“Time travel to the past is possible,” says Colin Stuart, author of The Big Questions in Science.
“In fact, you can travel back in time, arrive three months before you left - and buy yourself a Christmas present before you set off.”
Travelling into the past is “difficult,” Stuart admits, and there is one, crucial, limitation - you cannot travel back beyond the point when the first time machine is invented.
“In fact, the inventor of the first time machine will find it impossible to use,” says Stuart, “Lots of people will think, “Oh, I’ll go back and meet the inventor!’ So he’ll probably spend most of his time shaking their hands.”
Travelling forward in time is relatively easy, most scientists agree - to go forward in time, you simply need to accelerate to speeds close to the speed of light.
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As you get close to that speed, time slows down, but only for you, according to Einstein’s Theory of Special Relativity.
Travel far enough, and you could return thousands of years into the future. Travelling backwards, though, is much, much harder - but still, Stuart, says “possible”.
Stuart notes that we have, as yet, never seen a time-traveller - which argues that limitless travel through time is not possible.
Professor Stephen Hawking announced a “party” for time-travellers, with details of latitude and longitude, and invited visitors from the future. “None showed up,” says Stuart.
Stuart also points out that “time tourists” would visit moments such as the launch of the Titanic, or the assassination of JFK, “but they’re not in the photographs.”
Stuart’s method of travelling through time for a surprise Christmas gift is not easy, though - it requires travel to another star, a spacecraft that can travel at near light speed, and a gigantic amount of energy.
[William Hartnell: I didn't want to be Dr Who]
“What you would do is create a wormhole - you can use them to go backwards,” says Stuart. “What you would need is something really heavy - which bends space - or a huge amount of energy, to create a wormhole. There’s a rule in physics that you can borrow a huge amount of energy - as long as you pay it back quickly - it’s called the Heisenberg uncertainty principle.”
“You see it in particles in the Large Hadron Collider which pop in and out of existence. If you can somehow pay off that “debt”, you end up with a permanent wormhole in space - which would take you instantly to, say, another star.”
“To travel “back in time”, you simply have to attach one end of the wormhole to a spaceship, fly around at near the speed of light for a while (so time slows down for the spaceship), then jump through the wormhole.”
If the spaceship flew for five years, only six months would have passed within the wormhole - so if you jump through it to the alien star, then fly back to Earth (on yet another spaceship), you arrive three months before you left.
Because you rely on the wormhole, you can’t go further back than when the machine is invented - hence, perhaps, the reason we have never seen any time travellers. Either that, or the sheer amount of effort involved.
Stuart says that this method requires technology far beyond what we currently have, “This isn’t the time to do it,” he admits. “But there are stars that are billions of years older than ours - if there is life there, perhaps they can travel in time.”
Stuart says that while the idea is “possible” it does raise questions - “What happens if you arrive three months before you leave, then shoot yourself? We just don’t know.”
Thus far, the only things we can accelerate to near light speed are particles such as protons in accelerators such as the Large Hadron Collider.
Science writer Carl Sagan once offered the rather weak argument that time travellers might be among us already - but would “disguise themselves” to avoid disrupting the past.
Professor Brian Cox suggests, though, that the idea of a stable wormhole may not be realistic.
“In General Relativity, you can travel backwards in principle,” he said in a speech this year. “It's to do with building these things called wormholes; shortcuts through space and time. But most physicists doubt it. Hawking came up with the 'chronology protection conjecture' - physics we don't yet understand that means wormholes are not stable.”
The Big Questions in Science, published by Andre Deutsch, is out in November