For every action, there’s an equal and opposite reaction. This formulation of Newton’s third law has two very important modern consequences: one, that there’s a physical quantity that’s always conserved in the Universe (momentum), and two, that the laws of physics are the same irrespective of your position in space. This has a huge slew of implications, including that if you want to power a device to change its motion, you need to push against something. This could be exhaust from a rocket, tires pushing against a road, train wheels on a rail-line or even photons reflected off a sail. The one thing that’s forbidden is a reactionless drive: an action without a reaction. That’s exactly what the