The results should help scientists study the viscosity of neutron stars, plasma of the early universe, and other fluids that interact strongly.
To some, the sound of a “perfect stream” may be the gentle tapping of a forest creek or perhaps the gurgling of water poured from a pitcher. For physicists, a perfect flow is more specific, referring to a fluid flowing with the least amount of friction, or viscosity, allowed by the laws of quantum mechanics. Such perfect liquid behavior is rare in nature, but is thought to occur in the nuclei of neutron stars and in the soup plasma of the early universe.
now with physicists have created a perfect fluid in the laboratory and discovered that it sounds something like this:
This 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 pits that can be heard are the specific frequencies at which the gas resonates like a broken wire.
The researchers analyzed thousands of sound waves traveling through this gas to measure its “sound propagation,” or how quickly sound is dispersed in the gas, which is directly related to the viscosity of a material or its 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 the Planck constant and the mass of individual fermions in the fluid.
This fundamental value confirmed that the strongly interacting fermion gas behaves like a perfect liquid and is of a universal nature. The results, published today in the journal science, demonstrate for the first time that scientists have been able to measure the propagation of sound in a perfect fluid.
Scientists can now use fluid as a model for other more perfect, more complex flows to estimate plasma viscosity in the early universe, as well as 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.
“Quite it is quite difficult to hear one neutron star“Says Martin Zwierlein, Professor of Physics at Thomas A. Frank at MIT. “But now you can imitate it in a lab using atoms, shake that atomic soup and listen to it, and know what a neutron star would sound like.”
While a neutron star and team gas vary greatly in size and the speed at which sound travels, by some approximate calculations Zwierlein estimates that the resonant frequencies of the star would be similar to those of gas, and even audible – “if you can bring your ear closer without being torn apart by gravity,” he adds.
Zwierlein co-authors lead author Parth Patel, Zhenjie Yan, Biswaroop Mukherjee, Richard Fletcher and Julian Struck of the MIT-Harvard Center for Ultracold Atoms.
Knock, 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 semi-complete rotation, a property that prevents a fermion from receiving the same rotation as another nearby fermion. This exclusive nature is what enables the variety of atomic structures found in the periodic table of elements.
“If electrons were not fermions, but happy to be in the same state, hydrogen, helium and all the atoms, and ourselves, would look the same, like a terrible, boring soup,” says Zwierlein.
Fermions naturally prefer to stay away from each other. But when they are made to interact vigorously, they can behave like a perfect liquid, with very low viscosity. To create such a perfect liquid, the researchers first used a laser system to block 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 tuned in such a way that whenever the fermions hit the edges of the box they turned back to gas. Also, the interactions between the fermions were controlled to be as strong as allowed by quantum mechanics, so inside the box, the fermions had to collide with each other at each encounter. This turned the fermions into a perfect liquid.
“We had to make a liquid of 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 the 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 changing the brightness of one of the walls, to generate sound-like vibrations through the fluid at specific frequencies. They recorded thousands of photographs of the fluid as each sound wave reproduced.
“All of these images together give us a sonogram and it’s more or less like what is done when you do an ultrasound in the doctor’s office,” Zwierlein says.
In the end, they were able to watch the fluid density ripple in response to each type of sound wave. They then searched for the frequencies of sound that generated a resonance, or an amplified sound in the liquid, similar to singing in a glass of wine, and finding the frequency at which it splits.
“The quality of the resonances tells me about the viscosity of the liquid or the diffusion of sound,” Zwierlein explains. “If a liquid has a low viscosity, it can build a very strong sound wave and be very high if it is struck at the right frequency. If it is a very thick liquid, then there is no good resonance. “
From their data, the researchers observed clear resonances through the fluid, especially at low frequencies. By scattering these resonances, they calculated the propagation of the sound of the fluid. This value, they discovered, could also be calculated very simply through the Planck constant and the average fermentation mass in gas.
This told the researchers that gas was a perfect liquid and of a fundamental nature: Its sound propagation, 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 in understanding how certain materials can be made to exhibit a perfect, superconducting flow.
“This work is directly related to material resistance,” says Zwierlein. “Understanding what is the lowest resistance you can have from a gas tells us what can happen to electrons in materials, and how one can make materials where electrons can flow in a perfect way. “This is exciting.”
Reference: “Universal propagation of sound in a strongly interacting Fermi gas” by Parth B. Patel, Zhenjie Yan, Biswaroop Mukherjee, Richard J. Fletcher, Julian Struck and Martin W. Zwierlein, 4 December 2020, science.
DOI: 10.1126 / science.aaz5756
This research was supported, in part, by the National Science Foundation and the NSF Center for Ultracold Atoms, the Air Force Bureau of Scientific Research, the Office of Naval Research, and the David and Lucile Packard Foundation.