Dear 4B:

"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?

I'll let you do your own search. Those devices are called "radiometers".
If the envelope they are contained in contains any gas, then the black side
moves away from the light source. If there is no gas, the white side moves
away from the light source. I have never seen a radiometer with the white
side moving away from the light, so I have never seen a completely
evacuated one.

I wonder if any of the space missions ever took a non-enclosed radiometer
up with them?

David A. Smith