MIT researchers make major breakthrough; capture sound of 'perfect fluid'

The sound, which until now has only known to have been heard in neutron stars, makes this breakthrough a mammoth of an achievement

A team of physicists from the Massachusetts Institute of Technology on Friday successfully managed to capture the sound of a perfect fluid.

The sound, which until now was only known to have been heard in neutron stars, makes this breakthrough a mammoth of an achievement. The physicists, a team of six, a part of the MIT-Harvard Center for Ultracold Atoms published their findings in the journal 'Science'.

A perfect fluid is characterised by a perfect flow, which in simple terms, refers to a liquid that offers the least friction or viscosity to the flow of soundwaves. It offers the least heat or resistance to whatever pipe that it flows thorugh.

Speaking to MIT's news website, Martin Zwerlein expressed his elation and also the implications of his team's achievement. "It’s quite difficult to listen to a neutron star,” says Martin Zwierlein, the Thomas A. Frank Professor of Physics at MIT. “But now you could mimic it in a lab using atoms, shake that atomic soup and listen to it, and know how a neutron star would sound,” he said.

Massachusetts Institute of Technology (MIT) · MIT physicists find this is the sound of a perfect fluid

To do this, they first generated a gas of strongly interacting fermions. A fermion, like electrons and protons, is a type of elementary particle. They are, however, different from electrons and protons in that they have a half-integer spin, meaning that they assume a different state after a full spin, and that no two neighbouring fermions have the same spin, ensuring that they don't collide.

The researchers identified Lithium-6 atoms as suitable fermions and trapped them with the help of a laser system. The lasers form a box such that any of the atoms that try to exit it are bounced back in. They then induced the fermions to collide; interact with one another, and in the process form a perfect fluid.

“We had to make 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.”

They then sent sound waves from one side of the box and listened to it from another. This was repeated over a thousand times, varying the frequency and type of sound waves too, and in the process observing the different kinds of ripples that were produced. Zwerlein explained that the quality of the resonances gave them an idea of the fluid's viscosity or the sound's 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,” he said.

Their data pointed out that they were able to listen to clear resonances in the lowest of frequencies. This led them to infer and conclude that their gas was a perfect fluid and universal in nature. It also led them to believe that the sound diffusion, which is proportional to the fluid's viscosity, was at its lowest possible limit defined by quantum mechanics.

The scope is not solely limited to flashy neutron stars, however. “This work connects directly to resistance in materials,” Zwierlein says. “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,” he added.