Calculations show that wormholes should create a spectacular display of gamma rays that we could try to observe.
Despite his playful optimism, Hawking recognised that the undiscovered laws of physics may prevent time travel.
The pull created by a black hole is so strong that if you get too close to one – even if you are travelling away from it at the fastest speed it is possible to go – you will never be able escape.
A wormhole is like a tunnel connecting two places in space. They would be incredibly useful and are great for science-fiction stories.
The problem is we haven’t found any evidence of them existing.