Of the more bizarre things I learned some five or six decades ago was that everything has a wavelength.   Yes, a car that weighs about a ton has a wavelength; albeit a small one on the order of  5 X 10^-47 nm.   And, when compared to the wavelength of red light, which is 700 nm, you can see that the wave properties of a car are pretty darned small- especially in relation to the effect of its mass.

But, it did also mean that things that are waves have to have mass.   So, it’s not surprising that physicists finally found that to be true.  (Physical Review Letters  Angelo Esposito, Rafael Krichevsky, and Alberto Nicolis,all of  Columbia.  Gravitational mass carried by sound waves. ) And, that also means that sound – as it travels through air or water- should be affected by gravity!

Actually, last year, Drs.  Alberto Nicolis (Columbia) and Riccardo Penco (now at Carnegie Mellon) suggested that phonons (these are the packets of traveling sound waves; a photon is a packet of traveling light waves) should manifest mass when traveling in a superfluid.  Einstein’s theories predict this- it’s related to his famous equation that demonstrates that energy and mass are related. (You know: e=mc ² )  Until that article was published, physicists agreed that sound waves carry energy- but not mass.  (This meant sound waves would not generate any gravitational fields.)

But, I’m willing to bet that you won’t surprised that the mass of a phonon is going to be pretty darn small. Just like the wavelength of a car that was so small.  The mass of a phonon would be smaller still than the lightest material known to man- a hydrogen atom (which weighs all of 10^–24 grams).

So, the mass of an ordinary sound wave is equal to the energy of the sound wave multiplied by a factor that relates the medium’s mass density and the speed of the sound wave.

Except…

This means that the mass of phonons is negative- yes, that means they fall “up”.  They would travel away from gravitational fields.  But, not terribly far, since they have so little mass.  Moving at 1.5 km/s (the speed of sound in water), the phonon moves up at about 1 degree per second.  And, how easy do you think it will be to measure a 1 degree change over that distance? So, testing the theory is not exactly a snap.

So, the trick is to seek out media where the speed of sound is pretty slow.  As in supercritical fluids.  In superfluid Helium, for example, the speed of sound is on the order of 200 m/s.  Or, by studying a massive sound, like an earthquake, say one measuring 8 or 9 on the Richter scale,  would mean an atomic clock might detect the gravitational acceleration of the wave.

We’ll find out soon enough, I’m sure.

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