# Do objects lose momentum as space expands

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As far as I know, photons' wavelengths can be considered increasing as space expands, making them lose energy and momentum. Does the same apply to physical objects? I understand a photon's speed is the same regardless of the system of reference, and the same does not hold for physical objects, but is it possible to draw any kind of analogy, for instance, would the momentum of an object launched from Earth (neglect non-cosmological effects) be steadily decreasing when looked at from Earth?

GR doesn't have global frames of reference, so we can't say whether a projectile launched from galaxy A slows down relative to galaxy A due to cosmological expansion when it's at a cosmological distance. However, suppose that galaxy A and galaxy B are both at rest relative to the Hubble flow. We can ask whether the velocity of the projectile relative to B, when it gets to B, is lower than its velocity had been relative to A, when launched from A.

There are a couple of easy ways to see that the answer is yes.

One is to consider the fact that ultrarelativistic massive particles have to have same behavior as massless particles. For example, people didn't even used to know that neutrinos had mass. So an ultrarelativistic neutrino, just like a photon, has to lose momentum and energy by the time it gets to B. If this holds for ultrarelativistic particles that have mass, then we expect it to hold as well for lower-energy particles that have mass, because we expect the behavior to vary smoothly with energy.

Another way to see this is that we know the universe cooled down as it expanded. This means that massive particles must have lost energy. We can't blame this on interactions, because actually the matter in standard cosmological models is an ideal gas. So the result must be the same, on the average, for a particle that just travels freely. If there was no such tendency for motion to settle down to the Hubble flow, then we wouldn't have a Hubble flow now.

It's not true, however, that redshift factors are the same for ultrarelativistic particles as for nonrelativistic ones. The effect is bigger for ultrarelativistic particles, which is why the universe is no longer dominated by radiation, even though it was at one time.

For simplicity I'll refer to the launched object as a probe and other cosmological objects as planets (one of which is Earth).

There's no good answer to what happens to the probe's speed relative to Earth, since there's no good way to define a notion of relative speed of very distant objects in cosmology.

The speed of the probe relative to the nearest planet will decrease over time. This happens simply because the planets are moving away from each other. If you imagine the planets and probe to have negligible mass so that there is no gravity (and also no cosmological constant), everything has a constant speed, and the probe will eventually pass every planet that has a lower speed, and will never pass any planet that has a higher speed, so ultimately it will end up permanently in between planets with slightly higher and slightly lower speeds, with a small speed relative to them. To put it another way, objects that initially have a high peculiar speed relative to the Hubble flow end up moving with the Hubble flow, if you wait long enough.

Cosmological redshift happens for the same reason. You can imagine that the light is absorbed/detected by each planet it reaches and then reemitted at the same frequency. The reemitted light will be detected by the next planet with a redshift or a blueshift depending on the relative motion of those two planets. On average, the planets are moving apart, so the longer the chain of planets, the larger the accumulated redshift.

The energy density of the universe in the present era is very low, so this gravityless model is pretty accurate out to distances of hundreds of millions of light years. At larger scales, you can no longer ignore spacetime curvature, but spacetime curvature doesn't fundamentally change what happens, it only deforms it a bit. It's a misconception that the loss of momentum is due to some peculiar general-relativistic property of spacetime, like curvature or intrinsic expansion. It's simply due to the fact that the planets (and stars and galaxies) are moving away from each other.

## Question Edgeless universe?

Hypothesizing as to whether there is an edge to the universe is like suggesting there is a center to the universe. It speculates that the universe is some sort of contained volume of evolutionary activity rather than as a dimensional perspective in which reality plays out. You either go along with the concept of an isotropic and homogeneous universe, wherein there is no center or edges, or you don’t. If you go along with the concept, then the notion of infinity provides the description that best describes its infinite nature: limitless, boundless, and endless in spacetime wherein extent or size is impossible to measure or calculate.

The closest scientists have come to in a discussion which may represent a more limited universe is the debate over cosmological multipole patterns, as presented - "In fact, that pattern can be fitted to a quadrupole alignment with a much higher probability than chance suggesting that the early universe as a whole could have been spinning like a giant galaxy." A previous article noted that scientists had detected a massive rotating galaxy-like disk from the early forming universe. Almost provides for a sense of logic to the hypothesis of 'Selfish Biocosm'. Ever since Newton, scientists have tried to understand existence by discovering its underlying rules. The result of this hypothesis has been a massive edifice of natural law, and biology has been seen as a consequence of the universe’s construction, rather than an instigator. Isaac Newton's First Law of Motion describes the behavior of a massive body at rest or in uniform linear motion, i.e., not accelerating or rotating. The First Law states, "A body at rest will remain at rest, and a body in motion will remain in motion unless it is acted upon by an external force." Back then, most people believed that the natural state of a body was to be at rest.

But what if the cosmos was always spinning or rotating upon its very conception? This property of massive bodies to resist changes in their state of motion is called inertia, and this leads to the concept of inertial reference frames. An inertial reference frame can be described as a 3-dimensional coordinate system that is neither accelerating nor rotating however, it may be in uniform linear motion with respect to some other inertial reference frame. Newton showed that these laws of motion, combined with his law of universal gravitation, explained Kepler's laws of planetary motion. In modern physics, the laws of conservation of momentum, energy, and angular momentum are of more general validity than Newton's laws, since they apply to both light and matter, and to both classical and non-classical physics. This can be stated simply, "Momentum, energy and angular momentum cannot be created or destroyed."

In the book, 'The Evolutioning of Creation: Volume 2', the author implies of motion that the forced distortion of spacetime interacts with mass in a way that is reflective of his modeling of existence in the pre-proposed model of immersive liquid inversion. While the author was actually working to derive the forces required for the evolution of mass densities, he also tripped upon the notion of the evolutionary creation of the elements. The evolutionary creation of the elements is comparable to the evolutionary development of cell division in that both are dependent on the purpose of direction in the form of electromagnetic and/or gravitational poles.

Considering a directional attitudinizing upon the creation of the universe suggests that evolutionary existence itself, whether elemental or biological, relies on motion. Motion requires changes over time. The concept of motion requires a dimensional framework of convergence for the fabric of spacetime in which case there must have been an unpopulated spacetime fabric that preceded the notion of creation. It has always been my premise that dark energy, being the largest distribution of total energy, represents the foundation for space-time and provides for a net zero inclusion of matter as a whole, then it starts as 100% of the total energy. Considering the 'Big Bang' theory from a singular point as modeled after a gravitational singularity, rather try thinking of the 'Big Bang' theory from a pre-existing fabric of space-time without any real matter, as a the proposed one dimensional determinant. Then start unfolding this dimensional perspective so space-time fabric into existence first into a two dimensional space-time fabric, which is an expansion from our one dimensional space-time, and then into a three dimensional space-time fabric and so on. The expectation is that ordinary matter creation took place within a pre-existing medium of space-time that pre-existing medium which is responsible for our expanding universe: dark energy. Indeed, the existence of matter would only warp the pre-existing fabric of space-time. Take away the positive density matter and you would still have a vessel in which the matter once existed. It would only be logical for this vessel to be one of dark matter, as dark matter would be unaffected by the force of dark energy.

