You Can Now Listen To The Sound Of A “Perfect" Fluid

Christine Daniloff, MIT

Physicists have measured for the first time how soundwaves travel through the closest thing we have to a "perfect" fluid, an ideal case where a fluid is defined by its density and pressure alone. This could help researchers to study and better understand extreme environments such as the interior of neutron stars and the first few fractions of a second after the Big Bang.

You may think of fluid as only a liquid, but a fluid is any substance that conforms to its container (a study into whether cats are liquid won the Ig Nobel Prize in 2017), including gases and plasma. All three fluid states experience friction in their layers; the greater the friction, the thicker or more viscous the fluid.

Reporting in Science, the team from MIT created a perfect fluid using a Fermi gas. This is a particular state of matter where every constituting element is fermions, a particular class of particles that include electrons, protons, and neutrons. Fermions do not like to interact with each other due to quantum mechanical laws, but it is possible to force them to interact.

When this happens, the resulting fluid has very low viscosity, or the smallest amount of friction, one of the crucial steps toward what is known in physics as a perfect fluid. The team used strongly interacting fermionic atoms and created an optical box trapping the gas by using lasers. If an atom moved towards one of the sides of the box, the light from the laser would bounce it back in.

To create the soundwaves the team simply varied the brightness of one of the sides of the box. This generated mechanical waves through the gas akin to sound. They recorded thousands of these snapshots allowing them to understand which soundwaves propagated best, the resonance frequencies.

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“All these snapshots together give us a sonogram, and it’s a bit like what’s done when taking an ultrasound at the doctor’s office,” senior author Professor Martin Zwierlein said in a statement.

“The quality of the resonances tells me about the fluid’s viscosity or sound diffusivity. If a fluid has low viscosity, it can build up a very strong sound wave and be very loud, if hit at just the right frequency. If it’s a very viscous fluid, then it doesn’t have any good resonances.”

The distribution of the resonance frequencies gives them a way to calculate the fluid's sound diffusion. This is where things got very interesting. The same value is found using the mass of the individual atoms and the Planck Constant, one of the fundamental physical constants crucial to quantum mechanics. The properties of the fluid were only dependent on the limits imposed by quantum mechanics. This gas was as perfect fluid as can get it.

“This work connects directly to resistance in materials,” Zwierlein added. “Having figured out what’s the lowest resistance you could have from a gas tells us what can happen with electrons in materials, and how one might make materials where electrons could flow in a perfect way. That’s exciting.”

The reproduction of the sound is certainly something: a bit old-school sci-fi and a bit like an alarm. And it definitely needs the full explanation to be appreciated. 

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