[Baron Munchausen]

Originally posted by Wanderer:
A photon in a laser-bLast (for example) may not have any mass but it still has momentum. Beacuse of conservation of momentum you'll get recoil.

EDIT: For example, consider how a comet's tail always faces away from the sun because of the solar wind (i.e. light) pushing the recently-melted ice particles directly away from the sun.

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Actually, 'solar wind' really is a wind. There is a stream of super-hot gas coming from the sun and streaming out into deep space at all times. It's very, very thin compared to the atmosphere that we live in but it still exerts far more force than light. This is the main reason for comet tails.

There was a discussion thread here about solar sails some time back that covered the two different types, too. The shiny/silvery ones are indeed relying on light, but there is also a 'magnetic' sail in development that uses the pressure of solar wind on a cloud of plasma. This has the double advantage of not being degraded by micro-meteor punctures and not losing effectiveness with range from the sun! The density of light is of course reduced at a considerable rate as you get further from the sun (hmm, there's an exact formula that I've spaced). The solar wind also gets less with greater distance from the sun, but the magnetic sail expands as you get further away. It reaches it's 'equilibrium point' based on the pressure of the solar wind itself and produces the same force no matter where you are in the solar system! It would only become ineffective at the 'heliopause' where the solar wind fails completely and you hit interstellar space (which is far beyond pluto).

[ December 08, 2002, 19:27: Message edited by: Baron Munchausen ]