We have seen how an interval of time depends on your perspective
in relativity. Less time will elapse for someone who accelerates to
very high speeds or spends some time in a strong gravitational
field. I have discussed how this provides us with a way
of travelling into the future. Of course this cannot really be
considered as cheating time. All we are doing is getting to the
future more quickly. Think of it like those clever adverts on TV
where someone is slowly munching through a chocolate bar while
the rest of the world whizzes past at high speed. I have described
how this sort of time travel is quite normal for subatomic particles,
since they are the only objects capable of getting close to the speed
of light. Thus the muons that are produced by cosmic rays are time
travelling into the future (by a tiny fraction of a second) during
their shortened journey through the Earth’s atmosphere.
The problem with this sort of time travel is that it is one way.
We may well be able to one day travel into the future, maybe
even the distant future, but the only way to get back to our own
time again would be by travelling back into the past. This is an
altogether trickier problem. When scientists talk about time travel
they tend to mean time travel into the past. Throughout this
chapter, whenever time travel is mentioned it will refer to time
travel into the past.
There are two ways of going back to the past. One is by going
backwards through time, during which the hands on your watch would be moving round anticlockwise. Of course you would not
be aware this is happening. This would require faster-than-light
speeds which are not accessible to us, and so is not the sort of
time travel I intend to discuss here. The other way is by travelling
forward in time (your local time runs forwards) but by moving
along a warped path through spacetime that takes you back to your
past (like looping the loop on a roller coaster). Such a loop isknown
in physics as a ‘closed timelike curve’ and has been the subject of
intense theoretical research during the 1990s. What may come as
a surprise to you is that it has been known for half a century that
Einstein’s equations of general relativity allow such closed timelike
curves. The Austrian-born American mathematician Kurt G¨odel
showed in 1949 that such time travel into the past was theoretically
possible.
So what is all the fuss about? Time travel to the future is easy
and time travel to the past, while difficult, is not yet ruled out by
theory. What are we waiting for? Why haven’t we built a time
machine yet? The reason is that not only would it be exceedingly
difficult to create a closed timelike curve in spacetime, but that
we are not sure whether it is even possible in theory. As things
stand, general relativity tells us that we cannot rule out time travel,
but many physicists are hoping that a better understanding of the
mathematics will eventually lead to the conclusion that it is totally
forbidden. And the reason physicists feel so strongly about this is
that time travel leads to a number of strange paradoxes. In this
chapter I will take a look at some of these time travel paradoxes
and see if there is any way out.
As a scientist, I find it hard to just sit and watch a science
fiction film that involves time travel. Instead of taking it with the
requisite pinch of salt as I am supposed to, and just enjoying the
(usually) daft story line, I tend to pick holes in the logic. I’ll say
something like: hang on a minute; if he just went back in time
and did such and such then surely he has meddled with history
and . . . well, you probably know what I mean.
It’s quite sad really.
Most of these films are rather silly and I should just ‘go with
the flow’ and appreciate the millions of dollars spent on the special effects. If you’ve ever seen Star Trek IV: The Voyage Home you’ll
know what I mean. In that film, the best of the original Star
Trek movies, Captain Kirk and his crew travel back in time to
the twentieth century. It contains some amusing moments, like
when Scotty tries to talk to a computer, is told that he must use the
mouse and so picks it up and talks into it! Well it’s my favourite
bit.
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