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This is super cool. I was tempted to say it's no big deal because, of course, sound waves are pressure waves and therefore they are oscillations of mass density; apparently the oscillation is nonlinear resulting in transport of a slightly lower density region ("negative mass")./1
2/ We see something like this in asteroid impacts on the Moon. The sound wave of impact is actually a shock wave because the asteroids hit faster than the speed of sound in the Moonrock, causing the sound to "bunch up" into a large shock. Rocks behave nonlinearly to these shocks.
3/ So when the shockwave (the sound) passes through the rock it squeezes then unsqueezes the rock but not equally, so the rock ends up with some momentum. In other words, some of the sound energy is converted into the transport of mass.
4/ That residual momentum is actually what makes the crater form. The rock is moving a lot slower than the sound (the shockwave) was moving, but it has a lot of inertia and just keeps on flowing until there is a giant hole, the crater.
"So of course sound transports mass, no big deal," I was thinking. However, it's not so trivial to discover negative mass flow in ordinary sound waves in normal life. Most processes at the amplitude of normal soundwaves should be a lot more linear than shockwaves in rock!
6/ Of course we already knew they are not completely linear. Example: soundwaves in carbon dioxide cause molecules to start spinning which dissipates the sound wave into heat. (So it will be hard to hear on Mars.) Wherever the air is hotter it is less dense, "negative mass".
7/ We call that effect "bulk viscosity" (not ordinary viscosity). Most fluids like the air we breathe has negligible bulk viscosity so we don't notice it in ordinary life. Carbon dioxide has a lot of bulk viscosity because the molecules are made of 3 atoms in a straight line.
8/ And that gives them weird rotational properties. But this new result is found in the mathematics of sound transport in gravity, so it is not a material property of nonlinear rock compression or rotation of gas molecules. I am sorry I can't say more now about what causes it...
9/ ...but it is cool that sound has an inherent property of making the region containing the sound lower density than the surrounding matter, so sounds "floats" like a helium balloon in air. That is SO COOL!
