Scientists find upper limit for the speed of sound

Until now it was not known whether sound waves have an upper speed limit when travelling through solids or liquids.

A research team has discovered the fastest possible speed of sound. The result- about 36 km per second—is around twice as fast as the speed of sound in diamond, the hardest known material in the world. It is dependent on two dimensionless fundamental constants: the fine structure constant and the proton-to-electron mass ratio.

The new findings suggest that these two fundamental constants can also influence other scientific fields, such as materials science and condensed matter physics, by setting limits to specific material properties such as the speed of sound.

The scientists tested their theoretical prediction on a wide range of materials and addressed one specific prediction of their theory that the speed of sound should decrease with the mass of the atom.

This prediction implies that the sound is the fastest in solid atomic hydrogen. However, hydrogen is an atomic solid at very high pressure above 1 million atmospheres only, pressure comparable to those in the core of gas giants like Jupiter.

At those pressures, hydrogen becomes a fascinating metallic solid conducting electricity just like copper and is predicted to be a room temperature superconductor.

Sound waves in solids are already hugely important across many scientific fields. For example, seismologists use sound waves initiated by earthquakes deep in the Earth interior to understand the nature of seismic events and the properties of Earth composition. They're also of interest to materials scientists because sound waves are related to important elastic properties including the ability to resist stress.