Physicists capture the sound of a “perfect” fluid

For some, the sound of a “perfect stream” might be the gentle lapping of a forest stream or perhaps the tinkling of water being poured from a pitcher. For physicists, a perfect flow is more specific, referring to a fluid that flows with the least amount of friction, or viscosity, allowed by the laws of quantum mechanics. Such perfectly fluid behavior is rare in nature, but is thought to occur in the nuclei of neutron stars and in the soupy plasma of the early universe.

Now MIT physicists have created a perfect fluid in the lab and have listened to how sound waves pass through it. The recording is the 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 the speed at which sound dissipates in the gas, which is directly related to a material’s viscosity, or internal friction.

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

This fundamental value confirmed that the strongly interacting fermion gas behaves like 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 the diffusion of sound in a perfect fluid.

Scientists can now use the fluid as a model for other more complicated perfect flows, to estimate the viscosity of plasma in the early universe, as well as the quantum friction within neutron stars – properties that would otherwise be impossible to calculate. Scientists may even be able to roughly predict the sounds they make.

“It is quite difficult to hear a neutron star,” says Martin Zwierlein, physics professor Thomas A. Franck at MIT. “But now you could imitate it in a laboratory using atoms, shake that atomic soup and listen to it, and know what a neutron star would sound like.”

While a neutron star and the team’s gas differ widely in terms of the size and speed at which sound travels, by some rough calculations Zwierlein estimates that the resonant frequencies of the star would be gas-like, and even audible – “if I could bring my ear closer 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 for Ultracold Atoms.

Touch, listen, learn

To create a perfect fluid in the laboratory, Zwierlein’s team generated a gas of strongly interacting fermions: elementary particles, such as electrons, protons and neutrons, which are considered the building blocks of all matter. A fermion is defined by its half-integer spin, a property that prevents a fermion from taking on the same spin as another nearby fermion. This unique nature is what allows for the diversity of atomic structures found in the periodic table of elements.

“If electrons weren’t fermions, but happy to be in the same state, hydrogen, helium and all atoms, and ourselves, would look the same, like a terrible and boring soup,” says Zwierlein.

Fermions naturally prefer to keep separate from each other. But when they are made to interact strongly, they can behave like a perfect fluid, with very low viscosity. To create such a perfect fluid, the researchers first used a laser system to trap a gas of lithium-6 atoms, which are considered fermions.

The researchers precisely configured the lasers to form an optical box around the fermion gas. The lasers have been tuned in such a way that whenever the fermions hit the edges of the box they bounce into the gas. Furthermore, the interactions between fermions were controlled to be as strong as quantum mechanics allowed, so that inside the box the fermions had to collide with each other at each encounter. This caused the fermions to turn into a perfect fluid.

“We had to produce a fluid with uniform density, and only then could we tap on one side, listen to the other side and learn from it,” says Zwierlein. “It was actually quite difficult to get to this place where we could use sound in this seemingly natural way.”

“Flow perfectly”

The team then sent sound waves through one side of the optical box by simply varying the brightness of one of the walls, to generate sound-like vibrations through the fluid at particular frequencies. They recorded thousands of snapshots of the fluid as each sound wave rippled.

“All these snapshots together give us a sonogram, and it’s kind of like what you do when you do an ultrasound in the doctor’s office,” says Zwierlein.

Eventually, they were able to see the density of the fluid sway in response to each type of sound wave. They then looked for sound frequencies that would generate a resonance, or amplified sound in the fluid, similar to singing to a glass of wine and finding the frequency with which it shatters.

“The quality of the resonances tells me about the viscosity of the fluid, or diffusivity of the sound,” explains Zwierlein. “If a fluid has a low viscosity, it can create a very strong sound wave and be very loud if hit at the right frequency. If it’s a very viscous fluid, it doesn’t have good resonances.”

From their data, the researchers observed clear resonances through the fluid, particularly at low frequencies. From the distribution of these resonances, they calculated the sound diffusion of the fluid. This value, they found, could also be calculated very simply from Planck’s constant and the mass of the mean fermion in the gas.

This told the researchers that the gas was a perfect fluid and of a fundamental nature: its sound diffusion, and therefore its viscosity, was at the lowest possible limit set by quantum mechanics.

Zwierlein says that in addition to using the results to estimate quantum friction in more exotic matter, such as neutron stars, the results can be useful for understanding how some materials could be made to show perfect superconducting flux.

“This work ties directly to strength in materials,” says Zwierlein. “Understanding what the lowest resistance you could have from a gas tells us what can happen with electrons in materials and how you could create materials where electrons could flow perfectly. It’s exciting.”

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This research was supported, in part, by the National Science Foundation and the NSF Center for Ultracold Atoms, the Air Force Office of Scientific Research, the Office of Naval Research, and the David and Lucile Packard Foundation.

Written by Jennifer Chu, MIT News Office