Surprising Discovery Hints Sonic Waves Carry Mass

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.


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Source: *FS – All – Science News 2 Net
Surprising Discovery Hints Sonic Waves Carry Mass