Surprising Discovery Hints Sonic Waves Carry Mass (scientificamerican.com)

jbmartin6 shares a report from Scientific American: In a paper published in Physical Review Letters, a group of scientists has theorized that sound waves possess mass, meaning sounds would be directly affected by gravity. They suggest phonons, particle-like collective excitations responsible for transporting sound waves across a medium, might exhibit a tiny amount of mass in a gravitational field. "You would expect classical physics results like this one to have been known for a long time by now," says Angelo Esposito from Columbia University, the lead author on the paper. "It's something we stumbled upon almost by chance."

Esposito and his colleagues built on a previous paper published last year, in which Alberto Nicolis of Columbia and Riccardo Penco from Carnegie Mellon University first suggested phonons could have mass in a superfluid. The latest study, however, shows this effect should hold true for other materials, too, including regular liquids and solids, and even air itself. And although the amount of mass carried by the phonons is expected to be tiny -- comparable with a hydrogen atom, about 10^-24 grams -- it may actually be measurable. Except, if you were to measure it, you would find something deeply counterintuitive: The mass of the phonons would be negative, meaning they would fall "up." Over time their trajectory would gradually move away from a gravitational source such as Earth. "If their gravitational mass was positive, they would fall downward," Penco says. "Because their gravitational mass is negative, phonons fall upwards." And the amount they would "fall" is equally small, varying depending on the medium the phonon is traveling through. In water, where sound moves at 1.5 kilometers per second, the negative mass of the phonon would cause it to drift at about 1 degree per second. But this corresponds to a change of 1 degree over 15 kilometers, which would be exceedingly difficult to measure.

Comments

The only competent comment here.

[Anonymous Coward]

Given the idiocies all the other commenters wrote at this point, including TFS, you're the only one here who hit the nail on the head.

It seems people don't get that infomation is not a physical object (matter/energy) itself, but only the *structure* of matter/energy. So it's a meta level. In a medium. With different meta laws.
Hence the whole "intellectual property" oxymoron confusion.

TL;DR: Sound does not have mass. The particles that form the medium of sound, do.

Sound is a meta level, so it can only have meta mass. Its "mass" isn't real, just as phonons aren't real. They are only a useful construct.

Re:Vacuum

[Pfhorrest]

"Mass" isn't the same thing as "matter".

The kinds of particles, like for example electrons, that travel through vacuum, are waves in quantum fields. There is an electron field everywhere, some amount of "electron-ness" everywhere, and an electron particle is an excitation of that field. That particle would be massless, like all particles would be, if it weren't for some of its kinetic energy being bound up in interactions with other fields; in the case of free-travelling electrons, the Higgs field. Mass is just energy that's bound up doing something other than moving; most of the mass of a proton, for instance, is the binding energy of the color force holding its quarks together, way way way more than the rest-mass of those quarks (again, from the Higgs field) contributes.

Phonons are "quasiparticles" in that they are excitations of something other than a quantum field; they're compression waves in a medium like air or water. Quantum fields are everywhere, but air and water aren't everywhere, so phonons can't travel through a vacuum. To say that they have mass is, most likely (not having read all this new research yet), to say that some of their energy is bound up doing something other than moving the constituent particles of their medium. Or perhaps, since their mass is negative, that they are constantly drawing energy from their medium? In any case, it's definitely not to say that they are made of some kind of matter, which can then carry itself through the vacuum.

FWIW though, sound can travel through what we normally think of as "vacuum", since true vacuum doesn't actually exist. The space between planets is filled with a thin gas called the interplanetary medium; the space between stars is likewise filled with an even thinner interstellar medium; and the space between galaxies with an even thinner intergalactic medium. A very high-amplitude long-wavelength compression wave in this medium can travel through it, just so long as the wave moves the constituent particles hard enough and far enough that they can actually reach their nearest neighbor particles, quite some ways away in such a thin medium, and induce a similar motion in those.

Re:Theoretical physics isn't science

[Anonymous Coward]

Dude, Phys. Rev. Lett. is quite substantial. Physicist here.

15km in 1 second at 1.5km/s?

[Walking The Walk]

In water, where sound moves at 1.5 kilometers per second, the negative mass of the phonon would cause it to drift at about 1 degree per second. But this corresponds to a change of 1 degree over 15 kilometers, which would be exceedingly difficult to measure

Uh, if sound moves at 1.5 km/s, and drifts by 1 degree/s, then in 1 second it should have drifted by 1 degree and travelled 1.5 km, not 15km? After 10 seconds it will have travelled 15 km and drifted by 10 degrees, which surely would be measurable. (PS: I read the article, the summary quotes the article correctly.)

Re:Makes sense to me

[Baloroth]

Sound travels through matter, so consists of, well, "phonons" that are really just the slightly altered movements of the matter the sound travels through. Sound exists for as long as that extra movement exists, and for it to exist, the matter needs to be excited, ie possess energy, over and above ambient. So that means sound waves traveling perpendicular to a gravity field have a tendency to be a little less affected by that field than ambient matter. So it looks like phonons have negative mass.

So this apparent mass is an artifact of the way you look at it.

This is explicitly not what the paper is saying. I'll just quote the introduction:

Now, this effect is completely equivalent to standard refraction: in the presence of gravity, the pressure of the superfluid depends on depth, and so does the speed of sound. As a result, in the geometric acoustics limit sound waves do not propagate along straight lines. Because of this, one might be tempted to dismiss any interpretation of this phenomenon in terms of “gravitational mass”. However, since in the formalism of [1] the effect is due to a coupling with gravity in the effective Lagrangian of the phonon, the same coupling must affect the field equation for gravity: the (tiny) effective gravitational mass of the phonon generates a (tiny) gravitational field. The source of this gravitational field travels with the phonon.

In other words, if you look at the phonons path, the effect of gravity on it looks just like standard refraction because, well, this is a sound wave. But the phonon itself couples to gravity, which means the phonon produces a gravitational field (albeit an extremely tiny one) as if it has negative mass. That is interesting (although probably not very interesting, as phonons are still quasiparticles, not real particles: a real particle with negative mass would revolutionize physics. A quasiparticle with negative mass might revolutionize a few scientists CVs).