Comment Re:Possible explanations (Score 1) 416
AFAIK, the main theories bandied about for how a reaction-less drive might work come down to basically harnesses some sort of Mach's principle effect (e.g, Woodward effect) which would be analogous to using "friction" from fields (usually EM) setup by the rest of the universe and/or somehow exploit the creation of quantum vacuum virtual particles to supply some local reaction mass and using a form of magnetohydrodynamics for propulsion.
The physics loophole that they seem to exploit is that in our description of physics, not all vacuums are created equal (e.g., a vacuum in one frame inertial of reference is generally not actually a vacuum in another frame of reference when a vacuum is thought of as a volume of space where distant fields cancel each other out). This indirectly questions the nature of the frame of reference in the asymptotic limit of space from which we might define an "absolute" vacuum. You might also think of it as asserting that maybe there is actually an aether of some sort?
To provide a car analogy, people are suggesting that wiggling in an asymmetric way and effectively using vanishingly small amount of friction supplied by the rest of the universe can get you moving in one direction kind of like getting your car moving when it's stuck in snow with (almost) no traction. It doesn't take much traction to get you going in the right direction as long as you are wiggling the right way...