Astronomy

Do space objects stay in orbit once they get sucked in? Can they escape?

Do space objects stay in orbit once they get sucked in? Can they escape?


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Think of my question with respect to 2020 CD$_3$ minimoon that's been recently discovered.

If an object gets caught into a planet's orbit, can it ever escape it? Obviously it may escape on the first pass without making any full revolution. But if one or two revolutions took place, can it ever escape the planet's orbit on its own momentum it came in with?


If it can get captured, it can escape: we know this because the law of gravity is time-symmetric. If there is a trajectory that leads to a capture, then by running time backwards, there is a trajectory that leads to escape.

What happens with small objects like 2020CD3, is that they pass into the space between the Earth and moon, and it is the interaction between the Earth and Moon (and sun) that slows them down and allows them to enter into a long looping orbit.

But this orbit is too close to the moon to be really stable. Usually they go around in a complex orbit that changes each time, as they are affected by the gravity of the Earth, moon, (and sun). Usually after a few orbits they will have another interaction with the moon, but this time it will speed the object up, and eject it from the Earth's gravity well. Interactions that lead to a permanent capture are rare.

There are no objects in Earth orbit that have been captured. Asteroids are rather rare in the inner solar system, and the moon probably does a good job of ejecting any that do get into an orbit around the Earth. There are two captured asteroids around Mars, and many of Jupiter and Saturn's moons may have started out as asteroids. Triton is also probably a captured body. So it is possible for a body to get into a stable orbit from capture, but it is rare.

There is no sucking effect. Instead the orbit of the asteroid around the sun gets tangled up in the gravity field of the Earth and moon for a while, but eventually it will almost always escape.


How Long Will Space Junk Take to Burn Up? Here’s a Handy Chart

If the Roman Empire had been able to launch a satellite in a relatively high Low Earth Orbit – say about 1,200 km (750 miles) in altitude – only now would that satellite be close to falling back to Earth. And if the dinosaurs had launched a satellite into the furthest geostationary orbit – 36,000 km (23,000 miles) or higher — it might still be up there today.

While we’ve *really* only launched satellites since 1957, those examples show how long objects can stay in orbit. With the growing problem of accumulating space junk in Earth orbit, many experts have stressed for years that satellite operators must figure out how to responsibly dispose of derelict satellites at the end of their lives.

The European Space Agency (ESA) and the United Nations Office for Outer Space Affairs (UNOOSA) have collaborated for a new infographic to show how long it would take satellites at different altitudes to naturally fall back to Earth.

Credit: ESA & UNOOSA

While the natural de-orbit process can be relatively fast for satellites flying at low altitudes — taking less than 25 years — for satellites launched into orbits tens of thousands of kilometers away, it can be thousands of years before they return.

Gravity has little effect on a satellite’s return to Earth. The biggest factor in satellites decreasing their orbit is the amount of drag they encounter from Earth’s atmosphere. A satellite can remain in the same orbit for a long period of time as the gravitational pull of the Earth provides a balance to the centrifugal force satellites experience in orbit. For satellites in orbit outside the atmosphere, there is no air resistance, and therefore, according to the law of inertia, the speed of the satellite is constant resulting in a stable orbit around the Earth for many years.

“If we look at our statistics, we have about 300 objects per year returning to Earth, burning up in the atmosphere,” said Francesca Letizia, a space debris engineer at ESA, in a podcast on space debris. “Below 500 km, the effect of the atmosphere, the spacecraft can reenter within 25 years. At 800 km above Earth, it will take about 100-150 years to fall back to Earth.”

Letizia said the biggest risk for old satellites that aren’t currently operating is the risk they pose for exploding and creating more fragments, or for colliding with other satellites and either causing damage or destruction and also creating additional objects in Earth orbit.

This means that as we launch satellites to space we must consider how they will be removed at the end of their lives, or else the area around Earth will be filled with old, defunct spacecraft at risk of collision, explosion, and the near-certain creation of vast amounts of space debris.


Space Junk: Tracking & Removing Orbital Debris

Although outer space is often imagined to be a desolate, empty place, the region around Earth swarms with millions pieces of man-made debris that create potential hazards for their functioning neighbors. Where did all of this junk come from? Will it ever go away? What kind of problems might it create for people stationed on Earth? Let's take a look.

