In 2018, a trio of researchers at Columbia University proposed that sound waves—just ordinary sound waves—might generate negative gravity. That theory, covered at the website phys.org, has to do with something called a “phonon.” The phonon (not photon) is a concept from quantum mechanics that represents an elementary unit of vibrational motion. In terms that would surely drive a quantum physicist to distraction, it might be thought of as the smallest possible “particle of sound.”
As with photons and light waves, sound waves (or any “excitation”) within matter may display both wave-like and particle-like behaviors. And as with all things quantum, there are fixed scales at which things not only do, but can, occur. But while phonons may be “particle-like,” physicists have generally treated this behavior as a kind of convenience for dealing with how sound interacts with other forces. It hasn’t been treated like a “real particle.”
But the Columbia researchers did the math on what it would mean if a phonon was more than just particle-like, and out of the math of that theory sprang a prediction that phonons would interact with gravitational fields to produce negative mass and negative gravity. Unfortunately, the predicted phenomenon would be very difficult to detect using current technology. Except … as the March edition of Physics reports, the three researchers have extended their math, and found that the theory applies to sound waves traveling through materials such as liquids and solids, which means it potentially can be detected.
On a pure science-as-Einstein-explained it basis, it makes sense that sound waves should carry mass. After all, E=MC2, so the energy of a sound wave should be equivalent to a small (very small) amount of mass. But that’s not what the researchers are talking about. The phenomenon they calculated in 2018 may be small, but it is still much larger than the mass that would result from converting the tiny amounts of energy into extraordinarily tiny amounts of mass through relativity.
According to their theory, a one-second-long sound wave passing through water with a power of one watt carries with it a mass of negative 0.1 milligram. And now it looks like they might be getting close to proving it.
The size of the phenomenon—which should be true even of sound waves passing through solid materials—should produce results that can be measured consistently, carrying this experiment outside of the realm of quirky theory. And in fact, the researchers have built on their existing paper with proposals on how it may be tested. In a sound wave moving through water, the negative gravity generated by phonons would cause the wave to move upward about 1 degree for every 15 kilometers of travel. That’s a large-scale experiment, but far from impossible.
If true, the results are exciting on a purely scientific basis: This is basic behavior of an everyday phenomenon showing a relationship to action happening at the quantum level—results that were completely unexpected a year ago.
It seems unlikely, unfortunately, that this phenomenon is going to to result in turning sound into propulsion: no sound-powered flying cars.