In this view, in one alternative history, there would not have been any return from the future, before the rocket set off, and so no possibility of it
being blown up. But when the traveler returns from
the future, he enters another alternative history. In
this, the human race makes a tremendous effort to
build a space ship, but just before it is due to be
launched, a similar space ship appears from the other
side of the galaxy, and destroys it.
David Deutsch claims support for the alternative
histories approach, from the sum over histories
concept, introduced by the physicist, Richard
Feinman, who died a few years ago. The idea is that
according to Quantum Theory, the universe doesn’t
have just a unique single history.
Instead, the universe has every single possible history,each with its own probability. There must be a possible history in which there is a lasting peace in the Middle East, though maybe the probability is low.
In some histories space-time will be so warped, that objects like rockets will be able to travel into their pasts. But each history is complete and self contained, describing not only the curved space-time, but also the objects in it. So a rocket can not transfer to another alternative history, when it comes round again. It is still in the same history, which has to be self consistent. Thus, despite what Deutsch claims, I think the sum over histories idea, supports the consistent histories hypothesis, rather than the alternative histories idea.
It thus seems that we are stuck with the consistent histories picture. However, this need not involve problems with determinism or free will, if the probabilities are very small, for histories in which space-time is so warped, that time travel is possible over a macroscopic region. This
is what I call, the Chronology Protection Conjecture: the laws of physics conspire to prevent time travel, on a macroscopic scale.
It seems that what happens, is that when space-time gets warped almost enough to allow travel into the past, virtual particles can almost become real particles, following closed trajectories. The density of the virtual particles, and their energy, become very large. This means that the probability of these histories is very low. Thus it seems there may be a Chronology Protection Agency at work, making the world safe for historians. But this subject of space and time warps is still in its infancy. According to string theory, which is our best hope of uniting General Relativity and Quantum Theory, into a Theory of Everything, space-time ought to have ten dimensions, not just the four that we experience. The idea is that six of these ten dimensions are curled up into a space so small, that we don’t notice them. On the other hand, the remaining four directions are fairly flat, and are what we call space-time. If this picture is correct, it might be possible to arrange that the four flat directions got mixed up with the six highly curved or warped directions. What this would give rise to, we don’t yet know. But it opens exciting possibilities.
The conclusion of this lecture is that rapid space-travel, or travel back in time, can’t be ruled out, according to our present understanding. They would cause great logical problems, so let’s hope there’s a Chronology Protection Law, to prevent people going back, and killing our parents. But science fiction fans need not lose heart. There’s hope in string theory.
Since we haven’t cracked time travel yet, I have run out of time. Thank you for listening.