Photon rockets are not actually reactionless, because the emitted photons carry momentum and energy out of the system. This makes conservation laws to hold for

Photon rockets are not actually reactionless, because the emitted photons carry momentum and energy out of the system. This makes conservation laws to hold for such systems, cf. https://arxiv.org/abs/1506.00494 https://arxiv.org/abs/1506.00494 Now, the emdrive people are apparently claiming that it is truly reactionless. If so, you can build a perpetual motion machine of the first kind (produces energy from nothing) out of it --- which should demonstrate the degree of plausibility of such claims. For photon rockets, there's apparently a hard upper bound on the efficiency, which limits the practical usability.