Frequently Asked Questions About Black Holes

The answer involves the gravity and the internal pressure within the star. These two things oppose each other -- the gravitational force of the star acting on a chunk of matter at the star's surface will want to cause that matter to fall inward, but the internal pressure of the star, acting outward at the surface, will want to cause the matter to fly outward. When these two are balanced (i.e., equal in strength) the star will maintain its size: neither collapse not expand. Such is the case for the Sun at the moment, and even, for that matter, for the Earth.

However, when a star runs out of nuclear fuel, and therefore continues to lose energy from the surface (it is emitting light energy), while not replacing the lost energy through nuclear fusion (no more nuclear fuel), gravity will win out over internal pressure and the star will contract slowly or collapse quickly depending upon the details of the internal structure and composition. Gravity wins out over the internal pressure of the star, because that pressure was produced by a normal, hot gas, and that gas is losing energy as the star radiates energy from the surface.

The star may thus end up as a black hole. It just depends upon whether or not the collapse is stopped at some smaller size once another source of pressure (other than what is produced by a normal, hot gas) can become sufficiently strong to balance the inward gravitational force. There are other forms of pressure besides that produced by a hot gas. Pressing your hand upon a desk top will let you experience one of these other pressures --- the desk pushes up against you, indeed it can support your weight (gravitational force)! The pressure that keeps the desk rigid against your weight is caused by forces between the atoms in the desk.

Furthermore, electrons within atoms must avoid each other (for example, they cannot all be in the same atomic "orbit" --- this is called "the exclusion principle"). Therefore, if we had a collection of freely moving electrons they would also avoid each other: the harder you compress the collection (the smaller the volume they are confined in) the more they rebel against the squeeze --- a pressure opposes your confinement of the electrons.

This "electron avoidance" pressure can only become strong enough to oppose the gravitational forces within a star of about the mass of the Sun when the star is compressed by gravity to about the diameter of the Earth. Thus a star as massive as the Sun can be prevented from becoming a black hole when it collapses to the size of the Earth, and the internal "electron avoidance" pressure (called the "degenerate electron pressure") becomes strong enough to hold the star up. This sort of pressure does not depend upon the energy content of the star ---- even if the star continues to lose energy from its surface, the pressure will continue to hold the star up. Our Sun can never become a black hole.

However, if the star is more massive than something like 3 to 5 solar masses, its gravitational forces will be larger, and its internal degenerate electron pressure will never be sufficient to stop its collapse. It turns out that neutrons can also obey the exclusion principle and neutrons will be produced in abundance when a massive star collpses, but even neutron degeneracy cannot stop the collapse of massive stars --- anything over 3 to 5 solar masses cannot be stopped, it will become a black hole according to current thinking.

How is time changed in a black hole?

Well, in a certain sense it is not changed at all. If you were to enter a black hole, you would find you watch ticking along at the same rate as it always had (assuming both you and the watch survived the passage into the black hole). However, you would quickly fall toward the center where you would be killed by enormous tidal forces (e.g., the force of gravity at you feet, if you fell feet first, would be much larger than at you head, and you would be stretched apart).

Although your watch as seen by you would not change its ticking rate, just as in special relativity (if you know anything about that), someone else would see a different ticking rate on your watch than the usual, and you would see their watch to be ticking at a different than normal rate. For example, if you were to station yourself just outside a black hole, while you would find your own watch ticking at the normal rate, you would see the watch of a friend at great distance from the hole to be ticking at a much faster rate than yours. That friend would see his own watch ticking at a normal rate, but see your watch to be ticking at a much slower rate. Thus if you stayed just outside the black hole for a while, then went back to join your friend, you would find that the friend had aged more than you had during your separation.

Does the E=mc^2 equation apply to a black hole?

E=mc^2 is always true. In the case of a black hole, for instance, there has been some speculation that black holes can, through a quantum mechanical trick, radiate energy, and in the process their mass would therefore decrease.

If nothing travels at the speed of light, except light, how can a black hole also pull light into itself?

The path that a light ray follows can be bent by a gravitating body, even the Earth (although the bending in that case is very small). This effect has been measured for light from a star as it passed the Sun during a solar eclipse. This bending of the light rays increases as the strength of the gravitational field increases. A black hole is simply a region where the effect on light is so great that light cannot escape the region.

What is the best evidence for the existence of black holes? Is it all really just a theory?