The only problem with such a discussion is that it appears this interpretation of the data presents more questions than answers. If the universe was once spinning, then we need to explain how it continues to influence mass spin when the universe is no longer spinning. We would also need to explain the event that forced the universe to stop spinning, because without such an event the first law of motion implies that it should still be spinning, or rotating. And if it is still rotating, then why is there not a center to this rotating universe?

Scientists from University College London and Imperial College London have put this assumption through its most stringent test yet and found only a 1 in 121,000 chance that the universe is not the same in all directions. This study considered the widest possible range of universes with preferred directions or spins and determined what patterns these would create in the CMB. The results, published in the journal Physical Review Letters in 2016, show that no patterns were a match, and that the universe is most likely directionless, stating "We have put this assumption to its most exacting examination yet, testing for a huge variety of spinning and stretching universes that have never been considered before. When we compare these predictions to the Planck satellite's latest measurements, we find overwhelming evidence that the universe is the same in all directions. If this assumption is wrong, and our universe spins or stretches in one direction more than another, we'd have to rethink our basic picture of the universe."

So there are still problems with how this all would fit into the current view of our evolutionary universe.

## How do we know that the universe is expanding and light is not just losing momentum?

We know that the universe is expanding since light coming from distant galaxies are redshifted. How do we know that the redshift isn't the result of light losing momentum over incredibly long periods of time? (As momentum decreases, wavelength increases (p=h/λ))

Are there any other methods to verify that the universe is expanding other than observing redshifts of light?

This question is extremely common, there are likely many good threads with further discussion if you wish to search for it.

Much of the observational evidence for the big bang comes from studying its ⟬ho', the cosmic microwave background (CMBR). There are many measurements you can do with a quality map of the CMBR and each one we do matches our predictions for an expanding universe.

Another good piece of evidence is the relative ratios of primordial elements. We know roughly how much of each stuff was in the universe, 75% H 24%He and some lithium. We know how long the universe was at a certain temperature and we know fusion rates and decay rates of unstable isotopes so we should be able to predict this ratio. Our predictions match our observations.

This question is extremely common, there are likely many good threads with further discussion if you wish to search for it.

And if you want to look for more information beyond Reddit, this idea is called tired light. It was a serious proposal in the early years of cosmology, and crops up again from time to time, but it just doesn't seem to work as well as an expanding universe, especially as more and more data comes in.

When we say the universe is "expanding", do we mean to say that it is confined to a finite space that's "getting bigger" and that there is a defined limit to "traversable volume" by any 3 dimensional object? or simply that everything that is in existence is merely traveling farther away from each other in an infinite space?

There are other methods of confirming the expansion of the universe, one example being the measurement of density of mass/energy at different points in the history of the universe.

A lot of the evidence deals with the laws of special relativity and its predictions. Since its predictions and observations both match, the expansion of the universe is therefore given immense evidence.

There is no way light could lose momentum that would fit to the measurements. We also see that the galaxy structures change over time, and tracing that back leads to a phase of a very hot and compact universe.

The reason we know it works is the lack of a way for light to lose momentum. It can't be affected by any electromagnetic force or nuclear force because it isn't an atom and doesn't have electrons or protons. Gravity is the only force that can effect it and the only objects that can slow down light in such a significant manner as to redshift them are black holes, which we could detect due to light curving around them. Dark energy wouldn't do anything to the particle/wave itself ,because space only expands within itself, but makes the gap between galaxy clusters grow larger over time. This gives the galaxy it came from a momentum in the opposite direction as the light which causes the light to redshift.

So I know this thread is quite old but there wasn't really a great amount of specific evidence explained. The idea you're referring to is called "Tired Light" and originally it was more popular than the expanding universe as an explanation for Hubble's Law.

Originally to rule out tired light people looked at the so called Tolman Test, which measures the surface brightness of galaxies at different distances. Surface brightness is just how much energy you receive from a given area on the. In an expanding universe you expect the surface brightness to fall of very quickly, because of the combined effects of redshift (light loosing energy), geometry (inverse square law) and "cosmic time dilation". In a tired light scenario you don't have the latter and the distance can be different. The problem with this is that it strongly depends on how galaxies evolve.

Beyond that a nice test of cosmological time dilation is to look at supernovea, particularly type 1a which are very regular. As they get more distant you find that actually they take longer to brighten up and then fade. Link.

However the most damning bit of evidence is the spectrum of the Cosmic Microwave Background. The spectrum (over the sky) is a perfect blackbody to within the precision of the measurement. This means it's spectrum very actually follows the Planck distribution that is expected from ideal blackbody emitters. With tired light however the CMB is not emitted by the early universe as the free-electrons cleared but is usually due just to normal galaxies being extremely redshifted. The problem in tired light the CMB has to be emitted by galaxies over cosmic time but galaxies aren't good blackbodies. Additionally the lack of expansion means the density of photons doesn't drop so matching the CMB spectrum is incredibly hard.

In the somewhat distant future people hope to measure "redshift drift", that is as the universe expands objects move to greater distance and as they move to higher distance their redshift increases. Redshift is just a measure of how much the light has stretched. It's an incredibly difficult measurement to make with the change in "velocity" being something like 0.5 centimetres per second per year. It's incredibly hard to detect and it will take decades if things go to plan. The European Extremely large Telescope is well placed to do this first using quasar sightlines but it's very demanding on the instruments. The velocity accuracy needed is way beyond the needs of say exoplanet radial velocities. The Square Kilometre Array also hopes to do this but it will trade precision for vast numbers of galaxies measured, again it will take decades. This would be the ultimate nail in the coffin for static cosmologies.