The source of space junk

With the launch of the Soviet satellite Sputnik in 1957, mankind began its journey to reach the stars. But although the first probe in space returned to Earth after only three short months, it kicked off a series of launches that not only inspired people around the world but also filled the region with large chunks of inert metal.

Inactive satellites, the upper stages of launch vehicles, discarded bits left over from separation, and even frozen clouds of water and tiny flecks of paint all remain in orbit high above Earth's atmosphere. When one piece collides with another, even more debris is released. Over 21,000 pieces of space trash larger than 4 inches (10 centimeters) and half a million bits of junk between 1 cm and 10 cm are estimated to circle the planet. And the number is only predicted to go up.

There are also millions of pieces of debris smaller than a third of an inch (1 cm). In Low Earth-orbit, objects travel at 4 miles (7 kilometers) per second. At that speed, a tiny fleck of paint packs the same punch of a 550 pound object traveling at 60 miles per hour. Not only can such an impact damage critical components such as pressurized items, solar cells, or tethers, they can also create new pieces of potentially threatening debris.

For fifty years, the primary source of all of the junk came from objects that exploded by accident. However, in 2007, the intentional destruction of the Chinese weather satellite Fengyun-1C as part of an anti-satellite missile test created a significant field of space debris. Two years later, a defunct Russian military satellite struck an operational American Iridium satellite over northern Siberia, blowing even more trash into space. [Worst Space Debris Events of All Time]

An ounce of prevention

Despite the small size of most of the objects in space, the U.S. and Russian military are able to keep track of a great deal of the mess. Objects as small as 4 inches (about 10 cm) can be seen by radars or optical telescopes on Earth. When preparing a launch, mission controllers screen the predicted post-launch orbit for potential collisions to avoid as much damage as possible. Similarly, crafts such as the space shuttle and the International Space Station can change their orbits if a larger object approaches.

But everything sent into space still faces potential collisions with smaller, untrackable objects that can pit or damage them. Satellites and space craft are heavily shielded to protect vital components. At NASA's Hypervelocity Impact Technology Facility in Texas, new protective materials can be tested by shooting objects from a Light-Gas gun to simulate space junk collisions.

Watch for falling objects

Earth's orbit is segregated into three distinct regions. Low Earth-orbit (LEO), covers the area 125-1,250 miles (200-2000 km). Pieces of space junk in this region are impacted by the atmosphere, which degrades their orbit, dragging them back to Earth sooner. This is a prime realm for piloted spacecraft due to its easy access. Navigation and communication satellites tend to prefer a semi-synchronous orbit 6,000 to 12,000 miles (10,000 to 20,000 km) above the surface. Satellite telecommunication and weather satellites orbit in geosynchronous Earth orbit, over 22,000 miles (36,000 km) high, and can remain aloft for millions of years. The lower the orbit, the less time the object is likely to remain in space before returning to Earth.

Bits and pieces of trash constantly fall from the sky, but nearly everything larger than 4 inches (10 cm) survives in some form, likely in smaller fragments. In the last five decades, an average of one piece of debris fell to the Earth each day. Most of the trash raining down burns up in the atmosphere before it ever reaches the surface. Those that survive often fall into water remember, the ocean makes up approximately 70 percent of the Earth's surface. According to NASA's Orbital Debris Program Office, no serious injury or significant property damage from falling debris has been confirmed.

Video: An SM-3 missile launched from the USS Lake Erie hits a wayward satellite Wednesday, Feb. 20, 2008.

Adopt an atmosphere

In recent years, various space organizations have worked to reduce the amount of trash added to Earth's orbit by implementing better designs. Russia, China, Japan, France, and the European Space Agency have also issued guidelines on how to cut back on the potential impactors in orbit.

Cleaning the debris that already exists is a completely different challenge. Specific trips to larger objects could remove them from orbit, but at a high financial cost. Other proposals include the use of a laser to provide a path-shifting push that wouldn't damage the object. [Photos: Space Debris Images & Cleanup Concepts]


'Warp-Speed' Planets Flung Out of Galaxy on Wild Ride

Planets in tight orbits around stars that get ejected from our galaxy may actually themselves be tossed out of the Milky Way at blisteringly fast speeds of up to 30 million miles per hour, or a fraction of the speed of light, a new study finds.

