"Sound Wave" by Luis Lima89989 / CC BY-SA 3.0

Sound by the Pound: Surprising Discovery Hints Sonic Waves Carry Mass

Mar 11, 2019

By Jonathan O’Callaghan

You might be forgiven for thinking our understanding of classical physics had reached its peak in the four centuries since Isaac Newton devised his eponymous laws of motion. But surprising new research shows there are still secrets waiting to be found, hidden in plain sight—or, at least in this case, within earshot.

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, particlelike 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.

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