalars, with g = constant 9.8m/sec/sec. This is Galileo's finding and the basis for the law of falling bodies. --Ancheta Wis 01:20, 11 June 2007 (UTC) I don't understand what you're getting at - whether it's written F = mg or F = ma makes no odds. The equation F = ma applies universally. It would (or could) still apply even if gravity didn't obey Newton's inverse-square law. The law of gravitation is a completely different concept. It explains how the force of gravity arises from the masses and distances involved. You cannot get from the law of gravitation to F = ma, or vice versa, just by a process of "simplification". They are, as far as I can see, two completely different things, that can be combined to work out the acceleration of a falling body, say. That's quite different from "simplification". Matt 11:14, 11 June 2007 (UTC). I rephrased it. Two different laws say F = mg and F = ma, from which we can conclude a = g. --Patrick 11:42, 11 June 2007 (UTC) We're talking at cross-purposes here. I was not quibbling about whether it should be "a" or "g", I was questioning the word "simplify". However, having just read this paragraph again it makes perfect sense - it's just saying that when "r" in the law of gravitation is constant, the formula F = m1*m2*G/r^2 becomes F = m1*g for some suitable constant g. I understood this perfectly well all along, so I really don't know what was confusing me. Brain clearly not engaged! Thanks anyway, and btw I have just tweaked the formatting there. Matt 23:41, 11 June 2007 (UTC)

Please forgive me if this was adressed in the article, but I couldn't find a specific enough answer within the body of the article.

Why, exactly, does gravity exist? Everyone knows that an object of sufficient enough mass (a planet for instance) will exert a pull on other objects, but why is this? What is it about mass that creates a force? The force of gravity seems to come from nowhere.

Is this "just the way things are" or is there a specific scientific explanation for why mass causes gravity? EvaXephon 03:30, 11 June 2007 (UTC)

It's a profound question. Not even Newton could answer it. All he could give was what. The mechanism of gravitation (the how of gravitation) is current research. The Higgs field is a potential answer for the origin of mass. But that only answers how. So we might invert this and say that Newton was smart enough to know he couldn't answer why. Perhaps in another 400 years, after another Newton develops. --Ancheta Wis 10:44, 11 June 2007 (UTC) Very encompassing and informative! Thanks for your time. EvaXephon 10:05, 12 June 2007 (UTC)

You're asking the chicken/egg question. However, let's go with James Clerk Maxwell who wrote in the 9th edition of the Ency. Brit. that no one has ever seen a force and as far as I know he's still right. What you do like Galileo and Newton is note the actions of real physical entities and develope causitive concepts and rules of behavior from those observations.WFPMWFPM (talk) 02:13, 10 June 2008 (UTC)

I noticed at the bottom of the article someone posted a thing about his theory of gravity, which to mean seemed a very stupid theory trying to say magnetism has the same properties as gravity, but anyway thats not the point - should it even be there? It was just posted today according to the history.

Sicewa 20:02, 16 July 2007 (UTC)Sicewa

sorry to but in but i have many questions which may seem very simple to you guys what keeps a neutron spinning does the neutron leave a void in the space it has just left + could it pull another atom towards it do neutrons show any syncrinisation to each other when together first time on wiki no idea if this is what it is for or how to spell cheers johnny fabioyiuopouuuuuuuuuuuu 09/908/088 —Preceding unsigned comment added by 69.15.65.50 (talk) 21:02, 7 February 2008 (UTC) You have asked a great question and aren't going to get many answers. However let's say there are a lot of small particles (gravitons) that are being attracted by gravity into a very small volume of space. but they cant get together because their line of motion isn't aimed at the central point of attraction. So then they kind of mix with each other and acquire a more or less uniform property of angular motion around the central point. Then maybe the central particles of this "particle cloud" manage to "cohere" or achieve fixed positions relative to each other and then grow in size to some volume restriction limit. That moght do for a stert.WFPMWFPM (talk) 17:33, 11 June 2008 (UTC)

I've encountered online several people who believe that gravity is caused by spinning. They think that if the Earth stopped rotating, gravity will stop and everything will start floating into space. I have pointed out a least one web site that disputes this idea, but they seem set on ways of thinking. —Preceding unsigned comment added by 63.164.202.130 (talk) 15:32, 11 September 2007 (UTC)

Since the total mass of the sun is known (1.9891 × 10 30 kg or 332,946 Earths), 99.86% of the Solar System's total, wouldn't it then be possible to calculate were the gravitational influence of the sun ends, in other words: at what distance would an object no longer be pulled into an orbit around the sun? Currently, the edge of our solar system is only estimated to end at the Oort cloud, at a distance of about 50,000 AU from the Sun (1000 times the distance from the Sun to Pluto or nearly a light year). Why isn't a precise calculation made yet, if possible?

I'm far from an expert on the subject, but I saw a pretty cool picture the other day on Wikipedia, but couldn't remember where. I think it might have had something to do with the Voyager or Pioneer Probes. It showed the borders of various things such as the Oort Cloud and the Termination Shock and the Heliosphere and such. Opinions on what constitutes the boundary of the Solar system is certainly one problem. Perhaps, like the boundary between the Earth's Atmosphere and Space, there really isn't one. My own opinion is that all objects in the universe exert some influence on all other objects depending on their Gravitational Mass and distance. If that were the case, then there would be no true border, only a point where its influence became infinitesimally negligible. Hopefully someone better informed will be able to give you a more satisfactory answer. --Demonesque 04:49, 2 September 2007 (UTC)

I'm familiar with the Termination Shock and that image. But as I stated, I want to calculate the endpoint of the Sun's gravitational pull, not were the solor wind ends, it's two different things. A mass can cause gravity to bend the space fabric, see this picture from the earth for example: http://science.nasa.gov/headlines/y2005/images/gpb/vortex1_crop.jpg Now at the end of the "gravity well", were the line is horizonzal, an object is not catched by the earth. But when it enters the well, it will start to orbit the earth as it's moon. The same applies for the Sun. So my question is: were ends the "gravity well" of the sun, it is calculatable? Please take a look at this movie from NOVA (The Elegant Universe 3/8, you need Quicktime): http://www.pbs.org/wgbh/nova/elegant/media2/3012_q_03.html --Patrick1982 18:33, 2 September 2007 (UTC) Okay for anyone still interested, I found an answer. It is related to the Inverse-square law see here: [4] at 340km heigth, the International Space Station experiences only 90.13% of Earth's gravitational pull (6378,137^2 / 340^2 *1 *100 = 90.13%). So at 20 million kilometers away from earth, the pull is only 0,00001%. However the line goes very slowly to 0% (actually it never reaches 0% and goes on to infinity!). At Mars, 78 million km from Earth, the pull will still be 0,00000067% so the two planets effect each other's orbits at verry low levels! Now going back to the Sun, the calculation is as follows when observing the distance of Pluto: 695000^2 / 5906376272^2 *27,94 *100 = 0,0000014%. So this number appears verry weak, but still it is enough to get Pluto to orbit around the Sun! The Oord-cloud starts at 20,000 AU, so the calculated pull is 0,000000000005%. Furthermore, even at the nearest star, Proxima centauri, the Sun still has a gravitational pull of 0,00000000000003%! Only now I can see the problems of my underlying question: n-body problem. --Patrick1982 20:41, 26 September 2007 (UTC)

Just thought I'd bring attention to some apparent pseudoscience. A new page has been created entitled Gravitational distribution. I think it should be deleted. The supposed effect is not explained in any reasonable way. The author him/herself admits that it hasn't been verified, and has no equations to describe the effect. I deleted some stuff the same author added to this article. Someone other than an IP want to take care of the deletion of the other article? --131.215.123.98 18:38, 4 November 2007 (UTC)

