[DennisP]

There's a claim that it's just basic physics of microwaves bouncing around. That one's nonsense; from the article, "Woodward likened explaining the results seen at NASA purely in terms of microwave pressure to arguing that you can accelerate a car by getting in the driver’s seat and pushing on the windshield."

As that suggests, the drive would violate conservation of momentum as we currently understand it. It would also violate conservation of energy. Because of relativity, there's no such thing as absolute velocity, so the thrust can't vary by velocity. Since kinetic energy is proportional to the square of the velocity, but the acceleration is constant for a constant energy input, there's some velocity at which you're gaining more energy than you're putting in.

So if it does work, it'd be radically new physics. But there are various ideas for how it might work, like Woodward's, which is derived from general relativity and involves an explanation of inertia; he's been working for decades on reactionless drive experiments based on that idea. We don't currently have a generally-accepted explanation of inertia, so who knows.

[jwatte]

I rather like the "everything is quantized and this is a result of accumulated truncation errors" theory.