Time Trip. Ever since Einstein showed it was theoretically possible, the quest to travel through time has appealed to both amateurs and brilliant scientists alike. The amateurs include Aage Nost , who demonstrates his time machine for the cameras, while Professor Frank Tipler has theoretically sound designs for a machine - but in practice it would weigh half the mass of the galaxy. There is, however, one way that time travel to the past could be possible - future scientists could use computers to create exact replicas of past times. But if that's true, who's to say we are not one of the replicas? Producer Malcolm Clark ; Editor Matthew Barrett (S)