Black Holes FAQ

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B lack H oles FAQ

(F requently Asked Q uestions )

L ist

by Ted Bunn

What is a black hole? How big is a black hole? What would happen to me if I fell into a black hole? My friend Penelope is sitting still at a safe distance, watching me fall into the

black hole. What does she see? If a black hole existed, would it suck up all the matter in the Universe? What if the Sun became a black hole? Is there any evidence that black holes exist? How do black holes evaporate? Won't the black hole have evaporated out from under me before I reach it? What is a white hole? What is a wormhole? Where can I go to learn more about black holes?

What is a black hole?--------------------- Loosely speaking, a black hole is a region of space that has so much mass concentrated init that there is no way for a nearby object to escape its gravitational pull. Since our besttheory of gravity at the moment is Einstein's general theory of relativity, we have to delveinto some results of this theory to understand black holes in detail, but let's start of slow, by thinking about gravity under fairly simple circumstances.

Suppose that you are standing on the surface of a planet. You throw a rock straight upinto the air. Assuming you don't throw it too hard, it will rise for a while, but eventuallythe acceleration due to the planet's gravity will make it start to fall down again. If youthrew the rock hard enough, though, you could make it escape the planet's gravityentirely. It would keep on rising forever. The speed with which you need to throw therock in order that it just barely escapes the planet's gravity is called the "escape velocity."As you would expect, the escape velocity depends on the mass of the planet: if the planetis extremely massive, then its gravity is very strong, and the escape velocity is high. Alighter planet would have a smaller escape velocity. The escape velocity also depends onhow far you are from the planet's center: the closer you are, the higher the escapevelocity. The Earth's escape velocity is 11.2 kilometers per second (about 25,000 m.p.h.),while the Moon's is only 2.4 kilometers per second (about 5300 m.p.h.).
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Now imagine an object with such an enormous concentration of mass in such a smallradius that its escape velocity was greater than the velocity of light. Then, since nothingcan go faster than light, nothing can escape the object's gravitational field. Even a beamof light would be pulled back by gravity and would be unable to escape.

The idea of a mass concentration so dense that even light would be trapped goes all theway back to Laplace in the 18th century. Almost immediately after Einstein developedgeneral relativity, Karl Schwarzschild discovered a mathematical solution to theequations of the theory that described such an object. It was only much later, with thework of such people as Oppenheimer, Volkoff, and Snyder in the 1930's, that peoplethought seriously about the possibility that such objects might actually exist in theUniverse. (Yes, this is the same Oppenheimer who ran the Manhattan Project.) Theseresearchers showed that when a sufficiently massive star runs out of fuel, it is unable tosupport itself against its own gravitational pull, and it should collapse into a black hole.

In general relativity, gravity is a manifestation of the curvature of spacetime. Massive

objects distort space and time, so that the usual rules of geometry don't apply anymore. Near a black hole, this distortion of space is extremely severe and causes black holes tohave some very strange properties. In particular, a black hole has something called an'event horizon.' This is a spherical surface that marks the boundary of the black hole. Youcan pass in through the horizon, but you can't get back out. In fact, once you've crossedthe horizon, you're doomed to move inexorably closer and closer to the 'singularity' at thecenter of the black hole.

You can think of the horizon as the place where the escape velocity equals the velocity of light. Outside of the horizon, the escape velocity is less than the speed of light, so if youfire your rockets hard enough, you can give yourself enough energy to get away. But if

you find yourself inside the horizon, then no matter how powerful your rockets are, youcan't escape.

The horizon has some very strange geometrical properties. To an observer who is sittingstill somewhere far away from the black hole, the horizon seems to be a nice, static,unmoving spherical surface. But once you get close to the horizon, you realize that it hasa very large velocity. In fact, it is moving outward at the speed of light! That explainswhy it is easy to cross the horizon in the inward direction, but impossible to get back out.Since the horizon is moving out at the speed of light, in order to escape back across it,you would have to travel faster than light. You can't go faster than light, and so you can'tescape from the black hole.

(If all of this sounds very strange, don't worry. It is strange. The horizon is in a certainsense sitting still, but in another sense it is flying out at the speed of light. It's a bit likeAlice in "Through the Looking-Glass": she has to run as fast as she can just to stay in one place.)

Once you're inside of the horizon, spacetime is distorted so much that the coordinatesdescribing radial distance and time switch roles. That is, "r", the coordinate that describes


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how far away you are from the center, is a timelike coordinate, and "t" is a spacelike one.One consequence of this is that you can't stop yourself from moving to smaller andsmaller values of r, just as under ordinary circumstances you can't avoid moving towardsthe future (that is, towards larger and larger values of t). Eventually, you're bound to hitthe singularity at r = 0. You might try to avoid it by firing your rockets, but it's futile: no

matter which direction you run, you can't avoid your future. Trying to avoid the center of a black hole once you've crossed the horizon is just like trying to avoid next Thursday.

Incidentally, the name 'black hole' was invented by John Archibald Wheeler, and seemsto have stuck because it was much catchier than previous names. Before Wheeler camealong, these objects were often referred to as 'frozen stars.' I'll explain why below.

Back to Black Hole Question List

How big is a black hole?------------------------

There are at least two different ways to describe how big something is. We can say howmuch mass it has, or we can say how much space it takes up. Let's talk first about themasses of black holes.

