Wikipedia describes solar sailing as
a form of spacecraft propulsion using a combination of light and high speed ejected gasses from a star to push large ultra-thin mirrors to high speeds.
I understand the part where ejected gasses bump into the sail pushing the spacecraft. On the other hand, I don't understand how light can do this, since light has no mass.
How does that work? Does this mean that if I have a mirror balancing on a needle I would be able to push it over with my flashlight?
Answer
I think you are really asking "how can light deliver an impulse to the sail". The answer is that although light has no mass it does carry momentum. When light is reflected off the sail, conservation of momentum requires that the sail changes momentum by twice the momentum of the light. The extra kinetic energy of the sail comes from the red shift of the reflected light.
This question has several answers that discuss the momentum of light in some detail.
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