"These warp-speed planets would be some of the fastest objects in the galaxy, aside from photons and particles like cosmic rays," said Avi Loeb, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. "In terms of large, solid objects, they would be the fastest. It would take them 10 seconds or so to cross the diameter of the Earth."

In 2005, astronomers found evidence of a runaway star that was flying out of the Milky Way galaxy at a speed of 1.5 million mph (2.4 million kph). This hypervelocity star was part of a double-star system that wandered too close to the supermassive black hole at the center of the galaxy.

The strong gravitational pull at the galactic center ripped the stars apart, sending one hurtling through space at high speeds, while capturing the other to stay in orbit around the massive black hole.

In the seven years since, 16 of these hypervelocity stars have been found, and Loeb and his colleagues began to wonder whether planets could also be sent tearing through space at such extreme velocities.

Cosmic pinball

The researchers created simulations that examined what would happen if each star had at least one planet orbiting nearby. They found that up to 10 percent of planets tightly orbiting one of the stars could stay bound as the star is flung outward. The other star that is captured by the black hole could also have its planet ripped away from it, and this planet would then be pitched into interstellar space at intensely fast speeds as well.

"We were trying to predict, if you have planets around each of the stars in the binary system, what fraction of the planets might go along with the hypervelocity star for the ride," Loeb told SPACE.com. "What we found is that some of them get expelled at high speeds, up to a few percent of the speed of light. Some of the planets get ripped apart from the host stars and get sent out at high speeds, and they also become hypervelocity planets this way." [Gallery: The Strangest Alien Planets]

These planets would travel through space at unparalleled velocities, the researchers said.

"Other than subatomic particles, I don't know of anything leaving our galaxy as fast as these runaway planets," lead author Idan Ginsburg of Dartmouth College in Hanover, N.H., said in a statement.

A typical runaway planet would likely dash outward at 7 to 10 million mph (11.3 to 16.1 million kph), but given the right circumstances, a small fraction could have their speeds boosted to up to 30 million mph (48.3 million kph).

"It's like a pinball machine," Loeb said. "Things are kicking around, and if things happen to move in just the right way, a planet could get kicked out at a much higher speed than other planets."

Eventually, these hypervelocity planets will escape the Milky Way and travel through interstellar space on a wild ride, he added.

"If there is a civilization on such a planet, they would have a very exciting journey," Loeb said. "It would start at the center of the densest environment of the galaxy, and the planet would traverse through the galaxy, seeing it from different directions before eventually exiting from the Milky Way. Once the planet exits from the local group of galaxies, it will be accelerated away by cosmic expansion. So, within 10 billion years, it would go from the center of the galaxy to all the way to the edge of the observable universe."

Runaway stars as planetary hosts?

The researchers are now hoping other astronomers will use these findings to look for potential signs of these planets around hypervelocity stars. A planet that tightly orbits a runaway star will cross in front and cause its brightness to dim slightly in what astronomers call a "transit."

"Simply because it moves around the star, it may pass in front and then block some of the light emitted from the surface of the star," Loeb explained. "By monitoring the brightness of the star, we might see evidence of dimming."

To hitch a ride on a hypervelocity star, a planet would have to be locked in a tight orbit, which ups the odds of witnessing a transit to around 50 percent, the researchers said.

"With one-in-two odds of seeing a transit, if a hypervelocity star had a planet, it makes a lot of sense to watch for them," Ginsburg said in a statement.

In fact, some existing large telescopes could have instruments sensitive enough to detect this slight dimming.

"This is the first time someone is talking about searching for planets around hypervelocity stars," Loeb said. "It's possible with large telescopes, but observers need to put it on the agenda. The purpose of the paper was to propose this."

The detailed results of the study will be published in an upcoming issue of the journal Monthly Notices of the Royal Astronomical Society.


