Surprising Discovery Hints Sonic Waves Carry Mass

jbmartin6 shares a report from Scientific American: In a paper published in Physical Review Letters, a group of scientists has theorized that sound waves possess mass, meaning sounds would be directly affected by gravity. They suggest phonons, particle-like collective excitations responsible for transporting sound waves across a medium, might exhibit a tiny amount of mass in a gravitational field. 'You would expect classical physics results like this one to have been known for a long time by now,' says Angelo Esposito from Columbia University, the lead author on the paper. 'It's something we stumbled upon almost by chance.'


Esposito and his colleagues built on a previous paper published last year, in which Alberto Nicolis of Columbia and Riccardo Penco from Carnegie Mellon University first suggested phonons could have mass in a superfluid. The latest study, however, shows this effect should hold true for other materials, too, including regular liquids and solids, and even air itself. And although the amount of mass carried by the phonons is expected to be tiny -- comparable with a hydrogen atom, about 10^-24 grams -- it may actually be measurable. Except, if you were to measure it, you would find something deeply counterintuitive: The mass of the phonons would be negative, meaning they would fall 'up.' Over time their trajectory would gradually move away from a gravitational source such as Earth. 'If their gravitational mass was positive, they would fall downward,' Penco says. 'Because their gravitational mass is negative, phonons fall upwards.' And the amount they would 'fall' is equally small, varying depending on the medium the phonon is traveling through. In water, where sound moves at 1.5 kilometers per second, the negative mass of the phonon would cause it to drift at about 1 degree per second. But this corresponds to a change of 1 degree over 15 kilometers, which would be exceedingly difficult to measure.

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