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Q&A: Black Holes

Q:

Could you explain the last announcement from Stephen
Hawking about black holes?

A:

Stephen Hawking's recent announcement was that information about objects that fall into black holes is not lost. This is a difficult topic, not the least because Hawking has not yet published a paper so that we can understand his results. The brief summary is as follows. Almost 30 years ago Hawking asked whether information is lost in black holes. As an example, let's say that you throw a red sports car into a black hole. The black hole gains mass and we never see the car again. Can a person looking at that black hole at any time in the future figure out that a red sports car fell into it, or is that information lost forever? According to quantum mechanics (the laws of physics describing the world on very small scales) this information cannot be lost. The only thing that "comes out" of a black hole is radiation known as "Hawking radiation," which allows the black hole to evaporate over a very very long time (longer than the age of the Universe for even a small, say the mass of the Sun, black hole). In order for quantum mechanics to be correct, it may be that somehow the information about the car is somehow encoded in this Hawking radiation. It seems that Hawking himself believed that the information was lost forever, and that quantum mechanics was wrong.

Hawking has done a calculation that shows, under certain conditions, that the black hole does contain the information about what it has eaten, and can transmit that information to an outside observer. However! The crucial question is, under which conditions? Are they realistic? From the readings we've done, it looks like one of his assumptions in order to do this calculation is that the Universe has a cosmological constant. Please see our Chandra pages on dark energy, which might be exactly this cosmological constant:

http://chandra.harvard.edu/photo/2004/darkenergy/

and an interview with a leading cosmologist which explains the cosmological constant:

http://chandra.harvard.edu/photo/2004/darkenergy/turner_interview.html

The problem is that the dark energy observations imply a small positive number for the cosmological constant, whereas Hawking has assumed a small negative number. It is very difficult, mathematically, to go from a negative to a positive cosmological constant. So, it is not at all clear whether his calculations correspond to the Universe we live in. We'll have to wait until he publishes to learn more, but our guess is that the question of what happens to information in a black hole is still open.