Originally it was thought it was due to the momentum of photons, reflected off the silver surface resulting in an impulse of 2mv, and absorbed on the black surface resulting in an impulse of mv, so there was a net rotating force. This does occur, but produces insufficient torque at the light intensities used to overcome the friction. Furthermore, this explanation would cause it to rotate in the opposite direction to that observed. Studies showed the effect was maximum at a gas pressure of 7.5×10^−3 Torr, and it is caused by the black face of the paddles being heated to a greater degree than the polished silver faces. When air molecules hit the black faces they are bumped by the fast vibrating surface atoms, and bounce off with a greater velocity than the molecules colliding with the silver surface's slow vibrating surface atoms. Since every reaction has an equal and opposite reaction, a greater force is produced against the black surface than the silver surface. The reason there is a narrow gas pressure range, under which the effect is observed, is that the aerodynamic drag increases with pressure preventing perceived rotation at higher pressures, and the effect is completely lost under hard vacuum due to insufficient numbers of colliding air molecules.
That looks like a good place for it to sitting.I remember reading about them in university. But I still do not quite understand what causes the plates to spin. Well photographed. V