For the first time, physicists have recorded the fluid sound of a ‘perfect’ fluid



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For the first time, physicists have recorded sound waves moving through a perfect fluid with the lowest viscosity possible, as allowed by the laws of quantum mechanics, an ascending glissando of the frequencies at which the fluid resonates.

This research can help us understand some of the Universe’s most extreme conditions – the interiors of ultra-dense neutron stars and the quark-gluon plasma “soup” that filled the Universe in the years after the Big Bang.

“It’s quite difficult to listen to a neutron star,” said physicist Martin Zwierlein of 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.” (You can listen to the recording here.)

Fluids encompass a range of states of matter. Most people probably think of them as liquids, but a fluid is any incompressible substance and conforms to the shape of its container: gases and plasmas are also fluids.

These three states of fluid – liquid, gas, and plasma – experience internal friction between the layers of the fluid, which creates viscosity or thickness. Honey, for example, is very viscous. The water is less viscous. In supercooled liquid helium, a fraction of the fluid becomes a superfluid of zero viscosity. But it’s still not necessarily a perfect fluid.

“Helium-3 is a Fermi gas, so you would think it’s close to the situation we have. But instead, it turns out that helium-3 is very sticky, even when “It becomes superfluid. Helium-3 is actually a weakly interactive Fermi system, and it displays very high viscosities – even if it becomes superfluid,” Zwierlein told ScienceAlert.

“The viscosity of superfluid helium-3 is a thousand times the quantum limit!”

A perfect fluid, according to quantum mechanics, is one with the lowest possible friction and viscosity, which can be described with equations based on the mass of the average fermionic particle of which it is composed, and a fundamental constant of physics called Planck’s constant.

And, because the viscosity of a fluid can be measured by how sound dissipates through it – a property called sound diffusion – a team of researchers designed an experiment to propagate sound waves through a fluid of fermionic particles. to determine its viscosity.

Fermions are a class of particles that include the building blocks of atoms, such as electrons and quarks, as well as particles made up of fermions, such as neutrons and protons, made up of three quarks.

Fermions are related by Pauli’s principle of exclusion of quantum mechanics, which states that none of these particles in a system (like an atom) can occupy the same quantum state. This means that they cannot occupy the same space as each other.

Cool a bunch of fermions, like 2 million lithium-6 atoms, down to a mustache above absolute zero and place them in a laser cage, and their quantum blur will allow them to jostle each other in waves. which barely have any friction – the perfect fluid.

The experiment had to be designed to maximize the number of collisions between the fermions, and the lasers were set so that the fermions running within the limits bounced back in the gas. This gas has been maintained at temperatures between 50 and 500 nanoKelvin (-273.15 degrees Celsius or -459.67 degrees Celsius).

“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,” Zwierlein said. “It was actually quite difficult to get to this place where we could use sound in this seemingly natural way.”

To “tap” on the side of the container, the team varied the light intensity at one end of the cylindrical container. This, depending on the intensity, sent vibrations like different types of sound waves through the gas, which the team recorded through thousands of images – much like ultrasound technology.

This allowed them to find ripples in the density of the fluid similar to a sound wave. In particular, they were looking for acoustic resonances – an amplification of the sound wave that is produced when the frequency of the sound wave matches the frequency of the natural vibration of the medium.

“The quality of the resonances tells me about the viscosity of the fluid or the sound diffusivity,” Zwierlein said. “If a fluid has a low viscosity, it can create a very loud sound wave and be very loud, if it is hit just at the right frequency. If it is a very viscous fluid, then it doesn’t. has no good resonances. “

The researchers found very sharp resonances in their gas, especially at low frequencies. From these, they calculated the sound diffusion of the fluid. This was the same value that could be derived from the mass of fermionic particles and Planck’s constant – indicating that lithium-6 gas indeed behaved like a perfect fluid.

This has some pretty interesting implications. The interiors of spinning neutron stars, although many orders of magnitude higher in temperature and density, are also considered perfect fluids. They also have many oscillation modes, in which sound waves propagate through the star.

We could use fluids such as the team’s lithium-6 gas to understand the diffusivity of neutron stars, which could, in turn, lead to a better understanding of their interiors and the gravitational wave signals generated by them. the fusion of neutron stars.

And it could help scientists better understand superconductivity, in which electrons can flow freely through materials.

“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 you can make materials where electrons could flow perfectly. That’s exciting.

The research was published in Science.

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