Use one of the templates from WP:CSD. I think it won't survive :-) DVdm 18:56, 4 November 2007 (UTC) It's not obvious to me that this qualifies for speedy deletion. It's clearly not vandalism. Also, while WP:CSD does mention patent nonsense, it specifically says that doesn't include implausible theories. In any case, I edit anonymously because I don't want to be drawn into these behind-the-scenes struggles. I just want to bring it to the attention of people who can stomach these fights. (I know that's a wimpy cop-out, but it's a decision I've made. Sorry.) --131.215.123.98 19:16, 4 November 2007 (UTC) P.S. It looks like User:Glacialvortex is back at it, though claiming to have sources. Like I say, I don't want to get drawn in. --131.215.123.98 19:19, 4 November 2007 (UTC) I have added the tag. I will revert the new attempt on this article. DVdm 19:25, 4 November 2007 (UTC) Thanks. --131.215.123.98 19:35, 4 November 2007 (UTC) The author placed a <> tag and wrote on the talk page of the article that he was the creator of the theory. After a quick look at a few relevant passages I pointed to on WP:NOR, the author understood about Wiki policy and was comfortable with the idea that the article should be deleted. By the time I could say thanks and good luck, the article and its talk page were already deleted. Cheers, DVdm 20:21, 4 November 2007 (UTC) Very nice work! Well done. --131.215.123.98 20:52, 4 November 2007 (UTC)

I find the sentence in the introduction "Gravitation is also the reason for the very existence of the Earth, the Sun, and most microscopic objects in the universe without it, matter would not have coalesced into these large masses and life, as we know it, would not exist." somewhat (not a lot) philosophical and dogmatic. Anyways, my view would be that matter is infinitely more a reason for the very existence of the Earth, the Sun, etc. and that gravitation plays an important but secondary role. Maybe I am reading too much into the sentence. I think it is the “very existence” and the “life … would not exist” parts that get me. I will admit to a bias that I have I feel a twinge whenever scientific thought, effort and knowledge is presented as we absolutely (or for that matter, even remotely) know what happened and how the world works as opposed to our best understanding to date is . Along the same lines, I do not subscribe to the infallibility of science, scientific theory, and um, scientists (which would include myself). Irrespective my flaws and bias, I think that the article would be improved if the coalescence of matter and the formation of the Earth, planets, Sun and other macroscopic objects would be added to the existing previous sentence which already lists the things for which (we currently understand) gravitation is (in part) responsible. AikBkj 17:06, 9 November 2007 (UTC) On the contrary, gravity would have destroyed the earth by making it part of the sun except for the fortuitous circumstance that the mass of the earth managed somehow to retain half of its lost potential energy with respect to the sun's gravitational field and convert that to an orbital velocity around the sun. And dont ask me how.WFPMWFPM (talk) 02:40, 10 June 2008 (UTC)WFPMWFPM (talk) 02:43, 10 June 2008 (UTC)

The article claims that "[the] value [of the strength of the gravitation field] at the Earth's surface, denoted g, is between 8.4 and 10.6 m/s²". I don't think that on the Earth's surface there is any place where it is that strong or that weak. In the Earth's gravity article, there are values in some cities, ranging from 9.779 m/s^2 in Mexico City to 9.819 in Helsinki and Oslo. On some high mountain near the Equator it'll be even less, and at the North Pole it'll be even more, but definitely not as much/less as 10.6 or 8.4. --Army1987 11:06, 11 November 2007 (UTC)

Yea, something odd there. In the article on Earth's gravity in the section on Altitude, I noticed this "In reality, the gravitational field peaks within the Earth at the core-mantle boundary where it has a value of 10.7 m/s², because of the marked increase in density at that boundary." Maybe that's where one of those numbers came from? The core starts at a depth of about 2890 km according to the article on Earth, not exactly part of the Earth's surface. AikBkj 15:13, 11 November 2007 (UTC)

I didn't see in the article that this point was brought up, but should it be mentioned that heavier objects do in fact fall more quickly than lighter objects?

I can understand the reasoning behind teaching to people who are new to the subject that objects fall at the same rate and will hit the ground at the same time when dropped from the same height. But, being an encyclopedia article, should it not be explained that this is actually a misconception? Just for technical accuracy, I mean. It's not really important for the average person to know this, but for those who choose to study gravity in further detail, the teaching that all objects fall to Earth at the same rate regardless of their masses introduces an early misunderstanding that, though such a belief is acceptable for most practical applications, it is actually contrary to the way gravity actually works. -=( Alexis (talk) 17:42, 24 November 2007 (UTC) )=-

Please do not post that statement to the article. An astronaut conducted this experiment on the moon. Galileo's experiment holds Aristotle was wrong and Galileo noted this in 1638. Air resistance will alter the rate of fall but the dependence on air was one of the factors that Galileo accounted for. If you have questions, you are welcome to google "Apollo astronaut feather experiment" .--Ancheta Wis (talk) 18:06, 24 November 2007 (UTC) Here is a video of the Apollo 15 experiment performed by Astronaut David Scott on the moon. The video demonstrates that the dropped feather and hammer hit the moon at the same time. --Ancheta Wis (talk) 18:42, 24 November 2007 (UTC) When you say that larger objects fall more quickly are you referring to the fact that the Earth is pulled ever so slightly more to the larger object? I do not believe this is worth mentioning as that difference is immeasurable. It would be confusing at best, misleading at worst. Ben Hocking (talk|contribs) 17:59, 24 November 2007 (UTC) The concept of relative displacement (both objects move) is discussed in the last few sentences in the section "Earth's gravity”. Also, Newton's law of universal gravitation is wikilinked in the section directly following the section on “Scientific revolution” (which mentions air resistance) for those who wish to learn more detail. The existing text would seem to address the concerns of Alexis from what I understand. By the way, for technical accuracy, if one (like Galileo) drops two objects of different masses, they will hit the ground closer to the same time than Alexis might think (ignoring air resistance, etc). Consider that the absolute acceleration and absolute displacement of the Earth will be identical for both (since the drops are occurring simultaneously). The Earth’s motion would be very slightly different only if the drops are done separately and timed. I would agree with Ben’s assessment that this type of detail in the article would be misleading considering the other effects which become important at this level. AikBkj (talk) 16:32, 28 November 2007 (UTC)

What would happen if you dropped 2 objects, both being the exact same size and shape, but one weight 1 pound on the moon, and the other weighed 10 pounds with no air resistance, then dropped both at the exact same time at the exact same height on exactly leveled ground? I am pretty certain that the 10 pound object will hit the ground first, but I also know that the difference in time will be really short.
I have been thinking about this since I came across something I did not like on a game that I was playing and have been thinking since. My question to myself was, "If someone could jump 100 feet into the air, should that person be able to land the fall without taking any damage? (This is not putting into consideration on how that person lands, or how their internal organs and such move around). I was trying to find the formula to see how much force would be required from the initial jump as well as the ending impact. I mean, how much force would somebody need to use in order to jump that high to begin with?
If anybody wants, email me at [email protected] Szayn (talk) 22:54, 8 December 2007 (UTC) —Preceding unsigned comment added by Szayn (talk • contribs) 22:50, 8 December 2007 (UTC)

See the video of the Apollo 15 experiment performed by Astronaut David Scott on the moon. This experiment on the moon showed that the feather and the hammer land at the same time. Galileo predicted this in 1638. Ancheta Wis (talk) 23:35, 8 December 2007 (UTC) If you did fall from 30.48 metres (100 feet) from the surface of the moon, after 6.13 seconds of falling you would hit the ground at 9.9 m/s (22 miles per hour). This is the same as falling from 5 metres (16.5 feet) on Earth in terms of the speed at which that you would hit the ground. It is highly likely that you would survive this jump, although you could twist or even break something. People with more mass are more likely to be injured. I'm not an expert on human durability, but if you don't weigh too much, don't have any joint problems, and know the best way to land you should be able to land without taking damage. By the way, the high jump world record holder Javier Sotomayor would only be able to jump about 15 metres (50 feet) on the moon. The force required to jump a certain distance is dependant upon the weight of the person jumping. You should be able to jump a little more than six times the height you can jump on Earth as long as you aren't burdened by a heavy oxygen tank or space suit. Out of curiosity, what game are you referring to? Jecowa (talk) 14:22, 19 December 2007 (UTC)

"In scientific usage gravitation and gravity are distinct. "Gravitation" is the attractive influence that all objects exert on each other, while "gravity" specifically refers to a force which all massive objects (objects with mass) are theorized to exert on each other to cause gravitation. Although these terms are interchangeable in everyday use, in theories other than Newton's, gravitation is caused by factors other than gravity. For example in general relativity, gravitation is due to spacetime curvatures which causes inertially moving objects to tend to accelerate towards each other."

However, the Fundamental interaction article implies throughout that in general relativity gravitation is a force, and in at least two places it explicitly says so:

"Gravitation is by far the weakest interaction, but at long distances is the most important force." "Thus large celestial bodies such as planets, stars and galaxies dominantly feel gravitational forces"

Something needs fixing here. Matt 14:43, 3 December 2007 (UTC).

