Time travel: a user guide
Time travel could be possible. We investigate the science behind it.
In 1915 in Berlin, at the height of WWI, Albert Einstein presented a revolutionary new theory of gravity – the General Theory of Relativity. It has since become one of the most successful theories ever, passing every observational test thrown at it and predicating cosmological phenomena such as the Big Bang, black holes and gravitational waves. But the theory has also given scientists sleepless nights because it makes one thing pretty much unavoidable: time machines…
…It comes down to the fact that, in Einstein’s theory, time is not absolute, ticked off by a universal clock with which everyone agrees, but instead is relative. “I can’t talk to you in terms of time – your time and my time are different,” wrote the English novelist Graham Greene. According to Einstein, the rate at which time flows for someone depends on how fast they’re moving relative to you and the strength of the gravity they’re experiencing. If you can find a way to jump from a region where time flows at one rate to a region where it flows at slower rate, you can go back in time – you’ll have made a time machine.
The recognition that time is not what you think it is goes back to the Special Theory of Relativity that Einstein published in 1905, and it all hinges on the unique properties of the speed of light. Einstein realised that nothing can travel faster than light – it is the cosmic speed limit of our Universe. This makes light uncatchable by anything. He also discovered that intervals of space and time stretch like elastic as massive objects move through them. By a cosmic conspiracy this means that everything measures exactly the same speed for a light beam, no matter how fast that thing is travelling or in which direction.
To be a little more precise, moving clocks run slow. So, if someone flies past you – and it has to be at a speed approaching 300,000 kilometres per second – then their clock will run slow compared to yours. If they could ever reach the speed of light – which is impossible for a material body, though possible for a massless entity such as a particle of light (a photon) – time would come to a complete standstill.
What if you could travel faster than light? If you could, then you would arrive at your destination before you set off. This so-called ‘violation of causality’ gives scientists sleepless nights since it’s synonymous with time travel. If you travelled faster than light from Earth to the nearest star system, Alpha Centauri, then a light beam leaving at the same time would still be on its way by the time you had arrived. This makes it possible for you to look back at Earth and see your spacecraft blasting off.
But how would you ever be able to travel faster than light if nothing material can even achieve the speed of light? Physicists have postulated the existence of hypothetical particles called tachyons. Tachyons are born moving faster than light, just as photons (particles of light) are born travelling at the speed of light. If we could convert the atoms of our bodies into tachyons, fire them across space, then change them back into atoms, we could travel faster than light. And some observers would see us going backwards in time.
But there’s a catch. In 1974, US physicist Richard Gott of Princeton University discovered that a tachyon radiates a cone of gravitational waves (ripples in space-time) that trails behind it, much like the way an aeroplane creates a sonic boom when exceeding the speed of sound. Thanks to the tachyon’s unusual properties, this cone would cause the particle to lose energy and speed up – the opposite of what you might expect – before finally colliding with an equally fast-moving anti-tachyon and annihilating. Although the collision would occur at infinite speed, Gott’s solutions suggest that most of a tachyon’s life would be spent moving at barely above the speed of light. However, this would make any time travel effect exceedingly small.
Time travel is possible, but you have to become a hypothetical particle called a tachyon. Probably not likely.