There is no limit in principle to how much or how little mass a black hole can have. Anyamount of mass at all can in principle be made to form a black hole if you compress it toa high enough density. We suspect that most of the black holes that are actually out therewere produced in the deaths of massive stars, and so we expect those black holes toweigh about as much as a massive star. A typical mass for such a stellar black hole would be about 10 times the mass of the Sun, or about 10^{31} kilograms. (Here I'm usingscientific notation: 10^{31} means a 1 with 31 zeroes after it, or

10,000,000,000,000,000,000,000,000,000,000.) Astronomers also suspect that manygalaxies harbor extremely massive black holes at their centers. These are thought toweigh about a million times as much as the Sun, or 10^{36} kilograms.

The more massive a black hole is, the more space it takes up. In fact, the Schwarzschildradius (which means the radius of the horizon) and the mass are directly proportional toone another: if one black hole weighs ten times as much as another, its radius is ten timesas large. A black hole with a mass equal to that of the Sun would have a radius of 3kilometers. So a typical 10-solar-mass black hole would have a radius of 30 kilometers,and a million-solar-mass black hole at the center of a galaxy would have a radius of 3million kilometers. Three million kilometers may sound like a lot, but it's actually not so big by astronomical standards. The Sun, for example, has a radius of about 700,000kilometers, and so that supermassive black hole has a radius only about four times bigger than the Sun.


What would happen to me if I fell into a black hole?----------------------------------------------------
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hopeless, since the singularity lies in your future, and there's no way to avoid your future.In fact, the harder you fire your rockets, the sooner you hit the singularity. It's best just tosit back and enjoy the ride.


My friend Penelope is sitting still at a safe distance, watching me fall into the black hole. What does she see?------------------------------------------------------------------- Penelope sees things quite differently from you. As you get closer and closer to thehorizon, she sees you move more and more slowly. In fact, no matter how long she waits,she will never quite see you reach the horizon.

In fact, more or less the same thing can be said about the material that formed the black hole in the first place. Suppose that the black hole formed from a collapsing star. As thematerial that is to form the black hole collapses, Penelope sees it get smaller and smaller,

approaching but never quite reaching its Schwarzschild radius. This is why black holeswere originally called frozen stars: because they seem to 'freeze' at a size just slightly bigger than the Schwarzschild radius.

Why does she see things this way? The best way to think about it is that it's really just anoptical illusion. It doesn't really take an infinite amount of time for the black hole toform, and it doesn't really take an infinite amount of time for you to cross the horizon. (If you don't believe me, just try jumping in! You'll be across the horizon in eight minutes,and crushed to death mere seconds later.) As you get closer and closer to the horizon, thelight that you're emitting takes longer and longer to climb back out to reach Penelope. Infact, the radiation you emit right as you cross the horizon will hover right there at the

horizon forever and never reach her. You've long since passed through the horizon, butthe light signal telling her that won't reach her for an infinitely long time.

There is another way to look at this whole business. In a sense, time really does passmore slowly near the horizon than it does far away. Suppose you take your spaceship andride down to a point just outside the horizon, and then just hover there for a while(burning enormous amounts of fuel to keep yourself from falling in). Then you fly back out and rejoin Penelope. You will find that she has aged much more than you during thewhole process; time passed more slowly for you than it did for her.

So which of these two explanation (the optical-illusion one or the time-slowing-down

one) is really right? The answer depends on what system of coordinates you use todescribe the black hole. According to the usual system of coordinates, called"Schwarzschild coordinates," you cross the horizon when the time coordinate t is infinity.So in these coordinates it really does take you infinite time to cross the horizon. But thereason for that is that Schwarzschild coordinates provide a highly distorted view of what'sgoing on near the horizon. In fact, right at the horizon the coordinates are infinitelydistorted (or, to use the standard terminology, "singular"). If you choose to usecoordinates that are not singular near the horizon, then you find that the time when you
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cross the horizon is indeed finite, but the time when Penelope sees you cross the horizonis infinite. It took the radiation an infinite amount of time to reach her. In fact, though,you're allowed to use either coordinate system, and so both explanations are valid.They're just different ways of saying the same thing.

In practice, you will actually become invisible to Penelope before too much time has passed. For one thing, light is "redshifted" to longer wavelengths as it rises away from the black hole. So if you are emitting visible light at some particular wavelength, Penelopewill see light at some longer wavelength. The wavelengths get longer and longer as youget closer and closer to the horizon. Eventually, it won't be visible light at all: it will beinfrared radiation, then radio waves. At some point the wavelengths will be so long thatshe'll be unable to observe them. Furthermore, remember that light is emitted inindividual packets called photons. Suppose you are emitting photons as you fall past thehorizon. At some point, you will emit your last photon before you cross the horizon. That photon will reach Penelope at some finite time -- typically less than an hour for thatmillion-solar-mass black hole -- and after that she'll never be able to see you again. (After

all, none of the photons you emit *after* you cross the horizon will ever get to her.)Back to Black Hole Question List

If a black hole existed, would it suck up all the matter in the Universe?--------------------------------------------------------------- Heck, no. A black hole has a "horizon," which means a region from which you can'tescape. If you cross the horizon, you're doomed to eventually hit the singularity. But aslong as you stay outside of the horizon, you can avoid getting sucked in. In fact, tosomeone well outside of the horizon, the gravitational field surrounding a black hole is nodifferent from the field surrounding any other object of the same mass. In other words, a

one-solar-mass black hole is no better than any other one-solar-mass object (such as, for example, the Sun) at "sucking in" distant objects.


