speed of sound

A research collaboration between the University of Cambridge, the Queen Mary University of London, and the Institute for High-Pressure Physics in Troitsk have discovered the fastest possible speed of sound.

The outcome around 36 kilometers per second is around twice as fast as the speed of sound in diamond, the hardest known material on the planet.

Waves, for example, light waves or sound, are disturbances that move energy from one place to another. Sound waves can travel through various mediums, for example, water or air, and move at various speeds depending on what they are traveling through. For example, they move through solids much faster than they would through gases or liquids, which is the reason why you can hear an approaching train much faster if you listen to the sound propagating in the rail tracks rather than through the air.

Einstein’s theory of special relativity sets the absolute speed limit at which a wave can travel which is the speed of light & is equal to about 300,000 kilometers per second. However, until now, it was not known whether sound waves also have an upper-speed limit when traveling through solids or liquids.

The study, published in the journal Science Advances, shows that predicting the upper limit of the speed of sound is dependent on two dimensionless fundamental constants: the fine structure constant and the proton-to-electron mass ratio.

These two numbers are already known to play an important role in understanding our Universe. Their finely-tuned values govern nuclear reactions such as proton decay and nuclear synthesis in stars and the balance between the two numbers provides a narrow ‘habitable zone’ where stars and planets can form and life-supporting molecular structures can emerge. However, 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. Therefore, researchers performed state-of-the-art quantum mechanical calculations to test this prediction and found that the speed of sound in solid atomic hydrogen is close to the theoretical fundamental limit.

Professor Chris Pickard, Professor of Materials Science at the University of Cambridge, said that “Soundwaves in solids are already hugely important across many scientific fields. such as, seismologists use sound waves initiated by earthquakes deep in the Earth interior to understand the nature of seismic events & the properties of Earth composition. They are also of interest to materials scientists because sound waves are related to important elastic properties including the ability to resist stress.”

More information: “Speed of sound from fundamental physical constants” Science Advances (2020). advances.sciencemag.org/lookup … .1126/sciadv.abc8662

Journal information: Science Advances