# A liquid has a negative mass

## Physics: do sound waves have a negative mass?

In physics textbooks, the matter is clear: sound waves carry energy and also have what researchers call an impulse. But they have no mass. At least not in the sense that a stone has. This creates its own gravitational field, which exerts a force of attraction on other bodies.

Such a force should not actually emanate from sound, at least that is what the laws of Newton's theory of gravity dictate in its applied form. According to this, sound waves are just periodic density fluctuations that move through a medium such as air or water and seem to merge completely with it.

But it may be too easy for yourself, argues a team of theorists in the journal "Physical Review Letters". They have calculated more precisely than before how sound waves react to tiny disturbances. According to this, the sound in solids or liquids can very well be assigned a mass - and according to the researchers, this is also negative. Such a sound wave would have to move slightly upwards in the earth's gravitational field.

The assumption of the authors is based on a precise analysis of the equations of motion of the density fluctuations. The scientists write that it is often assumed that sound is a "linear" phenomenon. The movement of particles through a sound wave would therefore always be proportional to the force exerted. In the vast majority of cases, this is also a permissible simplification, emphasizes the team led by Angelo Esposito from Columbia University.

If you look closely at the relevant equations, however, "nonlinear" terms also played a role. And these give sound waves in liquids and solids a mass that strongly depends on the energy of the density fluctuation. A 1 watt sound wave in water would therefore transport 0.1 milligrams per second.

Precision experiments must now show whether the researchers' prediction is correct. The physicists are particularly hoping for ultra-cold quantum gases here. Sound waves should also have a negative mass in them, as scientists calculated a year ago. And here it could be mathematically possible to measure the mass of a sound wave in the laboratory.