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The problem isn't that the drive uses photons to carry off momentum, but with the claim by the inventor that it DOESN'T work that way. He essentially claims the interior of the drive is a wave guide, and that forward moving photons in the wave guide have greater momentum by virtue of the spacecraft moving forward than photons moving backwards in the wave guide. This gives rise to a net greater momentum imparted to the front of the drive from reflected photons than the momentum imparted by backward moving photons bashing into the back of the drive, thus pushing the spacecraft forward. [In brief: photons don't "go out the back of the drive," but impart momentum by a currently questionable process.]