In superfluid helium, a vortex is like a tornado in a bottle, but instead of water, it's a quantum liquid that flows without friction. Such a vortex doesn't dissipate on its own, so the question of where its energy goes has long remained open. Now physicists have seen the answer for the first time: the vortex emits a quantum of sound — a roton.
In the experiment, a single vortex was attached to a nanostring — a tiny tuning fork vibrating in liquid helium at 10 millikelvin (which is 30 times colder than deep space). When the vortex was sped up, at a certain speed it began to lose energy. The measured loss per cycle exactly matched the energy needed to create a roton. So the vortex doesn't just slow down; it emits a particle of sound.
This discovery closes a long-standing puzzle: how turbulence decays in a liquid without viscosity. In superfluid helium, there's no friction, so the classical mechanism (turning energy into heat) doesn't work. Instead, energy goes into creating rotons, and now this is confirmed by direct measurement.
Such a mechanism could also work in other quantum liquids, for example, in neutron stars, where matter is in a superfluid state. Understanding how vortices lose energy helps model the rotation of these stars and predict their behavior.
A roton is a quantum of sound with a 'minimum price': to create one, you need to expend energy, as if sound can't be quieter than a certain level.
In Larry Niven's novel 'The Integral Trees,' superfluid vortices in neutron stars drive the weather — now we know they can indeed lose energy through rotons.