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.
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