The upper limit for the speed of sound found

It is around twice as fast as the speed of sound in diamond.

Einstein’s theory of special relativity sets the absolute speed limit at which a wave can travel, which is the speed of light and is equal to around 300,000 km for each second. Anyway, as of recently, it was unknown whether sound waves also have an upper-speed limit when going through solids or fluids.

A new study conducted by scientists at the Queen Mary University of London in collaboration with the University of Cambridge and the Institute for High-Pressure Physics in Troitsk has discovered the fastest possible sound speed. The study shows that predicting the upper limit of sound speed depends on two dimensionless fundamental constants: the fine structure constant and the proton-to-electron mass ratio.

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.

The two dimensionless fundamental constants are known to play a significant role in understanding our Universe. Their finely-tuned values govern nuclear reactions such as proton decay and nuclear synthesis in stars. The balance between the two numbers provides a narrow ‘habitable zone’ where stars and planets can form, and life-supporting molecular structures can emerge.

This new study suggests 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.

Scientists tested their theoretical predictions on a wide range of materials. They addressed one specific prediction of their theory that sound speed 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. 

Thus, scientists performed state-of-the-art quantum mechanical calculations to test this prediction and found that solid atomic hydrogen’s sound speed is close to the fundamental theoretical limit.

Professor Chris Pickard, Professor of Materials Science at the University of Cambridge, said“Soundwaves 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 Earth composition properties. They’re also of interest to materials scientists because sound waves are related to important elastic properties, including the ability to resist stress.”

Journal Reference:
  1. K. Trachenko et al. Speed of sound from fundamental physical constants. DOI: 10.1126/sciadv.abc8662

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