Astronomers have found a half-dozen or so binary star systems (two stars orbiting each other) where one of the stars is invisible, yet must be there since it pulls with enough gravitational force on the other visible star to make that star orbit around their common center of gravity AND the mass of the invisible star is considerably greater than 3 to 5 solar masses. Therefore these invisible stars are thought to be good candidate black holes. There is also evidence that supermassive black holes (about 1 billion solar masses) exist at the centers of many galaxies and quasars. In this latter case other explanations of the output of energy by quasars are not as good as the explanation using a supermassive black hole. (You see, when matter falls in a gravitational field, its speed and therefore energy, increases. If lots of matter is falling in at the same time, and swirling around the black hole in a disk resembling a traffic jam in a cul-de-sac, then friction between the various pieces of matter will turn much of that speed-energy picked up during the fall into heat, which than gets radiated away. In this way, the matter surrounding a supermassive black hole can radiate more energy per gram of fuel than can be released by any other mechanism we know, including nuclear fusion.)

I've heard that a black hole 'belches' light and radiation whenever something falls into its event horizon. What does that mean and why does that happen?

I'm am not sure what the person is referring to, but I will take a guess. They may be referring to what happens as material falls into a black hole through the action of an accretion disk. As large amounts of material approach a black hole, the material will generally find itself in an orbiting disk-like structure with the hole at the center (i.e., it will look a bit like an extremely crowded solar system). The disk will be extremely hot due to the friction between material with different orbital speeds at slightly different orbital radii. Thus the disk will radiate much light. Much of the incoming kinetic energy of the material is radiated away through this friction-heat-light process. This is what gives rise to the extreme brightness of quasars, and this process is what makes us able to (possibly) find stellar-mass black holes that are part of a double star system. In the latter case, infalling material from the neighbor star makes for the accretion disk around the black hole, and X-rays are emitted by the disk (X-rays are emitted by extremely hot matter, just like the not-so-hot filament of a light bulb emits visible light). In the quasar case, a supermassive black hole (a billion solar masses or so) lies at the center of a galaxy, and gas near the black hole forms an accretion disk around the hole; again X-rays, and other forms of light, are the result.

In none of these cases is light being emitted, and reaching us, from beneath the black hole's event horizon. Nothing can escape from beneath the event horizon.

Can you see a black hole? What does a black hole look like?

Not directly. Nothing, not even light can escape from a black hole.

On the other hand, you can see some of the fireworks going on near a black hole. As gas falls into a black hole (perhaps coming from a nearby star), the gas will heat up and glow, becoming visible. Typically, not only visible light, but also more energetic photons like X-rays will be emitted by the gas. What we would expect to see (if our telescopes could "zoom-in" enough) would be a glowing rotating disk of material, with the black hole down a the center of the disk. See the above answers.

How big can a black hole get?

There is no limit to how large a black hole can be. However, the largest blackholes we think are in existence are at the centers of many galaxies, and have masses equivalent to about a billion suns (i.e., a billion solar masses). Their radii would be a considerable fraction of the radius of our solar system.

How small can a black hole get?

According to General Relativity (the theory that predicts, and explains most of the features of black holes), there is no lower limit to the size of a black hole. But, a full theory of how gravity works must also include quantum mechanics, and such a theory has yet to be constructed. Some hints from recent work on this theory suggest that a black hole can be no smaller than about "10-to-the-(-33)" cm in radius --- 0.000000000000000000000000000000001 cm. On that small a size scale, even the apparently smooth nature of space will break down into a "rat-trap" of tunnels, loops, and other interwoven structures! At least, that's what current work suggests.

[In reference to the answer to question 1 above.] Why don't the internal electron forces of a star increase at the same rate as gravitational forces?

In short, the degenerate electron pressure in the star depends upon the density of the gas in a specific way that has no direct dependence upon how gravity and density are related. If you'd like a mathematical relationship, its: the pressure is proportional to the density raised to the 5/3 power. This power is determined by the properties of quantum mechanics (and has nothing to do with gravity). On the other hand, the gravitational force at the surface (for example) of the star is proportional to the mass of the star and inversely proportional to the square of its radius (because of Newton's universal law of gravity!) If I try to express this surface gravity in terms of the density of the star (it's average density), I find M/r^2 is proportional to density times r. So, you see, "density times r" is not anything like "density raised to the 5/3 power."

Will an observer falling into a black hole be able to witness all future events in the universe outside the black hole?

