A new theoretical study has revealed how sound waves transfer small amounts of mass as they travel. Angelo Esposito, Rafael Krichevsky and Alberto Nicolis at Columbia University in the US have calculated that the transfer occurs even when both quantum and relativistic effects are ignored. Their result implies that current interpretations of the properties of sound waves may need to be rethought.
Physicists had widely accepted that sound waves carry energy and momentum, but not mass. In 2018, however, Riccardo Penco at Carnegie Mellon University and Niciolis and made an astonishing discovery when observing particle-like sound waves (called phonons) propagating through superfluid helium, cooled close to absolute zero.
They found that the phonons moved in upward trajectories, against gravity. Contrary to classical models of sound waves, this implied that the phonons were coupled to gravity, allowing them to carry minuscule amounts of “negative effective gravitational mass” as they travelled.
Fresh point of view
Now, Nicolis and colleagues at Columbia have analysed this intriguing property through theoretical calculations of sound waves in solids and ordinary fluids. The team normally work on theories of particle physics and say that this expertise allowed them to approach the problem from a fresh point of view. They derived an equation relating the mass carried by a sound wave to the wave’s energy, the mass density of the material, and the speed of sound inside it.
True to Niciolis and Penco’s previous observation, the team’s equation showed that sound waves carry a negative mass, meaning they deplete mass as they travel. This also meant that sound waves must interact with Earth’s gravitational feel, moving upwards like a buoyant object in water.
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The result seems to imply that a small fraction of material travels in the opposite direction to the sound wave. While it is easy to imagine how this could occur in a gas or liquid, it is difficult to envision in a solid. Niciolis and colleagues suggest that waves of elastic compression could shift small amounts of matter in one direction, but more research would be needed to confirm this prediction.
Currently, physicists use linear models to study sound waves. This approximation is suitable for most applications, but Niciolis and colleagues believe that when more accurate results are needed, linear models should be adapted to account for mass transfer.
The team now hope to explore mass transfer in more detail in materials including Bose-Einstein condensates, and, on much larger scales, through the Earth as a result of earthquakes.
The research is described in Physical Review Letters.