What if the Sun became a black hole?------------------------------------ Well, first, let me assure you that the Sun has no intention of doing any such thing. Onlystars that weigh considerably more than the Sun end their lives as black holes. The Sun isgoing to stay roughly the way it is for another five billion years or so. Then it will gothrough a brief phase as a red giant star, during which time it will expand to engulf the planets Mercury and Venus, and make life quite uncomfortable on Earth (oceans boiling,atmosphere escaping, that sort of thing). After that, the Sun will end its life by becominga boring white dwarf star. If I were you, I'd make plans to move somewhere far away before any of this happens. I also wouldn't buy any of those 8-billion-year government bonds.

But I digress. What if the Sun *did* become a black hole for some reason? The maineffect is that it would get very dark and very cold around here. The Earth and the other


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planets would not get sucked into the black hole; they would keep on orbiting in exactlythe same paths they follow right now. Why? Because the horizon of this black hole would be very small -- only about 3 kilometers -- and as we observed above, as long as you staywell outside the horizon, a black hole's gravity is no stronger than that of any other objectof the same mass.


Is there any evidence that black holes exist?--------------------------------------------- Yes. You can't see a black hole directly, of course, since light can't get past the horizon.That means that we have to rely on indirect evidence that black holes exist.

Suppose you have found a region of space where you think there might be a black hole.How can you check whether there is one or not? The first thing you'd like to do ismeasure how much mass there is in that region. If you've found a large mass concentrated

in a small volume, and if the mass is dark, then it's a good guess that there's a black holethere. There are two kinds of systems in which astronomers have found such compact,massive, dark objects: the centers of galaxies (including perhaps our own Milky WayGalaxy), and X-ray-emitting binary systems in our own Galaxy.

According to a recent review by Kormendy and Richstone (to appear in the 1995 editionof "Annual Reviews of Astronomy and Astrophysics"), eight galaxies have beenobserved to contain such massive dark objects in their centers. The masses of the cores of these galaxies range from one million to several billion times the mass of the Sun. Themass is measured by observing the speed with which stars and gas orbit around the center of the galaxy: the faster the orbital speeds, the stronger the gravitational force required to

hold the stars and gas in their orbits. (This is the most common way to measure masses inastronomy. For example, we measure the mass of the Sun by observing how fast the planets orbit it, and we measure the amount of dark matter in galaxies by measuring howfast things orbit at the edge of the galaxy.)

These massive dark objects in galactic centers are thought to be black holes for at leasttwo reasons. First, it is hard to think of anything else they could be: they are too denseand dark to be stars or clusters of stars. Second, the only promising theory to explain theenigmatic objects known as quasars and active galaxies postulates that such galaxies havesupermassive black holes at their cores. If this theory is correct, then a large fraction of galaxies -- all the ones that are now or used to be active galaxies -- must havesupermassive black holes at the center. Taken together, these arguments strongly suggestthat the cores of these galaxies contain black holes, but they do not constitute absolute proof.

Two very recent discovery has been made that strongly support the hypothesis that thesesystems do indeed contain black holes. First, a nearby active galaxy was found to have a"water maser" system (a very powerful source of microwave radiation) near its nucleus.Using the technique of very-long-baseline interferometry, a group of researchers was able


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to map the velocity distribution of the gas with very fine resolution. In fact, they wereable to measure the velocity within less than half a light-year of the center of the galaxy.From this measurement they can conclude that the massive object at the center of thisgalaxy is less than half a light-year in radius. It is hard to imagine anything other than a black hole that could have so much mass concentrated in such a small volume. (This

result was reported by Miyoshi et al. in the 12 January 1995 issue of Nature, vol. 373, p.127.)

A second discovery provides even more compelling evidence. X-ray astronomers havedetected a spectral line from one galactic nucleus that indicates the presence of atomsnear the nucleus that are moving extremely fast (about 1/3 the speed of light).Furthermore, the radiation from these atoms has been redshifted in just the manner onewould expect for radiation coming from near the horizon of a black hole. Theseobservations would be very difficult to explain in any other way besides a black hole, andif they are verified, then the hypothesis that some galaxies contain supermassive black holes at their centers would be fairly secure. (This result was reported in the 22 June 1995

issue of Nature, vol. 375, p. 659, by Tanaka et al.)A completely different class of black-hole candidates may be found in our own Galaxy.These are much lighter, stellar-mass black holes, which are thought to form when amassive star ends its life in a supernova explosion. If such a stellar black hole were to beoff somewhere by itself, we wouldn't have much hope of finding it. However, many starscome in binary systems -- pairs of stars in orbit around each other. If one of the stars insuch a binary system becomes a black hole, we might be able to detect it. In particular, insome binary systems containing a compact object such as a black hole, matter is suckedoff of the other object and forms an "accretion disk" of stuff swirling into the black hole.The matter in the accretion disk gets very hot as it falls closer and closer to the black

hole, and it emits copious amounts of radiation, mostly in the X-ray part of the spectrum.Many such "X-ray binary systems" are known, and some of them are thought to be likely black-hole candidates.

