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For the first time, physicists have recorded sound waves moving through a perfect fluid with the lowest possible viscosity, 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 most extreme conditions in the Universe: the interior of ultra-dense neutron stars and the quark-gluon plasma “soup” that filled the Universe in the years immediately following the Big Bang.
“It is quite difficult to hear a neutron star,” said MIT physicist Martin Zwierlein.
“But now you could imitate it in a laboratory using atoms, shake that atomic soup and listen to it, and know what a neutron star would sound like.” (You can listen to the recording here.)
Fluids comprise a series of states of matter. Most people probably regard them as liquids, but a fluid is any substance that is incompressible and conforms to the shape of its container: gas and plasma are also fluids.
All three of these fluid states – liquid, gas, and plasma – experience internal friction between the layers of the fluid, which creates viscosity or thickness. Honey, for example, is highly viscous. Water is less viscous. In super cooled liquid helium, a fraction of the fluid becomes a zero viscosity superfluid.
A perfect fluid, according to quantum mechanics, is the one with the lowest possible friction and viscosity, which can be described with equations based on the mass of the average fermion particle of which it is composed and a fundamental constant of physics called Planck’s constant.
And since a fluid’s viscosity can be measured by how sound dissipates through it – a property called sound diffusion – a team of researchers devised an experiment to propagate sound waves through a fluid of fermion particles to determine viscosity.
Fermions are a class of particles that include the building blocks of atoms, such as electrons and quarks, as well as particles that are made up of fermions, such as neutrons and protons, which are made up of three quarks.
Fermions are bound by the Pauli exclusion principle of quantum mechanics, which states that two of these particles in a system (such as an atom) cannot 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 whisker above absolute zero and squeeze them into a cage of lasers, and their quantum blur will have them thrust into waves that barely have any friction: the perfect fluid.
The experiment was to be designed to maximize the number of collisions between the fermions, and the lasers were tuned so that the fermions entering the boundaries bounced off the gas. This gas was kept at temperatures between 50 and 500 nanoKelvin (-273.15 degrees Celsius or -459.67 degrees Celsius).
“We had to produce a fluid with 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 “touch” the side of the container, the team varied the intensity of the light at one end of the cylindrical container. This, based on intensity, sent vibrations as different types of sound waves through the gas, which the team recorded through thousands of images, somewhat like ultrasound technology.
This allowed them to find ripples in the fluid’s density analogous to a sound wave. In particular, they sought acoustic resonances, an amplification in the sound wave 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 diffusivity of the sound,” Zwierlein said. “If a fluid has a low viscosity, it can create a very strong sound wave and be very loud if hit at the right frequency. If it’s a very viscous fluid, it doesn’t have good resonances.”
The researchers found very clear resonances in their gas, particularly at low frequencies. From these, they calculated the sound diffusion of the fluid. This was the same value that could have been derived from the fermion particle’s mass and Planck’s constant, indicating that the lithium-6 gas was actually behaving like a perfect fluid.
This has some pretty interesting implications. The insides of rotating neutron stars are also believed to be perfect fluids, although many orders of magnitude higher in temperature and density. They also have many modes of oscillation, in which sound waves propagate through the star.
We could use fluids like the team’s lithium-6 gas to understand the diffusivity of neutron stars, which in turn could lead to a better understanding of their interior and the gravitational wave signals generated by the merger of neutron stars.
And it could help scientists better understand superconductivity, where electrons can flow freely through materials.
“This work ties directly to strength in materials,” says Zwierlein. “Understanding what the lowest resistance you could have from a gas tells us what can happen with electrons in materials and how you could create materials where electrons could flow perfectly. It’s exciting.”
The research was published in Science.
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