Crookes Radiometer

Crookes Radiometer
No motor. No air inlet. Yet the rotor spins in a partial vacuum

This is a Crookes radiometer, invented in 1873 by Sir William Crookes, a chemist after finding that during very accurate weighings of samples in partially evacuated chambers, the weight was disturbed when sunlight shone on the balance.

Each of the rotors has one white (or polished) and one black (or dull) side and is attached to a low friction spindle. Typically it operates best around 1 pascal (atmospheric pressure is 101,325 pascals).
It is not powered from the momentum of light photons hitting the panes* given it will not work in a hard vacuum. In fact it would spin in the opposite direction** if this were the case! 

But what exactly is going on?
Two theories explain what is happening here:

1. Einstein’s theory: The heat of the black sided pane causes the gas molecules to gain momentum, bouncing against the pane faster. Although, this deters other molecules from coming closer which would cancel it out. Einstein theorized that the pressures don’t cancel out on the edges of the plane due to the temperature difference.

2. Maxwell / Reynold’s theory: If a porous plate is hotter on one side interactions between the plate and the gas molecules allow the gas to flow to the hotter side. The plates are not porous but the space past the edges acts like pores. 

*Which exists although it is incredibly small, there have in fact been nano-scale light mills produced that operate on this principle.
More about this here:
http://phys.org/news197555841.html

**Given light reflects better off the white surfaces depositing momentum. Light actually does have momentum despite having no mass.
More about this:
http://van.physics.illinois.edu/qa/listing.php?id=1424

Sources and further reading:
http://www.colin-baxter.com/academic/bib/downloads/worrall82.pdf
http://en.wikipedia.org/wiki/Crookes_radiometer