Suppose you've found an X-ray binary system. How can you tell whether the unseencompact object is a black hole? Well, one thing you'd certainly like to do is to estimate itsmass. By measuring the orbital speed of visible star (together with a few other things),you can figure out the mass of the invisible companion. (The technique is quite similar tothe one we described above for supermassive black holes in galactic centers: the faster the star is moving, the stronger the gravitational force required to keep it in place, and sothe more massive the invisible companion.) If the mass of the compact object is found to be very large very large, then there is no kind of object we know about that it could beother than a black hole. (An ordinary star of that mass would be visible. A stellar remnantsuch as a neutron star would be unable to support itself against gravity, and wouldcollapse to a black hole.) The combination of such mass estimates and detailed studies of the radiation from the accretion disk can supply powerful circumstantial evidence that theobject in question is indeed a black hole.


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Many of these "X-ray binary" systems are known, and in some cases the evidence insupport of the black-hole hypothesis is quite strong. In a review article in the 1992 issueof Annual Reviews of Astronomy and Astrophysics, Anne Cowley summarized thesituation by saying that there were three such systems known (two in our galaxy and onein the nearby Large Magellanic Cloud) for which very strong evidence exists that the

mass of the invisible object is too large to be anything but a black hole. There are manymore such objects that are thought to be likely black holes on the basis of slightly lessevidence. Furthermore, this field of research has been very active since 1992, and thenumber of strong candidates by now is larger than three.


How do black holes evaporate?----------------------------- This is a tough one. Back in the 1970's, Stephen Hawking came up with theoreticalarguments showing that black holes are not really entirely black: due to quantum-

mechanical effects, they emit radiation. The energy that produces the radiation comesfrom the mass of the black hole. Consequently, the black hole gradually shrinks. It turnsout that the rate of radiation increases as the mass decreases, so the black hole continuesto radiate more and more intensely and to shrink more and more rapidly until it presumably vanishes entirely.

Actually, nobody is really sure what happens at the last stages of black hole evaporation:some researchers think that a tiny, stable remnant is left behind. Our current theoriessimply aren't good enough to let us tell for sure one way or the other. As long as I'mdisclaiming, let me add that the entire subject of black hole evaporation is extremelyspeculative. It involves figuring out how to perform quantum-mechanical (or rather

quantum-field-theoretic) calculations in curved spacetime, which is a very difficult task,and which gives results that are essentially impossible to test with experiments. Physicists*think* that we have the correct theories to make predictions about black holeevaporation, but without experimental tests it's impossible to be sure.

Now why do black holes evaporate? Here's one way to look at it, which is onlymoderately inaccurate. (I don't think it's possible to do much better than this, unless youwant to spend a few years learning about quantum field theory in curved space.) One of the consequences of the uncertainty principle of quantum mechanics is that it's possiblefor the law of energy conservation to be violated, but only for very short durations. TheUniverse is able to produce mass and energy out of nowhere, but only if that mass andenergy disappear again very quickly. One particular way in which this strange phenomenon manifests itself goes by the name of vacuum fluctuations. Pairs consistingof a particle and antiparticle can appear out of nowhere, exist for a very short time, andthen annihilate each other. Energy conservation is violated when the particles are created, but all of that energy is restored when they annihilate again. As weird as all of thissounds, we have actually confirmed experimentally that these vacuum fluctuations arereal.


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Now, suppose one of these vacuum fluctuations happens near the horizon of a black hole.It may happen that one of the two particles falls across the horizon, while the other oneescapes. The one that escapes carries energy away from the black hole and may bedetected by some observer far away. To that observer, it will look like the black hole has just emitted a particle. This process happens repeatedly, and the observer sees a

continuous stream of radiation from the black hole.Back to Black Hole Question List

Won't the black hole have evaporated out from under me before I reach it?--------------------------------------------------------------------- We've observed that, from the point of view of your friend Penelope who remains safelyoutside of the black hole, it takes you an infinite amount of time to cross the horizon.We've also observed that black holes evaporate via Hawking radiation in a finite amountof time. So by the time you reach the horizon, the black hole will be gone, right?

Wrong. When we said that Penelope would see it take forever for you to cross thehorizon, we were imagining a non-evaporating black hole. If the black hole isevaporating, that changes things. Your friend will see you cross the horizon at the exactsame moment she sees the black hole evaporate. Let me try to describe why this is true.

Remember what we said before: Penelope is the victim of an optical illusion. The lightthat you emit when you're very near the horizon (but still on the outside) takes a verylong time to climb out and reach her. If the black hole lasts forever, then the light maytake arbitrarily long to get out, and that's why she doesn't see you cross the horizon for avery long (even an infinite) time. But once the black hole has evaporated, there's nothingto stop the light that carries the news that you're about to cross the horizon from reaching

her. In fact, it reaches her at the same moment as that last burst of Hawking radiation. Of course, none of that will matter to you: you've long since crossed the horizon and beencrushed at the singularity. Sorry about that, but you should have thought about it beforeyou jumped in.


