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.
OPTION 1: Travel faster than light
This story is from the November 2018 edition of BBC Earth.
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This story is from the November 2018 edition of BBC Earth.
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