Black Holes Don&rsquot Suck

First, let&rsquos clear up a common misconception. Black holes get a bad reputation for sucking in their surroundings, like some sort of cosmic vacuum cleaner. In reality, the gravitational pull of a black hole is the same of that for a regular star&mdashjust a lot stronger.

So astronomical objects can easily stay in orbit around a black hole, just as we stay in orbit around our Sun, so long as they are moving fast enough to balance the black hole&rsquos gravitational pull. In fact, the hundreds of billions of stars in our galaxy orbit a central, super massive black hole with a mass of 4.6 million times that of our Sun crammed into a space of less than

100 million miles (about the distance between the Earth and the Sun).

If we were to get knocked off course by a particularly large collision (think galaxy scales here, not just a tap from an asteroid), we could in theory be sent careening towards our galaxy&rsquos central black hole and pass the point of no escape. However, this would require a pretty significant event and so we have much more likely things to worry about (like what happens to our Sun when it runs out of fuel).


NASA hoax ISS Actornaut Chris Cassidy accidentaly admits they are filming in the USA BUSTED

Summary

  • After 100km altitude it starts to get very hot. At 110km it is 200°C. At 500km it is somewhere between 500°C and 1500°C or more. This is the thermosphere.
  • The cause of this heat is the extra solar radiation above the ionosphere, closer distance to the Sun, and above all the vacuum of space which doesn’t allow the heat to radiate away fast enough or allow a lower pressure differential with increasing altitude.
  • Space machines are said to orbit between 120 and 35000km+ altitude making them traveling furnaces and obviously a pure fabrication if said orbital altitudes are correct.
  • Possible counterarguments against a hot thermosphere are: 1. Invisible stars at high altitude may be responsible for lower heat at same said height although possible white hot asteroids orbiting the Sun and the detection of the extra sunlight intensity make this unlikely. 2. Long time spans make heating objects very slow and unnoticeable although it only takes a few months to heat up convective air on the ground from one season to another – in space heat can only be radiated away.
  • Above 100km altitude, objects are said to freefall along the curve of the Earth if initially traveling laterally at over 28000 kph. Falling is an acceleration making those objects that have been orbiting for years travel many times the standard speed of light.
  • One model of the vacuum at 400km is estimated to be one trillion trillionth of the air density at sea level allowing for an extremely high terminal velocity.
  • The easiest way to detect fake NASA footage is to compare it to the control videos of high altitude weather balloons – if not similar then fake.
  • There are numerous red flags when analyzing space footage that is not similar to the control: 1. Conclusive bubbles in space. 2. Swimming astronauts kicking their legs. 3. Lady astronaut hair behaving in a totally different way than hair at zero gravity on an airplane. 4. Chris Hatfield caught with wires sticking out his shirt. 5. Chris Cassidy’s Freudian admission of real location.
  • There are very few genuine photos of the Earth as a globe, despite 3700 satellites having been launched over the decades (1100 still in operation, although 6,578 are said to have been ever launched into orbit). Any orbiting distance from 6200km away or more would show the whole ball Earth.
  • There is no video of the globe Earth, only animations of photo sets.
  • There are only two sets of photos of globe Earth (known to the author) said to be genuine: 1. Those taken from the Apollo missions, and 2. Those from the 1990 Galileo satellite.
  • The Blue marble 2012 globe Earth picture is a composite of much, much smaller and nearer to Earth satellite photos from various instruments, layered and tweaked.
  • The Apollo moon landings are a farce due to the thermosphere and common sense.

Would you be surprised if it turned out that Judith Resnik – “the first jewish woman in space” (and alleged Challenger-disaster-victim) is still alive and well? That she’s been involved in movies, such as Doug Liman’s “Fair Game” (a 2010 Hollywood blockbuster starring Sean Penn / Naomi Watts involving a female covert CIA agent and “yellowcake uranium for making nuuukular bombs”) which won the “Freedom of Expression Award” ? That she’s today a highly-honored academic and the ‘Arthur Liman Professor of Law’ at Yale Law School?

What about Challenger astronot Michael J. Smith?

During his 18 years in the college, Michael J. Smith has advised 80 master’s and PhD students. Recently, a group of those students honored him with a surprise party and an award for excellence in holistic education. “He respects you as an equal and gives you the freedom to explore your interests, challenge his ideas and talk to other professors. His door is always open,” says a former PhD student.