What is a white hole?--------------------- The equations of general relativity have an interesting mathematical property: they aresymmetric in time. That means that you can take any solution to the equations and

imagine that time flows backwards rather than forwards, and you'll get another validsolution to the equations. If you apply this rule to the solution that describes black holes,you get an object known as a white hole. Since a black hole is a region of space fromwhich nothing can escape, the time-reversed version of a black hole is a region of spaceinto which nothing can fall. In fact, just as a black hole can only suck things in, a whitehole can only spit things out.


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White holes are a perfectly valid mathematical solution to the equations of generalrelativity, but that doesn't mean that they actually exist in nature. In fact, they almostcertainly do not exist, since there's no way to produce one. (Producing a white hole is justas impossible as destroying a black hole, since the two processes are time-reversals of each other.)


What is a wormhole?------------------- So far, we have only considered ordinary "vanilla" black holes. Specifically, we have been talking all along about black holes that are not rotating and have no electric charge.If we consider black holes that rotate and/or have charge, things get more complicated. In particular, it is possible to fall into such a black hole and not hit the singularity. In effect,the interior of a charged or rotating black hole can "join up" with a corresponding whitehole in such a way that you can fall into the black hole and pop out of the white hole.

This combination of black and white holes is called a wormhole.The white hole may be somewhere very far away from the black hole; indeed, it mayeven be in a "different Universe" -- that is, a region of spacetime that, aside from thewormhole itself, is completely disconnected from our own region. A conveniently-located wormhole would therefore provide a convenient and rapid way to travel verylarge distances, or even to travel to another Universe. Maybe the exit to the wormholewould lie in the past, so that you could travel back in time by going through. All in all,they sound pretty cool.

But before you apply for that research grant to go search for them, there are a couple of

things you should know. First of all, wormholes almost certainly do not exist. As we saidabove in the section on white holes, just because something is a valid mathematicalsolution to the equations doesn't mean that it actually exists in nature. In particular, black holes that form from the collapse of ordinary matter (which includes all of the black holesthat we think exist) do not form wormholes. If you fall into one of those, you're not goingto pop out anywhere. You're going to hit a singularity, and that's all there is to it.

Furthermore, even if a wormhole were formed, it is thought that it would not be stable.Even the slightest perturbation (including the perturbation caused by your attempt totravel through it) would cause it to collapse.

Finally, even if wormholes exist and are stable, they are quite unpleasant to travelthrough. Radiation that pours into the wormhole (from nearby stars, the cosmicmicrowave background, etc.) gets blueshifted to very high frequencies. As you try to passthrough the wormhole, you will get fried by these X-rays and gamma rays.



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Where can I go to learn more about black holes?----------------------------------------------- Let me begin by acknowledging that I cribbed some of the above material from the articleabout black holes in the Frequently Asked Questions list for the Usenet newsgroupsci.physics. The sci.physics FAQ is posted monthly to sci.physics and is also available by

anonymous ftp from rtfm.mit.edu (and probably other places). The article about black holes, which is excellent, was written by Matt McIrvin. The FAQ contains other neatthings too.

There are lots of books out there about black holes and related matters. Kip Thorne's"Black Holes and Time Warps: Einstein's Outrageous Legacy" is a good one. WilliamKaufmann's "Black Holes and Warped Spacetime" is also worth reading. R. Wald's"Space, Time, and Gravity" is an exposition of general relativity for non-scientists. Ihaven't read it myself, but I've heard good things about it.

Both of these books are aimed at readers without much background in physics. If you

want more "meat" (i.e., more mathematics), then you probably start with a book on the basics of relativity theory. The best introduction to the subject is "Spacetime Physics" byE.F. Taylor and J.A. Wheeler. (This book is mostly about special relativity, but the lastchapter discusses the general theory.) Taylor and Wheeler have been threatening for about two years now to publish a sequel entitled "Scouting Black Holes," which should be quite good if it ever comes out. "Spacetime Physics" does not assume that you knowvast amounts of physics, but it does assume that you're willing to work hard atunderstanding this stuff. It is not light reading, although it is more playful and lessintimidating than most physics books.

Finally, if "Spacetime Physics" isn't enough for you, you could try any of several

introductions to general relativity. B. Schutz's "A First Course in General Relativity" andW. Rindler's "Essential Relativity" are a couple of possibilities. And for the extremelyvaliant reader with an excellent background in physics, there's the granddaddy of all books on general relativity, Misner, Thorne, and Wheeler's "Gravitation." R. Wald's book "General Relativity" is at a comparable level to "Gravitation," although the styles of thetwo books are enormously different. What little I know about black-hole evaporationcomes from Wald's book. Let me emphasize that all of these books, and especially thelast two, assume that you know quite a lot of physics. They are not for the faint of heart.


September 1995

Black hole


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In astrophysics, a black hole is a massive object whosegravitational fieldis so intensethat it prevents any form of matter or radiation from escaping. The term derives from the
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fact that its absorption of visible light renders the hole invisible and indistinguishablefrom the black space around it.

Black holes are predicted to exist throughsolutions of Einstein's field equationsof general relativity. They are not directly observable, but several indirect observation

techniques in differentwavelengthshave been developed and used to study the phenomena they induce in their environment. In particular, gases caught by thegravitational field of a black hole are heated to considerably high temperaturesbefore being swallowed, and thereby emit a significant amount of X-rays. Therefore, even if a black hole does not itself give off any radiation, it may nevertheless be detectable by itseffect on its surrounding environment. Such observations have resulted in the generalscientific consensus that, barring a breakdown in our understanding of nature, black holesdo exist in our universe.

