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.
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.
Difficult it might be, but such a measurement should still be possible. Esposito notes that to distinguish the phonons’ mass, one could look for them in a medium where the speed of sound was very slow. That might be possible in superfluid helium, where the speed of sound can drop to hundreds of meters per second or less, and the passage of a single phonon might shift an atom’s equivalent of material.
Alternatively, instead of seeking minuscule effects magnified by exotic substances, researchers might look for more obvious signs of mass-carrying phonons by closely studying extremely intense sound waves. Earthquakes offer one possibility, Esposito says. According to his calculations, a magnitude 9 temblor would release enough energy so that the resulting change in the gravitational acceleration of the earthquake’s sound wave might be measurable using atomic clocks. (Although current techniques are not sensitive enough to detect the gravitational field of a seismic wave, future advancements in technology might make this possible.)
Sound waves having mass are unlikely to have a major impact on day-to-day life, but the possibility something so fundamental has gone unnoticed for so long is intriguing. “Until this paper, it was thought that sound waves do not transport mass,” says Ira Rothstein from Carnegie Mellon University, who was not involved in this research. “So in that sense it’s a really remarkable result. Because anytime you find any new result in classical physics, given that it’s been around since Newton, you would have thought it would be completely understood. If you look carefully enough, you can find fresh [ideas] even in fields which have been covered for centuries.”
As for why this has never been spotted before, Esposito is uncertain. “Maybe because we are high-energy physicists, gravity is more our language,” he says. “It’s not some theoretical mumbo jumbo kind of thing. In principle people could have discovered it years ago.”