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Gravity and weightlessness

Our everyday lives involve such activities as sitting, walking, picking up things from the ground and lying in bed. None of these activities are possible in orbit.

Once a spacecraft reaches orbit, everything inside it appears to be weightless. Anything (or anyone) that is not tied down will float.

Astronauts first feel the effect of weightlessness when the rocket engines are turned off. Straight away, they begin to float, held down only by seatbelts. Weightlessness allows astronauts to appear superstrong. They can lift objects that would be far too heavy to move on Earth. But there are some drawbacks.

Without the effect of gravity, blood and other body fluids begin to flow towards the head. This can cause a feeling of stuffiness and headaches. With no gravity to push against, bones and muscles can become weak. To stay fit, they have to exercise several hours each day. This allows them to recover more quickly when they return to Earth.

In a shuttle or space station, there is no up or down. There is no difference between a floor and a ceiling. This can make astronauts feel sick until they get used to this strange arrangement.


'Spaghettification'

Black holes are blobs of unbelievably dense matter with a gravitational pull millions of times greater than the force we feel on Earth.

If you got too close, these gargantuan forces would pull your body apart.

As you got closer, the difference in gravity between your head and your feet would stretch you out like a piece of chewing gum.

Scientists affectionately call this process "spaghettification".

You eventually become a stream of subatomic particles that swirl into the black hole like water down a plug.

According to TV physicist Neil De Grasse Tyson: "As you get closer and closer, the force of gravity grows astronomically. You stay whole until the stretching force exceeds the molecular bonds of your body's flesh.

"At that moment, your body would snap into two segments. Everything of you that ever was gets funneled to the black hole's center.

"Not only have you been ripped in half – you've been extruded through the fabric of space and time like toothpaste through a tube."


How fast does air get sucked into space?

You see in movies when there is a hull breach or whatnot, air/object get sucked out of the hole extremely fast? How fast is it?

Air molecules move QUICKLY. At 20°C, an average air molecule will probably move around 500 meters per second. When they bounce off something, they push off that thing, they impart some momentum. The force experienced by an area with a lot of air molecules bouncing off it is air-pressure. Normally, youɽ have air molecules bouncing off either side of an object, and the net force imparted cancels out.

Air pressure in a spaceship would probably be around 1 atmosphere, which is 15 pounds per square inch of pressure. Now imagine you made a hole one square inch in size in your spaceship, and put say, say, a marble in the hole. That marble would feel 15 pounds of force pushing it out the hole. but because there's no air on the other side, nothing pushing back. And air molecules nearest the hole and coincidentally heading towards it would just fly out at 500m/s straight away. Others might take some bounces, but because they can't bounce off other air molecules that have already escaped, everything will tend to bounce towards the hole on average. Objects won't have that speed, because it takes a while to accelerate to it even at 15 pounds of force per square inch, and they'll be clear of the hole pretty quickly. How fast will depend on the object's size and mass. A 1 m 2 hole with a 15 psi pressure difference yields 23000 pounds-force which would in turn accelerate a 1 m 2 person at 1km/s 2 for a short time.. If we say that a person is 1 foot wide, they'll accelerate to about 25m/s in the time that they're in the hole, (using the last of these kinematic equations.)

So how fast will all the air empty? It'll depend how much there is, but it might be quite tricky to calculate. Luckily others have done this in the past, here's an example. Their result for a 30m 3 cabin:

We can see that a one square centimetre hole will reduce cabin pressure by 50% in 500 seconds (8.3 minutes). This value scales in inverse proportion to hole area. Thus a 10 sq cm hole will only take 50 seconds to halve the pressure, (. )

disclaimer: It's late, someone check my maths.

[edit] As Overunderrated says, this is not a complete picture and wantonly ignores fluid dynamics!

[edit 2] Well, it looks like this isn't quite the right answer, but I'll leave it up as an approximation until someone comes up with a better one.


Ask Me Anything AMA with Astrophysicist Dr. Joe Pesce!

