Einstein’s Special theory of relativity gave the speed of light in vacuum and it is the theoretical speed limit of the universe. But, what could be the absolute highest speed of sound is a question that remains trickier and till now, there has been no such value ascertained for sound waves.
Now, a team of scientists from Queen Mary University, London, and the University of Cambridge along with the Institute of High-Pressure Physics, Russia, have been able to estimate the value of the maximum speed of sound, which is 36 kilometres per second. The research was published in Science Advances.
Both sound and light travel as waves, but they have fundamentally different characters. Light is an electromagnetic radiation, meaning, a light wave has oscillating electric and magnetic fields. These fields produce a self-propagating wave and can travel through vacuum with a maximum speed of 300,000 kilometres per second. But, when a light wave travels through a medium like water or atmosphere it gets slowed down.
On the other hand, sound is a mechanical wave, a wave created by vibration in a medium. The disturbance in the medium causes the medium’s molecules to collide with each other transferring energy and the waves travel through it. The rigidity of the medium determines how fast a sound wave can propagate through it; tougher the medium, tougher it is to compress it, faster the sound wave travels.
Travelling of a sound wave in a rigid medium helps scientists to study the inside of the Earth, especially when seismic waves travel through it. Seismologists take sound waves created by earthquakes to study the properties of the composition of Earth’s interior. This property is even used to study the interior of the stars. Even the material scientists take the help of sound wave to study elastic properties of certain materials.
Till now, what was not possible is to determine an upper limit of speed of sound waves. The differences in speed of sound in different materials made it even tougher. It was simply impossible to study the speed of sound in all possible materials in the universe so that a maximum speed can be ascertained.
The current study took the advantage of fundamental constants of physics. They found that two such fundamental constants determine the maximum speed of sound. These are fine structure constant and the proton to electron mass ratio. The fine structure constant, in simpler words, characterises the strength of the electromagnetic force, which governs how electrically charged elementary particles, like electrons, and light interact.
The researchers argued in their paper—“The finely tuned values of the fine structure constant and the proton-to-electron mass ratio, and the balance between them, govern nuclear reactions such as proton decay and nuclear synthesis in stars, leading to the creation of the essential biochemical elements, including carbon. This balance provides a narrow 'habitable zone' in the space where stars and planets can form and life-supporting molecular structures can emerge.”
They further said, “We show that a simple combination of the fine structure constant and the proton-to-electron mass ratio results in another dimensionless quantity that has an unexpected and specific implication for a key property of condensed phases - the speed at which waves travel in solids and liquids, or the speed of sound.”
To confirm their estimate, they measured the speed of sound in different solid and liquid media. They found that their experimental measurement of the speed of sound in different media provide results consistent with their theoretical prediction.
One prediction made by them is that the speed of sound decreases with mass of atom. If this is to be true, then speed of sound should be maximum in solid atomic hydrogen. This form of hydrogen only exists at extremely high pressure, almost 1 million times higher than Earth’s atmospheric pressure at sea level. Verify their theory in such an extreme condition is highly difficult and the team relied on calculations that were based upon the properties of solid atomic hydrogen. Again, their prediction proved to be realistic.
Commenting on their findings, Kostya Trachenko of Queen Mary University and the lead author of the study was quoted to have said, “We believe the findings of this study 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.”