Comment by jjtheblunt
16 hours ago
I had a radiometer when i was a kid ... is that momentum what makes it spin, because i thought the light was heating the black surfaces more than the white surfaces, so the air molecules bounced more vigorously away from the black.
i didn't think about photons having momentum when i was 8, though.
Photon momentum is a negligible factor in the case of a Crookes radiometer. On a typical vane, it amounts to nano-Newtons of force which wouldn’t move the vane, and certainly wouldn’t overcome friction. Your other explanation is roughly correct for a first approximation.
To make a radiometer that runs on photon momentum, you’d need much lighter vanes, probably larger, a harder vacuum, mirrored surfaces, a stronger source of photons, like a laser, and probably most importantly, some way to reduce friction - like a torsion balance.
NASA does solar sail experiments along these lines, e.g. at the Space Power Facility at Plum Brook Station, Ohio - see e.g. https://www.sciencedirect.com/science/article/abs/pii/S00945...