After studying the phenomena of two sound waves in quantum liquids, scientists have now observed sound traveling 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 each sound twice: each individual sound carried by two different sound waves traveling at two different speeds.
This is an important development in the field of superfluidity – viscosityless fluids that can flow without any loss of energy.
Remarkably, the observed behavior in the gas in terms of densities and velocities matched the parameters set by Landau’s two-fluid model, a theory developed for superfluid helium in the 1940s. When it comes to quantum gas configurations, the same rules apply.
“These observations demonstrate all the key features of the two-fluid theory for a highly compressible gas,” the researchers write in their published paper.
We’d say don’t try this at home, but we doubt you can: In this experiment, scientists cooled a gas of potassium atoms to less than a millionth of a degree above absolute zero, trapping atoms in a vacuum. bedroom.
This partly formed what is called a Bose-Einstein condensate, where there is so little energy that the atoms barely move or interact. The interactions were then artificially increased so that the gas became hydrodynamic, that is, more like a fluid.
But since Bose-Einstein condensate always maintained high compressibility – the same as air – it was still a gas. Rather than two liquids with slightly different properties, the configuration created a condensed and uncondensed gas in one, capable of transmitting two sound speeds.
“We observed both the first and second sound in a 3D ultracold Bose gas that interacts strongly enough to be hydrodynamic, but is still highly compressible,” the researchers write.
“We found that Landau’s two-fluid theory captures all the essential features of this system, with the first and second sound modes, respectively, exhibiting mainly oscillations of the normal and superfluid component.”
Liquids and gases become quantum when they begin to exhibit quantum mechanical properties – they begin to obey a different set of laws than that which governs 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 of heat that act like particles.
All of this feeds into our knowledge of quantum hydrodynamics, essentially the study of liquids in this quantum state.
The quantum realm is difficult to understand, and information like this will be useful for future research and observation.
As is often the case, this remarkable first – the first time that sound travels at two different speeds in a quantum gas – will serve as a springboard for other types of research and experiments in the years to come.
“Experimental access to microscopic and hydrodynamic properties provides an excellent opportunity for further studies of Bose fluids. In particular, it would be interesting to explore lower temperatures,” the researchers write.
The research has been published in Physical examination letters.