Starlight carries not only energy but momentum. Comet tails, for example, are the result of light blowing dust off the comet's core. It's not a lot of juice, but it's enough.
The force on a solar sail is gentle, if not feeble, but unlike a rocket, which fires for a few minutes at most, it is constant. Over days and years a big enough sail, say a mile on a side, could reach speeds of hundreds of thousands of miles an hour, fast enough to traverse the solar system in 5 years. Riding the beam from a powerful laser, a sail could even make the journey to another star system in 100 years, that is to say, a human lifespan.
We're pretty damned far off from a person riding the light to whatever planet Avatar is set on, but it's still a pretty neat idea. [NY Times]
