Moved this question from the quantum physics section to this section, since... Well it fits this section better since electrons apparently do obey conservation of momentum in the 'classical' sense. Why not produce thrust in microgravity with electrons? Plenty of harnessable electricity in space. While I'm on it, I bet you're exponentially better at solid-state physics than I am, so I have no pressure of making a fool out of myself. ;) http://imgur.com/i9jWmFE Imagine the crudely presented structure in 3D (a metallic torus confined within a dielectric sarcofage, with holes left in it for electric current input + the photon-electron input-output/nozzle/whatever you want to call it. A direct current is fed onto the torus (which is confined within an electrostatically-repellent dielectric structure), while electrons are expelled off the inner rim of the torus with energetic-enough photons, giving them momentum in the direction of the movement vector of the photons. Could this kind of nanoscale device be effective (mass to thrust ratio wise) at producing thrust - or is this an obvious fluke somehow?