Physicists Caught Sound Moving at Two Different Speeds in 3D Quantum Gas

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Scientists have detected sound travelling at two distinct speeds in a quantum gas, after previously researching the phenomenon of two sound waves in quantum liquids.

Every sound would be heard twice if you were submerged in the three-dimensional gas utilized in this study: each individual sound would be conveyed by two distinct sound waves flowing at two different speeds.

This is a significant advancement in the realm of superfluidity, which refers to fluids that have no viscosity and can flow without losing energy.

Surprisingly, the gas's behavior in terms of densities and velocities matched the parameters established by Landau's two-fluid model, a hypothesis created in the 1940s for superfluid helium. To a considerable extent, it appears that the same laws apply to quantum gas settings.

"These observations demonstrate all the key features of the two-fluid theory for a highly compressible gas," the researchers wrote in their study.

We'd warn don't attempt this at home, but we doubt you'd be able to: the scientists cooled a gas of potassium atoms to less than a millionth of a degree above absolute zero, trapping the atoms in a vacuum chamber.

This partially generated a Bose-Einstein condensate, in which the atoms are scarcely moving or interacting due to the lack of energy. The interactions were then intentionally intensified, causing the gas to become hydrodynamic, or fluid-like.

However, because the Bose-Einstein condensate had a high compressibility – the same as air – it remained a gas. Instead of two liquids with somewhat differing characteristics, the configuration combined a condensed and non-condensed gas into one that could transmit sound at two distinct 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." 

When liquids and gases begin to display quantum mechanical qualities, they begin to obey a distinct set of laws than those that govern the classical physics of the Universe.

The quantum structure of the gas explains the pair of noises in this situation - one is a standard wave of compressed particles, while the other is fluctuations in heat that act like particles.

All of this contributes to our understanding of quantum hydrodynamics, or the study of liquids in their quantum state.

This kind of understanding will be important for future study and observations in the quantum world, which is a challenging subject to grasp.

This remarkable first - the first time sound has been seen flowing at two distinct speeds in a quantum gas – will, as is often the case, serve as a springboard for future study and experimentation.

"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," the researchers write.