A BRIDGE TOO FAR? For the aspiring time traveller, the wormhole offers some hope. Here the concept is illustrated by showing space and time imagined as a two-dimensional folded surface, allowing for a ‘bridge’ connecting two different points.
WHAT IT IS A distortion of the linear nature of time
WHY YOU WON’T DO IT See Back to the Future for what can go wrong
Aside from belligerent Martians, nothing has occupied the minds of science fiction writers as much as the possibility of time travel. So if you fancy reliving the past, seeing your future or popping back in time to meet your great-grandfather, how realistic are your chances?
In some ways, we’re all time travellers already – a second ago we were in the past, now we’re in the present and we’ll shortly be in the future. Okay, that’s cheating – what we really mean by time travel is journeying into the future more quickly than we do normally, or rewinding time and travelling to the past.
Accelerated travel into the future, at least, has a strong scientific basis. Albert Einstein’s famous theory of general relativity shows how time is affected by the gravity of large masses, moving at different speeds in different locations. This effect is called gravitational time dilation, and it’s been proved in many ways – for instance, the clocks of GPS satellites orbiting high above Earth pick up about a third of a billionth of a second per day over time compared to their ground-based equivalents.
A BRIDGE TOO FAR? For the aspiring time traveller, the wormhole offers some hope. Here the concept is illustrated by showing space and time imagined as a two-dimensional folded surface, allowing for a ‘bridge’ connecting two different points.
Obviously this is not something we would normally notice, but if we were able to, say, travel close to a huge black hole, such as Sagittarius A at the centre of our galaxy, we would experience time at half the speed of Earth. If we left Earth at the beginning of the year and spent a year circling the back hole, then by the time we returned home, two years would have passed on Earth. We would have travelled forwards in time.
Another form of time dilation effect happens when objects move at speeds close to the speed of light. Get close enough to the Universe’s ultimate speed limit, and you could travel for a week and miss a century of life on Earth. But to achieve this, you would need a vehicle 2,000 times faster than the fastest manned spacecraft ever built.
Confused yet? Well, we haven’t even started on travelling backwards in time. Our greatest minds are much less certain this is possible even in theory, but if it is, then some believe the answer again rests with black holes. Specifically, a ‘Kerr black hole’ is a theoretical construct created by rings of superdense neutron stars behaving in particular ways. This black hole could suck you in at one point and expel you at another, possibly in the future but perhaps in the past.
A slightly more familiar concept is the wormhole. This is a hypothetical tunnel through space and time connecting distant parts of the Universe. Scientists believe that wormholes exist in the submicroscopic ‘quantum foam’ of the Universe – but their entrance tunnels are less than a billion-billion-trillionth of a centimetre across. It’s clearly going to take more than a diet and some lycra to squeeze into that kind of opening, but some scientists believe that it might one day be possible to construct a giant version.
Elsewhere, physicist J. Richard Gott has outlined his theory of cosmic strings, objects that are narrower than an atom but exert massive gravitational pull affecting time dilation. By bringing two strings close together, it might be possible to form a time loop, propelling you into the future or back into the past.
If backward time travel is possible, however, many theorists are convinced that it would not be possible to go back further than the moment at which the method of travel was invented. This might explain why up we are not currently inundated with visitors from the future.
Of course, if it turns out that we can go back in time, we’ll still need to iron out causality. If one thing happens as a result of another, what happens if we go back and change the initial event? That’s the question posed by the ‘grandfather paradox’ – if you decide to go back in time to kill your grandfather when he is a child, your father won’t be born. And if he isn’t born, neither are you. So how can you be around to travel back through time in the first place? For more of this sort of thing, treat yourself to a box set of the Back to the Future movies.