The problem is with points of view. What Newton and Galileo viewed as an influence due to force is due to the geometrical curvature of a 'trajectory' (called a geodesic) in Einstein's view. So the sentences might be reframed "Gravitation is by far the weakest interaction, but at long distances gravitation is the most important factor (in the motion of objects). Thus large celestial bodies such as planets, stars and galaxies dominantly feel gravitation (rather than the strong, weak or electromagnetic forces)." You are touching on an important point, which is about inertia and how it is defined with respect to a reference frame. For Galileo, 'rest' was obvious, but how do you assert what that means for galaxies, galactic clusters or even larger contexts? --Ancheta Wis (talk) 23:48, 8 December 2007 (UTC) This point relates to a fundamental difference between how gravity is viewed in general relativity (where gravity is not viewed as a force), and how it is viewed in quantum mechanics (where it is, although a force so weak that it's almost always ignored). However, the statement seems to be proposing a distinction in terminology that is not commonly used by actual physicists. This statement "in theories other than Newton's, gravitation is caused by factors other than gravity" really does not reflect the way that the term is used by physicists, or by anybody else. It's just not the way the term is used, and I will see if I can rewrite the section to reflect this. Geoffrey.landis (talk) 03:46, 14 December 2007 (UTC) OK, so we seem to have established that in GR gravity/gravitation definitely isn't a force. So, in addition to the clarifications you have made to this article about the meaning of the terms "gravity" and "gravitation", should the Fundamental interactions article also be tweaked to make this clearer? That article states at the start that GR is the "current theory" of gravitation, and the implication is that the statements made there about gravitation pertain to the GR viewpoint. So, am I right in thinking that it shouldn't be referred to there as a force -- or there should at least be some caveats? Matt 01:30, 16 December 2007 (UTC). I wish there were a simple answer to this query. At one level, it's a question of definitions more than a question of real physics: a question of what you chose to define as a force. In General Relativity, it's convenient in many cases to just pick a freely-falling reference frame, and in that frame the "force" of gravity doesn't exist by definition, and since the GR theory has built into it the techniques to transform into any reference frame, this is straightforward. You can formulate GR in such a way as to treat gravity as a force-- this is perfectly acceptable, but has the conceptual disadvantage that it ends up with frame-dependent descriptions, which true relativists disdain as inelegant. In quantum mechanics, though, it's more convenient to deal with gravity, if you ever need to, as a force fundamentally no different than any other. Certainly in classical mechanics it's straightforward to think of gravity as a force. (Real physics tends to deal in potentials rather than forces at a fundamental level anyway-- in fact, Mermin argued in Physics Today that we shouldn't even teach F=ma anyway, we should just go right to potentials). Overall, I think that any "clarification" would probably end up muddying things and end up being more confusing than explanatory. Geoffrey.landis (talk) 02:26, 16 December 2007 (UTC)

Would be nice to include comparable graviation forces on our closest planets, and an example of what the actual difference recorded in gravity with height and negative height. —Preceding unsigned comment added by Seb-Gibbs (talk • contribs) 16:23, 21 December 2007 (UTC)

That was formerly on this page but was put on a child page to save space. Comparative gravities of_the Earth Sun Moon and planets can be found here --Ancheta Wis (talk) 18:58, 21 December 2007 (UTC)

There is a nice sentence in the main article (between other nice sentences):

Modern physics describes gravitation using the general theory of relativity, but the much simpler Newton's law of universal gravitation provides an excellent approximation in most cases.

While it is true that Newton's law is much simpler and sufficient in most cases because it does not require to understand how the gravitational force is generated but a bad part of it is that being simpler it's not true (for the lack of attractive force acting through vacuum). So why wikipedians keep around this sentence instead of explaining how the modern physics explains the gravitational force? Would the wikipedians didn't know how and would my professor be right maintaining that the physics students are too stupid to understand spacetime curvature (when I proposed to him to teach gravitation at the first year of physics instead of waiting till post gradute studies)? Jim (talk) 23:56, 21 December 2007 (UTC)

And the effect of gravitation on the atomic clocks of the satellites in the global positioning system is non-neglible as well, so we can't ignore Einstein in our daily Newtonian lives either. --Ancheta Wis (talk) 00:57, 22 December 2007 (UTC) We not only can ignore Einstein but we do. The wikipedia editors revert any attempt to explain the Einsteinian gravitation to high school students. The result is that virtually no physicist, who usually learns the gravitation at high school, knows the simple Einsteinian derivation of gravitational force and how it follows from curvature of space and time dilation. Or even heard about it. After more than 100 years after of Einteinian gravitation. Jim (talk) 04:10, 22 December 2007 (UTC) I'd say that physics is not about "being true", but about "being useful" and about providing tools to describe rather than to explain. Newton's law of gravitation is neither true, nor untrue. It is useful, simple, and it works - in most cases. That is a fact and i.m.o. not really open for debate. I think that the nice sentence is sufficient - at least for the average high school student. DVdm (talk) 11:07, 22 December 2007 (UTC) The point of view is called pragmatism. Unfortunately, the founders of the science would not have discovered what they did from pragmatism alone. They were driven by passion: Galileo, Newton, Einstein. They spent years of their lives on this and we reap the benefit, but it does disjustice to physics and physicists not to acknowledge their motivation, which transcends being useful. Physics is true and useful. --Ancheta Wis (talk) 15:07, 22 December 2007 (UTC) I propose to baptise this point of view scientific passionism, but I don't really see its relevance in the context of Jim's question. DVdm (talk) 15:59, 22 December 2007 (UTC) Physicists call "physics" only what is true. Otherwise it is called "math" or "magic". Good physics has to follow the truth verifying it through observations and experiments. Good math has to follow logic working with assumptions. The truth doesn't apply to it, only to its assumptions. The magic needs to follow neither the truth nor the logic just the prejudice. That's why 300 years of physics gave more than 3,000 years of math and 30,000 yeas of magic. And that's why disriminating against Einstein's physics by universities and wikipedia is silly even if it's good to professors and editors. Jim (talk) 21:21, 22 December 2007 (UTC)

The section originally had this line: "Several decades after the discovery of general relativity it was realized that general relativity cannot be the complete theory of gravity because it is incompatible with quantum mechanics". This statement implicitly promotes a point of view: the correct statement is that GR and quantum mechanics are incompatible, and hence either GR is an incomplete theory, or quantum mechanics is. (or both, of course). It's quite plausible that it may be possible to reformulate QM in a way to incorporate highly-curves spacetime, and that doing so would solve the problem with no changes in GR. I rewrote this section slightly to remove the implied statement that, of the two theories, the one that's in errror is GR, not QM. I'm also not at all sure that the formulation of gravity as a force involving virtual exchange of spin-2 gravitons postdates the (rather slow) realization that QM and GR are incompatible at the fundamental level in fact, without formulating GR in terms of quantum field theory, you can't really show that GR and QM are incompatible, so I removed a handful of words suggesting this timeline.Geoffrey.landis (talk) 16:06, 4 January 2008 (UTC)

Looks like this might be worth reading and mentioning in article:

"In a paper in the August 3 online edition of the Institute of Physics' peer-reviewed Journal of Cosmology and Astroparticle Physics, they put forth the idea that scientists were forced to propose the existence of dark energy and dark matter because they were, and still are, working with incorrect gravitational theory.

The group suggests an alternative theory of gravity in which dark energy and dark matter are effects – illusions, in a sense – created by the curvature of spacetime (the bending of space and time caused by extremely massive objects, like galaxies). Their theory does not require the existence of dark energy and dark matter.

“Our proposal implies that the 'correct' theory of gravity may be one based solely on directly observed astronomical data,” said lead author Salvatore Capozziello, a theoretical physicist at the University of Naples, to PhysOrg.com."

