In Uncle Al wrote:
"Geoffrey A. Landis" wrote:
Despite a recent article in New Scientist, a solar sail does not
break the laws of physics.
[snip]
Actually, it does as proposed. The sail will come into thermal
equilibrium with the radiation field and emit photons from its other
side, counter-thrusting.
Yes, but if reflectivity is R, the amount of photons absorbed is A=(1-R).
These are then radiated from both sides. The re-radiation contributes
no force. For a sail perpendicular to the sun, therefore, the force (
assuming thermal equilibrium) is
F = I (2R+A)/c
accounting for the fact that the reflected photons contribute momentum
once on arrival and once on departure, for a factor of 2, and the
absorbed ones contribute momentum only on arrival, not on departure.
...
--
Geoffrey A. Landis
http://www.sff.net/people/geoffrey.landis