For some, the sound of a “perfect flow” might be the soft lapping of a stream in the woods or perhaps the clink of water poured from a jug. 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. This perfectly fluid behavior is rare in nature, but is believed to occur in the cores of neutron stars and in the thick plasma of the early universe.
Now physicists at MIT have created a perfect fluid in the lab and discovered that it sounds like this:
This 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 tones that can be heard are the particular frequencies at which the gas resonates like a plucked string.
The researchers analyzed thousands of sound waves that travel through this gas, to measure its “sound diffusion” or how quickly sound dissipates in the gas, which is directly related to the viscosity or internal friction of a material. .
Surprisingly, they found that the sound diffusion from the fluid 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 like a perfect fluid and is universal in nature. The results, published in the magazine Sciences, 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 could even roughly predict the sounds they make.
“It is quite difficult to hear a neutron star,” says Martin Zwierlein, Thomas A. Frank Professor of Physics at MIT in a statement. “But now you can mimic it in a lab using atoms, stir 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 their size and the speed at which sound travels, from some rough calculations, Zwierlein estimates that the star’s resonance frequencies would be similar to those of the gas, and even audible: “if the ear can be brought closer without gravity breaking it,” 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.
Play, 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 entire mean spin, a property that prevents a fermion from assuming the same spin as another nearby fermion. This unique nature is what makes possible 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 the atoms, and ourselves, we would look the same, like a terrible, boring soup,” says Zwierlein.
Fermions naturally prefer to be kept 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 system of lasers 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 adjusted in such a way that every time the fermions hit the edges of the box, they bounced off 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 every encounter. This made the fermions a perfect fluid.
“We had to make 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.”
“It flows in a perfect way”
The team then sent sound waves through one side of the optical box simply by 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 passed.
“All these snapshots together give us an ultrasound, and it’s a bit like what you do when you do an ultrasound in the doctor’s office,” says Zwierlein.
In the end, they were able to observe the ripple of the fluid’s density in response to each type of sound wave. They then looked for the frequencies of sound that generated a resonance, or amplified sound in the fluid, similar to singing in a wine glass and found the frequency at which it breaks.
“The quality of the resonances tells me about the viscosity of the fluid or the diffusivity of the sound,” explains Zwierlein. “If a fluid has low viscosity, it can generate a very strong sound wave and be very strong, if it is hit with the correct frequency. If it’s a very viscous fluid, then 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 discovered, could also be calculated very simply by using 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 matter, such as neutron stars, the results can be helpful in understanding how certain materials can be made to exhibit a perfect superconducting flux.
“This work connects directly to the strength of materials,” says Zwierlein. “Having figured out what is the lowest resistance a gas could have, he tells us what can happen to electrons in materials and how you can make materials where electrons flow perfectly. That’s exciting.”
SHARE this fascinating story with friends on social media …