As a fellow ST:TOS fan, I was wondering how you feel about the "remastered"version of episodes vs the originals. Have to say that my favorite episode, "The Doomsday Machine" felt a lot cooler/smoother than the original version.

I value the remastered versions of ST:TOS for what they are. They provide a nifty look at digital graphics technology (of the mid 1990s), ultimately "enhancing" the production quality of the television show.

I watch the remastered version, certainly, but I pine for the original. Why? Because the show is a product of the 1960s. Why can't we see it as it was constructed and intended to be? AND the graphics used in the 1960s on ST:TOS were cutting edge - they ARE one of the things that made TOS the jewel that it is (and why it was so remarkable in its day). As I said, the remastered version is an interesting product worth viewing. It's not ST:TOS. So, yes, I feel strongly about this!

But you know what else bothers me? Editing that removed the original (four) commercial breaks. The storyline, the original editing, the soundtrack, were all designed to have these commercial breaks. Removing the original commercial breaks (or, more commonly, adding more), also changes the tone and flow of an original episode.

Helio

Ah ha. Yes, Huygens did introduce the kind of waves you, no doubt, prefer!

It's quite a story on how light bounced from particle to wave to particle to wave throughout history. I can't imagine, for practical use, much of a particle viewpoint taken in radio astronomy, right?

DrJoePesce

I would also note this member's question:

A great question that gets to a fundamental of astrophysics. On the local level, the movements of galaxies toward or away from us impart a doppler effect on light emitted from those galaxies (blueshift or redshift). On the larger scale, we have the cosmological redshift which is the stretching of a photon's wavelength as it travels through an expanding universe: the very fabric of the universe is stretching as the photon flies through it, and this increases the wavelength, leading to the cosmological redshift. Redshift is related to distance, and we can measure distances in several independent ways. And when we do, redshift is consistent with the picture I outlined above. There are alternatives, but I am not versed in them, and they haven't yet gained traction.

There is also a thing called gravitational redshift which is caused by photons near an intense gravitational field.

Helio

Einstein, from what I am trying to understand through some limited reading, had developed his GR theory initially on the equivalence principle. This allowed the then known solar redshifts and the precession of the orbit of Mercury to, finally, be explained. [Luckily, a new, smarter, Vulcan came along once tv was born.]

The solar redshifts, though the data was a bit muddy, became understandable as the solar limb presented, primarily, only the grav. redshift since on the limb the Doppler shifts are not so significant. [This may be one reason he spent more time, apparently, with solar experts than with Hubble when he visited Mt. Wilson, though the solar guys spoke German and he was still learning English at the time.]

But, he recognized he had left out the component involving spacetime effects (I think this is the right term). His starlight near the Sun deflection estimate was the same as the Newtonian deflection in his early model, and the one he expected to get from the German astronomer he helped get funding. [This eclipse expedition to Russia (IIRC) failed as the astronomer was put in a jail as a potential spy, so Einstein was lucky his incomplete theory wasn't falsified, and with some of his own money, apparently. ]

When he realized his shortcoming, he corrected the theory and doubled the deflection amount, which was confirmed by Eddington and Dyson (late 1919). [The better data came from Dyson's team, actually.]

So, my question is whether their is some relationship between the spacetime deflection factor (Einstein's second key element in GR) and the cosmological redshift? [I have struggled trying to see how feeble spacetime in empty space can suck so much energy from an EM particle given that feeble gravity for galaxies holds-up so well against the expansion.]

DrJoePesce

Einstein, from what I am trying to understand through some limited reading, had developed his GR theory initially on the equivalence principle. This allowed the then known solar redshifts and the precession of the orbit of Mercury to, finally, be explained. [Luckily, a new, smarter, Vulcan came along once tv was born.]

The solar redshifts, though the data was a bit muddy, became understandable as the solar limb presented, primarily, only the grav. redshift since on the limb the Doppler shifts are not so significant. [This may be one reason he spent more time, apparently, with solar experts than with Hubble when he visited Mt. Wilson, though the solar guys spoke German and he was still learning English at the time.]

