(ORDO NEWS) — For the first time, physicists have registered sound waves moving through an ideal fluid with the lowest possible viscosity, as allowed by the laws of quantum mechanics, an upward glissando of the frequencies at which the fluid resonates.
This study could help us understand some of the most extreme conditions in the universe – the bowels of superdense neutron stars and the quark-gluon plasma soup that filled the universe just after the Big Bang.
“Listening to a neutron star is quite difficult,” said physicist Martin Zwierlein of the Massachusetts Institute of Technology.
“But now you can simulate it in the laboratory using atoms, shake this atomic soup, listen to it and find out what a neutron star would sound like.
Fluids cover a range of states of matter. Most people probably think of them as liquids, but a liquid is any incompressible substance that matches the shape of its vessel: gases and plasma are also liquids.
All three states of a liquid – liquid, gas, and plasma – experience internal friction between layers of liquid, which creates viscosity or thickness. For example, honey is very viscous. The water is less viscous. In supercooled liquid helium, part of the liquid becomes a superfluid liquid with zero viscosity. But it doesn’t have to be a perfect liquid.
“Helium-3 is a Fermi gas, so one would think that it is very close to the situation we need. But instead, it turns out that helium-3 is very sticky, even when it becomes superfluid. Helium-3 is actually a weakly interacting Fermi gas, and it shows very high viscosity – even when it becomes superfluid, ”said Zwierlein.
“The viscosity of superfluid helium-3 is a thousand times the quantum limit!”
According to quantum mechanics, an ideal fluid is a fluid with the lowest possible friction and viscosity, which can be described by equations based on the mass of the average fermionic particle of which it is composed and a fundamental physical constant called Planck’s constant.
And because the viscosity of a liquid can be measured by how sound is scattered through it – a property called sound diffusion – a group of researchers devised an experiment to propagate sound waves through a fermionic particle to determine its viscosity.
Fermions are a class of particles that include the building blocks of atoms, such as electrons and quarks, and particles, such as neutrons and protons, made up of three quarks.
Fermions are bound by the Pauli quantum mechanical exclusion principle, which states that no two such particles in a system (for example, an atom) can be in the same quantum state. This means that they cannot occupy the same space.
Cool a bunch of fermions, such as 2 million lithium-6 atoms, to a level above absolute zero and place them in a cage of lasers, and quantum fuzziness will allow them to be pushed in waves that have almost no friction – an ideal fluid.
The experiment was to be designed to maximize the number of collisions between fermions and lasers tuned so that fermions passing through the boundaries bounce back into the gas. This gas was maintained at a temperature from -273.15 degrees Celsius to -459.67 degrees Celsius.
“We needed to create a fluid with a uniform density, and only then could we knock on one side, listen to the other side,” Zwierlein said. “It was actually quite difficult to get to this place where we could use sound in this seemingly natural way.”
To “tap” the side of the container, the team changed the intensity of the light at one end of the cylindrical container. This, depending on the intensity, sent vibrations through the gas, similar to the various types of sound waves that the team recorded with thousands of images – a kind of ultrasonic technology.
This allowed them to detect ripples in the density of the liquid, similar to a sound wave. In particular, they looked for acoustic resonances – the amplification of a sound wave that occurs when the frequency of the sound wave coincides with the frequency of the natural vibration of the environment.
“The quality of the resonances tells me about the viscosity of a liquid or the coefficient of sound propagation,” Zwierlein said. “If the liquid has a low viscosity, it can create a very strong sound wave and be very loud. If it is a very viscous liquid, then it does not have good resonances. ”
The researchers found very clear resonances in their gas, especially at low frequencies. From them, they calculated the propagation of sound in the liquid. This was the same value that could be obtained from the mass of a fermionic particle and Planck’s constant, indicating that lithium-6 gas does indeed behave like an ideal liquid.
This has some pretty interesting implications. The interiors of rotating neutron stars, although many orders of magnitude higher in temperature and density, are also considered ideal fluids. They also have many modes of vibration in which sound waves travel through the star.
We could use liquids such as lithium-6 gas to understand the diffusion coefficient of neutron stars, which in turn could lead to a better understanding of their interior and the gravitational wave signals generated by neutron star mergers.
And it could help scientists better understand superconductivity, in which electrons can freely flow through materials.
“This work is directly related to the strength of materials,” says Zwierlein. “Finding out what the lowest resistance a gas can have tells us what can happen to electrons in materials and how materials can be created in which electrons can flow in an ideal way. It’s exciting.”
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