[Spaceships] Your preferences regarding plausible/playable Reactionless Drives

[Anthony]

Originally Posted by teviet

Regarding the tractor/pressor beam, this will violate local energy/momentum conservation unless the beam itself has significant rest mass in the frame of whatever you're pushing against.

Pressor beams don't require rest mass, they just require the same amount of energy as creating photon pressure between two mirrors (which is a special case of the photon rocket, and can theoretically be vastly more efficient than a conventional photon rocket). Tractor beams are messier, because what they really require is an energy debt of the same magnitude.