sound waves
NASA used a U.S. Air Force Test Pilot School T-38 aircraft and cameras with a special filter on the ground to capture images of shockwaves coming off the aircraftNASA

Sound has mass, somehow this mass is negative, and it also carries mass. If that seems too confusing and highly unlikely, a new study has challenged all conventional understanding of sound and all that it constitutes.

According to a paper submitted takes a whole new angle at sound and sound waves. So far, sound waves are nothing but massless ripples that travel through matter, the denser the material, the faster it moves. Sound is just applying vibrations to molecules, moving it aside as it passes through, but the opposite and equal shove, balances out everything. While that is not entirely wrong and it does explain the properties of sound in most circumstances, actually it does not explain everything related to sound waves.

Particle-like unit of vibration is used to describe sound at small scales called a phonon, explain the researchers, as having a "negative mass". That simply means, according to a report, when sound waves travel, they move upward, and it is slight but measureable, said Rafael Krichevsky, from the Columbia University.

Shock waves
Shockwaves produced by a U.S. Air Force Test Pilot School T-38C banking at Mach 1.05NASA

The paper explains that Phonons are particles, but not like what one would imagine physical particles to be, like atoms and molecules, said Krichevsky. As sound moves through air, it vibrates molecules that are in contact with it, but that vibration is not easily explained by how the molecules actually move, said Krichevsky to Live Science in an email.

Phonons are hence described in comparison to photons—light particles which are both particles and waves. Sound particles, similarly are a result of complicated interactions of fluid molecules, explains Krichevsky. There are no actual physical particles as such, but they can be described mathematically.

In their new study, researchers showed how phonons, in fact, have "negative mass", which means that when gravity pulls on a phonon, it moves 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.

The concept was explained using the dynamics of normal fluids where gravity acts in a normal manner by pulling everything downward. Fluid particles compress particles directly below it, this in turn, causes a difference in density of the fluid. The upper layers are not as dense as the lower layers.

Sonic boom
A U.S. Air Force Test Pilot School T-38 is captured, using a special filtered camera on the ground, flying in front of the sun. Shockwaves are seen beginning to form as the vehicle transitions from a subsonic speed to supersonicNASA

It is a well-known fact that sound is a lot faster through denser objects, so going by this logic, sound that moves above a phonon will be slower than sound through the denser particles below it. This causes the particle to "deflect" upward, Krichevsky explained.

This process can be found even with the sounds made by a person when speaking, the paper found. So over a distance that is far enough, a person shouting would actually bend upward ever so slightly. The effect, say the researchers, is, as of now, too small to actually measure with current technology.

The paper was submitted to the journal arXiv.