Why does it not work to carry a laser and shine it on a light sail?

[Cato]

Reactionless drives are impossible. That means that a laser carried on a satellite would produce no thrust. Why, then, would shining that laser on a light sail not produce a net force? Using a fan to blow air on a sail does not work because the fan and the sail produce opposing forces. But that's not the case when using a laser. A 2018 thread discussing this subject has several contributors saying something like this: "You can create thrust with a laser (albeit inefficiently)...", but isn't that mistaken? A laser produces absolutely zero thrust. So what opposes the momentum transfer from light reflecting from the light sail?

 

[berkeman]

That means that a laser carried on a satellite would produce no thrust.

Why do you say that? The light from the laser source is leaving the satellite. What happens to that momentum?

 

[russ_watters]

Reactionless drives are impossible. That means that a laser carried on a satellite would produce no thrust.

No, that means it does produce thrust.

Why, then, would shining that laser on a light sail not produce a net force?

It does. Follow the momentum!

 

[Cato]

Maybe I'm wrong here. I thought the various types of reactionless drives were impossible.

Sample description of the impossibility of reactionless dives from Wikipedia:

A reactionless drive is a device producing motion without the exhaust of a propellant. A propellantless drive is not necessarily reactionless when it constitutes an open system interacting with external fields; but a reactionless drive is a particular case of a propellantless drive as it is a closed system presumably in contradiction with the law of conservation of momentum and often considered similar to a perpetual motion machine.[1] The name comes from Newton's third law, which is usually expressed as, "for every action, there is an equal and opposite reaction." A large number of infeasible devices, such as the Dean drive, are a staple of science fiction particularly for space propulsion.

 

[Ibix]

Light carries momentum, so a laser is a kind of (very low thrust) reaction drive, with the light being the exhaust/propellant. This is not a reactionless drive.

 

[Vanadium 50]

Why do you even need a sail? Just shine a laser out the back end.

 

[Ibix]

...so shining a laser off a sail will work, although there's no advantage (if the laser is carried by the ship) compared to just turning the laser round and using it without reflection. (Edit: beaten to it by V50, I see).

You can (in principle) drive a sail with a fan on the back of the boat because the air bounces off the sail and goes backwards. The efficiency is incredibly poor because the stream of air from the fan interacts with the reflected air and the air tends to go sideways rather than backwards. Light doesn't interact with itself, so there isn't this problem with a laser-and-light-sail - so it'll work. It's just over-complex.

 

[Dale]

I thought the various types of reactionless drives were impossible.

They are, but shining a laser off the back of a ship is not a reactionless drive. It is a rocket with an exhaust velocity of c.

 

[Cato]

OK, thanks all. That explains it. Fundamental misunderstanding of just what a "reactionless drive" is.

 

[Mister T]

A beam of light moving in one direction has no mass, but it does have momentum. A purely relativistic concept.

 

[Cato]

OK, to continue in my nonunderstanding: In what way is shining a laser, which you all say does in fact produce some thrust, different from the "EM Drive", which 1) does not work and 2) is provably physically impossible? Both emit electromagnetic radiation in an attempt to produce thrust.

 

[Ibix]

The EM drive doesn't emit a beam of radiation. It just bounces radiation around inside itself which causes teams of unicorns to emerge from the vacuum and start pushing it. Or something. If it emitted a stream of radiation it would just be a photon rocket, which would be fine.

A fan may push a sail inefficiently. A fan in an airtight box achieves nothing.

 

[Cato]

Thanks. Case closed. I do appreciate all of your insights.

 

[Redbelly98]

...so shining a laser off a sail will work, although there's no advantage (if the laser is carried by the ship) compared to just turning the laser round and using it without reflection. (Edit: beaten to it by V50, I see).

In fact, there is a disadvantage. The sail would have to be a 100% reflective mirror to have the same thrust as just firing the laser beam away from the ship. Even if that were the case, you then have the extra mass of the sail that must be accelerated along with the rest of the ship.