But, he recognized he had left out the component involving spacetime effects (I think this is the right term). His starlight near the Sun deflection estimate was the same as the Newtonian deflection in his early model, and the one he expected to get from the German astronomer he helped get funding. [This eclipse expedition to Russia (IIRC) failed as the astronomer was put in a jail as a potential spy, so Einstein was lucky his incomplete theory wasn't falsified, and with some of his own money, apparently. ]

When he realized his shortcoming, he corrected the theory and doubled the deflection amount, which was confirmed by Eddington and Dyson (late 1919). [The better data came from Dyson's team, actually.]

So, my question is whether their is some relationship between the spacetime deflection factor (Einstein's second key element in GR) and the cosmological redshift? [I have struggled trying to see how feeble spacetime in empty space can suck so much energy from an EM particle given that feeble gravity for galaxies holds-up so well against the expansion.]

DrJoePesce

I don't want to pose a question that can't be answered, but I have wondered for a long time whether or not our universe is everything in existence, including all the dimensions we have yet to discover. If this is true, I don't see the universe simply reversing course because it apparently is still accelerating. What I see is a space-time fabric possibly being stretched to its limits before quantum leaping into something with different properties and different laws of physics. From there, it may undergo more quantum leaps before reverting back to whatever it was before the Big Bang.

If the universe is only a fraction of everything in existence, it becomes a much more complicated picture, perhaps beyond the reach of the most intelligent beings to have ever existed. I realize there is a 0.0000000000001% chance we are the only intelligent beings to ever exist in the universe.

Oh, I loved you in "My Cousin Vinny." Just kidding. His surname is Pesci.

Cosmology is so thought-provoking, isn't it? The universe is everything in existence we can know about. There are some thoughts of multi-verses, and the like. But, for the most part, these views (maybe we can call them models) cannot be tested, and so are not scientific and merely speculation. That doesn't mean 1) we won't be able to conduct tests in the future and 2) there won't be new and wonderful models.

As to your point about the universe reversing: Up until the discovery of dark energy, the big open question was did the universe have enough mass (or mass density) to slow the universal expansion, and indeed even reverse it so that eventually the universe would collapse? The view was that the mass density of the universe was not sufficient to reverse the expansion, but it would slow it down. So, a universe expanding forever but at an ever-decreasing rate. Dark energy basically throws all that out, because not only is there not enough matter to stop the expansion, but the expansion rate is accelerating. It looks like the universe will expand forever.

P.S. I love "My Cousin Vinny" too!

Helio

Right, but I will assume you see where I have trouble with, say, a 12 billion-year old galaxy constantly being stretched yet never fully allowing the expansion to get the best of it, thanks to its self-gravity that overpowers that expansion.

Yet, if we know gravity is extremely weak relative to EMf, you can see my puzzlement in trying to see how the extremely strong (EMf) allows the expansion to stretch that which gravity seems to disallow. Perhaps you can suggest what I'm missing.

Rabsal

Rabsal

Right, but I will assume you see where I have trouble with, say, a 12 billion-year old galaxy constantly being stretched yet never fully allowing the expansion to get the best of it, thanks to its self-gravity that overpowers that expansion.

Yet, if we know gravity is extremely weak relative to EMf, you can see my puzzlement in trying to see how the extremely strong (EMf) allows the expansion to stretch that which gravity seems to disallow. Perhaps you can suggest what I'm missing.

JSNardello

DrJoePesce

Right, but I will assume you see where I have trouble with, say, a 12 billion-year old galaxy constantly being stretched yet never fully allowing the expansion to get the best of it, thanks to its self-gravity that overpowers that expansion.

Yet, if we know gravity is extremely weak relative to EMf, you can see my puzzlement in trying to see how the extremely strong (EMf) allows the expansion to stretch that which gravity seems to disallow. Perhaps you can suggest what I'm missing.

A terrific question, Helio, and it's one where that vastness of space gets us.

Currently, the universal expansion is on a very large scale. The expansion rate, Hubble's constant, is still being pinned down, but let's say it's 67 km/s/Mpc. That is, the expansion rate is 67 km/s BUT over 3.3 million light years. I will leave it as an exercise to determine the expansion rate on a human or galactic scale. The universe is expanding on a human scale, just at a miniscule rate. I started by saying "currently". In the far distant future, expansion becomes relevant even on the small scale, and eventually atomic nuclei will be pulled apart from their electrons, subatomic particles will be pulled apart, etc.