Dark Energy and Dark Matter – The Results of Flawed Physics? By Laura Mgrdichian, Copyright 2006 PhysOrg.com Discussion of J. Cosmol. Astropart. Phys. 08 (2006) 001.
http://www.physorg.com/news77190620.html

I think that this article [5] is about the Pioneer anomaly. Given the source (The Economist), I thought it could be discussed here since it's obviously gaining notability. --Childhood's End (talk) 21:27, 7 March 2008 (UTC)

Could someone please explain why the gravitation template should be removed? So far two editors seem to have expressed a desire to remove it, although I do not see any particular reason for it. Silly rabbit (talk) 14:30, 8 March 2008 (UTC)

The last editor to delete it explained: "this is a general public article", presumably suggesting that it was a bit complicated to be the lead picture. Would it be more acceptable to bring it back opposite the text to which it refers: Gravitation#Newton's theory of gravitation? --Old Moonraker (talk) 14:54, 8 March 2008 (UTC) That's a good idea. I also had another thought. Now that the template is gone (or at least moved), someone should find a suitable image to go alongside the first paragraph text. Initially I thought about using the solar system image which is already in the article, but it's a bit oversized. There are loads of nice images which could go here. I looked over at black hole for something very dramatic, although such images may not be appropriate. Silly rabbit (talk) 14:57, 8 March 2008 (UTC)

How does gravity effect space time when there is nothing ( or near nothing ( vacuum )) in that space-time?

When looking at gravitational effects specifically when applied to light travelling from distant stars can we be deceived that gravity is constant?

Does a gravitational field only exist when there is mass in that field, or does its gravitational well exist when there is no matter there? To take this a step further would it be possible that a gravitational field with respects to space time bending if it has no mass near it?

Is there an easy experiment to prove this wrong? ---- (Tommac2|Tommac2)

Please keep in mind that this is not the appropriate place to ask questions such as this. This is for discussion of the article itself. To answer you very briefly - gravity is not bending things in space-time but rather bending space-time itself. I recommend reading up on the subject in some introductory texts before going much further. PhySusie (talk) 15:25, 26 March 2008 (UTC)

I would propose that gravity shares many of the same characteristics as a superfluid. [6] —Preceding unsigned comment added by Rdailey1 (talk • contribs) 16:44, 1 April 2008 (UTC)

I believe that your idea has merit. I have been developing a hypothesis based on an idea similar to this, but because of time constraints I haven't been left the time to develop the mathematics that would be required to back such a strong claim. Stating this has many implications, such that G is not constant and similar things. Unfortunately research of this type doesn't belong on Wikipedia (see WP:OR) but would be more fit for a forum on similar topics. Infonation101 (talk) 04:53, 11 April 2008 (UTC)

What about the experiments on gravity done by Milhouse Van Houten? —Preceding unsigned comment added by 68.164.89.236 (talk) 05:57, 10 April 2008 (UTC)

Quote: "According to Kanada, founder of the Vaisheshika school".

Quote: "it would be Isaac Newton that gave the first correct description of gravity".

Comment: Galileo's description is correct for phenomena close to the surface of the earth where the acceleration of gravity is constant, just as Newton's description is correct for phenomena at low velocities and weak gravitational fields where the general relativistic corrections are small, and Einstein's description is correct at low energies when quantum phenomena are not important.

Suggestion: Remove the remark.

It should be said in the beginning that gravitation is the reason why the earth is round. Bo Jacoby (talk) 08:14, 27 April 2008 (UTC)

It's 3 dimensional space that makes the earth round. In one dimensional space the earth would be a straight line.WFPMWFPM (talk) 12:08, 10 June 2008 (UTC)

A counterfactual assumption! Still, asteroids having weak gravitation are not round. Bo Jacoby (talk) 22:30, 10 June 2008 (UTC).

They would be if they were fluid enough.WFPMWFPM (talk) 17:44, 11 June 2008 (UTC)

Another conterfactual assumption. But confirming that gravity is the force that makes the planets round. Bo Jacoby (talk) 19:10, 12 June 2008 (UTC).

For information: There is a thread on Talk:Fictitious force asking whether this article should be in the category "fictitious forces". --PeR (talk) 13:17, 15 May 2008 (UTC)

Gravity is weaker than the strong nuclear force, weak nuclear force and electromagnetic force. Why is this not mentioned in the article? There are some very interesting theories on where the energy of gravity goes to, such as other membranes. Altonbr (talk) 13:19, 9 June 2008 (UTC)

I'm not much of a math wizard, so I'll ask this question in more mechanical terms. Let's say, hypothetically, if someone could hollow out a room at the very center of the Earth, would he be pulled to the center of its mass, (ie: float in the middle), or would he be pulled toward the mass itself, (be able to walk across all the walls)? 216.67.92.66 (talk) 17:36, 11 June 2008 (UTC) CLewis Yep! Just like in the orbiting space laboratory.WFPMWFPM (talk) 17:48, 11 June 2008 (UTC)

OK, assuming that you answered yes to the first part of my question, gravity is a property of space, I would float in the center of this hypothetical "room" at the center of the Earth. But, unlike the spacestation, because of the Earth's vastly larger mass than my own, it seems to me that I would still be experiencing weight, (in way that would feel more like pressure than weight as we normally experience it). I would be stuck fast at the center, and if someone were to lower a ladder to me, my weight would be a great deal more than it is on the surface. Would this be correct? —Preceding unsigned comment added by 216.67.92.66 (talk) 19:35, 11 June 2008 (UTC) When you say center of the earth I assume you mean the gravitational center, which is where all the gravitational force of the surrounding matter balances out. At that point there would would be no net directional gravitational force and thus no weight.WFPMWFPM (talk) 22:41, 11 June 2008 (UTC)

Gravity is a property of matter moving through space. It arises entirely from the law of conservation of momentum. It exists simply because that's the way matter has moved and will continue to move in accordance with momentum conservation laws. As I see it, there's nothing mysterious about that. As for that "room in the center of the earth", the mass surrounds you from all directions has very little to do with adding to your weight. Much of it will cancel itself and would be far less than what is on the surface.Kmarinas86 (6sin8karma) 17:33, 12 June 2008 (UTC) Kmarinas86, Thank you for the citation. I quote from it "a mass moving at a constant acceleration does not radiate" which is very similar to the situation for charge at constant velocity. It suggests an experiment which must be in the literature: for any astronomical bodies which are rotationally accelerating (and therefore non-constant acceleration), what are the measurements for gravitational radiation? What comes to mind immediately are black holes under increasing rotational acceleration. --Ancheta Wis (talk) 04:35, 5 July 2008 (UTC)

Thank you for these responses. I agree, there does seem to be a direct corrolation between gravity and motion, (namely, accelleration), possibly linked to the speed of light, I don't know.

As the article states, General Relativity says that gravity is caused by curvature in spacetime, which is a hard concept for some of us to wrap our heads around. Teachers often use the analogy of a bowling ball on a trampoline to describe the "gravity well", (as a golf ball set on that trampoline will roll toward it). The section on Newton describes how weight decreases in proportion to an objects movement from the "center about which [the Earth] revolves". By this, it seems to me that my weight would increase the closer I get to the bottom of the gravity well, regardless of the amount of mass above me, and "up" would always be "away from center". That implies to me that gravity is a property of the universe interacting with matter.

The idea that there is a "balancing point", by which at the center I would be pulled equally in all directions, and therefore not pulled at all, tells me that if I were to move slightly off center, an imbalance would be created by which I would be pulled toward the walls of this fictional room. In which case, "up" would no longer be "away from the center", and my weight should actually decrease the deeper I travel into the Earth. Therefore, gravity would be a property of matter interacting with the universe. But I am no expert on the subject. I'm just trying to get a little more of a mechanical explanation, rather than just a mathamatical one. Thanks for the input. 216.67.92.66 (talk) 20:08, 13 June 2008 (UTC) CLewis

Gravity is a property of space-time, not space. Mass (actually energy) creates hills and valleys of space in the 4-dimensional space-time universe that Einstein conceived. The center of Earth is the bottom of one such 'space' valley. As you move from the center of earth you move out of that valley you start going uphill, or more accurately you start going 'up-space' -- accelerating away from the 'space valley'. In the Newtonian frame, you start feeling a force pulling you towards the center as you move away from the center. To continue in the Newtonian (illusory) frame, as you move away from the center of the earth, there becomes more mass behind you than ahead of you so you start to feel a 'gravitational force' pulling you back towards the center. The further away form the center that you get, the stronger becomes this apparent force because the amount of mass behind you keeps increasing and the mass ahead keeps decreasing. (Looking for references to use for this. anyone else got references for this?) -- Len Raymond (talk) 14:49, 21 June 2008 (UTC)

