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What science today says about how we might travel backwards and forwards in time

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Twisty-turny universe

Mathematician Kurt Gödel said time travel is possible in a rotating universe - a Gödel Universe, as we now call it. He said if we lived in a rotating universe, the universe could spin out loops and you could travel along them to meet yourself in the future.

Gödel’s theory is founded on one of Einstein’s core beliefs: that the universe is not expanding and is rotating at a constant speed.

Models have been created of a Gödel Universe but they don’t resemble what we see of the observable universe. Our space-time isn’t rotating at a constant speed, so the idea breaks down.

Another idea comes from theoretical physicist Miguel Alcubierre, who gave time-travel fans hope with his Alcubierre Warp Drive in 1996. Using the Alcubierre Warp Drive you create a bubble in space-time that contains you and your ship – let’s call it a Tardis. In front of the bubble you scrunch up space and behind you fold it flat. The upshot: you travel around faster than light does.

In October, physicists Benjamin Tippett and David Tsang published a paper outlining a conceptual Tardis that exploits the Aclubierre drive. Their Tardis – which stands for Traversable Achronal Retrograde Domain in Space-time, rather than Time and Relative Dimension in Space – would travel along a closed loop in space and time with a person safe within the bubble. You can read their paper here (non-technical version) and here (come wearing your propeller hat).

Space-time under general relativity is curved. It is curved because it’s distorted by objects with a mass. Imagine space-time is like a huge rubber sheet. Place a heavy ball in the middle of the sheet and the ball will create a dip.

Warp drive

Now scale that up. If you create a large enough gravitational force in space-time - swap the ball for a planet or a star - then you can bend space-time even further. If you can then rotate that object fast enough, up to and beyond speed of light, you can - potentially - travel in time through a trick called ‘frame dragging’.

This is presumably the theory of a time travelling Tardis: a ship with a tremendously powerful engine and energy source capable of distorting space-time enough to move faster than light, thereby letting an occupant travel in time. You can view the Tardis’ interior, held in a separate ‘dimension’ from the outside universe - a separate space-time, in other words - as the bubble of Alcubierre’s warp drive.

But could it ever generate enough power? Therein lies the rub and it’s the square-one Tippett and Tsang are pulled back to at the end of their paper.

“Unfortunately, just like the Alcubierre Warp Drive, generating the Tardis geometry would require exotic matter, violating the classical energy conditions. This matter would be gravitationally repulsive and would need to move faster than light,” they write.

Doctor Phil Bull, a post-doctoral research fellow at the University of Oslo, outlined the scale of the challenge for would-be time travellers. “You can can only get these extreme effects with really gigantic distortions in space-time. You’d need a planet to spin very close to the speed of light,” Bull tells The Reg.

The amount of energy required to even make a planet spin that fast would be huge. “Maybe you could set that up, but it’s still a gigantic engineering problem that’s out of our reach,” Bull says.

Exotic matter

Black holes are another option because they spin and they are hugely massive. But, Bull says, black holes don’t spin “in the right way to give us time travel”.

“Maybe in 200 years’ time space technology and the way we harness energy will have evolved to a point where we can manipulate things as big as stars and then we can start using general relativity to harness energy from black holes.”

Interestingly, this exactly what Rassilon, founder of Time Lord society, did to enable time travel. Just watch The Deadly Assassin.

Of course, all of the challenges outlined so far are put there by general relativity. But what if general relativity isn’t the all-encompassing framework that governs events in the universe that we think it is?

Indeed, general relativity is already under threat from the recent suggestion that there might be something out there in the universe which we now call ‘dark energy’.

This is the material theorised to be responsible for the continued and accelerating expansion of the observable universe. Einstein discounted the idea of an expanding universe and so, as a result, does general relativity. Einstein believed the universe to be static.

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