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Message: <blockquote data-quote="Pbartender" data-source="post: 5030503" data-attributes="member: 7533">It's called a radiometer.That's the conceptual problem you're having...  Collisions don't pull, they push.If you treat photons as particles, you can treat the problem as two different types of collision:On the white side, the photons are "ricocheting" off of the paddle.  Presuming an elastic collision in which kinetic energy is conserved, the massless reflected photon will retain practically all its energy and the paddle will gain virtually none.On the black side, the photons are being absorbed into the paddle.  Using the same inelastic model, the combined paddle plus absorbed photon have all the kinetic energy of the incoming photon.  While this doesn't translate to much momentum, since the energy of a single photon is small and the mass of the paddle is comparatively large, the paddle now has kinetic energy equal to that of the absorbed photon, and it the same direction as the photon was traveling (away from the source of light).So, you've effectively got nothing pushing on one side, and oodles of teeny tiny pushes on the other side.  Given the quantity of photons hitting the black side, it adds up to be enough to make the paddles move.</blockquote>