Impossible Concept#1: Reversing The Polarity Of The Neutron Flow
Maybe I'm being a bit unfair with including this one given that Jon Pertwee has already stated that the phrase is meaningless as the Third Doctor, but the idea continues to rustle a lot of jimmies with my not-so-inner-geek. Neutrons have no overall charge to reverse, so reversing it achieves nothing other than confining a few fist-shaking physics nerds to the couch with paralytic annoyance for an hour or two. Unless temporary and minor global domination was Terrence Dicks' intention all along, in which case: well played, Terrence, well played...
Possible Concept #1: Time Travel
Where would sci-fi be without the gratuitous use of theoretical physics? One of Doctor Who's main saviours is the faster than light tachyon proposed in 1967, four years after the classic series was first broadcast. Although Romana stated that the Time Lords "abandoned tachyonics when developed warp matrix engineering", the use of tachyons could still be a viable explanation for time travel. And why should we want to abandon it? After all, an idea like paradoxical time travel has the three hallmarks of entertainment to aspire to: excitement, terror and the faintest hint of an uneasy truth.