Physicists propose the top speed of sound

The fastest possible speed of sound is around 36 kilometres per second, according to a team of British and Russian physicists.

To put things into context, that’s around a hundred times faster than its speed through the air in normal circumstances – and around twice as fast as in diamond, the hardest known material in the world.

Sound is known to travel at different speeds through different materials (and more quickly through solids than liquids or gases) but it has not been clear whether there is an upper limit.

The researchers from Queen Mary University of London, the University of Cambridge and the Institute for High Pressure Physics in Troitsk have put their number on it in a paper in the journal Science Advances.

Doing so was dependent, they say, on two dimensionless fundamental constants: the fine structure constant and the proton-to-electron mass ratio.

These two numbers, and the balance between them, are known to play an important role in the Universe – governing nuclear reactions and providing a narrow “habitable zone” where stars and planets can form and life-supporting molecular structures can emerge.

The new findings suggest they also can influence other scientific fields.

“We believe [they] could have further scientific applications by helping us to find and understand limits of different properties such as viscosity and thermal conductivity relevant for high-temperature superconductivity, quark-gluon plasma and even black hole physics,” says Queen Mary’s Kostya Trachenko.

The scientists say they tested their theoretical prediction on a range of existing materials and even a theoretical one – solid metallic hydrogen, which may or may not exist at the centre of giant planets.

One argument holds that if the speed of sound decreases with the mass of the atom, then it should be fastest in solid atomic hydrogen. Quantum mechanical calculations show, the researchers say, that it is: close to the theoretical fundamental limit, but not above it.

That limit – if it’s correct (and, indeed, if such a limit does exist) – applies only at pressures typically found on Earth. Elsewhere sound might well travel at greater speeds.

But not at more than 300,000 kilometres per second – the speed of light and, according to Einstein’s theory of special relativity, the absolute speed limit at which a wave can travel.

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