Horizon's Time Trip is a thrilling journey deep into the strangeness of cutting-edge physics - a place where beautiful, baffling ideas are sometimes indistinguishable from the utterly crazy.

On this journey, we meet a time-travelling pizza, a brilliant mathematician in a ski mask and even God. The journey ends with a strange and dark conclusion - one which calls into question our very existence.

Ever since Einstein showed it was theoretically possible, the quest to travel through time has drawn eccentric amateurs and brilliant scientists in almost equal numbers. The amateurs include Aage Nost, who demonstrates his time machine in front of the cameras. The professionals include the likes of Professor Frank Tipler of Tulane University. His time machine sounds good - but it would weigh half the mass of the galaxy.

There is, however, one way that time travel to the past could be possible. And it would be much more convenient. Future civilisations could use computers to create exact replicas of the past. Unfortunately that idea has physics trembling in its socks. Because if you can generate a perfect virtual reality version of the past, who's to say we are not one of the replicas?