As for relative strengths of the forces: the difference between gravity and the other three fundamental forces is that gravity operates on the very large scales while the other three - though tremendously stronger than gravity - only operate on a very small (atomic or sub-atomic) scale.

DrJoePesce

DrJoePesce

David-J-Franks

Was excited to see you back Dr. Joe

I learnt from reading these forums that the speed of light has only been measured with a two-way trip ie it's always been reflected back from something. The reason given why a 1-way speed measurement cannot be made is that it is not possible to synchronise 2 clocks because when you move them apart special relativity says they will alter. I don't like it when someone says you can't do something so I came up with a thought experiment to overcome the synchronising clock problem. I'm sure it must be wrong because I'm not a scientist, so I would like to see what you think

If you take a clock with the dial on and then have a very long shaft with a similaur dial on the other end will that mean the two ends of the shaft are synchronised in time? So, therefore can you measure the one-way speed of light? Basically it is the same clock that both observers at each end are referring to. For the sake of symmetry you could even have the clock mechanism in the middle and a shaft going in each direction.

This need not be a thought experiment, in space it would theoretically be practical to have say a 10 km or even 100 km a long shaft so both ends would therefore be exactly synchronised in time, so you could perform lots of experiments with it. Obviously, you could replace the dial with some electrical triggers and make it digital.

Also you could have one end of the shaft nearer a strong gravitational body where time is supposed to slow down so what happens to the other end of the shaft?

Yet again you could also spin one end of the shaft round the other and the different velocities again should produce the different times, so what happens to each end of the shaft in that case?

I can't get my head around this, so your help would be very much appreciated

Helio

Assuming you meant 3.3 billion lyrs., is that our current SN study limit? Will we be bumping that distance soon, perhaps with the GMT or the Webb spacescope?

DrJoePesce

Was excited to see you back Dr. Joe

I learnt from reading these forums that the speed of light has only been measured with a two-way trip ie it's always been reflected back from something. The reason given why a 1-way speed measurement cannot be made is that it is not possible to synchronise 2 clocks because when you move them apart special relativity says they will alter. I don't like it when someone says you can't do something so I came up with a thought experiment to overcome the synchronising clock problem. I'm sure it must be wrong because I'm not a scientist, so I would like to see what you think

If you take a clock with the dial on and then have a very long shaft with a similaur dial on the other end will that mean the two ends of the shaft are synchronised in time? So, therefore can you measure the one-way speed of light? Basically it is the same clock that both observers at each end are referring to. For the sake of symmetry you could even have the clock mechanism in the middle and a shaft going in each direction.

This need not be a thought experiment, in space it would theoretically be practical to have say a 10 km or even 100 km a long shaft so both ends would therefore be exactly synchronised in time, so you could perform lots of experiments with it. Obviously, you could replace the dial with some electrical triggers and make it digital.

Also you could have one end of the shaft nearer a strong gravitational body where time is supposed to slow down so what happens to the other end of the shaft?

Yet again you could also spin one end of the shaft round the other and the different velocities again should produce the different times, so what happens to each end of the shaft in that case?

I can't get my head around this, so your help would be very much appreciated

Thank you David-J-Franks! Fascinating question and topic! I REALLY like that you are thinking about this and creating experiments! I will have to think more about your experiments but let me offer the following in the meantime.

Not all measurements are necessarily two-way. In the astronomical case, if we know distances to an astronomical object and we see something emitting light (for example, a blob in an accretion disk, and then reflection of that emitted light off something local to the source) we can measure the speed of light. (And/or we can measure distances at the local source given the speed of light.)

But we can also do this in the laboratory, creating a photon and then detecting it, and thus measuring the speed of light.

You also allude to relativity and changing clock speed in the presence/absence of a gravitational field. This has been measured astronomically, but also locally: Something that we use every day - GPS - must include a relativistic time-correction because the GPS satellites are in Earth orbit, but we are on the surface and there's a difference between our respective clocks. This has been measured in the laboratory too, given the incredibly accurate clocks we can produce.