[jiggawatts]

… which just heats up the sail.

To move, you want to absorb momentum, not energy.

[woleium]

Correct as per Newtonian physics, bit shit gets weird when you go quantum :)

[monocasa]

At the scale of individual particles, heat is movement.

[ianburrell]

Heat is random movement. Infrared photons go in random directions, not the direction wanting to go. Some go in the wrong direction.

Reflection means solar sail goes in direction you want to go.