Black Holes - do they exist?
| See Black Hole Evaporation - further reading for a list of books related to this subject |
Black Holes are now well known. They are popular subjects of science fiction, and are thought of as the driving force behind quite a few astronomical phenomena. However they haven't always been accepted, and it is my belief that they may not be for very much longer. I'll start with a little history....
Early ideas -disbelief
Soon after the development of Einstein's General Theory of Relativity, Karl Schwarzschild came up with a solution of the equations. Although this was meant as the solution external to a spherical body, it was seen that it could also be continued to smaller and smaller radius. But could anything really collapse this far? Subrahmanyan Chandrasekhar
worked on the equation of state of collapsing stars which indicated that it could. However, established opinion (i.e. Sir Arthur Eddington) was so against this that Chandrasekhar went to work on something else. Gradually it was seen that the collapse could not be resisted. However the time dilation effects in the strong gravitational fields lead to such possible objects being referred to as "Frozen Stars".
New thoughts
The use of Schwarzschild coordinates indicated that there was a singularity at what is now known as the event horizon. However this is not the only possible coordinate system, and using a different system showed that the singularity was in fact at the centre. If you were falling into the black hole then you would pass through the event horizon in finite proper time (i.e. time experienced by you).
Also, people had wondered if in the real world the fact that you would never have perfect spherical symmetry might mean that an object wouldn't actually collapse down to a singularity. In the 1960s Roger Penrose showed that this was not the case - a sufficiently compact object would always form a singularity. Hence people began to see Black Holes as possible objects, and it was in this era that they got their name.
Also, people had wondered if in the real world the fact that you would never have perfect spherical symmetry might mean that an object wouldn't actually collapse down to a singularity. In the 1960s Roger Penrose showed that this was not the case - a sufficiently compact object would always form a singularity. Hence people began to see Black Holes as possible objects, and it was in this era that they got their name.
A surprise
Black holes were thought of as objects into which things could fall in, but nothing could escape. However, some people began to think that this didn't fit in to the general ideas of thermodynamics - maybe black holes could have a non-zero temperature. So in the early 1970's Stephen Hawking did a calculation based on quantum field theory in curved spacetime, and found to most people's surprise that black holes would actually emit radiation. Strangely the smaller the black hole the more radiation it would emit.
A fallacy
| The standard view, that black holes don't evaporate before you fall in, is given at cosmology.berkeley.edu/Education/BHfaq.html#q9. Note that there have been many claims in the past that black holes cannot exist, which are debunked at math.ucr.edu/home/baez/RelWWW/wrong.html#holes. However none of the claims dealt with on that page involve Hawking radiation |
The question then arose: would black holes evaporate before you could fall in. There is an argument against this, but as far as I can tell it is fallacious. Essentially it says that it is proper time that should be considered, so that if an observer sent a probe into a black hole which would take ten minutes proper time to reach the centre, then after ten minutes he could say "Right, it's destroyed now", despite the fact that he might still be receiving signals from it.
But suppose you beamed a radio signal to a transponder Mars and received a reply 40 minutes later. When did the signal reach Mars? You wouldn't say it was after 40 minutes, but neither would you say that the proper time experienced by the signal is zero, and so it arrived immediately. No, you would say that in your frame of reference it took 20 minutes. Likewise the observer should not use the frame of reference of the probe but his own - that is Schwarzschild coordinates - in which the probe takes infinitely long to reach the event horizon (this has nothing to do with how long it takes to receive a signal from the probe). Using this frame the hole evaporates in a finite time, so nothing can ever fall in to form the black hole.
So do they exist?
Well just because an argument for something is fallacious, it doesn't prove the opposite. Space and time get so jumbled up close to a black hole that it's hard to be sure.
There is a related argument against black holes forming. If a star was collapsing then you would expect a small black hole to form at the centre first, and then grow. However a small black hole would evaporate immediately due to Hawking radiation, so the black hole would never get started. Here again, the argument isn't conclusive. It depends on the effective "equation of state" when Hawking radiation is taken into account. Alternatively a large volume might suddenly find itself forming a black hole rather than starting at a point.
There is a related argument against black holes forming. If a star was collapsing then you would expect a small black hole to form at the centre first, and then grow. However a small black hole would evaporate immediately due to Hawking radiation, so the black hole would never get started. Here again, the argument isn't conclusive. It depends on the effective "equation of state" when Hawking radiation is taken into account. Alternatively a large volume might suddenly find itself forming a black hole rather than starting at a point.
Information
| You may then ask "What is at the centre of our galaxy". The answer is "A very dense object". This is only identified as a black hole because the theory says that such an object must form a black hole. If the theory were different, then it wouldn't be thought of as one. |
The fundamental laws of physics have been found to be reversible in time. This means that information is never truly lost. However black holes don't seem to fit in with this. They seem to be highly irreversible objects, and any information falling into a black hole is lost forever. This paradox has been prominent in the minds of theorists for a while. Now Hawking claims to have solved it. It's a very difficult area to understand, but combined with the above misgivings about the existence of black holes, I think that this might show not only that black holes don't destroy information, but that black holes never form in the first place. Of course, it is unlikely that black holes will disappear from the public imagination very quickly but:
My feeling is that this could be the beginning of the end for Black Holes