Thanks, but this view seems to contradict itself. If gravity is caused by "hills and valleys in spacetime", then wouldn't all matter within the Earth's gravitational field only add to that field, in a direction that would cause matter to be pulled to the center with an increasing degree of curvature until reaching the bottom of the well. Then Newton's math should still work, (in reverse of course), all the way to the bottom, which would make the center of the Earth a singularity of such, (no, not a black hole), but in the way that all directions from the center would be up? 216.67.92.66 (talk) 21:31, 21 June 2008 (UTC) CLewis

As I move towards the center, the mass behind me keeps increasing and the mass ahead keeps decreasing. This causes the curvature to get flatter (decrease, not increase). At the center, the curvature becomes completely flat (weightless) -- zero gravity in the Newtonian model. Realize that matter adds in a localized, directional fashion to the gravity field. When you stick a clump of matter to the side of a ball the curvature increases towards the point at which you added the clump, not towards the point at the (old) center of the ball. Important note: Adding this mass actually shifts the center of mass -- towards the added clump -- so the old center-of-mass no longer has zero G's. The old center now has a slight gravity pull towards the new center. Your room analogy has a another issue. To simplify, imagine one inside a hallow ball. In that case, a single piece of loose matter (such as a yourself inside the ball) would, I expect, be 'attracted' to the matter in the wall -- net effect: there is a small gravitational effect towards the nearest wall (towards the nearest matter). But the Earth is not hallow. Any point in the Earth is fully packeted with matter on all sides, all exerting a 'pull'. The net effect: The side of you that is surrounded by the most mass is the side one is 'pulled' towards unless, of course, you are at the center of mass where all sides pull equally. -- Len Raymond (talk) 13:48, 23 June 2008 (UTC)

I'm not sure this is the proper place for a discussion on the subject. I just thought my question seemed rather simple. "Pulled toward center, or pulled toward the mass?" There should be some easy experiments that would show this, (ie: an atomic clock/deep cave experiment, or a drop of mercury in a space at the center of a large lead ball, while free falling). 216.67.92.66 (talk) 21:31, 21 June 2008 (UTC) CLewis

• Good exercise on wording for spacetime curvature concepts

Thank you very much. It never occured to me that way, but that answer makes perfect sense. The bottom of the valley is not a point in space, (as a black hole would be), but a nice "round" basin. I hope this helps other readers understand. As for helping improve the article, all I can say is that the language of math is a very precise language, which can make it very difficult to translate into everyday speech. Thanks again for answering my question. 216.67.92.66 (talk) 16:20, 24 June 2008 (UTC) CLewis

There are some people who believe that gravity dose not exist and that there is some logical explanation for the planets to be orbiting the sun and for us to come back down when we jump there is even an explanation for why we even stay on the ground. The planets are able to stay in orbit around the sun because space is like a trampoline pulled tight and you can imagine the planets to be like ants. Now if you were to drop a bowling ball (sun) onto the "trampoline" it would make the "trampoline" have a hole like imprint. Now if the "ants" were to come anywhere near the "bowling ball" they would slide towards it. This is how I believe the planets stay in orbit. For more info check out the book: "The Quantum Zoo: A Tourists Guide To The Never Ending Universe" By: Marcus Chown. When we jump we come back down because the Earth's core is made up of what most people think is hot, molten lava and this lava pushes against the underside of the Earth's surface now when we jump we are in free fall for a small amount of time and since the Earth is blocking our path to continue we land on it. We stay on the Earth's surface because we are pushing against the lava on the inside. This is what causes us to stay on the surface of the Earth. You must remember that this is just a theory and it can be incorrect or correct and the same goes for the theory of gravity existing. Theoretical Physicist (talk) 15:19, 15 July 2008 (UTC)

What you describe sounds like an analogy that is often used to qualitatively describe the relationship between spacetime curvature and orbits in General Relativity. Indeed, in General Relativity, there is no force of gravity -- but gravitation as described in this article does exist, because it's just the term for why massive objects "tend to come together" and has no prejudice about the mechanism. So if you're trying to describe GR, we've got you covered -- and if you're describing something else, then you'll need a reliable source to add it to the article. By the way, your material was removed earlier because it probably seemed to be a using Wikipedia as a soapbox rather than making suggestions for the article. -- SCZenz (talk) 15:36, 15 July 2008 (UTC) It was also deleted because you replaced the box at the top of this talk page with your theory rather than putting the comment at the bottom of the page. Besides that, this talk page is for discussing the article, not presenting alternate theories. There are more appropriate places for that. PhySusie (talk) 15:43, 15 July 2008 (UTC)

I agree with -- SCZenz (talk). This is correct with GR. But have you ever wondered if there actually is no such thing as gravity? It sounds kind of weird to have a force that makes us stay on the ground, I wonder if there is a way to prove the theory wrong. Think about it it's the only fundamental force that isn't unified with the other three fundamental forces.

Folks, the purpose of this talk page is to discuss proposed changes to the article, not to propose alternative theories of/to gravity. There are plenty of other forums where you can do that. Can we please close this discussion now. Thanks, Gwernol 17:01, 15 July 2008 (UTC)

If you pass a magnetic object through a coil you produce an electric current, correct? It is basis of traditional generators. But what if the driving force for said generator was gravity.

Now, hypothetically speaking, if you constructed a coil of truly colossal dimensions and placed it around the earth (or a simmilar body), and then placed in orbit an object with magnetic qualities in such a way that it grew no closer or further from the earth, and said object orbited within the coil, wouldent you have on your hand a generator run purely by gravitation?

If gravity is NOT energy, then this senario defys the conservation of energy, but I cant see why it wouldn't work. If gravity IS a form of energy, then that is one enormous and non poluting souce of power.

Call me a Quack or call me a Genious, I just want to know if this would work.--Hsan007 (talk) 07:46, 16 July 2008 (UTC)

## III. What do asteroid look like?

### III.1. Means of study

Lightcurves: The main way to study the rotation period and orientation of asteroids is the acquisition of lightcurves: the variation of the asteroid brightness as function of time. Because asteroids only reflect the sunlight (they do not produce light by themselves at visible wavelength) the quantity of light we measure (called photometry) is function of the apparent surface area of the asteroids. Over its rotation, this projected surface on the plane of the sky changes, and draws a sinusoidal curve (Figure 7). The rotation period can be directly measured on that curve and can be thus achieved in a night of observation. The rotation period is known for about 15,000 asteroids as of today (July 2016).

Determining the orientation of the spin axis requires more observations. To each Sun-asteroid-observer configuration corresponds a different lightcurve. It is easy to see that if the Earth is aligned with the spin axis the lightcurve will present no variation. On the contrary, its variation will be maximized if the Earth lays within the equatorial plane of the asteroid. Lightcurves taken in different geometry will have reduced amplitude, and different shapes. It is therefore requiered to collect lightcurves of the same body, under different Sun-asteroid-observer configurations, to determine the orientation of the spin axis. This is why this knowledge is still limited to a few hundreds of asteroids.

The shapes of the lightcurves being dictated by the apparent surface area of the asteroid over time, they can be used to reconstruct the 3-D shape of the asteroid (Kaasalainen et al. 2002, Durech et al. 2015). A single lightcurve is of course not enough. Many lightcurves, acquired under many different geometries must be collected. It is a complex problem that can lead to ambiguous solution. Yet, since early 2000s, it is the main source of 3-D shape models of asteroids. There are current a thousand asteroids with a shape model (see the DAMIT database, Durech et al. 2010).

Figure 7: How to determine the rotation period of asteroid with lightcurves. Credits: B. Carry (IMCCE).

Direct imaging: Small bodies of the solar system are. small! Even the largest asteroid, (1) Ceres, has an angular diameter below the arcsecond (1°/3600). This is the angular size of a 2 Euros coin placed at 4 km. It is thus extremely difficult to directly image asteroids. Until the 1990s and the first flyby of an asteroid by a spaceprobe (Ida by Galileo on its way to Jupiter) asteroids had remained point-like sources.

