There exists a SF short story about anti-gravitation called the 'Mother of Invention' (AFAIK) A bunch of scientists are shown a (fake) movie in which an anti-gravity device is demonstrated by an inventor. The scientists are told the inventor's notes were lost/destroyed and they have to start from scratch. The bottom line - there was no anti-grav invention, the scientists needed to believe it had been invented so they would think 'outside the square'. 
Anyone know who wrote the short story, and has anyone got a copy?