Scientists have captured the sound of a “perfect liquid” that flows with the least amount of friction permitted by the laws of quantum mechanics. Photo credit: Christine Daniloff, MIT
The results are intended to help scientists study the viscosity in neutron stars plasma of the early universe and other strongly interacting fluids.
For some, the sound of a “perfect river” could be the gentle lapping of a forest stream, or perhaps the clink of water poured from a jug. For physicists, a perfect flow is more specific and refers to a liquid that flows with the least 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 WITH Physicists made a perfect liquid in the laboratory and found that it sounds something 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 pitches that can be heard are the specific 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 to determine how quickly sound dissolves in the gas, which is directly related to the viscosity or internal friction of a material.
Surprisingly, they found that the sound diffusion of the liquid was so low that it could be described by “quantum” friction, which is given by a natural constant known as Planck’s constant and the mass of the individual fermions in the liquid.
This basic value confirmed that the strongly interacting fermion gas behaves like a perfect liquid and is universal in nature. The results, published today in the journal scienceshow for the first time that scientists were able to measure sound diffusion in a perfect liquid.
Scientists can now use the fluid as a model for 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 may even be able to roughly predict the sounds they will make.
“It’s quite difficult to hear a Neutron star“, Says Martin Zwierlein, Thomas A. Frank Professor of Physics at MIT. “But now you could mimick it in a laboratory with 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 greatly in terms of their size and the speed at which the sound travels through, Zwierlein estimates after some rough calculations that the resonance frequencies of the star would be similar to those of the gas and even audible – “if you could Close your ear without being torn apart by gravity, ”he adds.
Zwierlein’s co-authors are the lead author Parth Patel, Zhenjie Yan, Biswaroop Mukherjee, Richard Fletcher and Julian Struck from the MIT-Harvard Center for Ultracold Atoms.
Typing, listening, learning
To create a perfect liquid in the laboratory, Zwierlein’s team created a gas from strongly interacting 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 half-integer spin, a property that prevents one fermion from adopting the same spin as another nearby fermion. This exclusive nature enables the variety of atomic structures in the periodic table of the elements.
“If electrons weren’t fermions, but happy to be in the same state, hydrogen, helium and all atoms and we ourselves would look the same like a terrible, boring soup,” says Zwierlein.
Fermions, of course, prefer to be separate from one another. However, when they interact strongly, 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 capture a gas made up of lithium-6 atoms, known as fermions.
The researchers precisely configured the lasers so that they form an optical box around the fermion gas. The lasers were tuned so that whenever the fermions hit the edges of the box, they bounced back into the gas. In addition, the interactions between fermions were controlled as tightly as quantum mechanics would allow, so that fermions had to collide with each other at every encounter within the box. This made the fermions a perfect liquid.
“We had to produce a liquid with a uniform density, and only then could we tap 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.”
“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 create sonic vibrations through the liquid at specific frequencies. They recorded thousands of snapshots of the liquid as each sound wave passed through.
“All of these snapshots together give us a sonogram, and it’s a bit like what is done during an ultrasound examination in the doctor’s office,” says Zwierlein.
In the end, they could watch the density of the liquid ripple in response to any type of sound wave. Then they looked for the frequencies of sound that would create a resonance or amplified sound in the liquid, much like singing over a wine glass and finding the frequency at which it breaks.
“The quality of the resonances tells me something about the viscosity or sound diffusivity of the liquid,” explains Zwierlein. “When a liquid is low in viscosity, it can build up a very strong sound wave and be very loud if hit at just the right frequency. If it is a very viscous liquid, it will not have good resonances. ”
From their data, the researchers observed clear resonances through the liquid, especially at low frequencies. From the distribution of these resonances, they calculated the sound diffusion of the liquid. They found that this value could also be calculated very easily using Planck’s constant and the mass of the average fermion in the gas.
This told the researchers that the gas was a perfect liquid and of a fundamental nature: its sound diffusion, and therefore its viscosity, were at the lowest possible limit set by quantum mechanics.
Zwierlein says the results not only use the results to estimate quantum friction in more exotic matter such as neutron stars, but can also be helpful in understanding how certain materials can exhibit perfect superconducting flux.
“This work is directly linked to the durability of materials,” says Zwierlein. “After we have found out what the lowest resistance a gas can have, we learn what can happen to electrons in materials and how to make materials in which electrons can flow perfectly. That is exciting.”
Reference: “Universal sound diffusion in a strongly interacting Fermigas” by Parth B. Patel, Zhenjie Yan, Biswaroop Mukherjee, Richard J. Fletcher, Julian Struck and Martin W. Zwierlein, December 4, 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 Office of Scientific Research, the Office of Naval Research, and the David and Lucile Packard Foundation.
