Page 80 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 10
regarded as reliable witnesses? If they are trying to warn us of some great danger, they are not being very
effective.

A possible way to explain the absence of visitors from the future would be to say that the past is fixed because
we have observed it and seen that it does not have the kind of warping needed to allow travel back from the
future. On the other hand, the future is unknown and open, so it might well have the curvature required. This
would mean that any time travel would be confined to the future. There would be no chance of Captain Kirk and
the Starship Enterprise turning up at the present time.
This might explain why we have not yet been overrun by tourists from the future, but it would not avoid the
problems that would arise if one were able to go back and change history. Suppose, for example, you went
back and killed your great-great-grandfather while he was still a child. There are many versions of this paradox
but they are essentially equivalent: one would get contradictions if one were free to change the past.

There seem to be two possible resolutions to the paradoxes posed by time travel. One I shall call the consistent
histories approach. It says that even if space-time is warped so that it would be possible to travel into the past,
what happens in space-time must be a consistent solution of the laws of physics. According to this viewpoint,
you could not go back in time unless history showed that you had already arrived in the past and, while there,
had not killed your great-great-grandfather or committed any other acts that would conflict with your current
situation in the present. Moreover, when you did go back, you wouldn’t be able to change recorded history.
That means you wouldn’t have free will to do what you wanted. Of course, one could say that free will is an
illusion anyway. If there really is a complete unified theory that governs everything, it presumably also
determines your actions. But it does so in a way that is impossible to calculate for an organism that is as
complicated as a human being. The reason we say that humans have free will is because we can’t predict what
they will do. However, if the human then goes off in a rocket ship and comes back before he or she set off, we
will be able to predict what he or she will do because it will be part of recorded history. Thus, in that situation,
the time traveler would have no free will.

The other possible way to resolve the paradoxes of time travel might be called the alternative histories
hypothesis. The idea here is that when time travelers go back to the past, they enter alternative histories which
differ from recorded history. Thus they can act freely, without the constraint of consistency with their previous
history. Steven Spiel-berg had fun with this notion in the Back to the Future films: Marty McFly was able to go
back and change his parents’ courtship to a more satisfactory history.

The alternative histories hypothesis sounds rather like Richard Feynman’s way of expressing quantum theory
as a sum over histories, which was described in Chapters 4 and 8. This said that the universe didn’t just have a
single history: rather it had every possible history, each with its own probability. However, there seems to be an
important difference between Feynman’s proposal and alternative histories. In Feynman’s sum, each history
comprises a complete space-time and everything in it. The space-time may be so warped that it is possible to
travel in a rocket into the past. But the rocket would remain in the same space-time and therefore the same
history, which would have to be consistent. Thus Feynman’s sum over histories proposal seems to support the
consistent histories hypothesis rather than the alternative histories.

The Feynman sum over histories does allow travel into the past on a microscopic scale. In Chapter 9 we saw
that the laws of science are unchanged by combinations of the operations C, P, and T. This means that an
antiparticle spinning in the anticlockwise direction and moving from A to B can also be viewed as an ordinary
particle spinning clockwise and moving backward in time from B to A. Similarly, an ordinary particle moving
forward in time is equivalent to an antiparticle moving backward in time. As has been discussed in this chapter
and Chapter 7, “empty” space is filled with pairs of virtual particles and antiparticles that appear together, move
apart, and then come back together and annihilate each other.

So, one can regard the pair of particles as a single particle moving on a closed loop in space-time. When the
pair is moving forward in time (from the event at which it appears to that at which it annihilates), it is called a
particle. But when the particle is traveling back in time (from the event at which the pair annihilates to that at
which it appears), it is said to be an antiparticle traveling forward in time.

The explanation of how black holes can emit particles and radiation (given in Chapter 7) was that one member

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