## Contents

The first HH object was observed in the late 19th century by Sherburne Wesley Burnham, when he observed the star T Tauri with the 36-inch (910 mm) refracting telescope at Lick Observatory and noted a small patch of nebulosity nearby. [1] It was thought to be an emission nebula, later becoming known as Burnham's Nebula, and was not recognized as a distinct class of object. [2] T Tauri was found to be a very young and variable star, and is the prototype of the class of similar objects known as T Tauri stars which have yet to reach a state of hydrostatic equilibrium between gravitational collapse and energy generation through nuclear fusion at their centres. [3] Fifty years after Burnham's discovery, several similar nebulae were discovered with almost star-like appearance. Both Haro and Herbig made independent observations of several of these objects in the Orion Nebula during the 1940s. Herbig also looked at Burnham's Nebula and found it displayed an unusual electromagnetic spectrum, with prominent emission lines of hydrogen, sulfur and oxygen. Haro found that all the objects of this type were invisible in infrared light. [2]

Following their independent discoveries, Herbig and Haro met at an astronomy conference in Tucson, Arizona in December 1949. Herbig had initially paid little attention to the objects he had discovered, being primarily concerned with the nearby stars, but on hearing Haro's findings he carried out more detailed studies of them. The Soviet astronomer Viktor Ambartsumian gave the objects their name (Herbig–Haro objects, normally shortened to HH objects), and based on their occurrence near young stars (a few hundred thousand years old), suggested they might represent an early stage in the formation of T Tauri stars. [2] Studies of the HH objects showed they were highly ionised, and early theorists speculated that they were reflection nebulae containing low-luminosity hot stars deep inside. But the absence of infrared radiation from the nebulae meant there could not be stars within them, as these would have emitted abundant infrared light. In 1975 American astronomer R. D. Schwartz theorized that winds from T Tauri stars produce shocks in the ambient medium on encounter, resulting in generation of visible light. [2] With the discovery of the first proto-stellar jet in HH 46/47, it became clear that HH objects are indeed shock-induced phenomena with shocks being driven by a collimated jet from protostars. [2] [4]

Stars form by gravitational collapse of interstellar gas clouds. As the collapse increases the density, radiative energy loss decreases due to increased opacity. This raises the temperature of the cloud which prevents further collapse, and a hydrostatic equilibrium is established. Gas continues to fall towards the core in a rotating disk. The core of this system is called a protostar. [5] Some of the accreting material is ejected out along the star's axis of rotation in two jets of partially ionised gas (plasma). [6] The mechanism for producing these collimated bipolar jets is not entirely understood, but it is believed that interaction between the accretion disk and the stellar magnetic field accelerates some of the accreting material from within a few astronomical units of the star away from the disk plane. At these distances the outflow is divergent, fanning out at an angle in the range of 10−30°, but it becomes increasingly collimated at distances of tens to hundreds of astronomical units from the source, as its expansion is constrained. [7] [8] The jets also carry away the excess angular momentum resulting from accretion of material onto the star, which would otherwise cause the star to rotate too rapidly and disintegrate. [8] When these jets collide with the interstellar medium, they give rise to the small patches of bright emission which comprise HH objects. [9]

Electromagnetic emission from HH objects is caused when their associated shock waves collide with the interstellar medium, creating what is called the "terminal working surfaces". [10] The spectrum is continuous, but also has intense emission lines of neutral and ionized species. [6] Spectroscopic observations of HH objects' doppler shifts indicate velocities of several hundred kilometers per second, but the emission lines in those spectra are weaker than what would be expected from such high-speed collisions. This suggests that some of the material they are colliding with is also moving along the beam, although at a lower speed. [11] [12] Spectroscopic observations of HH objects show they are moving away from the source stars at speeds of several hundred kilometres per second. [2] [13] In recent years, the high optical resolution of the Hubble Space Telescope has revealed the proper motion (movement along the sky plane) of many HH objects in observations spaced several years apart. [14] [15] As they move away from the parent star, HH objects evolve significantly, varying in brightness on timescales of a few years. Individual compact knots or clumps within an object may brighten and fade or disappear entirely, while new knots have been seen to appear. [8] [10] These arise likely because of the precession of their jets, [16] [17] along with the pulsating and intermittent eruptions from their parent stars. [9] Faster jets catch up with earlier slower jets, creating the so-called "internal working surfaces", where streams of gas collide and generate shock waves and consequent emissions. [18]

The total mass being ejected by stars to form typical HH objects is estimated to be of the order of 10 −8 to 10 −6 M per year, [16] a very small amount of material compared to the mass of the stars themselves [19] but amounting to about 1–10% of the total mass accreted by the source stars in a year. [20] Mass loss tends to decrease with increasing age of the source. [21] The temperatures observed in HH objects are typically about 9,000–12,000 K, [22] similar to those found in other ionized nebulae such as H II regions and planetary nebulae. [23] Densities, on the other hand, are higher than in other nebulae, ranging from a few thousand to a few tens of thousands of particles per cm 3 , [22] compared to a few thousand particles per cm 3 in most H II regions and planetary nebulae. [23]

Densities also decrease as the source evolves over time. [21] HH objects consist mostly of hydrogen and helium, which account for about 75% and 24% of their mass respectively. Around 1% of the mass of HH objects is made up of heavier chemical elements, including oxygen, sulfur, nitrogen, iron, calcium and magnesium. Abundances of these elements, determined from emission lines of respective ions, are generally similar to their cosmic abundances. [19] Many chemical compounds found in the surrounding interstellar medium, but not present in the source material, such as metal hydrides, are believed to have been produced by shock-induced chemical reactions. [7] Around 20–30% of the gas in HH objects is ionized near the source star, but this proportion decreases at increasing distances. This implies the material is ionized in the polar jet, and recombines as it moves away from the star, rather than being ionized by later collisions. [22] Shocking at the end of the jet can re-ionise some material, giving rise to bright "caps". [6]

HH objects are named approximately in order of their identification HH 1/2 being the earliest such objects to be identified. [24] More than a thousand individual objects are now known. [7] They are always present in star-forming H II regions, and are often found in large groups. [9] They are typically observed near Bok globules (dark nebulae which contain very young stars) and often emanate from them. Several HH objects have been seen near a single energy source, forming a string of objects along the line of the polar axis of the parent star. [7] The number of known HH objects has increased rapidly over the last few years, but that is a very small proportion of the estimated up to 150,000 in the Milky Way, [25] the vast majority of which are too far away to be resolved. Most HH objects lie within about one parsec of their parent star. Many, however, are seen several parsecs away. [21] [22]

HH 46/47 is located about 450 parsecs (1,500 light-years) away from the Sun and is powered by a class I protostar binary. The bipolar jet is slamming into the surrounding medium at a velocity of 300 kilometers per second, producing two emission caps about 2.6 parsecs (8.5 light-years) apart. Jet outflow is accompanied by a 0.3 parsecs (0.98 light-years) long molecular gas outflow which is swept up by the jet itself. [7] Infrared studies by Spitzer Space Telescope have revealed a variety of chemical compounds in the molecular outflow, including water (ice), methanol, methane, carbon dioxide (dry ice) and various silicates. [7] [26] Located around 460 parsecs (1,500 light-years) away in the Orion A molecular cloud, HH 34 is produced by a highly collimated bipolar jet powered by a class I protostar. Matter in the jet is moving at about 220 kilometers per second. Two bright bow shocks, separated by about 0.44 parsecs (1.4 light-years), are present on the opposite sides of the source, followed by series of fainter ones at larger distances, making the whole complex about 3 parsecs (9.8 light-years) long. The jet is surrounded by a 0.3 parsecs (0.98 light-years) long weak molecular outflow near the source. [7] [27]

