Physics Professor's Research Sheds Light on the Mass of Sound

July 10, 2019

Most of us normally think that sound travels through the air without shape or substance. A recent study building on research conducted by Carnegie Mellon University Assistant Professor of Physics Riccardo Penco has shown that sound waves actually have a small amount of mass that is in a possibly exotic form. The paper was published in the journal Physical Review Letters and featured in Scientific American.

"This is something that started with a paper published when I was at the University of Pennsylvania," Penco said of his contribution to the research as a postdoctoral fellow before joining the faculty of Carnegie Mellon. That study, published in the journal Physical Review B, looked at the behavior of phonons, which are akin to particles of sound waves like photons are to particles of light waves under quantum mechanics.

Penco and his former postdoctoral supervisor Alberto Nicolis, an associate professor of physics at Columbia University, were inspired by Archimedes' famous bathtub observation of how objects react when put in a fluid. "The object will receive a push upwards and sink or float," Penco noted. "There are other things that exist in fluids, such as sound waves. "Do they sink or do they float?"

It has long been believed that sound waves do not carry a net mass of their own, but instead are transferred through solids, liquids or air by displacing matter locally, Penco explained. However, using a theoretical model for a superfluid at zero temperatures, which has zero viscosity, Penco and Nicolis found that sound waves "tend to float," meaning that, unlike light waves, they must have mass that can be acted upon by gravity.

That floating effect also suggests that this net mass that sound waves carry as they travel may even be an example of "negative mass." In Newtonian physics, when force is applied to normal mass, it moves away from the force. Negative mass moves towards the direction of the force.

Building on Penco's 2018 paper, Nicolis and collaborators at Columbia University found that this modeled effect can be extended beyond superfluids to regular fluids and even solids. They reported these findings in the Physical Review Letter paper.

"It's surprising in this day and age that it is still possible to find new results in classical Newtonian physics," Carnegie Mellon Professor of Physics Ira Rothstein, who did not participate in the study, told Inside Science about this research.

"With modern analytical techniques, we can still uncover these new nuggets," Penco noted.

While Penco was not directly involved in the Physical Review Letter paper, he said he is excited to see what further results come from it, results which he believes will intersect with his research on condensed matter systems.

"There is value in having cross-pollination among subfields," Penco said.