In article <1119332552.800095.73010@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
> Chris L Peterson wrote:
>> There have been solar wind driven sails developed (on paper). But
>> AFAIK the purpose of this spacecraft is to test a photon pressure
>> driven sail.
> OK, color me dense. If photons are massless, how can they be
> perturbed by gravity (one of the tests of Einstein's theories)
Because perturbation by gravity is independent on the mass of
the perturbed object - and there's no exception for zero mass.
Btw it's only the rest mass of the photons which is zero. But
photons are never at rest -- they always travel at precisely the
speed of light. As you may recall from textbooks in special
relativity, the moving mass of an object travelling at the speed
of light is its rest mass multiplied by infinity -- that's why
objects with non-zero rest mass can never get accelerated all the
way to the speed of light: their masses would become infinite,
and it would thus require an infinite amount of energy to accelerate
them all the way to the speed of light.
Photons have zeo rest mass though, so their "moving mass" becomes
zero multiplied by infinity, which is an indefinite number, i.e.
it can have any value. Therefore, photons appear with a lot of
different "moving masses", or, as we prefer to express it, a lot
of different energies (and, through Plank's law, a lot of different