Here is the maximum speed of sound

Here is the maximum speed of sound

An Anglo-Russian collaboration has calculated the upper limit of the speed of sound in a vehicle: 36 kilometers per second

(image: Getty Images) 36 kilometers per second. This is the maximum speed at which a sound wave, in theory, can travel. At least according to the researchers of an Anglo-Russian collaboration, who put on paper in the journal Science Advances the equations that allowed the calculation. Even in the best possible propagation medium, sound can't go faster than that.

How does sound travel?

Let's give a couple of essential physics notions. Sound is a mechanical wave. It propagates through a medium, through matter: the energy is transferred to the molecules, which move and collide perpetuating the transfer. The characteristics of the medium determine the speed with which vibrations propagate: sound travels faster in solids, slower in liquids and even slower in gases.

The upper speed limit of the sound

But according to Kostya Trachenko of Queen Mary University of London and his colleagues there is a way to calculate the upper limit of the speed of sound, the value beyond which it cannot go. It is enough to use two fundamental constants: the fine structure constant and that of the proton-electron mass ratio. From a simple combination of them - write the authors of the work - we obtain "another dimensionless quantity that has an unexpected and specific implication for a key property of the condensed phases: the speed with which the waves travel in solids and liquids, or velocity of sound ". The value obtained was, in fact, 36 kilometers per second, twice what sound can reach in a diamond, that is, the most rigid solid we know.

Half by medium

To confirm the validity of the equation, the researchers also experimentally measured the speed of sound in different materials: measurements and predictions, they say, are consistent.

The speed of sound should thus also be influenced by the size of the atoms of the medium in which it propagates. Therefore, the best medium should be solid hydrogen, a material that can only exist at pressures a million times higher than that of the Earth's atmosphere, in stars for example. Getting experimental confirmation is more than unlikely, but if theoretical estimates of the properties of solid hydrogen are used, the computation approaches precisely the upper speed limit returned by the new equation.

A valuable tool

The calculation of the upper limit of the speed of sound is not an exercise in itself, but it could help scientists better understand our planet (sound waves are already used to study the interior of the Earth and earthquakes, for example) and the Universe. “We believe that the results of this study,” Trachenko concluded, “could have further scientific applications by helping us to find and understand the limits of several properties such as viscosity and thermal conductivity relevant to high-temperature superconductivity, quark plasma- gluons and even the physics of black holes ".

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