The stars from which HH jets are emitted are all very young stars, a few tens of thousands to about a million years old. The youngest of these are still protostars in the process of collecting from their surrounding gases. Astronomers divide these stars into classes 0, I, II and III, according to how much infrared radiation the stars emit. [28] A greater amount of infrared radiation implies a larger amount of cooler material surrounding the star, which indicates it is still coalescing. The numbering of the classes arises because class 0 objects (the youngest) were not discovered until classes I, II and III had already been defined. [29] [28]

Class 0 objects are only a few thousand years old so young that they are not yet undergoing nuclear fusion reactions at their centres. Instead, they are powered only by the gravitational potential energy released as material falls onto them. [30] They mostly contain molecular outflows with low velocities (less than a hundred kilometres per second) and weak emissions in the outflows. [17] Nuclear fusion has begun in the cores of Class I objects, but gas and dust are still falling onto their surfaces from the surrounding nebula, and most of their luminosity is accounted for by gravitational energy. They are generally still shrouded in dense clouds of dust and gas, which obscure all their visible light and as a result can only be observed at infrared and radio wavelengths. [31] Outflows from this class are dominated by ionized species and velocities can range up to 400 kilometres per second. [17] The in-fall of gas and dust has largely finished in Class II objects (Classical T Tauri stars), but they are still surrounded by disks of dust and gas, and produce weak outflows of low luminosity. [17] Class III objects (Weak-line T Tauri stars) have only trace remnants of their original accretion disk. [28]

About 80% of the stars giving rise to HH objects are binary or multiple systems (two or more stars orbiting each other), which is a much higher proportion than that found for low mass stars on the main sequence. This may indicate that binary systems are more likely to generate the jets which give rise to HH objects, and evidence suggests the largest HH outflows might be formed when multiple–star systems disintegrate. [32] It is thought that most stars originate from multiple star systems, but that a sizable fraction of these systems are disrupted before their stars reach the main sequence due to gravitational interactions with nearby stars and dense clouds of gas. [32] [33]

The first and currently only (as of May 2017) large-scale Herbig-Haro object around a proto-brown dwarf is HH 1165, which is connected to the proto-brown dwarf Mayrit 1701117. HH 1165 has a length of 0.8 light-years (0.26 parsec) and is located in the vicinity of the sigma Orionis cluster. Previously only small mini-jets (≤0.03 parsec) were found around proto-brown dwarfs. [34] [35]

HH objects associated with very young stars or very massive protostars are often hidden from view at optical wavelengths by the cloud of gas and dust from which they form. The intervening material can diminish the visual magnitude by factors of tens or even hundreds at optical wavelengths. Such deeply embedded objects can only be observed at infrared or radio wavelengths, [36] usually in the frequencies of hot molecular hydrogen or warm carbon monoxide emission. [37] In recent years, infrared images have revealed dozens of examples of "infrared HH objects". Most look like bow waves (similar to the waves at the head of a ship), and so are usually referred to as molecular "bow shocks". The physics of infrared bow shocks can be understood in much the same way as that of HH objects, since these objects are essentially the same – supersonic shocks driven by collimated jets from the opposite poles of a protostar. [38] It is only the conditions in the jet and surrounding cloud that are different, causing infrared emission from molecules rather than optical emission from atoms and ions. [39] In 2009 the acronym "MHO", for Molecular Hydrogen emission-line Object, was approved for such objects, detected in near infrared, by the International Astronomical Union Working Group on Designations, and has been entered into their on-line Reference Dictionary of Nomenclature of Celestial Objects. [38] The MHO catalog contains over 2000 objects.

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## Do objects lose momentum as space expands - Astronomy

Do asteroids hit the Sun like they hit the planets and moons?

No asteroids have ever been observed to hit the Sun, but that doesn't mean that they don't! Asteroids are normally content to stay in the asteroid belt between Mars and Jupiter, but occasionally something nudges them out of their original orbits, and they come careening into the inner solar system. The "something" that changes the asteroid orbits is often thought to be the Yarkovsky effect (illustrated here). It is known that Jupiter has a strong effect on the asteroid belt. Jupiter's gravity interacts with the Belt to form the Kirkwood gaps. Orbits within in a Kirkwood gap are not stable, and any asteroid whose orbit wanders into such a region will eventually get pulled into a different orbit, which may take it into the inner solar system. Therefore the Kirkwood gaps have almost no asteroids. In addition to Jupiter's influence, occasional random impacts within the belt probably send asteroid pieces flying in toward the inner solar system.

Once they are on their way in toward the Sun, you might think that they should be guaranteed to hit the Sun, but that's not the case! It is actually difficult for something that is orbiting to fall all the way into the Sun. This is because of a property of orbiting objects called angular momentum. Angular momentum is a sort of measure of how much something is rotating around a central point. The reason that this is important is that one of the fundamental principles of physics is that angular momentum must be conserved. For something to fall into the Sun, it has to lose nearly all of its angular momentum somehow, so that it is falling straight at the Sun. If it is off just slightly, instead of falling in, the asteroid will just fall very close, and then slingshot back out far from the Sun. It is probably quite rare for an asteroid to lose all of its angular momentum and fall straight into the Sun. However, there might be quite a few that lose enough to get close to the Sun and vaporize.

As I mentioned, we have never seen an asteroid come close to the Sun and vaporize. That's because asteroids are small rocks or pieces of metal, and even when they are being vaporized they are hard to see. Comets, on the other hand, give off huge glowing plumes of gas when they get close to the Sun, making them very easy to spot. The SOHO satellite has detected more than 1100 comets known as "sun grazers." These are comets that get close enough to the Sun to glow very brightly and show up in the SOHO images. Some of them disintegrate while others survive the close call and sail back out to the outer solar system until their next orbit brings them back. Check the SOHO Comets website for more information.

#### Ryan Anderson

Ryan is a research fellow at USGS in Flagstaff, AZ and is a member of the Curiosity ChemCam team. He also loves explaining all aspects of astronomy. Check out his blog!

## Shared Flashcard Set

- in the absence of net force, an object will move with constant velocity.

*ie: a spaceship needs no fuel to keep moving in space

- as long as an object is traveling at a constant velocity, no net force is acting on it.

NEWTON'S SECOND LAW OF MOTION

- tells us what happens to an object when a net for is present

- FORCE = MASS x ACCELERATION

- explains why you can throw a rock further than you can throw a brick

- more massive planets exert a stronger gravitational force? (pg. 92, paragraph 2)

NEWTON'S THIRD LAW OF MOTION

- Every force is always paired with an equal and opposite reaction force

* explains rocket propulsion - a rocket engine generates a force that drives hot gas out the back, which creates an equal and opposite force that propels the rocket forward.

