Just one word of warning though before anyone attempts to build a spaceship and steer for the nearest wormhole: they may be real, they may be common and they may bridge time and space, but having a wormhole is one thing, having one you can use is quite another. Even Professor Hawking, who admits to being “obsessed by time” and to wanting to believe time travel is possible, points out that wormholes are only thought to exist in the “quantum foam” right down below even the scale of atoms. That’s way too small to squeeze a spaceship through. Or Arnold Schwarzenegger. Or even Michael J Fox.

There are those who argue that given enough technology, theoretical physicists and, er, time we might eventually develop a way to snare some of these infinitesimally tiny wormholes and then make them billions of times bigger so we could go where and when we wanted. It’s all colossally speculative at the moment, but just supposing such a traversable wormhole were someday constructed, and you carefully avoided all deliberate interference with the past, you would still run smack into another prohibitive paradox.

Beware the Butterfly Effect

It’s one nicely illustrated by Ray Bradbury’s classic early 50s short story A Sound of Thunder, where time travellers to Earth’s prehistoric past are kept on a levitating walkway to minimise their chance of any contact with the past. Someone falls off and accidentally crushes a single butterfly. When they return to their present all sorts of things, from spellings of words to the outcome of elections, are different and they have created an alternative reality.

Bradbury’s story is the first incarnation of the ‘Butterfly Effect” often evoked in chaos theory: the idea that one tiny change now can result in all manner of large and often unforeseeable changes later. And that’s a real obstacle for going back in time. If anyone could overcome the enormous challenge of how to do it, they would still face the equally great challenge of how to do it without risk of affecting the past in the slightest. Alter one thing, and you could alter everything and end up rewriting reality.