Could Phonons Have a Negative Mass?
Sound has negative mass, and all around you it's drifting up, up and away — albeit very slowly.
That's the conclusion of a paper submitted on July 23 to the preprint journal arXiv, and it shatters the conventional understanding that researchers have long had of sound waves: as massless ripples that zip through matter, giving molecules a shove but ultimately balancing any forward or upward motion with an equal and opposite downward motion. That's a straightforward model that will explain the behavior of sound in most circumstances, but it's not quite true, the new paper argues. [The Mysterious Physics of 7 Everyday Things]
A phonon — a particle-like unit of vibration that can describe sound at very small scales — has a very slight negative mass, and that means sound waves travel upward ever so slightly, said Rafael Krichevsky, a graduate student in physics at Columbia University.
Phonons aren't particles of the sort most people typically imagine, like atoms or molecules, said Krichevsky, who published the paper along with Angelo Esposito, a graduate student in physics at Columbia University, and Alberto Nicolis, an associate physics professor at Columbia.
When sound moves through air it vibrates the molecules around it, but that vibration can't be easily described by the movement of the molecules themselves, Krichevsky told Live Science in an email.
Instead, just as light waves can be described as photons, or a particles of light, phonons are a way to describe sound waves that emerge from the complicated interactions of the fluid molecules, Krichevsky said. No physical particle emerges, but researchers can use the mathematics of particles to describe it.
And it turns out, the researchers showed, these emergent phonons have a tiny mass — meaning that when gravity tugs on them, they move in the opposite direction.
"In a gravitational field phonons slowly accelerate in the opposite direction that you would expect, say, a brick to fall," Krichevsky said.
To understand how this might work, imagine a normal fluid in which gravity acts downward. Fluid particles will compress the particles below it, so that it's slightly denser lower down. Physicists already know that sound typically moves faster through denser media than through less-dense media — so the speed of sound above a phonon will be slower than the speed of sound through the slightly denser particles below it. That causes the phonon to "deflect" upward, Krichevsky said.
This process happens with large-scale sound waves, too, Krichevsky said. That includes every bit of sound that comes out of your mouth — albeit only very slightly. Over a long-enough distance, the sound of you saying "hello" would bend upward into the sky.
The effect is too tiny to measure with existing technology, the researchers wrote in the new paper, which has not been peer-reviewed.
But it's not impossible that, down the road, a very precise measurement could be made using super-precise clocks that would detect the slight curvature of a phonon's path. (The New Scientist suggested heavy-metal music would be a fun candidate for such an experiment in their original report on the subject.)
And there are real consequences to this discovery, the researcher wrote. In the dense cores of neutron stars, where sound waves move at nearly the speed of light, an anti-gravitational sound wave should have real effects on the whole star's behavior.
For now, though, this is entirely theoretical — something to ponder as sound falls upward all around us.
Originally published on Live Science.