In 1980, rocket engineer G. Harry Stine, writing under the pen name Lee Correy, published a quite good SF novel called “Star Driver“. In the story, an unemployed astronomer/pilot helps a small aerospace company invent and debug a “skyhook”: a device that converts energy into linear thrust without having anything to thrust against. Stine envisions it as a rod-shaped special alloy resonator fed by a careful synthesized waveform generator. The first prototype is built into a small plane that uses the device to attain record altitude.

Now there’s news of an invention which promises to do just that. (The article may not be available for long; there’s also a PDF explaining the theory of the device.) It’s basically a cone-shaped hollow resonator fed by a microwave generator. Current models generate up to 300mN of thrust – enough to levitate about one ounce of weight against gravity – from a 1KW generator. This sounds puny but is already better than current ion thrusters used in some space probes.

Roger Shawyer, the inventor, plans to modify superconductors now used in particle accelerators to make the thruster more efficient. He says 30KN per KW might be possible if the considerable practical obstacles can be overcome; this would be enough to levitate a 3-ton car.

The WikiPedia article lists many objections to the device’s theory, and it may turn out not to be practical or widely usable. Still, it’s always interesting to see how life can imitate fiction.

Update: Yes, I know that the device violates conservation of momentum; but I think it’d be cool that it might work by what Terry Pratchett calls “something quantum”, in spite of the obvious theoretical faults.