Time Travel
Time travel is a staple of fiction. Hardly surprising, it's such a wonderful idea. Who wouldn't like to see the future or possibly change the past?Unfortunately once you scratch the surface, the notion of "time travel" is full of complexity. For a start, what does the term actually mean?
What Is Time Travel?
At some level time travel exists and can be shown to exist - Einsteinian phsyics predicts time dilation as the speed of light is approached. Using such a method an observer could theoretically visit the future - however it would only be time travel in the sense that being cryogenically suspended for a thousand years would be time travel. And it would be strictly one way unless we could cross the light barrier.Some people have even argued for "virtual time travel" - entering into an accurate computer simulation of the future or the past. This raises an interesting, age-old philosophical question: what is reality? For me neither virtual reality nor time dilation would be "real" time travel.
In order for us to truly "travel" in time this suggests that different times exist in the same way that different places do - that time is literally a "fourth dimension". Yet even though the laws of physics often treat it as such, our intuition tells us that time is different to the other three dimensions. Entropy, for example, is a feature of time - some people argue it is responsible for the arrow of time. Is time in reality just some way of measuring change? Maybe there is no past or future to "travel to"?
Paradoxes
If the past and future do exist, are they fixed? If the past is fixed then any visit we make has already happened and we can change nothing, only observe. If we can change the past then we introduce the well-known time travel paradoxes. What happens if you go back and kill your grandfather? Or even yourself?As to the future, if it is fixed then we have no free will and determinism rules - which would seem in conflict with current scientific theories. If the future is undetermined then any future we visit can only be a possible one - and which one would it be? Could we "choose"?
Where Are They Now?
Another well-known issue concerning time travel is simply that we have not seen time travellers. If at some point in the future we will discover time travel, why has no-one yet visited us from the future? The idea of "time tourists" was amusingly addressed in John Wyndham's story "Pawley's Peepholes".One World - Or Many?
Perhaps the "multiple universes" theory is literally true and by travelling backwards we cause the universe to branch. This raises obvious questions about conservation of matter and energy.Or perhaps all there is no branching and all these many worlds exist in parallel. Perhaps time travel is simply crossing a yet higher dimension to visit an alternative time track - in which case, could we ever get back to our own world?
These are questions as well suited to philosophy as to
physics. As is so often the case with time there are no
easy answers.