What Did Stephen Hawking Think About Time Travel?

Stephen Hawking found time travel a serious and irresistible question, and his answer was nuanced: travelling forwards in time is real and proven, while travelling backwards is probably forbidden by the laws of physics, though he could not entirely rule it out.

Forward time travel is real

This part is not science fiction. Einstein's theory of relativity shows that time runs at different rates depending on speed and gravity. Travel fast enough, or sit near a strong enough gravitational field, and time passes more slowly for you than for everyone else, so you effectively jump forward into their future. Astronauts experience this in tiny amounts already. Hawking liked to point out that a fast enough spaceship would be a genuine, if impractical, machine for visiting the future.

Backward time travel is another matter

Going back is far harder, and Hawking suspected nature does not allow it. The equations of general relativity do permit exotic structures such as wormholes that might in principle connect different times, but Hawking argued that any attempt to use them would likely be destroyed by runaway feedback before it could work. He summed this up in his chronology protection conjecture: the idea that the laws of physics conspire to prevent travel into the past, keeping the universe "safe for historians," as he put it.

The party for time travellers

Hawking tested his scepticism with one of his best-loved jokes. In 2009 he held a party, complete with balloons and champagne, for time travellers, but sent out the invitations only after the event had taken place. The logic was playful but pointed: if backward time travel were ever possible, a guest from the future could have read the invitation and come back to attend. Nobody showed up. It was not proof, as he cheerfully admitted, but it captured his hunch that the past stays firmly out of reach.

Explore the effect for yourself with the time dilation calculator.