Page 60 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 7
gamma rays from space have been detected by satellites originally constructed to look for violations of the Test Ban
Treaty. These seem to occur about sixteen times a month and to be roughly uniformly distributed in direction across
the sky. This indicates that they come from outside the Solar System since otherwise we would expect them to be
concentrated toward the plane of the orbits of the planets. The uniform distribution also indicates that the sources are
either fairly near to us in our galaxy or right outside it at cosmological distances because otherwise, again, they would
be concentrated toward the plane of the galaxy. In the latter case, the energy required to account for the bursts would
be far too high to have been produced by tiny black holes, but if the sources were close in galactic terms, it might be
possible that they were exploding black holes. I would very much like this to be the case but I have to recognize that
there are other possible explanations for the gamma ray bursts, such as colliding neutron stars. New observations in
the next few years, particularly by gravitational wave detectors like LIGO, should enable us to discover the origin of the
gamma ray bursts.
Even if the search for primordial black holes proves negative, as it seems it may, it will still give us important
information about the very early stages of the universe. If the early universe had been chaotic or irregular, or if the
pressure of matter had been low, one would have expected it to produce many more primordial black holes than the
limit already set by our observations of the gamma ray background. Only if the early universe was very smooth and
uniform, with a high pressure, can one explain the absence of observable numbers of primordial black holes.
The idea of radiation from black holes was the first example of a prediction that depended in an essential way on both
the great theories of this century, general relativity and quantum mechanics. It aroused a lot of opposition initially
because it upset the existing viewpoint: “How can a black hole emit anything?” When I first announced the results of
my calculations at a conference at the Rutherford-Appleton Laboratory near Oxford, I was greeted with general
incredulity. At the end of my talk the chairman of the session, John G. Taylor from Kings College, London, claimed it
was all nonsense. He even wrote a paper to that effect. However, in the end most people, including John Taylor, have
come to the conclusion that black holes must radiate like hot bodies if our other ideas about general relativity and
quantum mechanics are correct. Thus, even though we have not yet managed to find a primordial black hole, there is
fairly general agreement that if we did, it would have to be emitting a lot of gamma rays and X rays.
The existence of radiation from black holes seems to imply that gravitational collapse is not as final and irreversible as
we once thought. If an astronaut falls into a black hole, its mass will increase, but eventually the energy equivalent of
that extra mass will be returned to the universe in the form of radiation. Thus, in a sense, the astronaut will be
“recycled.” It would be a poor sort of immortality, however, because any personal concept of time for the astronaut
would almost certainly come to an end as he was torn apart inside the black hole! Even the types of particles that were
eventually emitted by the black hole would in general be different from those that made up the astronaut: the only
feature of the astronaut that would survive would be his mass or energy.
The approximations I used to derive the emission from black holes should work well when the black hole has a mass
greater than a fraction of a gram. However, they will break down at the end of the black hole’s life when its mass gets
very small. The most likely outcome seems to be that the black hole will just disappear, at least from our region of the
universe, taking with it the astronaut and any singularity there might be inside it, if indeed there is one. This was the
first indication that quantum mechanics might remove the singularities that were predicted by general relativity.
However, the methods that I and other people were using in 1974 were not able to answer questions such as whether
singularities would occur in quantum gravity. From 1975 onward I therefore started to develop a more powerful
approach to quantum gravity based on Richard Feynrnan’s idea of a sum over histories. The answers that this
approach suggests for the origin and fate of the universe and its contents, such as astronauts, will be de-scribed in the
next two chapters. We shall see that although the uncertainty principle places limitations on the accuracy of all our
predictions, it may at the same time remove the fundamental unpredictability that occurs at a space-time singularity.

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