Contents

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1 Introduction and Terminology 2 History 3 Properties and Features

o 3.1 Classification3.1.1 By physical properties3.1.2 By mass

o 3.2 Event horizono 3.3 Singularityo 3.4 Photon sphereo 3.5 Ergosphere

4 Formation and evolutiono 4.1 Gravitational collapse

4.1.1 Creation of primordial black holes in the big bango 4.2 High energy collisionso 4.3 Growtho 4.4 Evaporation

5 Observationo 5.1 Accretion disks and gas jetso 5.2 Strong radiation emissionso 5.3 Gravitational lensingo 5.4 Orbiting objectso 5.5 Determining the mass of black holes

6 Black hole candidateso 6.1 Supermassiveo 6.2 Intermediate-masso 6.3 Stellar-masso 6.4 Micro
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permassivehttp://en.wikipedia.org/wiki/Black_hole#Intermediate-mass%23Intermediate-masshttp://en.wikipedia.org/wiki/Black_hole#Stellar-mass%23Stellar-masshttp://en.wikipedia.org/wiki/Black_hole#Micro%23Micro


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7 Advanced Topicso 7.1 Information loss paradoxo 7.2 Worm Holeso 7.3 Holographic worldo 7.4 Entropy and Hawking radiationo 7.5 Black hole unitarity

8 References 9 Further reading

o 9.1 Popular readingo 9.2 University textbooks and monographso 9.3 Research papers

10 External links

Introduction and Terminology

A black hole is often defined by itsescape velocity, the minimum velocity that must beattained to escape from thegravitationalpull of some object. For example, the escapevelocity of an object onEarth is equal to 11 km/s. No matter the object, be it a rocket or baseball, it must go at least 11 km/s to avoid falling back to the Earth's surface due toEarth's gravity. The escape velocity of an object depends on howcompactit is; that is,the ratio of its mass to radius. A black hole is an object so compact that, within a certaindistance of it, even thespeed of lightis not fast enough to escape.

A black hole's mass is concentrated at a point called asingularity. Surrounding thesingularity is a theoreticalspherecalled theevent horizon. The event horizon marks the' point of no return', a boundary that, if crossed, inevitably leads falling matter andradiation towards the singularity. This sphere is a kind of spatial extension of the black hole.

For a black hole of mass equal to that of the Sun, its radius is about 3 km. At a distance of about 106 km, a black hole has no more attraction than any other body of the same mass.For example, if the sun were replaced by a black hole of the same mass, the orbits of the planetswould remain unchanged.

There are several types of black holes. When they form as a result of the gravitationalcollapse of a star , they are calledstellar black holes. Black holes found at the center of galaxies have a mass up to several billionsolar masses and are calledsupermassive black holes simply because of their size. Between these two scales, it is believed that there areintermediate black holes with a mass of several thousand solar masses. Black holes withvery small masses, believed to have formed early in the history of the Universe, duringthe Big Bang, are also considered, and are referred to as primordial black holes. Their existence is, at present, not confirmed.

It is impossible to directly observe a black hole. However, it is possible to infer its presence by its gravitational action on the surroundingenvironment, particularly with
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microquasarsand active galactic nuclei, where material falling into a nearby black hole issignificantly heated and emits a large amount of X-ray radiation. This observationmethod allowsastronomersto detect their existence. The only objects that agree withthese observations and are consistent within the framework of general relativityare black holes.

History

Simulation of Gravitational lensingby a black hole which distorts a galaxyin the background.

The idea of a body so massive that even light could not escape was put forward bygeologistJohn Michell in a letter written to Henry Cavendish in 1783 to the RoyalSociety:

If the semi-diameter of a sphere of the same density as the Sun were to exceed that of the Sun inthe proportion of 500 to 1, a body falling from an infinite height towards it would have acquiredat its surface greater velocity than that of light, and consequently supposing light to be attracted by the same force in proportion to its vis inertiae, with other bodies, all light emitted from such a body would be made to return towards it by its own proper gravity.

John Michell[2]

In 1796, mathematicianPierre-Simon Laplacepromoted the same idea in the first andsecond editions of his book Exposition du systme du Monde (it was removed from later editions).[3][4] Such "dark stars" were largely ignored in the nineteenth century, since lightwas then thought to be a massless wave and therefore not influenced by gravity. Unlikethe modern black hole concept, the object behind the horizon is assumed to be stableagainst collapse.

In 1915,Albert Einsteindeveloped his general theory of relativity, having earlier shownthat gravity does in fact influence light's motion. A few months later,Karl Schwarzschild gave thesolutionfor the gravitational field of a point mass and a spherical mass,[5]showing that a black hole could theoretically exist. The Schwarzschild radiusis nowknown to be the radius of theevent horizonof a non-rotating black hole, but this was not
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well understood at that time, for example Schwarzschild himself thought it was not physical. Johannes Droste, a student of Hendrik Lorentz, independently gave the samesolution for the point mass a few months after Schwarzschild and wrote more extensivelyabout its properties.

