Crookes radiometer
The light mill that fooled physicists. Drag the light source around the glass bulb and watch the vanes spin — black sides retreating from the light. The common explanation (radiation pressure pushes the shiny side) is wrong. The real mechanism is thermal transpiration: gas molecules near the hot black surface gain extra momentum, pushing the vanes away. Crucially, it only works at low — but non-zero — pressure.
The radiometer paradox
When William Crookes invented this device in 1873, he believed the vanes were driven by radiation pressure — photons bouncing off the shiny side should push it away, while photons absorbed by the black side deliver less momentum. If that were true, the shiny side would retreat from the light. But the vanes spin the other way: the black side retreats.
James Clerk Maxwell and Osborne Reynolds independently showed that the real mechanism involves the rarefied gas inside the bulb. The black surface absorbs more light and becomes hotter. Gas molecules near the hot edge gain kinetic energy and rebound with greater momentum, creating a net force that pushes the black side away from the light.
Thermal transpiration
More precisely, the effect is thermal transpiration (also called thermal creep). At the edges of each vane, there is a temperature gradient between the hot black side and the cool white side. Gas molecules flow along this gradient from cold to hot at the surface, creating a pressure differential that drives rotation. This mechanism was worked out by Reynolds and refined by Einstein in 1924.
The pressure sweet spot
At high vacuum (near zero pressure), there are too few gas molecules to transfer momentum — the vanes barely move. At atmospheric pressure, the gas is too dense: molecules collide with each other before reaching the other side of the vane, equalizing pressure. The radiometer works best at a low but non-zero pressure (around 0.01–1 Pa), where the mean free path of gas molecules is comparable to the vane size. This is why Crookes needed a partial vacuum.
Why radiation pressure fails
Radiation pressure is real but far too weak. Sunlight exerts about 4.6 µPa of pressure. To get measurable force, you would need a near-perfect vacuum. In any real Crookes radiometer with residual gas, thermal effects dominate radiation pressure by many orders of magnitude. If you could achieve a perfect vacuum, the shiny side would indeed be pushed — but you would need extraordinarily sensitive apparatus to detect it.