Travelling in time might sound like a flight of fancy, but some physicists think it might really be possible. BBC Horizon looked at some of the most promising ideas for turning this staple of science fiction into reality.
Ron Mallett has a dream: He wants to travel in time.
This isn't mere fantasy - Mallett is a respected professor of physics.
"I think of myself as being an ordinary person with a passion, and my passion is the possibility of time travel," he says.
Prof Mallett has wanted to build a time machine for most of his life. His passion, he explains, can be traced to a tragic event early in his life.
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