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.