A group of scientists have written a paper that theorizes that sound waves possess mass, which would mean sound is affected by gravity. The paper suggests that phonons might have 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.”
Sounds fall up
The work of Esposito and his colleagues build from previous research that suggested phonos have mass in superfluids. The latest work takes this further, offering the idea that they could exhibit mass in other materials too, like regular liquids and solids, and even air itself.
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The actual mass of the phonons is likely to be very very small. But big enough to be measurable. However measuring it would reveal something really weird: the mass of the phonons would be negative, meaning instead of falling down, they would fall “up.” This “upwards” fall means they would 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.” Like positively massed objects the amount of “fall” depends on what material they are traveling through.
For example in water 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 broken down this drift is almost impossible to measure as it correlates to a change of just 1 degree over 15 kilometers.
How can we measure that?
A better way to measure the mass of sound might be in a medium where sound travels very slowly. Superfluid helium might do the trick, in such an environment the sound of speed can be as slow as hundreds of meters per second or less, and the passage of a single phonon might shift an atom’s equivalent of material.
The researchers suggest another way to try and measure the proposed mass might be to ignore being subtle and look for obvious signs of mass-carrying phonons by looking into extremely intense sound waves.
In this case, you’ll need a big sound creating an event, like an earthquake. According to Esposito’s calculations, a magnitude 9 temblor would release enough energy so that atom clocks could be used to measure the change in the gravitational acceleration of the earthquake’s sound wave.
However, this is a forward-looking dream as such sensitive technology does not exist yet. The understanding of soundwaves having mass is not likely to affect our daily lives too much but its recent discovery is puzzling to physicists but proves there is always something new to learn about the world around us.
Inspirational new discovery
“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.”