To fans of Doctor Who, which returns to BBC1 next month, the good doctor couldn't possibly exist without his Tardis. After all, time travel makes the programme tick. But isn't Doctor Who stretching reality a bit too far? What about all those futuristic baddies and technology?A close parallel between real science and fiction has remained with the programme for more than 40 years - ever since the first episode went out on November 23, 1963, the day after President John F Kennedy was shot.The doctor has fought genetically modified Daleks; encountered nanorobots that can heal your every ill; owned a robot dog (20 years before Sony); and explored a virtual-reality world called the Matrix back when Keanu Reeves was knee-high to a memory card.The new series, with Scottish actor David Tennant playing the eponymous Time Lord's 10th incarnation, sees the return of his old adversaries the Cybermen. These hybrids of man and machine were created in 1966 by the show's then science adviser, Dr Kit Pedler of University College London. Fascinated by new developments in transplant surgery, Pedler imagined the Cybermen as a race that had given themselves so many cybernetic implants that little of their original bodies remained.So what about the science in the show - time travel, sonic screwdrivers, cosmic empires? And what is the big deal about hiding behind the sofa…?