For those of you that dream of one day owning your very own X-34 Land Speeder, the Force is indeed with you, for there is new hope that such a levitating craft might actually be possible.
At first blush, the idea would seem to violate what we know about Newton's laws and the "equal and opposite reactions" that have historically driven traditional rocket engines. But according to this theoretical paper by Roger Sawyer, Einstein's rules of relativity can be exploited to generate thrust without a traditional propellant. The general idea is that the momentum transfers calculations between the trapped resonating microwaves must be done in the photons' frame of reference. When you work out the math around the truncated cone cavity shape, you end up with thrust in the cavity's frame of reference despite the fact that nothing is emerging from the cavity.
European scientists are generally skeptical, but the initial laboratory tests have demonstrated greater thrusts than the recently launched ESA Smart satellite's ion drive. Better yet, the so-called EM drive reached this performance level without requiring any fuel other than electricity to power a magnetron (a source of microwaves, common to any microwave oven), and all that from an engine that weighs about one tenth of even the very latest ion-engine alternative plus propellant payload.
Indeed, it sounds too good to be true, but NASA is taking a serious look. Here is a diagram from the article:
While a great deal of work would certainly remain in order to improve the new engine efficiency and output to levels useful on Earth, the prospect of having electronically controllable thrust without any emitted blast or heat is an awesome prospect that could change the face of transportation overnight.
If feasible, I would expect the principle challenges to be in managing high energy densities inside a resonant cavity without expansion, warping, slagging, or other mechanical failures that would ruin the Q (or peak resonance and energy storage capability) of the cavity.
Definitely worth a look.