The main issue is the atmospheric turbulence, which "blurs" the images, degrading their resolution. The measure of this degradation is called the seeing. Even in favored sites, like the Mauna Kea on Hawai`i or the Atacama desert in Chile where the world's largest telescopes have been built, the seeing remains generally above one arcsecond. It is therefore impossible to observe small details. Thanksfully, a real-time correction has been invented in the late 1980s, called adaptive optics. With this technique, the first images of (1) Ceres and (2) Pallas were obtained in 1991 with the ESO 3.6m telescope (Saint-PÃ© et al. 1993). These were followed by many others once 10m class telescopes were operational: ESO VLT, Keck, Gemini. The Hubble space telescope, non affected by atmospheric turbulence thanks to its location above the atmosphere was also successfully used to image the two largest asteroids, (1) Ceres and (4) Vesta (Thomas et al. 1997).

The main interest of direct imaging is its repeatability. The second is the fact that it represents a direct measurement. Lightcurves allow to reconstruct the 3-D shape of an asteroid but only by stock-piling data for years. A few images taken over a night can provide the same result, each image providing the size and shape on the plane of the sky. Such images have been obtained on several tens of asteroids.

Figure 8: Example of direct imaging. Credits: NASA/ESA/JAXA.

Stellar occultations: Another direct way to directly measure the apparent size and shape of asteroids consists in recording the duration of the disapparition of a star when an asteroid occults it. This duration can be converted into a length on the apparent disk of the asteroid, called a chord, by knowing its apparent speed, which is a direct by-product of our knowledge on its orbit. If several observers are located on the occultation path, several chords are measured and draw the apparent shape at the time of the stellar occultation.

This technique relies on a timing measurement, and can therefore be much more precise than direct imaging. Moreover, the observability of the occultations depends more on the star characeteristcs (mainly its apparent brightness) than of the occulting asteroid. Many targets too small angularly to be imaged (intrinsically small or very distant) can thus be studied by stellar occultations. However, because a given asteroid will only seldom occult a bright star, stellar occultations must be used in combination with other types of observations. Indeed, only a few asteroids have multiple recorded occultations with several chords, although more than 2000 stellar occultations were observed (mainly by amateur astronomers). Stellar occultations are a very good complement to lightcurves and can be used to reconstrct 3-D shapes (Carry et al. 2010, Kaasalainen 2011).

Figure 9: How to measure the size and shape of asteroids by stellar occultations. Top: Plane of the sky view of negative chords (gray), and positive on the asteroid (blue) and its satellite (green). Bottom: Lightcurve of the occulted star showing the passage of the asteroid and its satellite. Each drop in flux is due to a body, and its length indicate its size. Credits: B. Carry (IMCCE).

Interferometry: the angular resolution of images is limited by the atmospheric turbulence as discussed above. These effect can be corrected if the telescope or the camera are equipped with an adaptive-optics system. However, even in this case, the angular resolution will be limited by the diffraction, inverse proportional to the mirror size with the formula R = l/d, where l is the wavelength in meters, D the diameter of the primary mirror in meters, and R is the angular resolution in radians. As an example, the Hubble space telescope has the same angular resolution as the VLT telescopes, of about 0.05 arcsecond.

It is thus necessary to increase the mirror size D to improve the resolution. This becomes quickly expense and technically diffcult. A workaround consists in using two telescopes simultaneously, separated by a baseline B, and to combine the light they receive. Interference fringes appears, like in the Young experiment. The smallest detail observable is then l/B. The potential improvement is straightfoward: the VLT telescopes being separated by several tens of meters, their joint use provides an angular resolution ten times better than used separetedly.

This resolution is achieved along the line between the two telescopes only. There is no spatial information in other directions: interferometers do not take images. Moreover, this technique is limited to very bright objects: the combination of the two light beams is also disturbed by atmospheric turbulence, and only short integration times can be used. To date, only a few asteroids were studied using interferometry (Delbo et al. 2009, Carry et al. 2015). The technical aspect of interferometry are lower at longer wavelengths, and this technique seems highly promising for obsevratories like ALMA, collecting millimetric and sub-millimetric wavelength.

Figure 10: How to measure an asteroid diameter with inteferometry. Instead of being sent the the Cassegrain or Nasmyth foci, the light is conveyed through the CoudÃ© focus to the interferometric laboratory where it is recombined. The fringe pattern, contrast and separation, is linked with the apparent size of the target on the plane of the sky. Credits: B. Carry (IMCCE).

Radar echoes: Traditionnally, the measure of the position of asteroids to determine their orbit is done on images of stellar fields. The position is this a position on the plane of the sky, without information on the distance. By recording the time between the emission and reception of a radar signal echoed by an asteroid it is possible to measure its distance, and its velocity thanks to the Doppler-Fizeau effect .

Going further, by splitting the signal into delay and Doppler shift, provides informations on the rotation, size, and 3-D shape of the astroide. This information is not direct, but can reach a resolution of a few tens of meters only. Such a resolution by direct imaging requieres to be extremely close to the objects, as it occurs during spaceprobe flyby. Unfortunately, the energy of a radar echo being inverse proportional to the distance at the forth power, only objects coming very close to Earth ca be studied with radar echoes. Hence, only NEAs are studied with radar, when they close closer than 0.01 ua from Earth.

Radiometry in the thermal infrared: At visible wavelengths, the vast majority of the light we receive from asteroids comes from the reflection of the sunlight. In the infrared, the situation is different. Asteroid surfaces have typically temperatures of 200-300 K, and their blackbody radiation peaks around 10 microns. Measuring the flux at these wavelengths, and knowing the surface temperature, provides an estimate of the apparent diameter of the target.

This technique is the largest provider of asteroid diameters. The first catalogue of 2000 asteroid diameters was built in the 1980s using IRAS spaceprobe. The recent NASA WISE mission harvested a large amount of high-precision thermal fluxes, allowing the determination of the diameter of more than 150,000 asteroids from the NEAs to Jupiter Trojans. (ex. Masiero et al. 2011).

One of the limits of this technique comes from the modeling of the surface temperature. Many models exist, from simplistic models in which asteroids are represented by non-rotating sphere (e.g., the Standard Thermal Model, Lebofski et al. 1989), to complex models in which the 3-D shape, the rotation state, the granular aspect of the surface are taken into account (these are called thermophysical models). Depending on which model is used, the resulting diameter can be different. Results are thus unfortunately potentially biased, up to 10-20% in some pathological cases. The value of such catalogues is thus somehow limited for individual objects, also they are gold for statistical purposes.

Figure 12: Radiometry in the thermal infrared. As opposed to visible wavelengths, the brightness of an asteroid in the mid-infrared only depends of its temperature (that we computed) and of its size (which we want to estimate). Credits: WISE/NASA.

### III.2. Rotation period and orientation

The distribution of the rotation period and of the spin axis orientation of asteroids is not uniform, not isotrop. These two parameters bring many information, from the internal structure of asteroids, to non-ravitational forces, to the gas-pebble interactions within the proto-planetary nebula/

Spin-axis orientation: Among asteroids larger than 60 km in diameter, there is an exces of prograde rotator (spinning in the same sense as their motion around the Sun). This exces has been linked to the interaction between the disk of dust and gas in which they accreted (Johansen & Lacerda, 2010).

In asteroids smaller than 20 km in diameter, there is a concentration of the spin-axis orientation perpendicularly to their orbital plane. This is the result of the YORP effect (Yarkovsky-O'Keefe-Radzievskii-Paddack, see Figure 13). This effect is similar to the Yarkovsky effect described above in its mechanism (see II.2): the delayed thermal emission by the asteroid of the heat provided by the sunlight will preferentially orient asteroids in a "spinning top" configuration, either prograde or retrograde, but always with an obliquity close to 0° (Hanus et al., 2011, 2013).

Rotation period: The vast majority of asteroids spin in 4 to 10h. There are however some extremely slow rotators (rotation period of several days) and also extremely fast rotators (rotation period of a few minutes only!). The YORP effect described above is most likely responsible for these extreme periods. Another feature is the spin barrer, at about 2h that only monolithic asteroid can cross and spin faster. This barrer corresponds at a rotation state at which the surface particules are ejected: the rotation speed matches the liberation speed. Hence, only very small and solid asteroids, in which no matter can be ejected, are observed with shorter periods (Scheeres et al. 2015).

### III.3. Size

The largest known asteroid, (1) Ceres, has a diameter of 935 km. By extending the term "asteroid" to all the small bodies of the solar system (the different names of the different population is mainly historical: they are all remnants of the planetary formation), the largest, (136108) Eris and (134340) Pluton, have diameters of about 2400 km. At the other end, the smallest bodies considered as asteroids have diameters of a few centimeters or tens of centimeters. Smaller bodies are generally refered to as dust. Such small bodies have never been observed directly, but the meteor showers are a direct evidence of their existance.