- one of Newton's conservation laws

-the total momentum of all interacting objects always stays the same

- an individual object can gain or lose momentum only when a force causes it to exchange momentum with another object.

- conservation of angular momentum and conservation of energy are other conservation laws

- a special type of momentum used to describe objects turning in circles or going around curves

Angular Momentum of Earth = m x v x r

THE LAW OF CONSERVATION OF ANGULAR MOMENTUM

- total angular momentum can never change

- and individual object can only change its angular momentum by transferring some angular momentum to or from another object

What are the 2 key facts about earth's orbit that can be explained by understanding The Law of Conservation of Angular Momentum and the equation for Earth's angular momentum (angular momentum = m x v x r)

1. Earth will keep orbiting the sun as long as nothing comes along to take away its angular momentum

- Earth needs no fuel or push to orbit the sun

2. Earth's angular momentum at any point depends on the product of its speed and orbital radius (distance from the sun)

- therefore: Earth's orbital speed must be faster when it is nearer to the sun (radius is shorter) and slower when it is farther from the sun (radius is longer).

- THIS IS CONFUSING: remember that angular momentum of the earth ALWAYS STAYS THE SAME, so if r (the radial orbit) is smaller, but the momentum is still the same, there must be an increase in the velocity (v) in order for the total product to remain the same.

THE LAW OF CONSERVATION OF ENERGY

- energy cannot appear out of nowhere or disappear into nothingness

- objects gain or lose energy only be exchanging energy with other objects

3. Potential (stored energy)

- examples: falling rocks, orbiting planets and molecules moving in the air

ie: light can change molecules in our eyes, allowing us to see

or warm the surface of the planet

- stored energy which may later be converted into kinetic or radiative energy

ie: a rock on a ledge has gravitational potential energy, because it may fall

g asoline has chemical potential energy that may be converted into kinetic energy to make a car move

- a subcategory of kinetic energy

- represents the collective kinetic energy of the many individual particles moving within a substance

- not the same thing as temperature

- depends on temperature (higher average kinetic energy must lead to a higher total kinetic energy)

-depends on number and density of particles

Thermal Energy: measures the total kinetic energy

Temperature: measures the average kinetic energy

GRAVITATIONAL POTENTIAL ENERGY

- an object gravitational potential energy depends on its mass and how far it can fall as a result of gravity

- an object has more gravitational potential energy when it is higher than when it is lower

- kinetic energy increases as gravitational potential energy decreases

- mass is a form of potential energy

- a form of potential energy

e: amount of potential energy

m: that mass of the object

- small amount of mass contains a huge amount of energy

-mass can be converted into energy and energy can be converted into mass

- the total energy content of the universe was determined in the big bang

NEWTON'S UNIVERSAL LAW OF GRAVITATION

- every mass attracts every other mass through the force called gravity

- the strength of the gravitational force attracting any two objects is directly proportional to the product of their masses

- doubling the mass of one object doubles the force of gravity between the two objects

-the strength of gravity between two objects decreases with the square of the distance between their centers

- gravitational force follows an inverse square law

- doubling the distance between two objects weakens the force of gravity by 2 to the second power (4)

The idea of "counter all of this motion" does not make sense, I'm afraid. There is no way to define a "fixed point in space" - you can only define motion relative to something else.

Basically, then, it doesn't matter what you do you have the same experience of the laws of physics. This is the point of the "principle of relativity" that underlies the theory of relativity - physics is the same for all observers who are not accelerating.

Hi @Nathan991
The answers already given are good. There's no universal reference frame, so you should give up on trying to define anything as "motionless" unless you specify with respect to what.

What could you compare your motion against?
One of the more sensible things you could choose is the Cosmic Microwave background radiation (CMBR). You could try to adjust your motion so that the CMBR looked the same in every direction. This is a special class of motions that is (arguably) most like being motionless in space. It is described as being "co-moving".

The Earth is spinning and orbiting the Sun, and that is orbiting the center of the Milky Way, and that too moving in some way.

Say I were to hop in a spaceship and travel in such a way that it counters all of this motion, leaving me and the spaceship completely motionless.

Well, if you were stationary with respect to the centre of our galaxy then I think you'd be moving at around 80km/s relative to everything else around you in the solar system, so it certainly wouldn't 'look' like you are stationary from where you are sitting.

If you were to be stationary wrt the solar system you're in, as you'd not be in a stable orbit, you'd get pulled in towards the Sun (and/or nearest planet/moon) and as you crash and burn up you can then reflect on why your quest for motionless ended up causing you to crash into something.

I mean, the only real world comparison is that being motionless in the middle of the highway is not actually a very safe place to be. Best to keep up with the motion of the solar system/rotation of the earth/etc. would be my advice. Keep up with the prevailing astronomical-body traffic speed.

## As The Universe Expands, Does Space Actually Stretch?

It’s been almost 100 years since humanity first reached a revolutionary conclusion about our Universe: space itself doesn’t remain static, but rather evolves with time. One of the most unsettling predictions of Einstein’s General Relativity is that any Universe — so long as it’s evenly filled with one or more type of energy — cannot remain unchanging over time. Instead, it must either expand or contract, something initially derived independently by three separate people: Alexander Friedmann (1922), Georges Lemaitre (1927), Howard Robertson (1929), and then generalized by Arthur Walker (1936).

Concurrently, observations began to show that the spirals and ellipticals in our sky were galaxies. With these new, more powerful measurements, we could determine that the farther away a galaxy was from us, the greater the amounts its light arrived at our eyes redshifted, or at longer wavelengths, compared to when that light was emitted.

But what, exactly, is happening to the fabric of space itself while this process occurs? Is the space itself stretching, as though it’s getting thinner and thinner? Is more space constantly being created, as though it were “filling in the gaps” that the expansion creates? This is one of the toughest things to understand in modern astrophysics, but if we think hard about it, we can wrap our heads around it. Let’s explore what’s going on.

The first thing you have to understand is what General Relativity does, and doesn’t, tell us about the Universe. General Relativity, at its core, is a framework that relates two things that might not obviously be related:

• the amount, distribution, and types of energy — including matter, antimatter, dark matter, radiation, neutrinos, and anything else you can imagine — that are present all throughout the Universe,
• and the geometry of the underlying spacetime, including whether and how it’s curved and whether and how it will evolve.

If your Universe has nothing in it at all, no matter or energy of any form, you get the flat, unchanging, Newtonian space you’re intuitively used to: static, uncurved, and unchanging.

If instead you put down a point mass in the Universe, you get space that’s curved: Schwarzschild space. Any “test particle” you put into your Universe will be compelled to flow towards that mass along a particular trajectory.