The normal presentation of these gravitational time dilation effects can lead one to a mistaken conclusion. It is true that if an observer (A) is stationary near the event horizon of a black hole, and a second observer (B) is stationary at great distance from the event horizon, then B will see A's clock to be ticking slow, and A will see B's clock to be ticking fast. But if A falls down toward the event horizon (eventually crossing it) while B remains stationary, then what each sees is not as straight forward as the above situation suggests.

As B sees things: A falls toward the event horizon, photons from A take longer and longer to climb out of the "gravtiational well" leading to the apparent slowing down of A's clock as seen by B, and when A is at the horizon, any photon emitted by A's clock takes (formally) an infinite time to get out to B. Imagine that each person's clock emits one photon for each tick of the clock, to make it easy to think about. Thus, A appears to freeze, as seen by B, just as you say. However, A has crossed the event horizon! It is only an illusion (literally an "optical" illusion) that makes B think A never crosses the horizon.

As A sees things: A falls, and crosses the horizon (in perhaps a very short time). A sees B's clock emitting photons, but A is rushing away from B, and so never gets to collect more than a finite number of those photons before crossing the event horizon. (If you wish, you can think of this as due to a cancellation of the gravitational time dilation by a doppler effect --- due to the motion of A away from B). After crossing the event horizon, the photons coming in from above are not easily sorted out by origin, so A cannot figure out how B's clock continued to tick.

A finite number of photons were emitted by A before A crossed the horizon, and a finite number of photons were emitted by B (and collected by A) before A crossed the horizon.

You might ask What if A were to be lowered ever so slowly toward the event horizon? Yes, then the doppler effect would not come into play, UNTIL, at some practical limit, A got too close to the horizon and would not be able to keep from falling in. Then A would only see a finite total of photons form B (but now a larger number --- covering more of B's time). Of course, if A "hung on" long enough before actually falling in, then A might see the future course of the universe.

Bottom line: simply falling into a black hole won't give you a view of the entire future of the universe. Black holes can exist without being part of the final big crunch, and matter can fall into black holes.

For a very nice discussion of black holes for non-scientists, see Kip Thorne's book: Black Holes and Time Warps.

Could black holes be used as an energy source?

There a great deal of information on the potential use of a black hole as a source of energy. (Of course, it should be mentioned that one must first acquire a black hole! At least in the case of the Sun, we already have the Sun!) An excellent source of information on black holes, written for the layperson, is Kip Thorne's excellent book: Black Holes and Time Warps. I suggest you consult it for "all the information [I] could possibly give" you.

In brief, a rotating black hole can store a huge amount of energy in its rotation. This energy is actually accessible since the rotation is imposed on the space outside the hole. In principle, therefore, energy can be extracted from the rotation of the black hole. Exactly what mechanism is used is a potentially complicated story.

I read somewhere that in the VERY distant future black holes could leak and disperse. Can that happen? If it can, how?

As yoy probably know, any object falling into a black hole cannot get out. However, over a very long time, particles of matter "leak" out of a black hole. So, even if all of the objects in the universe were to end up in black holes, after a long, long time, the holes would gradually lose their matter, and the matter would disperse througout the universe (as a thin gas of particles).

The process by which black holes lose matter is called Hawking radiation, after Stephen Hawking, the person who first figured out how it might happen. How it happens is a complicated story. One way of looking at the story uses concept of "virtual particles." At any moment, particle-antiparticle pairs are appearing and disappearing at any location, even just near the event horizon ("surface") of a black hole. These pairs exist for a short time, so short that we cannot measure their masses accurately enough to even know that they are there (however, we do know of their presence by the other effects they cause). But, for a pair near a black hole, one of the particles may fall into the hole, leaving the other without a partner; the particle left behind can't be quickly annihilated by its now missing partner (which is what happens normally). So the lonely particle left behind finds itself no longer "virtual," but now "real," just like any particle in your body. Since this particle is now real, it contains some amount of mass, and that mass has been supplied by the energy of the black hole (through the hole's gravity): the now real particle exists because it has taken mass from the black hole. Thus, gradually, mass leaves the black hole in the form of new particles appearing outside the hole. This process by which black holes lose mass is very slow (at least for massive black holes made from stars), so the time it would take for a typical black hole to eventually disappear is very long. (For a black hole of a mass equal to the mass of the Sun, the entire process would take about 10**66 years, or 1 with 66 zeros after it.)

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