Perfect fluid? I like the sound of that

Physicists in the US say they have been able to measure sound diffusion in a “perfect fluid” for the first time.

That’s not just technically impressive. It could, they suggest, be used as a model for more complicated perfect flows to estimate the viscosity of the plasma in the early Universe and even the quantum friction within neutron stars.

“It’s quite difficult to listen to a neutron star,” says Martin Zwierlein from Massachusetts Institute of Technology (MIT), “but now you could mimic it in a lab using atoms, shake that atomic soup and listen to it, and know how a neutron star would sound.”

For physicists, “perfect flow” refers to a fluid that flows with the smallest amount of friction, or viscosity, allowed by the laws of quantum mechanics. It is rare in nature, but is thought to occur in the cores of neutron stars and in the early Universe.

In a paper in the journal Science, Zwierlein and MIT colleagues describe how they were able to create such a “perfect fluid” in the lab and listen to how sound waves travel through it.

The recording is a product of a glissando of sound waves that the team sent through a carefully controlled gas of elementary particles known as fermions. The pitches that can be heard are the particular frequencies at which the gas resonates like a plucked string.

The researchers analysed thousands of sound waves travelling through this gas, to measure its sound diffusion – how quickly sound dissipates in the gas – which is related directly to a material’s viscosity, or internal friction.

Surprisingly, they say, they found that the fluid’s sound diffusion was so low as to be described by a “quantum” amount of friction, given by a constant of nature known as Planck’s constant, and the mass of the individual fermions in the fluid.

This fundamental value confirmed that the strongly interacting fermion gas behaves as a perfect fluid, and is universal in nature.

While Zwierlein’s gas and a neutron star are very different, from some rough calculations he estimates that the star’s resonant frequencies would be similar to those of the gas, and even audible — “if you could get your ear close without being ripped apart by gravity”.

Closer to Earth, the results might also be helpful in understanding how certain materials could be made to exhibit perfect, superconducting flow.

“This work connects directly to resistance in materials,” Zwierlein says. “Having figured out what’s the lowest resistance you could have from a gas tells us what can happen with electrons in materials, and how one might make materials where electrons could flow in a perfect way. That’s exciting.”