Heavy metal music isn’t heavy after all – it is actually the opposite. Sound waves have mass and can interact via gravity, but that mass is negative. In other words, sound floats upwards.
Angelo Esposito at Columbia University in New York and his colleagues calculated the relationship between sound and gravity, taking into account complicated particle interactions that had previously been ignored. They found that, although the effect is small, sound waves should have negative gravitational mass.
“It’s almost like antigravity,” says Ira Rothstein at Carnegie Mellon University in Pennsylvania. “The atoms that are moved by the wave are still being pulled down by Earth, but the sound wave itself is being repelled.”
Advertisement
Listen up
This means that as a sound wave moves, its path will bend upwards, away from the pull of gravity. “Heavy metal music, given a long enough time, is probably going to start floating in air, so it is probably not that heavy,” says Riccardo Penco at the University of Pennsylvania. Of course, the same is true no matter the genre of music.
In some sense this is a trick of mathematics, because a sound wave is simply an excitation of a medium and not a particle with a true mass, but the sound really is being affected by gravity.
“If you were to weigh a sound wave using a scale, it wouldn’t be mass in that sense,” says Walter Goldberger at Yale University. “But it is mass in the sense that this thing gravitates.”
The sound wave’s negative mass is not absolute, but rather relative to the density of the medium. That means that this negative mass can’t exist in a vacuum, where sound has no medium to travel in, which is handy because such a thing would break a number of laws of physics. “A negative potential in a vacuum would be rather insane since it would require truly negative masses,” says Esposito.
Performing an experiment to measure this predicted effect will be difficult, because observing it requires extremely strong gravitational fields or an extremely dense medium for the sound wave to travel through. It could be done using clouds of molecules cooled to near absolute zero, or an extremely strong sound wave like that created by a particularly powerful earthquake, Esposito says.
It could also be seen in exotic materials like superfluid helium, which has zero viscosity when chilled to near absolute zero. A sound wave travelling through this weird liquid in normal gravity would have a negative mass equivalent to the positive mass of a single helium atom. After travelling 100 metres, it would have floated upwards by about one metre.
There are other strange consequences of sound having negative mass. Just as two objects with positive masses will attract one another, two waves with negative masses will do the same. So if two people were screaming while facing the same direction, given enough time the two screams would bend towards one another and merge, Esposito says.
“But they would have to be screams with really large energies or in an extremely dense area like the centre of a neutron star,” he says. “Don’t try it at home.”
Reference:
