Physics

Physicists caught sound moving at two different speeds in 3D quantum gas

After previously studying the phenomena of two sound waves in quantum liquids, scientists have now observed sound moving at two different speeds in a quantum gas.

If you were somehow immersed in the three-dimensional gas used for this study, you would hear every sound twice: each individual sound carried by two different sound waves moving at two different speeds.

This is an important development in the field of superfluidity – fluids with no viscosity that can flow without any loss of energy.

Remarkably, the behavior observed in the gas in terms of densities and velocities matched the parameters set down by Landau’s two-fluid model, a theory developed for superfluid helium in the 1940s. To a large extent, it seems that when it comes to quantum gas setups, the same rules apply.

“These observations demonstrate all the key features of the two-fluid theory for a highly compressible gas,” write the researchers in their published paper.

We would say do not try this at home, but we doubt you would be able to: in this experiment, the scientists cooled a gas of potassium atoms down to less than a millionth of a degree above absolute zero, trapping the atoms in a vacuum chamber.

This partly formed what is known as a Bose-Einstein condensate, where there is so little energy that the atoms are barely moving or interacting. The interactions were then artificially increased so that the gas became hydrodynamic – in other words, more like a fluid.

But as the Bose-Einstein condensate still maintained a high compressibility – the same as air – it was still a gas. Rather than two liquids with slightly different properties, the setup created a condensed and a non-condensed gas in one, capable of transmitting two sound speeds.

“We observed both first and second sound in a 3D ultracold Bose gas that is sufficiently strongly interacting to be hydrodynamic, but is still highly compressible,” write the researchers.

“We found that Landau’s two-fluid theory captures all the essential features of this system, with the first and second sound mode, respectively, predominantly featuring oscillations of the normal and the superfluid component.”

Liquids and gases become quantum when they start exhibiting quantum mechanical properties – they start obeying a different set of laws compared with those that govern the classical physics of the Universe.

In this case, the quantum nature of the gas explains the pair of sounds – one a typical wave of compressed particles, the other, fluctuations in heat that act like particles. 

All of which feeds into our knowledge of quantum hydrodynamics, essentially the study of liquids in this quantum state.

The quantum realm is a difficult one to get your head around, and insights like this will be useful for future research and observations. 

As is often the case, this notable first – the first time sound has been shown moving at two different speeds in a quantum gas – will act as a springboard for other types of research and experiments in the years to come.

“The experimental access to both microscopic and hydrodynamic properties offers an excellent opportunity for further studies of Bose fluids. In particular, it would be interesting to explore lower temperatures,” write the researchers

The research has been published in Physical Review Letters.

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