In 1930,astrophysicist Subrahmanyan Chandrasekhar calculated using general relativitythat a non-rotating body above 1.44 solar masses (theChandrasekhar limit) wouldcollapse. His arguments were opposed byArthur Eddington, who believed that somethingwould inevitably stop the collapse. Eddington was partly correct: a white dwarf slightlymore massive than the Chandrasekhar limit will collapse into aneutron star . But in 1939,Robert Oppenheimer and others predicted that stars above approximately three solar masses (theTolman-Oppenheimer-Volkoff limit) would collapse into black holes for thereasons presented by Chandrasekhar.[6]

Oppenheimer and his co-authors usedSchwarzschild's system of coordinates(the onlycoordinates available in 1939), which producedmathematical singularities at the

Schwarzschild radius, in other words the equations broke down at the Schwarzschildradius because some of the terms wereinfinite. This was interpreted as indicating that theSchwarzschild radius was the boundary of a "bubble" in which time "stopped". Thecollapsed stars were briefly known as "frozen stars",[citation needed ] as the calculationsindicated that an outside observer would see the surface of the star frozen in time at theinstant where its collapse takes it inside the Schwarzschild radius. But many physicistscould not accept the idea of time standing still inside the Schwarzschild radius, and therewas little interest in the subject for over 20 years.

In 1958,David Finkelsteinbroke the deadlock over "stopped time" and introduced theconcept of theevent horizonby presenting Eddington-Finkelstein coordinates, which

enabled him to show that "The Schwarzschild surface r = 2 m is not a singularity but actsas a perfect unidirectional membrane: causal influences can cross it in only onedirection".[7] All theories up to this point, including Finkelstein's, covered only non-rotating, uncharged black holes.

In 1967, astronomers discovered pulsars,[8] [9] and within a few years could show that theknown pulsars were rapidly rotatingneutron stars. Until that time, neutron stars were alsoregarded as just theoretical curiosities. So the discovery of pulsars awakened interest inall types of ultra-dense objects that might be formed by gravitational collapse.

Physicist John Wheeler is widely credited with coining the termblack hole in his 1967 public lectureOur Universe: the Known and Unknown , as an alternative to the morecumbersome "gravitationally completely collapsed star". However, Wheeler insisted thatsomeone else at the conference had coined the term and he had merely adopted it asuseful shorthand. The term was also cited in a 1964 letter by Anne Ewing to theAAAS:

According to Einsteins general theory of relativity, as mass is added to a degenerate star asudden collapse will take place and the intense gravitational field of the star will close in on itself.Such a star then forms a "black hole" in the universe.
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Ann Ewing, letter to AAAS [10]

Properties and Features

Main article: Properties and features of black holes

According to the"No Hair" theorema black hole has only three independent physical properties:mass, electric chargeand angular momentum.[11] Any two black holes thatshare the same values for these properties are indistinguishable. This contrasts with other astrophysical objects such as stars, which have very manypossibly infinitely many parameters. Consequently, a great deal of information is lost when a star collapses toform a black hole. Asinformation is preserved in most physical theories, the loss of information in black holes is puzzling. Physicists refer to this as the black holeinformation paradox.

The No Hair theorem makes assumptions about the nature of our universe and the matter within. Other assumptions lead to different conclusions. For example, if nature allowsmagnetic monopolesto existwhich appears to be theoretically possible, but has never been observedthen it should also be possible for black holes to have magnetic charge.If the universe has more than four dimensions (asstring theories, a controversial butapparently possible class of theories, would require), or has a global anti-de Sitter structure, the theorem could fail completely, allowing many sorts of "hair". However, inour apparently four-dimensional, very nearly flat universe[12], the theorem should hold.

Classification

By physical properties

The simplest black hole is one that has mass but neither charge nor angular momentum.These black holes are often referred to as Schwarzschild black holesafter the physicistKarl Schwarzschild who discovered thissolutionin 1915.[5] It was the first non-trivialexact solutionto the Einstein equations to be discovered, and according toBirkhoff'stheorem, the onlyvacuum solutionthat is spherically symmetric. For real world physicsthis means that there is no observable difference between the gravitational field of such a black hole and that of any other spherical object of the same massfor example aspherical star or planetonce one is in the empty space outside the object. The popular notion of a black hole "sucking in everything" in its surroundings is therefore incorrect;the external gravitational field, far from the event horizon, is essentially like that of ordinary massive bodies.

Electrically charged non-rotating black holes are described by theReissner-Nordstrmsolution, developed byHans Reissner [13] and Gunnar Nordstrm.[14]

MathematicianRoy Kerr presented the Kerr solution in 1963, showing how this could beused to describe arotating black hole.[15] In addition to its theoretical interest, Kerr's work
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made black holes more believable for astronomers, since black holes are formed fromstars and all known stars rotate.

The most general knownstationary black hole solution is theKerr-Newman metric,discovered by physicistEzra Newmanet al[16][17] having both charge and angular

momentum. All these general solutions share the property that they converge to theSchwarzschild solution at distances that are large compared to the ratio of charge andangular momentum to mass (innatural units).