And if you make it a little more complicated, by putting down a point mass that also rotates, you’ll get space that’s curved in a more complex way: according to the rules of the Kerr metric. It will have an event horizon, but instead of a point-like singularity, the singularity will get stretched out into a circular, one-dimensional ring. Again, any “test particle” you put down will follow the trajectory laid out by the underlying curvature of space.

These spacetimes, however, are static in the sense that any distance scales you might include — like the size of the event horizon — don’t change over time. If you stepped out of a Universe with this spacetime and came back later, whether a second, an hour, or a billion years later, its structure would be identical irrespective of time. In spacetimes like these, however, there’s no expansion. There’s no change in the distance or the light-travel-time between any points within this spacetime. With just one (or fewer) sources inside, and no other forms of energy, these “model Universes” really are static.

But it’s a very different game when you don’t put down isolated sources of mass or energy, but rather when your Universe is filled with “stuff” everywhere. In fact, the two criteria we normally assume, and which is strongly validated by large-scale observations, are called isotropy and homogeneity. Isotropy tells us that the Universe is the same in all directions: everywhere we look on cosmic scales, no “direction” looks particularly different or preferred from any other. Homogeneity, on the other hand, tells us that the Universe is the same in all locations: the same density, temperature, and expansion rate exist to better than 99.99% precision on the largest scales.

In this case, where your Universe is uniformly filled with some sort of energy (or multiple different types of energy), the rules of General Relativity tell us how that Universe will evolve. In fact, the equations that govern it are known as the Friedmann equations: derived by Alexander Friedmann all the way back in 1922, a year before we discovered that those spirals in the sky are actually galaxies outside of and beyond the Milky Way!

Your Universe must expand or contract according to these equations, and that’s what the mathematics tells us must occur.

But what, exactly, does that mean?

You see, space itself is not something that’s directly measurable. It’s not like you can go out and take some space and just perform an experiment on it. Instead, what we can do is observe the effects of space on observable things — like matter, antimatter, and light — and then use that information to figure out what the underlying space itself is doing.

For example, if we go back to the black hole example (although it applies to any mass), we can calculate how severely space is curved in the vicinity of a black hole. If the black hole is spinning, we can can calculate how significantly space is “dragged” along with the black hole due to the effects of angular momentum. If we then measure what happens to objects in the vicinity of those objects, we can compare what we see with the predictions of General Relativity. In other words, we can see if space curves the way Einstein’s theory tells us it ought to.

And oh, does it do so to an incredible level of precision. Light blueshifts when it enters an area of extreme curvature and redshifts when it leaves. This gravitational redshift has been measured for stars orbiting black holes, for light traveling vertically in Earth’s gravitational field, from the light coming from the Sun, and even for light passing through growing galaxy clusters.

Similarly, gravitational time dilation, the bending of light by large masses, and the precession of everything from planetary orbits to rotating spheres sent up to space has demonstrated spectacular agreement with Einstein’s predictions.

But what about the Universe’s expansion? When you think about an expanding Universe, the question you should be asking is: “what, observably, changes about the measurable things in the Universe?” After all, that’s what we can predict, that’s what’s physically observable, and that’s what will inform us as to what’s going on.

Well, the simplest thing we can look at is density. If our Universe is filled with “stuff,” then as the Universe expands, its volume increases.

We normally think about matter as the “stuff” we’re thinking about. Matter is, at its simplest level, a fixed amount of massive “stuff” that lives within space. As the Universe expands, the total amount of stuff remains the same, but the total amount of space for the “stuff” to live within increases. For matter, density is just mass divided by volume, and so if your mass stays the same (or, for things like atoms, the number of particles stays the same) while your volume grows, your density should go down. When we do the General Relativity calculation, that’s exactly what we find for matter.

But even though we have multiple types of matter in the Universe — normal matter, black holes, dark matter, neutrinos, etc. — not everything in the Universe is matter.

For example, we also have radiation: quantized into individual particles, like matter, but massless, and with its energy defined by its wavelength. As the Universe expands, and as light travels through the expanding Universe, not only does the volume increase while the number of particles remains the same, but each quantum of radiation experiences a shift in its wavelength towards the redder end of the spectrum: longer wavelengths.

Meanwhile, our Universe also possesses dark energy, which is a form of energy that isn’t in the form of particles at all, but rather appears to be inherent to the fabric of space itself. While we cannot measure dark energy directly the same way we can measure the wavelength and/or energy of photons, there is a way to infer its value and properties: by looking at precisely how the light from distant objects redshifts. Remember that there’s a relationship between the different forms of energy in the Universe and the expansion rate. When we measure the distance and redshift of various objects throughout cosmic time, they can inform us as to how much dark energy there is, as well as what its properties are. What we find is that the Universe is about ⅔ dark energy today, and that the energy density of dark energy doesn’t change: as the Universe expands, the energy density remains constant.

When we put the full picture together from all the different sources of data that we have, a single, consistent picture emerges. Our Universe today is expanding at somewhere around 70 km/s/Mpc, which means that for every megaparsec (about 3.26 million light-years) of distance an object is separated from another object, the expanding Universe contributes a redshift that’s equivalent to a recessional motion of 70 km/s.

That’s what it’s doing today, mind you. But by looking to greater and greater distances and measuring the redshifts there, we can learn how the expansion rate differed in the past, and hence, what the Universe is made of: not just today, but at any point in history. Today, our Universe is made of the following forms of energy:

• about 0.1% neutrinos, which now behave like matter but behaved like radiation early on, when their mass was very small compared to the amount of (kinetic) energy they possessed,
• about 4.9% normal matter, which includes atoms, plasmas, black holes, and everything that was once made of protons, neutrons, or electrons,
• about 27% dark matter, whose nature is still unknown but which must be massive and clumps, clusters, and gravitates like matter,
• and about 68% dark energy, which behaves as though it’s energy inherent to space itself.

If we extrapolate backwards, based on what we infer about today, we can learn what type of energy dominated the expanding Universe at various epochs in cosmic history.

10,000 years of the Universe after the Big Bang. (E. SIEGEL)

Notice, very importantly, that the Universe responds in a fundamentally different way to these differing forms of energy. When we ask, “what is space doing while it’s expanding?” we’re actually asking which description of space makes sense for the phenomenon we’re considering. If you consider a Universe filled with radiation, because the wavelength lengthens as the Universe expands, the “space stretches” analogy works very well. If the Universe were to contract instead, “space compresses” would explain how the wavelength shortens (and energy increases) equally well.

On the other hand, when something stretches, it thins out, just like when something compresses, it thickens up. This is a reasonable thought for radiation, but not for dark energy, or any form of energy intrinsic to the fabric of space itself. When we consider dark energy, the energy density always remains constant. As the Universe expands, its volume is increasing while the energy density doesn’t change, and therefore the total energy increases. It’s as though new space is getting created due to the Universe’s expansion.

