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?

Imagine a world where disease could be eradicated by an injection of tiny robots the size of molecules. That is the hope offered by nanotechnology - the science of microscopically small machines. But others fear nanotechnology could lead to a non-biological cancer - where swarms of tiny nanobots come together and literally devour human flesh.

Sounds like science fiction? It certainly did until a brilliant young scientist called Hendrik Schön seemed to bring it a step closer.

Schön's great breakthrough was to make a computer transistor out of a single organic molecule. It was an achievement of almost incalculable brilliance. Some speculated this technology could spell the end of the entire silicon chip industry.

Crucially, Schön's transistor was organic. Suddenly, this seemed to be the first step towards true nanotechnology, where minute computers could grow as living cells.

Scientists speculated about how these tiny machines could be used to target diseases with astonishing precision. Others wondered - could the military use them as a new weapon? Others, including Prince Charles, were terrified. If these machines can grow by themselves, how do we stop them from growing?

What happened next would destroy reputations and shatter lives - because there was more to Hendrik Schön's discovery than anyone knew.