Page 47 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 6
Suppose an intrepid astronaut on the surface of the collapsing star, collapsing inward with it, sent a signal every
second, according to his watch, to his spaceship orbiting about the star. At some time on his watch, say 11:00, the star
would shrink below the critical radius at which the gravitational field becomes so strong nothing can escape, and his
signals would no longer reach the spaceship. As 11:00 approached his companions watching from the spaceship would
find the intervals between successive signals from the astronaut getting longer and longer, but this effect would be very
small before 10:59:59. They would have to wait only very slightly more than a second between the astronaut’s 10:59:58
signal and the one that he sent when his watch read 10:59:59, but they would have to wait forever for the 11:00 signal.
The light waves emitted from the surface of the star between 10:59:59 and 11:00, by the astronaut’s watch, would be
spread out over an infinite period of time, as seen from the spaceship. The time interval between the arrival of
successive waves at the spaceship would get longer and longer, so the light from the star would appear redder and
redder and fainter and fainter. Eventually, the star would be so dim that it could no longer be seen from the spaceship:
all that would be left would be a black hole in space. The star would, however, continue to exert the same gravitational
force on the spaceship, which would continue to orbit the black hole. This scenario is not entirely realistic, however,
because of the following problem. Gravity gets weaker the farther you are from the star, so the gravitational force on our
intrepid astronaut’s feet would always be greater than the force on his head. This difference in the forces would stretch
our astronaut out like spaghetti or tear him apart before the star had contracted to the critical radius at which the event
horizon formed! However, we believe that there are much larger objects in the universe, like the central regions of
galaxies, that can also undergo gravitational collapse to produce black holes; an astronaut on one of these would not be
torn apart before the black hole formed. He would not, in fact, feel anything special as he reached the critical radius,
and could pass the point of no return without noticing it However, within just a few hours, as the region continued to
collapse, the difference in the gravitational forces on his head and his feet would become so strong that again it would
tear him apart.

The work that Roger Penrose and I did between 1965 and 1970 showed that, according to general relativity, there must
be a singularity of infinite density and space-time curvature within a black hole. This is rather like the big bang at the
beginning of time, only it would be an end of time for the collapsing body and the astronaut. At this singularity the laws
of science and our ability to predict the future would break down. However, any observer who remained outside the
black hole would not be affected by this failure of predictability, because neither light nor any other signal could reach
him from the singularity. This remarkable fact led Roger Penrose to propose the cosmic censorship hypothesis, which
might be paraphrased as “God abhors a naked singularity.” In other words, the singularities produced by gravitational
collapse occur only in places, like black holes, where they are decently hidden from outside view by an event horizon.
Strictly, this is what is known as the weak cosmic censorship hypothesis: it protects observers who remain outside the
black hole from the consequences of the breakdown of predictability that occurs at the singularity, but it does nothing at
all for the poor unfortunate astronaut who falls into the hole.

There are some solutions of the equations of general relativity in which it is possible for our astronaut to see a naked
singularity: he may be able to avoid hitting the singularity and instead fall through a "wormhole” and come out in another
region of the universe. This would offer great possibilities for travel in space and time, but unfortunately it seems that
these solutions may all be highly unstable; the least disturbance, such as the presence of an astronaut, may change
them so that the astronaut could not see the singularity until he hit it and his time came to an end. In other words, the
singularity would always lie in his future and never in his past. The strong version of the cosmic censorship hypothesis
states that in a realistic solution, the singularities would always lie either entirely in the future (like the singularities of
gravitational collapse) or entirely in the past (like the , big bang). I strongly believe in cosmic censorship so I bet Kip
Thorne and John Preskill of Cal Tech that it would always hold. I lost the bet on a technicality because examples were
produced of solutions with a singularity that was visible from a long way away. So I had to pay up, which according to
the terms of the bet meant I had to clothe their nakedness. But I can claim a moral victory. The naked singularities were
unstable: the least disturbance would cause them either to disappear or to be hidden behind an event horizon. So they
would not occur in realistic situations.

The event horizon, the boundary of the region of space-time from which it is not possible to escape, acts rather like a
one-way membrane around the black hole: objects, such as unwary astronauts, can fall through the event horizon into
the black hole, but nothing can ever get out of the black hole through the event horizon. (Remember that the event
horizon is the path in space-time of light that is trying to escape from the black hole, and nothing can travel faster than
light.) One could well say of the event horizon what the poet Dante said of the entrance to Hell: “All hope abandon, ye
who enter here.” Anything or anyone who falls through the event horizon will soon reach the region of infinite density
and the end of time.

General relativity predicts that heavy objects that are moving will cause the emission of gravitational waves, ripples in
the curvature of space that travel at the speed of light. These are similar to light waves, which are ripples of the
electromagnetic field, but they are much harder to detect. They can be observed by the very slight change in separation
they produce between neighboring freely moving objects. A number of detectors are being built in the United States,

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