Neither explanation works universally well: it’s that one works to explain what happens to radiation (and other energetic particles) and one works to explain what happens to dark energy (and anything else that’s an intrinsic property of space, or a quantum field coupled directly to space).

Space, contrary to what you might think, isn’t some physical substance that you can treat the same way you’d treat particles or some other form of energy. Instead, space is simply the backdrop — a stage, if you will — against or upon which the Universe itself unfolds. We can measure what the properties of space are, and under the rules of General Relativity, if we can know what’s present within that space, we can predict how space will curve and evolve. That curvature and that evolution will then determine the future trajectory of every quantum of energy that exists.

The radiation within our Universe behaves as though space is stretching, although space itself isn’t getting any thinner. The dark energy within our Universe behaves as though new space is getting created, although there’s nothing we can measure to detect this creation. In reality, General Relativity can only tell us how space behaves, evolves, and affects the energy within it it cannot fundamentally tell us what space actually is. In our attempts to make sense of the Universe, we cannot justify adding extraneous structures atop what is measurable. Space neither stretches nor gets created, but simply is. At least, with General Relativity, we can accurately learn “how” it is, even if we can’t know precisely “what” it is.

## University of California, San Diego Center for Astrophysics & Space Sciences

The General Theory of Relativity is an expansion of the Special Theory to include gravity as a property of space. Start with this Gravity Tutorial.

The Theory of Special Relativity has as its basic premise that light moves at a uniform speed, c = 300,000 km/s, in all frames of reference. This results in setting the speed of light as the absolute speed limit in the Universe and also produced the famous relationship between mass and energy, E = mc 2 . The foundation of Einstein's General Theory is the Equivalence Principle which states the equivalence between inertial mass and gravitational mass.

Inertial Mass is the quantity that determines how difficult it is to alter the motion of an object. It is the mass in Newton's Second Law: F = ma

Gravitational mass is the mass which determines how strongly two objects attract each other by gravity, e.g. the attraction of the earth:

It is the apparent equivalence of these two types of mass which results in the uniformity of gravitational acceleration -- Galileo's result that all objects fall at the same rate independent of mass:

Galileo and Newton accepted this as a happy coincidence, but Einstein turned it into a fundamental principle. Another way of stating the equivalence principle is that gravitational acceleration is indistinguishable from other forms of acceleration. According to this view a student in a closed room could not tell the difference between experiencing the gravitational pull of the earth at the earth's surface and being in a rocketship in space accelerating with a = 9.8 m/s 2 .

nor could students in a similar room distinguish between free-fall under gravity and the weightlessness of space.

Curved Spacetime

 The second fundamental principle of General Relativity is that the presence of matter curves space. In this view, gravity is not a force, as described by Newton, but a curvature in the fabric of space, and objects respond to gravity by following the curvature of space in the vicinity of a massive object. The description of the curvature of space is the mathematically complicated part of general relativity involving "metrics", which describe the way that matter curves space, and tensor calculus. The Curvature of Space caused by a Massive Object.

The figure above represents a two-dimensional slice through three-dimensional space showing the curvature of space produced by a spherical object, perhaps the sun. Einstein's view is that the planets follow the curvature of space around the sun (and produce a tiny amount of curvature themselves).

Here are two fine astronomy course pages on General Relativity from Dr. Terry Herter at Cornell, from whom I stole the above images, and astronomers at the University of Tennessee.

Deflection of Light by Gravity: A direct consequence of the equivalence principle is that light should be deflected or bent by gravity. Einstein twice calculated the amount that light would be deflected passing by the sun, the largest "nearby" mass. His first calculation used only the Equivalence Principle and the equivalent mass-energy of a visible photon. In his second calculation, published in 1916, he included the space-time metric, which describes the curvature of space and time caused by gravity and got an answer twice as large as his first calculation. The second calculation predicts that light from a distant star passing by the limb of the sun would be deflected by 1.75 arcseconds (less than 1/2000th of a degree).

The first opportunity to test Einstein's calculation came with the Solar Eclipse of 1919. British Astrophysicist Sir Arthur Eddington mounted a pair of expeditions to West Africa and Brazil to observe the shift in position of the Hyades cluster stars behind the occulted sun. Eddington's measurements, though not perfectly precise clearly showed a deflection and favored the larger value. The result made Einstein world-famous. The test can now be made with greater precision. Every year the radio source 3C279 is occulted by the sun. Because the sun is only a modest radio-emitter, Radio Astronomers do not need to wait for an eclipse. Radio interferometry of 3C279 as it passes behind the sun has confirmed Einstein's calculation to better than 1%.

An exciting and only very recently verified prediction of the bending of light by gravity is the existence of gravitational lenses an optical lens focuses light be refraction, bending of light due to the change of the speed of light as it passes through a refractive medium. Because gravity can bend light, massive objects can act as lenses, focusing and amplifying images of distant objects. Gravitational lenses have rather different properties than "normal" lenses producing multiple images such as the Einstein Cross, a case of a distant quasar imaged by a galaxy between us and the quasar, discovered by J. Huchra & colleagues, shown to the left. If the alignment between us, the lensing galaxy, and the distant object, an Einstein Ring is produced. Distant galaxy clusters may also act as gravitational lenses. Astronomers are beginning to make use of the gravitational lensing phenomenon to study very distant galaxies and quasars. More about this in Lecture #17.

Twins Bill and Jill, born within minutes of each other, take differing career paths. Jill becomes an astronaut and Bill becomes a ground-based astronomer. On their 21st birthday Jill sets out on a space mission to Aldebaran, 32 light years away. Travelling at 99.5% of the speed of light, Jill measures a time of 3.2 years for her trip to Aldebaran and another 3.2 years for her return. (Incideltally, while she is travelling near the speed of light she also sees the distance to Aldebaran contracted to a mere 3.2 light years.) Bill finds that it takes her 32 years and 2 months for each leg. Upon Jill's return, she is 27 while her sibling is 85! Bizarre as these effects appear to us slow moving mortals, relativistic time dilation has been repeatedly confirmed in high energy particle accelerators, where particles travel near the speed of light, and by atomic clock on supersonic aircraft.

A similar process occurs in the presence of strong gravity a timekeeper in a strong gravitational field will measure a slower time than one in the absence of gravity. It is not just clocks, by the way, all physical processes: clocks ticking (however they measure their ticks), hearts beating, aging, etc., must slow down, but the only one who notices is the distant timekeeper. Everything seems "normal" to the person measuring the duration of events in his own frame of reference. Light waves travelling past the sun are slowed down by this time dilation by a small but measurable amount. In 197X the Viking Mars Lander performed the initial confirming experiment of gravitational time dilation by relaying radio signals back to earth from the Martian surface on the other side of the solar system. Although the effects of the intervening solar wind complicate the experiment, NASA scientists demonstrated clearly that the radio signals took longer on their round trip by just the amount predicted by the predicted slowing of time.

