Hello PF members, I was wondering whether newtons third law was exclusive to spacecraft thrust; space being empty and without enough particles for a force to push against. That in order to produce thrust in space, you would need to carry the fuel that you plan to eject with you (disregarding the solar sail; good luck if it gets punctured during a microscopic meteor shower). Is this notion correct or are there other methods of space flight. I had an idea some time ago, that instead of ejecting fuel out of an engine, you built a device which emitted vibrations in a certain direction much like a stereo speaker. would something like this work in space, or are there not enough particles in space to generate a vibration?