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For some, the sound of a “perfect flow” might be the gentle lapping of a forest stream or perhaps the tinkle of water poured from a pitcher. For physicists, 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 believed to occur in the nuclei of neutron stars and in the soupy plasma of the early universe.
Now physicists at MIT have created a perfect fluid in the lab and found it to look like this:
This recording is the product of a glissando of sound waves the team sent through a carefully controlled gas of elementary particles called 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 passing through this gas, to measure its “sound diffusion”, or the speed at which sound dissipates in the gas, which is directly related to the viscosity of a material or to friction. internal.
Surprisingly, they discovered that the sound diffusion of the fluid was so small that it could be described by a “quantum” amount of friction, given by a constant of nature known as the Planck constant, and the mass of the fermions. individual 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 sound diffusion in a perfect fluid.
Scientists can now use the fluid as a model of other, more complex, perfect flows to estimate the viscosity of plasma in the early universe, as well as quantum friction in neutron stars – properties that would otherwise be impossible to calculate. Scientists might even be able to roughly predict the sounds they make.
“It’s quite difficult to listen to a neutron star,” says Martin Zwierlein, Thomas A. Franck professor of physics at MIT. “But now you can 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 considerably in terms of the size and speed at which sound travels, Zwierlein estimates, based on some rough calculations, that the star’s resonant frequencies would be similar to gas, and even audible – “if you can get close to the ear without being torn by gravity,” he adds.
Zwierlein’s co-authors are lead author Parth Patel, Zhenjie Yan, Biswaroop Mukherjee, Richard Fletcher, and Julian Struck from the MIT-Harvard Center for Ultracold Atoms.
Tap, listen, learn
To create a perfect fluid in the lab, Zwierlein’s team generated a gas of strongly interactive fermions – elementary particles, such as electrons, protons, and neutrons, which are considered to be the building blocks of all matter. A fermion is defined by its full half-spin, a property that prevents a fermion from taking the same spin as another nearby fermion. This exclusive nature is what enables the diversity of atomic structures found in the Periodic Table of the Elements.
“If the electrons were not fermions, but happy to be in the same state, the hydrogen, helium and all the atoms, and ourselves, would look the same, like a terrible and boring soup” , Zwierlein says.
Fermions naturally prefer to keep themselves separate from each other. But when 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 were set so that whenever the fermions hit the edges of the box, they bounced back into the gas. Additionally, 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 encounter. This transformed the fermions into a perfect fluid.
“We had to make a fluid with a 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.”
“Flows 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 vibrations through the fluid at particular frequencies. They recorded thousands of snapshots of the fluid as each sound wave propagated.
“All of those snapshots put together gives us an ultrasound, and it’s kind of like what is done when taking an ultrasound at the doctor’s office,” Zwierlein says.
Ultimately, they were able to observe the density ripple of the fluid in response to each type of sound wave. They then looked for the sound frequencies that generated a resonance, or amplified sound in the fluid, akin to singing over a wine glass and finding the frequency at which it shatters.
“The quality of the resonances tells me about the viscosity of the fluid or the diffusivity of the sound,” Zwierlein explains. “If a fluid has a low viscosity, it can create a very loud sound wave and be very loud if hit at the right frequency. If it is a very viscous fluid then it does not have good resonances.
From their data, the researchers observed clear resonances through the fluid, especially 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 via Planck’s constant and the mass of the average fermion in the gas.
This told the researchers that the gas was a perfect fluid and fundamental in 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 materials, such as neutron stars, the results can be useful in understanding how certain materials might be caused to exhibit perfect superconducting flow.
“This work is directly related to the strength of the materials,” says Zwierlein. “Determining what the lowest resistance you could have from a gas tells us what can happen with electrons in materials, and how we can make materials where electrons could flow perfectly.” . That’s exciting.”
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.
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