> Terry Blanton wrote: > >>> From: Harry Veeder >>> >>> >> >> >> >>> Solar sails work on a relatively simple principle -- that beams of light >>> bouncing off a reflective surface will transmit a push to the surface, >>> driving it forward. >>> >>> >> >> Are you sure of this?
Well, I did not write the news story so *I* am not sure of this. ;-) >> A photon's relative mass is given by the ratio of >> momentum to light speed. To impart momentum to the solar sail, how is this >> ratio altered? >> >> Stephen A. Lawrence wrote: > The ratio isn't altered. The sail is a mirror; when photons bounce off, > their momentum vectors "flip" -- they change direction by roughly 180 > degrees. The momentum of each photon changes from +p to -p when its > direction is reversed. That's a net change in momentum of -2*p, so the > sail must pick up momemtum of +2*p to balance it. > If that were precisely the case, the sail would have infinite mass and it would be impossible to accelerate it. In reality the wavelength (and consequently momentum) of the reflected photon is slightly less than the wavelength ( momentum ) of the incoming photon. Harry