The size distribution of asteroids tells us about the dynamical processes that reign over their evolution. The size distribution of each population is well described by a power law. In the asteroid belt, this power law corresponds that of a theoretical power law for a population in collisional cascade (Dohnanyi 1969). Said differently, the collisions are the main processus which sculpt the main belt. The power law for Jupiter trojans and irregular satellites of giant planets are very different, and highlight a different history: these were captured during planetary migration (Morbidelli et al. 2005).

### III.4. 3-D shape

Far from being spherical, asteroids present a large variety of shapes. The largest, like (1) Ceres and (4) Vesta, have reached hydrostatic equilibrium and their shape is close to an oblate sphere, like the Eeath. The shape of asteroids is however generally dictacted by the collisions they suffered over eons. Large craters have been imaged on every asteroid visited by spacecraft, sometimes with diameters as large as the asteroid!

The overall shape and internal structure is hence revelatory of the past of each asteroid. For instance, two impact bassins have been detected at the South pole of (4) Vesta, first with the Hubble space telescope (Thomas et al. 1997), then by the NASA Dawn mission (Russell et al. 2012). The most recent impact is the source of the asteroid family linked with Vesta, the Vestoids.

For the smallest asteroids, formed through collisions, the shape is highly irregular. These objects are likely the reaccumulation of a large number of fragments, hold together by gravity. These objects are called "rubble pile", such as the NEA (25143) Itokawa, visited by the Japanese mission Hayabusa.

### III.5. Masse

The mass is the most difficult parameter to measure for an asteroid. These bodies are so small that their gravitational influence on other bodies is tiny compared to that of planets. The largest asteroids nevertheless perturb the orbit of Mars satellite, and small asteroids can be deflected during close encounters with more massive asteroids.

To measure the mass of an asteroid, its gravitational influence on another body must be detected, Close encounters between asteroids occur often and allowed to estimate the mass of 300 asteroids (see, e.g., Fienga et al. 2011). The accuracy of such determinations is however limited to date, owing to the complexity of the dynamical system to model, and of the limited quality of the astrometric positions of asteroids.

A much more accurate mass can be determined from the orbital study of a satellite, either natural or articifial (Figure 14). This is how the mass of asteroid visited by spacecrafts was measured with high accuracy. On the other hand, natural satellites are discovered almost monthly around asteroids. Their mass should logically be therefore determined. Practically, only a fraction of these systems have been characterize, owing to the challenge in observing these satellites. Large ground-based telescopes equipped with adaptive-optics system, large radio-telescopes, or multi-year photometric surveys are required (see III.1)

Figure 14: How to measure the mass of an asteroid by studying its companion orbit A. Image of an asteroid obtained at the ESO VLT. B. The same image, after data processing, showing the presence of a satellite (top-right dot). C. The positions of the satellite measured at different epochs (blue and red markers) allow to determine its orbit, and hence the asteroid mass. Credits: B. Carry (IMCCE).

### III.6. Density and meteorites

Once the mass is measured, the density can be computed. The density reflects the quantity of matter, the mass, contained within a given volume. The tricky question "What's the heaviest? A kg of leed, or a kg of feathers?" plays with that notion. In that question, the leed and the feathers of course have the same weight (1 kg), but the density of the leed being much higher than that of feathers (11300 agains 20 kg.m -3 ), the kilogram of feathers will occupy a much larger volume. Knowing the density of asteroids help us in constraining their composition.

Furthermore, if the surface composition is known (see IV below), the comparison of the asteroid density with that of its constituant allows to guess its internal structure (Figure 15).

• If the asteroid is denser than its surface components, some denser compounds are located in its interior. It may indicates that the asteroid is differentiated, like (4) Vesta for instance
• If the asteroid is less dense than its surface compounds, its interior may contain less dense compounds, like ices, or large voids. Some asteroids are indeed so little dense that their density can be hardly explained without the presence of large cracs or voids in the interior.
• If the asteroid has the same density as its surface material, it is likely homogeneous.
Figure 15: Comparaison of meteorite densities and possible internal structures of an asteroid. Credits: B. Carry (IMCCE).

## CARGO FOR CALLISTO

The big rocket freighter was speeding through the star dust of outer space. It was carrying supplies to Callisto (one of the twelve moons of Jupiter) and the Shannons, on another space adventure.

Steve and Sue looked out a window of the freighter at the airless world growing in size. Callisto was a gigantic roughened rock, but it was a globe larger than the planet Mercury. It reminded Steve of a giant cockle-burr hanging in the sky.

Suddenly the children heard a tiny voice behind them say, &ldquoRocket away!&rdquo

They turned and Sue exclaimed, &ldquoIt&rsquos Bud!&rdquo

The blue parakeet, a budgy, blinked lazily at them. The twins had met Mr. Whittle&rsquos pet a week ago. He had taken a liking to them from the very start. They didn&rsquot know that a few hours from now their very lives would depend on this little fellow.

&ldquoWe&rsquod better take him back to Mr. Whittle,&rdquo Steve said.

The budgy kept studying them with his flat face and blinking his tiny button eyes. Then he squawked again, &ldquoRocket away!&rdquo

&ldquoIt&rsquoll be &lsquorocket away&rsquo for you, young fellow!&rdquo Steve said sternly. &ldquoUp on my finger, Bud!&rdquo

The bird did as he was ordered. They took him down the hall to Mr. Whittle&rsquos room. Bud&rsquos owner, off duty now, was a tall, spidery crewman with a big Adam&rsquos apple. He always gave his pet full run of the ship.

Mr. Whittle whistled to the parakeet, but the bird stayed on Steve&rsquos finger.

Mr. Whittle chuckled. &ldquoHey, I believe he likes you two better than his master!&rdquo

&ldquoWe like him, too,&rdquo Sue told the crewman.

&ldquoYou can keep him for a few days if you want to,&rdquo Mr. Whittle said. &ldquoI&rsquom going to be pretty busy after we land.&rdquo

&ldquoGee, we&rsquod like to look after him!&rdquo Steve answered.

&ldquoIf you take him outside on Callisto, you&rsquoll have to put him in that air-tight cage over there I had made. It&rsquos sort of like a space suit for him.&rdquo

Sue and Steve played with Bud in the room they used for games until it was time to &ldquostrap down&rdquo for landing. Then they went to the couch hall and lay down on cots like the other space travelers were doing. They buckled straps across their bodies to keep them in place.

For a long time, Steve and Sue lay there as the big freighter began cutting its rushing speed. It felt to Steve as if a giant anvil were crushing downward on his chest. Take-off and landing were always the roughest moments in space travel, as the twins had already found out on other space trips.

At last the ship set down on Callisto. The young Shannons went back to the game room. Then with the bird on Steve&rsquos shoulder, the twins looked out the window at the strange new world.

They saw a land bathed in ghostly twilight. Very little light was coming from the sun. It was so far away that it was only a small circle. Most of the light came from a huge shape that looked like somebody&rsquos lost beach ball resting on the ground. Its bottom edge just touched the horizon.

Sue and Steve were joined by their father, who worked for the space freight company.

&ldquoThat&rsquos His Majesty, Jupiter&mdashthe king of planets,&rdquo Mr. Shannon told them. &ldquoHe&rsquos over a million miles away and yet he looks close enough to touch, doesn&rsquot he?&rdquo

&ldquoLet&rsquos go outdoors, Dad!&rdquo Steve begged.

&ldquoNo reason why we can&rsquot,&rdquo Mr. Shannon replied.

After they had put on their space clothes, Steve popped Bud into his warm, air-tight cage.

As they all went outside, they saw the crewmen unloading the cargo.

&ldquoThere&rsquos the colony over there,&rdquo Mr. Shannon said, pointing to a high framework that looked something like an oil derrick.

&ldquoThey mine here for a mineral called magna. It&rsquos very valuable, because without it we couldn&rsquot have atomic engines. Magna is what keeps our rocket tubes from melting under the terrific heat that goes through them.&rdquo

&ldquoWe&rsquoll see if we can,&rdquo said his father.

