We just got more evidence that sound waves really do carry mass

Physics

Traditionally when we think of sound waves, we think of invisible vibrations moving weightless through the air – not carrying any mass.

That might be about to change. Physicists have just provided further evidence that particles of sound really can carry tiny amounts of mass. And that means they can produce their own gravitational fields – which could be a big deal for our understanding of space.

But let’s back up for a second and go back to the basics. Kick a ball, and you put energy into it. Einstein would tell you you’ve also contributed a tiny bit of mass by making it accelerate.

If that ball is a tiny particle, and the kick is a wave of sound, you might imagine the same thing. Yet for decades, physicists have argued over whether the momentum within a surge of jiggling particles adds up to a net amount of mass.

Last year, physicist Alberto Nicolis from Columbia University in New York worked with a colleague from the University of Pennsylvania in Philadelphia to investigate how different waves decay and scatter in a super cold fluid of helium.

Not only did they show that sounds can actually generate a non-zero value for mass, but they might also weirdly ‘float’ along gravitational fields in an anti-gravity sense.

This might not make much difference for relatively quiet booms and squeaks on Earth, but for the star-quaking roars that pulse through dense objects like neutron stars, interactions between massive sound waves and gravity could be important.

While the pair affirmed the possibility, it was limited to a specific set of conditions. So Nicolis has now used a different set of techniques to show that sounds have mass inside ordinary fluids and solids, and even create their own faint gravitational field.

Their new conclusion contradicts views that phonons are massless. Their movements don’t just respond to a gravitational field in strange ways, but are a source of a field in their own right.

In a Newtonian sense, this is the very definition of mass.  

So why is there so much confusion over this issue?

The core of the problem is in how waves move through a medium. Just as a wave of light is called a photon, a wave of vibration can be thought of as a unit called a phonon.

Imagine standing still at a rock concert, enjoying the show. Your body’s mass is the same it was in the morning when you stepped on the scales. Then, a killer track starts and your neighbour shoves you, accelerating your body.

Einstein’s law – the one that says energy equals mass times the speed of light squared – says the tiny bit of energy you gain from the push is also mass. Colliding with the next person, the energy transfers into them along with the imperceptibly small bit of mass. 

In this metaphor, the chain of body slams going back and forth through the crowd is the phonon, and since it’s a transfer of energy, you might be forgiven for immediately thinking it’s also a movement of mass.

Under such simple conditions, the perfect back-and-forth movement of the bodies and direct transfer of momentum can be described as a form of linear dispersion.

While energy levels might fluctuate during the back-and-forth jostle, your body resets to give the whole phonon cycle no mass overall. This linear dispersion gives each phonon a net mass of naught, just as with photons of light.

But reality isn’t always so straightforward. 

Light waves moving through a vacuum and phonons in a theoretically perfect material might well be linear, but solids and fluids jostling with one another obey a variety of other laws according to certain fields and influences.

Those are a little complicated, arising from the medium’s state and components.

So using approximations known as effective field theory, Nicolis and Columbia University colleagues Angelo Esposito and Rafael Krichevsk got a broad sense of how the phonon travels through such media and how to calculate their response to a gravitational field.

And what they showed was that even in these messy ‘real world’ conditions, the sound waves could carry mass.

To be clear, that mass isn’t exactly huge, as you’d expect. We’re talking roughly the same as the amount of energy in the phonon divided by the square of the speed of light. So … small.

It’s also important to keep in mind the mathematics behind the claim haven’t actually been put to the test. Sound foundations aside, somebody now needs to measure gravitational shifts in atoms chilled to near zero, something which just might be possible as we explore such condensates in space.

Alternatively, the researchers suggest it might be easier to weigh an earthquake. The sound generated by a large tremor could amount to billions of kilograms of mass.

Anybody up to that challenge?

This research was published in Physical Review Letters.

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