On Thu, 14 Oct 2004, Mike Hord wrote: > That implies that laser light could be used as a propulsive force, > you just need something to push off of. The question I have is, > was the "equal and opposite" force pushing the laser itself down, > or were the photons the ones losing the energy. In the first case, > you could "zap" the Earth with a laser to push the spacecraft along > (seems unlikely), or hit the spacecraft with a laser. In the second > case, a large sail with a laser emitter being towed along behind it > could push the whole works forward. The trick with the laser and the mirror won't work as you think, but if you lose the mirror and just use the laser it should work (you can use the mirror for thrust vectoring). Producing thrust is really simple: take something with mass and toss it away from you. Do not look where it is going (you don't really care). Photons have mass so it works. It works with anything that has mass. The interesting part is that the efficiency is greatest when the speed of what you toss is equal and opposite to your vehicle's speed. Iow, using a rocket to start from standstill or fly at low speed in general is extremely inefficient. So if a photon powered spaceship would approach c then its ejecta would approach standstill, i.e. frozen photons (think of it as extreme redshift). Then, would they still have mass ? Imho this is an interesting paradox (knowing that the ship would increase in mass as it would approach c). Peter _______________________________________________ http://www.piclist.com View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist