How to make a sound wave spin? Hit it with a pipe

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It's a question I'm sure was keeping you up at night: can you make an object spin with a sound wave? The answer, generally speaking, used to be no. Now, though, mechanical engineers have taken a look at what their colleagues who play with lasers can do, and having seen the light, they copied it. And with that, spinning objects with sound waves has been achieved... but only in simulations.

Is it really that hard to make things spin?

To get an idea of why making things spin with sound waves is difficult, picture a tube that holds a turbine. Normally, to make the turbine spin, a fluid would flow past the blades of the turbine. The force of the fluid on the blades imparts a torque, which sets the turbine spinning. If we replace that flow with a pressure wave (like a sound wave), the fluid moves back and forth. So the local motion will first impart a torque that is clockwise and then one that is counterclockwise. The result is a rocking motion.

More fundamentally, the wave carries linear momentum but not angular momentum (specifically, it's orbital angular momentum, but we'll drop the "orbital"). Something that spins has angular momentum. In the turbine example, the total angular momentum cannot change. If the wave has no angular momentum and the turbine has no angular momentum, nothing will change.

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