[mikeash]

That makes it a photon rocket, which is nothing new. It's a logical consequence of the fact that light carries momentum, which is part of Maxwell's electromagnetic theory from 150 years ago. It's not very practical unless you have an extremely dense power source (nuclear fission won't cut it, you need fusion or antimatter) so it's more of a theoretical toy at this point.

What's interesting about the EmDrive's claims is that it supposedly produces far more thrust for the power that goes into it than would be possible from a photon rocket.

(The Pioneer anomaly wasn't because nobody realized that thermal radiation would act as a photon rocket, but rather that the effect wasn't correctly calculated. The theory is pretty simple, but it gets complex when you apply it to a real spacecraft with complex shapes, non-blackbody materials, and temperature gradients.)