The laws of classical physics require a rocket's thrusters to push against something, creating acceleration by expelling matter (burned fuel) in the opposite direction.

Thus, so-called space drives -- engines that don't require fuel, but use alternative mechanisms to thwart the laws of physics and create thrust out of thin air (really thin space air) -- have always been considered "impossible."

But last week, scientists with NASA's Eagleworks Laboratories in Houston, Texas, presented a paper

at a conference in Cleveland, Ohio, that suggests the impossible may be possible. The scientists say they tested an engine that created a small amount of thrust without burning or expelling any traditional fuels -- the acceleration created by microwaves bouncing around inside.

More specifically, the engine musters up a bit of thrust by bouncing microwaves from one end to the other of an unevenly-shaped container, creating a difference in radiation pressure and generating drive. Though the engine generated only enough thrust to compel only the tiniest of movement, that any sort drive was created -- subverting one of the central laws of physics, "the conservation of momentum" -- is potentially revolutionary.

The experimental drive engine produced "a force that is not attributable to any classical electromagnetic phenomenon," the scientists claim in their paper.

But a number of science journalists and experts are understandably skeptical. Chris Lee of Ars Technica points to the fact that the experiment involved two test engines -- one that was designed to create thrust and another that was not. But both created thrust, meaning the experiments negative control also worked, calling into question the expected thrust-creating mechanism and the experiment overall.

"All in all," Lee concluded, "it will take a lot more information before we can judge whether the thrust is really a thrust or not."