What is the incredible force that turns the Crookes radiometer?

The Crookes radiometer is a very interesting thing. It works, it seems, simply and the mechanism is obvious. A turntable is placed in a tightly sealed flask with a rarefied gas. If you shine a flashlight on the blades of the turntable, then it starts spinning. And here is the main question...Why?

😏 The question remained controversial for a long time and the physicist did not understand "why" themselves, but in the end there was some general opinion. Everyone immediately remembers Lebedev's experiments and light pressure. But that's not the effect here, and if the conversation was about light pressure, then we wouldn't call this thing a radiometer. The effect that makes the turntable spin is much more interesting. This is a radiometric effect.

The radiometric effect is the phenomenon of the movement of unevenly heated bodies placed in rarefied gases in the direction from the more heated side to the less heated side. The unevenness of heating is usually carried out by one-sided illumination of the body, which is why the name of the effect is connected. The forces that set the body in motion are called radiometric.

✅ Each blade has two sides. One with a dark coating, and the other with a light coating. The part with a dark coating will move away from the heat source as it heats up, and the part with a light coating moves towards the heat source. Cooling of the blades causes movement. This movement is connected with the uneven flow around the rarefied gas blade. About how to divide two streams of water with different intensities with a board.

Do not confuse this effect with the "solar sail" effect. In a solar sail, photons, when interacting with the material of the sail, transmit their momentum to it and the whole cuttlefish flies forward. Here the effect is different.