"4B" wrote in message ...
What about those small propels that "mysteriously" spin when they are put
close to a source of light. Is not the momentum of the photons that make
them spin?

The usual setup for such a critter is a small "turbine" with,
most often, four blades. Each blade is black on one side and
silvery on the other. The "glib" explanation is that the silvery
side reflects light, and so gets double the push from each photon
that hits it.

The problem with the glib explanation is, the typical device of
this type that you can buy in a novelty store or such, turns the
wrong way. They spin such that there would appear to be more
push on the dark sides of the turbine than the light. Then you
find out that the bulb this thing is built in does not contain
a vacuum, but only a very thin gas.

See, what's really happening is, at least in the "novelty store"
version of this thing, the black sides are warmer. And warmer
means the gas next to them is warmer. And that means the gas
molecules move faster. So it's a little gas turbine, powered by
the energy of the light involved.

Let's think about the approximate force that would be involved
from light pressure. Say you've got bright sunlight, which is
roughly 1 killowatt/m^2. (Depends on lattitude, clouds, etc.,
but we are just doing an estimate here.) Suppose the fins were
2 mm across, giving an area of 4E-6 m^2. So you've got a power
of 4E-3 Watts. The force is power/c or a bit more than
1E-11 Newtons. What does the turbine in one of these gizmos mass?
Say it's .01 grams, or 100,000 of them to the kilo. Probably
masses more than that, but let's play. So you've got F/m to
give you the approx acceleration. You'd need to work out the
torque and know the shape of the silly thing to get it correct.
This will probably get us within a factor of 3 or so. So F/m
gives 1E-11/1E-5 or 1E-6m/s^2. So to get the thing going to
1 cm/s would take about 1E4 seconds, or about 3 hours. And that
is neglecting any friction. And my experience with these gadgets
is that they speed up very quickly in bright sun, and slow down
again fairly quickly when you put them in less light.

So it's probably not light momentum turning the fins.
Socks