While the mass of a black hole can take any positive value, the charge and angular momentum are constrained by the mass. In natural units , the total chargeQ and the totalangular momentum J are expected to satisfyQ2+( J /M )2 M 2 for a black hole of massM .Black holes saturating this inequality are calledextremal. Solutions of Einstein'sequations violating the inequality do exist, but do not have a horizon. These solutionshave naked singularitiesand are deemedunphysical , as thecosmic censorship hypothesis rules out such singularities due to the generic gravitational collapse of realistic matter .[18]

This is supported by numerical simulations.[19]

Due to the relatively large strength of the electromagnetic force, black holes formingfrom the collapse of stars are expected to retain the nearly neutral charge of the star.Rotation, however, is expected to be a common feature of compact objects, and the black-hole candidate binary X-ray source GRS 1915+105[20] appears to have an angular momentum near the maximum allowed value.

By mass

Black holes are

commonly classifiedaccording to their mass, independent of angular momentum J .The size of a black hole, as determined bythe radius of the event horizon, or Schwarzschild radius, is proportional to the massthrough where is the Schwarzschild radius and is the mass of the Sun. A black hole's sizeand mass are thus simply relatedindependent of rotation. According to this criterion, black holes are classed as:

Supermassive- contain hundreds of thousands to billions of solar masses. and arethought to exist in the center of most galaxies,[21][22] including theMilky Way.[23]They are believed to be responsible for active galactic nuclei, and presumablyform either from the coalescence of smaller black holes, or by the accretion of stars and gas onto them. The largest known supermassive black hole is located inOJ 287weighing in at 18 billion solar masses.[24]

Class Mass Size

Supermassive black hole ~105 - 109 MSun ~0.001 - 10 AUIntermediate-mass black hole ~103 MSun ~103 km = R EarthStellar-mass ~10 MSun ~30 kmPrimordial black hole up to ~MMoon up to ~0.1 mm
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Intermediate- contain thousands of solar masses. They have been proposed as a possible power source for ultraluminous X-ray sources.[25] There is no knownmechanism for them to form directly, so they likely form via collisions of lower mass black holes, either in the dense stellar cores of globular clusters or galaxies.[citation needed ] Such creation events should produce intense bursts of gravitational

waves, which may be observedsoon. The boundary between super- andintermediate-mass black holes is a matter of convention. Their lower mass limit,the maximum mass for direct formation of a single black hole from collapse of amassive star, is poorly known at present.

Stellar-mass black holes- have masses ranging from a lower limit of about 1.5 3.0 solar masses (theTolman-Oppenheimer-Volkoff limit for the maximum massof neutron stars) up to perhaps 1520 solar masses. They are created by thecollapse of individual stars, or by the coalescence (inevitable, due togravitationalradiation) of binary neutron stars. Stars may form withinitial masses up to 100solar masses, or possibly even higher, but these shed most of their outer massive

layers during earlier phases of their evolution, either blown away in stellar windsduring thered giant, AGB, and Wolf-Rayetstages, or expelled insupernovaexplosions for stars that turn into neutron stars or black holes. Being knownmostly by theoretical models for late-stage stellar evolution, the upper limit for the mass of stellar-mass black holes is somewhat uncertain at present. The coresof still lighter stars formwhite dwarfs.

Micro (also mini black holes ) - have masses much less than that of a star. Atthese sizes, the effects of quantum mechanics are expected to come into play.There is no known mechanism for them to form via normal processes of stellar evolution, but certaininflationary scenarios predict their production during the

early stages of the evolution of the universe.[citation needed ]

According to some theoriesof quantum gravitythey may also be produced in the highly energetic reaction produced bycosmic rayshitting theatmosphereor even in particle acceleratorssuch as theLarge Hadron Collider .[citation needed ] The theory of Hawking radiation predicts that such black holes will evaporate in bright flashes of gamma radiation. NASA's Fermi Gamma-ray Space Telescopesatellite (formerly GLAST)launched in 2008 is searching for such flashes.[26]
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Event horizonMain article: Event horizon

The defining feature of a black hole is the eventhorizon, a surface inspacetimethat marks a point of no return. Oncean object crosses this surface, it cannot return to the other side. Consequently, anythinginside this surface is completely hidden from outside observers. Other than this, the eventhorizon is a completely normal part of space with no special features that would allowsomeone falling into the black hole to know when they would cross the horizon. Theevent horizon is not a solid surface, and does not obstruct or slow down matter or radiation that is traveling towards the region within the event horizon.

Outside the event horizon, thegravitational fieldis identical to the field produced by anyother spherically symmetric object of the same mass. The popular conception of black holes as "sucking" things in is misleading: objects canorbit black holes indefinitely, provided they stay outside the photon sphere (described below), and also ignoring theeffects of gravitational radiation which causes orbiting objects to lose energy (similar tothe effect of electromagnetic radiation).

SingularityMain article: Gravitational singularity

According togeneral relativity, there is a space-time singularity at the center of aspherical black hole, which means an infinite space-time curvature. It means that, fromthe viewpoint of an observer falling into a black hole, in a finite time (at the end of hisfall) a black hole's mass becomes entirely compressed into a region with zero volume, soits density becomesinfinite. This zero-volume, infinitely dense region at the center of a black hole is called a gravitational singularity.

The singularity in a non-rotating black hole is a point, in other words it has zero length,width, and height. The singularity of arotating black holeis smeared out to form aringshape lying in the plane of rotation. The ring still has no thickness and hence no volume.

The appearance of singularities in general relativity is commonly perceived as signalingthe brea

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Black Holes FAQ

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