Predicted sources of strong gravitational waves in the Galaxy are supernova explosions, collapsing stellar cores as they form neutron stars or black holes, compact binary star systems, collisions of neutron stars & black holes, or possibly material falling into the blavk hole which may reside in the Galactic Center. Gravitational waves have not yet been detected directly, but we believe that they have been detected indirectly by radio astronomers in the binary pulsar system 1913+16. As the pulsar is accelerated around its companion, orbiting every 8 hours in this compact system, General Relativity predicts that gravitational waves should be produced. Although these waves are far too faint to be detected directly, the binary pulsar system is losing energy through this radiation, and the pulsar/neutron star and its companion are predicted to be slowly spiralling together. The rapid radio pulses permit precise timing of the pulsar orbit by doppler shifts of the pulse period as the pulsar moves toward or away from us. Since the discovery of the binary pulsar in 1974, timing of the pulsar has shown that the stars are indeed spiralling together just as predicted. In 300 million years the stars will coalesce - that should produce gravitational radiation that can be easily detected!

All of this amounts to pretty spectacular confirmation of General Relativity Theory.

So, Einstein was right and Newton was wrong!

1. developing theories or hypotheses,
2. testing them repeatedly by experiment and observation,
3. using them where they are shown to be applicable, and
4. revising & improving them when they are shown to disagree with experiment.
• Spacetime Wrinkles - National Center for Supercomputer Applications at U. Illinois has put together a superb Relativity site which includes History, Special Relativity, General Relativity, Tests of Relativity, Black Holes, Gravitational Waves, Relativistic Astrophysics, Relativistic Astronomical Objects, Spacetime Movies, and more. Many of the above links are to pages at this site. Strongly recommended!
• Jillian's Guide to Gravitational Waves

The French mathematician LaPlace first speculated about the existence of an object so compact that the escape speed would be greater than the speed of light. The first relativistic calculation was performed by Karl Schwarzschild (1916) shortly after Einstein published his theory. Curiously, Schwarzschild's result is the same as that of LaPlace an object with mass M which has a size

will have an escape speed equal to the speed of light. We call such an object a Black Hole. (Note that for the sun to be a black hole it would have to be compressed by a quarter of a million times down to a radius less than 3km.) A black hole is an object so compact that nothing can escape its gravity, not even light. Mathematically, a black hole is an object of zero size and infinite density (but finite mass) - a singularity. Schwarzschild's calculation shows that the gravitational radius, also called the Schwarzschild radius or event horizon, provides an effective size for a black hole because nothing can escape from inside the gravitational radius and there can be no communication from objects inside Rgrav and the outside world.
Curved Spacetime around a Black Hole.
Inside the horizon or gravitational radius space
is so strongly curved that nothing can escape.

First, perhaps we should dispel a prime misapprehension about black holes: Black holes are not gigantic vacuum cleaners sucking everything in the Universe into their darkness. And you would have to be pretty foolish to get caught in the strong gravity of a black hole hopefully our interstellar astronauts will get better training than the hapless space explorers in so many bad sci-fi stories. This is because black holes have finite mass and because everything in the Universe is so far apart. Black holes are produced by massive stars as a natural part of the stellar evolutionary process. A black hole from a collapsed 10M stellar core will have a mass of 10 solar masses. It will produce gravitational effects on neighboring stars just like a normal 10M star would. You need to get close to black hole (i.e. near the gravitational radius) for its strong gravity to "suck you in" or for General Relativistic effects to be important.

Similarly, if you were on a planet orbiting a star which became a black hole, you would not be sucked in by the Black Hole's gravity. If the star loses no mass, you would feel no change in the gravity and would continue to stay in the same orbit. (Lots of other bad things would happen, particularly if the star goes through a supernova explosion. In that case, cosmic rays & gamma rays would extinguish life on the planet and the mass lost in the explosion would decrease the gravitational pull of the remnant causing your planet to fly off into space.)

 We believe that we have found black holes in our galaxy in the form of X-Ray Binary Stars. In these star systems material may be transferred from a main sequence or red giant companion onto the black hole. (Remember that massive stars live fast & die young.) When a binary star system is formed, the more massive star will complete its life cycle first, becoming a black hole (or perhaps a neutron star). When the lower mass companion begins to expand, evolving toward the red giant phase, material may be pulled toward the black hole. Because of the angular momentum from the stars mutual orbits, the material cannot fall directly down the black hole, but spirals inward forming an accretion disk. The release of gravitational energy as material spirals into the black hole heats the accretion disk to millions of degrees so that it emits x-rays. Artists Conception of the Black Hole Binary Star System, Cygnus X-1. Material is pulled from the Companion into an Accretion Disk (shown in red) which is heated to millions of degrees as material spirals into the Black Hole.

Neutron stars in binary star systems may also be x-ray binaries. Material falling from a companion onto a compact neutron star may release just about as much gravitational energy as material falling into a black hole. Neutron stars will probably be pulsars in x-rays just like in the radio. Here is a JAVA x-ray pulsar animation courtesy of the Chandra X-Ray Observatory.

The best known black hole candidate is Cygnus X-1, an x-ray binary in Cygnus and one of the brightest x-ray sources in the sky. In 1972 Cygnus X-1 was identified with a 9th magnitude O supergiant, catalogued as HDE226868. HDE226868 is orbiting an unseen companion which orbital analysis indicates has a mass of about 20M , far too massive to be a neutron star or white dwarf. Cygnus X-1 also has unusual x-ray properties which lend support to the idea that this must be a black hole.

Stellar black holes have masses in the range of a few times the mass of the sun, up to a few tens of solar masses, but other processes may produce very massive black holes. There is increasing evidence that there may be a million solar mass black hole in the center of our Milky Way galaxy, and black holes with masses up to a billion times the sun's mass in the cores of other galaxies. Many astronomers also believe that black holes power quasars and other active galaxies.

• The best book about black holes is Kip Thorne's "Black Holes & Time Warps: Einstein's Outrageous Legacy" (W.W. Norton, 1994). This book is challenging, but worth the effort.
• Astronomy Pictures of the Day of Black Holes.
• Newton's Apple PBS series Black Hole Page.
• Black Holes from U. Tenn. Violence in the Universe Pages. from "Into the Cosmos".
• A Black Hole Tutorial from John Blondin, N. Carolina State U.
• Virtual Trip to a black hole which depicts distortion effects in the vicinity of a compact object.
• Jillian's guide to black holes
• Falling into a black hole
• Movies in a variety of formats of a Black Hole bending light.

Prof. H. E. (Gene) Smith
CASS 0424 UCSD
9500 Gilman Drive
La Jolla, CA 92093-0424

Last updated: 9 March 2000