Motie "Light Sails"

Someone mentioned a Crookes radiometer which is a working example of a
light sail( though small and constrained to revolving in a vacuum
inside a glass bulb) . There's no doubt that light can and does cause
things to move. As I understand it if the vacuum is empty enough the
reflective side moves away from the light. However if the vacuum is
not sufficient the dark side moves away because of something called
thermal transpiration which involves interaction with gas molecules (
the dark side gets warmer and the gas molecules near the surface are
warmed too and their motion impacts the surface more strongly) . The
ones I've seen must have had sufficient vacuum as the reflective faces
moved away from the light source but I've seen radiometers advertised
that have the dark side move away as did Crookes original one. It's
the former that would be the effect used by a light sail. To claim it
can't work when most school science departments have Crookes
radiometers that do work seems a poor argument. ( A bit like the
claim I'd heard that says a dragonfly can't fly due to the bad
aerodynamics of it's wings and wing motions. Luckily dragonflies
haven't read that!) How and why reflected light imparts motion is
beyond my knowledge but I hope someone does build a light sail driven
space craft- No fuel for propulsion needed and even small
accelerations if continuous can build up to very high velocities.
Ken

Ken wrote:

 > Someone mentioned a Crookes radiometer which is a working example of a
 > light sail( though small and constrained to revolving in a vacuum
 > inside a glass bulb) .

It isn't. Radiometers rotate, but not due to photon pressure. If they
did, they'd rotate in the opposite direction. Photon pressure is real,
but it's much weaker than the thing that makes radiometers move: heat.

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