For some, the sound of a “perfect flow” might be the gentle lapping of a forest brook or perhaps the tinkling of water poured from a pitcher. For physicists, a perfect flow is more specific, referring to a fluid that flows with the smallest amount of friction, or viscosity, allowed by the laws of quantum mechanics. Such perfectly fluid behavior is rare in nature, but it is thought to occur in the cores of neutron stars and in the soupy plasma of the early universe.
Now MIT physicists have created a perfect fluid in the laboratory, and listened 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 analyzed thousands of sound waves traveling through this gas, to measure its “sound diffusion,” or how quickly sound dissipates in the gas, which is related directly to a material’s viscosity, or internal friction.
Surprisingly, 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. The results, published today in the journal Science, demonstrate the first time that scientists have been able to measure sound diffusion in a perfect fluid.
Scientists can now use the fluid as a model of other, more complicated perfect flows, to estimate the viscosity of the plasma in the early universe, as well as the quantum friction within neutron stars—properties that would otherwise be impossible to calculate. Scientists might even be able to approximately predict the sounds they make.
“It’s quite difficult to listen to a neutron star,” says Martin Zwierlein, the Thomas A. Franck Professor of Physics at 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.”
While a neutron star and the team’s gas differ widely in terms of their size and the speed at which sound travels through, from some rough calculations Zwierlein 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,” he adds.
Zwierlein’s co-authors are lead author Parth Patel, Zhenjie Yan, Biswaroop Mukherjee, Richard Fletcher, and Julian Struck of the MIT-Harvard Center