As they walked toward the mining place, Mr. Shannon said, &ldquoUnderneath us are pockets of poisonous gas like that found in Jupiter&rsquos atmosphere. Sometimes it leaks into the mining tunnels causing danger from suffocation.&rdquo

&ldquoI sure hope the gas stays where it belongs while we&rsquore down there!&rdquo Steve said and swallowed the lump of fear in his throat.

They turned their attention to Jupiter. It looked even more like a beach ball now with its stripes of beautiful colors. Mr. Shannon said the bands were floating ice bergs of the poisonous gases he was talking about.

&ldquoNo ship can land on Jupiter,&rdquo he said. &ldquoIts gravity would crush a spaceman flat. Gravity pull is much stronger on the larger planets, you know. Jupiter&rsquos atmosphere is many thousands of miles deep. Raging storms are going on beneath it all the time.&rdquo

&ldquoOoo!&rdquo Sue gasped. &ldquoI guess we&rsquore close enough to it then!&rdquo

Other wonders of the sky were the round beacons of Jupiter&rsquos other moons, three of which were about the same size as Callisto. They hung like bright searchlights in the starry heavens.

The men at the mining place greeted the Shannons warmly. They had not seen anyone from Earth for so long that they had grown very lonely.

The chief mining engineer said he would be glad to take the visitors on an underground tour. His name was Dr. Harding. He was plump and short and wore black-rimmed glasses inside his space helmet.

He led them into an elevator and it sank into the darkness. Steve remembered about the poisonous gases that crept about underground and it made him shiver to think about it.

Dr. Harding watched Bud hopping around uncomfortably inside his small space cage. &ldquoDo you remember, Mr. Shannon,&rdquo he asked over his suit radio, &ldquowhen they used to use canary birds in mines to warn about leaking gas? The birds would notice it first and give the miners time to get out.&rdquo

&ldquoNow we have automatic warning machines in the tunnels to do that,&rdquo the chief engineer told Sue and Steve.

Deeper and deeper below the soil of Callisto the elevator sank. At last the cage reached the bottom, and the riders found themselves in a large cavern. There were machines and men all about, working busily. Tracks led off into tunnels and ore cars were running on them. Some were going empty into the tunnels while others were coming out full of rock and gravel.

&ldquoThe magna is separated from the rock in that big machine over there,&rdquo Dr. Harding explained. &ldquoWant to ride an ore car into one of the tunnels?&rdquo

&ldquoThe mine is air-conditioned,&rdquo the chief engineer said, &ldquoso we can take off our helmets.&rdquo

This done, Steve let Bud out of his cage. The little bird hopped up on his gloved finger, saying, &ldquoRocket away!&rdquo several times. His two-word language seemed to do for everything.

One worker controlled all the cars at a main switch in the middle of the cavern. The Shannons and their guide climbed into an empty ore car and it rolled into a tunnel.

Glistening dark rock crowded in on Sue and Steve from all sides. Steve hoped the walls were strong enough so they would not come crashing down on their heads! There were lights along the way to help brighten the gloom.

After clicking along like a trolley for awhile, the car came to the end of the line. It was a large room with more machines and workmen. The men were digging magna ore out of the wall with drills.

As Dr. Harding explained about the work, Bud began flitting about as though sight-seeing on his own. He was shy of the workers at first, but then made friends with them. He spoke to them with his favorite two words and the men laughed in great fun to hear him.

Then a few minutes later, Bud began acting queerly. He flew back to Steve&rsquos finger and started wobbling as though dizzy.

&ldquoWhat&rsquos the matter with him?&rdquo Steve asked.

&ldquoHe&rsquos sick or something!&rdquo Sue cried out. She took the budgy from Steve and cuddled him in her own gloves. But the little blue bird seemed to be no better.

Dr. Harding walked over to look at the bird. Then he ordered, &ldquoEverybody into the ore car! We have to get out of here fast! Sue, hold the bird up close to your suit!&rdquo

The workers dropped their tools as if they were red hot and climbed into the car. Mr. Shannon helped Sue and Steve on, then jumped on himself.

Dr. Harding pressed the electric button that was the signal to the operator in the main cavern to move the car. The car began to roll down the track. It picked up speed as Dr. Harding kept pressing the button.

&ldquoLeaking gas, Dr. Harding?&rdquo Mr. Shannon asked worriedly.

The chief engineer nodded. He sniffed the air like a hunting dog after a scent. &ldquoTake a deep breath, everyone, then hold it!&rdquo

Steve thought his lungs would burst, but finally Dr. Harding let them take another deep breath. By the time they had taken one more, the car had reached the main cavern. As it rolled to a stop, Dr. Harding jumped down and ran over to the car operator.

Steve saw a door slide down and close off the tunnel where they had come out. Then the little man gave a deep sigh and took off his black-rimmed glasses to wipe them.

Sue and Steve watched Bud hopefully. He was standing more steadily on Sue&rsquos finger now.

&ldquoI think he&rsquoll be all right,&rdquo the chief engineer said. &ldquoWe sure owe Bud a lot for warning us the way he did. Something must have happened to the warning machine. It was supposed to set off a siren.&rdquo

&ldquoIf it weren&rsquot for Bud we might have been overcome before we could have gotten out of there!&rdquo Mr. Shannon added.

&ldquoYou&rsquore so right!&rdquo Dr. Harding said. &ldquoThe men will go back in there in gas masks to find the leak and see what&rsquos wrong with the warning machine.&rdquo

&ldquoWe&rsquore plenty lucky!&rdquo Steve sighed, his spine still prickly from their narrow escape.

Sue kissed the budgy. &ldquoYou&rsquore a hero, Bud,&rdquo she told him, &ldquoand we love you!&rdquo

Bud blinked lazily. Then as if to show that he was all right again, he squawked, &ldquoRocket away!&rdquo

### The Birth of the Twin Comets

Are we off-track? I don't think so. Just as "all roads lead to Rome," so too do all ancient astronomy events lead to a double comet, one, soaring over the earth, with a tail full of debris from the disintegrating nova, until it finally fell to into the ocean/sea/well/river or on top of homes and forests.
Just as it was in the earlier post:

a man of clay "that the water could dissolve" died.

### Orion's Belt or the Summer Triangle (Continued)

A Persian monument [found in Rome possibly], that of Mithras killing the bull, one of three versions illustrated in the book of G. Sesti, (1991) called The Glorious Constellations, History and Mythology . One panel contains the image of that nebula that NASA called the "Hand of God," as meteorites sped away from it. The correct panel of the three illustrated by G. Sesti is shown by the position of the "Twins" on the left side. They are holding their torches correctly, one is held up and the other is facing downwards. The animal with the “ear” is not a Bull nor is it Taurus. It is only an imaginary animal with a “forked tail."

Behind the nose of that bull being "sacrificed" by Mithras, under a tree, leans a telescope of the ancient variety, a thin shaft of metal with two or more lens inside of it. It looks more like the first known gun, illustrated in L. Spague de Camp's book , Ancient Engineers. A gun that had to be jammed into the earth and leaned on a "Y" stick. It was only effective as a sudden blast of fire to frighten horses and extremely dangerous for the "gunner" himself.

## When the asteroid hit the earth 65 mya did the earth's gravity pull change? By how much? - Astronomy

I am a middle school teacher. Today I did a lesson in which students determined what their weight would be on the other planets in our solar system. We used a formula I found on the internet in another teacher's lesson Plan the formula is:

Mass (weight on earth) x gravity (different for each planet) = weight on that planet.
The gravity chart I used looked like this:
Earth: 1
Our Moon 0.17
Venus .90
Mars 0.38
Mercury 0.38
Jupiter 2.36
Saturn 0.92
Uranus 0.89
Neptune 1.13
Pluto 0.07.

My students quickly filled in the chart and discovered how heavy they would be on Jupiter, how light they'd be on our Moon, and Pluto, and Mars and Mercury. Since they finished so quickly I asked them to find their Planetary Average Weight. We decided to throw out the Moon (not a planet) and Pluto (not a real planet anymore.) We found that every single student had an average planetary weight that was within one pound of their weight on Earth! I was in awe of this "co-incidence." I tried more weights, and the result was always the same: 8 planets, total weight on each, divided by eight ALWAYS equals Earth weight. So here are my questions:

My understanding /conclusion is that eventually and according to laws of Physics, Earth at some point will stop spinning for a moment in time, and then start spinning clockwise instead of counter-clockwise. Is this possible? Thank you.