where vacuum fluctuations can have any
wavelength. The reduction in the number of virtual
particles between the plates means that they don’t hit the plates so often, and thus don’t exert as much pressure on the plates, as the virtual particles outside. There is thus a slight force pushing the plates together. This force has been measured experimentally. So virtual particles actually exist, and produce real effects.
Because there are fewer virtual particles, or vacuum fluctuations, between the plates, they have a lower energy density, than in the region outside. But the energy density of empty space far away from the plates, must be zero. Otherwise it would warp space-time, and the universe wouldn’t be nearly flat. So the energy density in the region between the plates, must be negative.
We thus have experimental evidence from the bending of light, that space-time is curved, and confirmation from the Casimir effect, that we can warp it in the negative direction. So it might seem possible, that as we advance in science and technology, we might be able to construct a wormhole, or warp space and time in some other way, so as to be able to travel into our past.
If this were the case, it would raise a whole host of questions and problems. One of these is, if sometime in the future, we learn to travel in time, why hasn’t someone come back from the future, to tell us how to do it.
Even if there were sound reasons for keeping us in
ignorance, human nature being what it is, it is difficult to believe that someone wouldn’t show off, and tell us poor
benighted peasants, the secret of time travel. Of course,
some people would claim that we have been visited from the
future. They would say that UFO’s come from the future, and that governments are engaged in a gigantic conspiracy to
cover them up, and keep for themselves, the scientific
knowledge that these visitors bring. All I can say is, that if governments were hiding something, they are doing a pretty
poor job, of extracting useful information from the aliens.
I’m pretty skeptical of conspiracy theories, believing the
cock up theory is more likely. The reports of sightings of UFO’s can’t all be caused by extra terrestrials, because they are mutually contradictory. But once you admit that some are mistakes, or hallucinations, isn’t it more probable that they all are, than that we are being
visited by people from the future, or the other side of the galaxy? If they really want to colonize the Earth, or warn us of some danger, they are being pretty ineffective.
A possible way to reconcile time travel, with the fact that we don’t seem to have had any visitors from the future, would be to say that it can occur only in the future. In this view, one would say space-time in our past was fixed, because we have observed it, and seen that it is not warped enough, to allow travel into the past. On the other hand, the future is open. So we might be able to warp it enough, to allow time travel. But because we can warp space-time only in the future, we wouldn’t be able to travel back to the present time, or earlier.
This picture would explain why we haven’t been over run by tourists from the future.
But it would still leave plenty of
paradoxes. Suppose it were possible to
go off in a rocket ship, and come back
before you set off. What would stop you
blowing up the rocket on its launch pad,
or otherwise preventing you from setting
out in the first place. There are other
versions of this paradox, like going back,
and killing your parents before you were born, but they are essentially equivalent. There seem to be two possible
resolutions.
One is what I shall call, the consistent histories approach. It says that one has to find a consistent solution of the equations of physics, even if space-time is so warped, that it is possible to travel into the past. On this view, you couldn’t set out on the rocket ship to travel into the past, unless you had already come back, and failed to blow up the launch pad. It is a consistent picture, but it would imply that we were completely determined: we couldn’t change our minds. So much for free will. The other possibility is what I call, the alternative histories approach. It has been championed by the physicist David Deutsch, and it seems to have been what Stephen Spielberg had in mind when he filmed, Back to the Future.