Jones Beene wrote:
In the Crookes tube, the white side moves forward because it is colder but not because of any kind of "photon pressure" or counter effect, since photons are massless.
Eh ... hold on. They're massless but none the less they carry momentum, and like everything else that carries momentum, if they bounce off of something they'll transfer more momentum than if they "stick" (are absorbed). Momentum per photon has magnitude h nu, IIRC, give or take a factor of C.
If photons didn't carry momentum, solar sails, laser confinement fusion, and fission-fusion-fission bombs wouldn't work. (In the last two explosive ablation no doubt provides a big kick, but the main momentum transfer is from the radiation pressure ... or so I understand.)
However, there are two problems with light-pressure-as-mechanism in a Crook's tube.
First, a practical issue: A radiometer powered by photon pressure would spin the "wrong way" (thrust from the white side), which makes it clear that's not the dominant mechanism in the ones you can buy at your local Science Museum gift shop.
Second, a theoretical issue: The photon pressure on a small object due to ordinary sunlight is entirely negligible.
As to the warm-side cold-side explanation -- I've been led to believe that a careful analysis leads to the conclusion that, in a large sheet with one black side and one white side, there won't be any pressure difference. Supposedly the "thrust" in a radiometer is entirely due to edge effects; the gas at the edge of the vane is given a boost in the direction of the black side. The gas in the "middle" of each vane, on the other hand, doesn't do anything for it. I can't claim to have analyzed it myself so I don't know if this is actually correct.
If that last claim is correct, then you might be able to make a "faster" radiometer by either giving it sawtooth edges on its vanes, or by punching a lot of little holes in the vanes.
