system
Given the following post, respond as if you're commenting on Reddit:
human
If a laser can impart momentum to a light sail, does it mean it has recoil?

Using space-based lasers to accelerate (or decelerate) ships is a near-future tech that's often taken as granted by futurists.

It sure sounds awesome, but if it can impart momentum doesn't that mean that it will be submitted to some sort of recoil? Would they need to carry chemical fuel or have a backwards-shooting laser to compensate?

If so, is it a significant challenge to manage, or pretty easy once you already have high-power lasers in space?
gpt
Yes, a space-based laser being shot at a light sail would gain exactly as much momentum (but in the opposite direction) as the momentum of the light it sends out. This is true of any photon source; the momentum is in most cases completely negligible, but here we're talking a laser capable of pushing a light sail, so presumably it's very powerful (or over a long period of time).

Laser + light-sail setups are not widely regarded as practical for a number of reasons, therefore there's no single consistent approach to how they would be (hypothetically) set up. The solution to "recoil" would depend on the setup. You could use a sufficiently massive laser base that the momentum it gains is negligible - in the extreme case you just stick it onto an asteroid or moon, blurring the line between a "space laser" and a "ground laser". You could use some alternate kind of thrusters, including chemical thrusters, to compensate.