Which can be done, because the sail is not just being pushed by the air, it's also redirecting the flow of air. The momentum of the fan pulling the air in does cancel any momentum imparted to the sail as the air pushes it, but there is also momentum gained by the air molecules rebounding back, away from the sail. It's (partially) the same reason you can sail perpendicular to the wind (the shape of the keel is also important), or sail faster than wind. Sailing is not air pushing on a sail, it's air rebounding away from the sail.

> Sailing is not air pushing on a sail, it's air rebounding away from the sail.

To add just a tiny amount of nuance - sailing is redirecting airflow with a sail.

Surface and flow effects mean that very little air should be bouncing off the sail. If there is you probably have a lot of turbulence, and a 'bubble' will form around the sail around which most of the wind will simply divert, rather than redirect.

Or to put it another way, think of an entire parcel of air rebounding away from the sail, not individual air molecules.

Right, a sail has more in common with a wing than a parachute. The force that is propelling the sailboat is basically lift—turned sideways—not collision.

Which, to bring it back to the origin of this thread of armchair physicists incorrecting each other, means that an LED mounted on a solar sail would not work, because the only force it imparts is collision. Which makes the sail analogy kind of a bad one for the use of solar pressure as a driving force.

Edit: Hmmm, but what if the surface material of the sail were phosphorescent and could gain energy state from the solar pressure, then kick it a photon when dropping energy state? And then what if there were a fly wheel (magnetic axel, in the vacuum of space, extremely low friction) that could act as the keel... Hmmmmmmm

Edit 2: I wonder if a lens that refracted light similarly to how air flows over an airfoil would create a "solar lift" effect? Unfortunately, lenses are heavy.

I mean, it's only the size of a boxing ring, not a football field. I think LEDs could produce enough light to have a net effect.

No. Think of the photons as producing little pushes. When they are emitted by the LED, they produce a little push in one direction, and then the sail pushes a little bit back in the other direction.

If it were possible to create a significant volume of thrust from photons emitted by LEDs, then you would be better off just pointing the LEDs away from where you wanted to go (like a rocket).

I don't know if the strength of an LED is enough to move the craft significantly, but if it is you'd need to point it at anything _but_ the ship itself, otherwise the two forces would cancel out. Just shine it in the opposite direction of where you want to go.

No, the two forces don't cancel out, I can't remember exactly why but elastic collisions don't work like that. That's why you can point a fan into your sailboat's sail and it will still go forward, mythbusters proved it.

Mythbusters demonstrated something different, and something much less efficient than simply pointing the fan backwards like a normal fan boat (no sail required). Here: https://mythresults.com/blow-your-own-sail