# Physicists discovered the fastest possible speed of sound and Liquids

Physicists discovered the fastest possible speed of sound and Liquids

It’s impossible to measure the speed of sound in every single material in existence, but scientists have now managed to pin down an upper limit based on fundamental constants, the universal parameters by which we understand the physics of the Universe.

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

Using two dimensionless fundamental constants, an international team of physicists has calculated the fastest possible speed of sound in condensed phases (solids and liquids): 36 km per second (22.4 miles per second). 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.

Sound is a mechanical wave, which is caused by a vibration in a medium. As the wave travels through the medium, that medium’s molecules collide with each other, transferring energy as they go. Waves, such as sound or light waves, are disturbances that move energy from one place to another. Sound waves can travel through different mediums, such as air or water, and move at different speeds depending on what they’re traveling through.

For example, they move through solids much faster than they would through liquids or gases, which is why you’re able to 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 gave us the speed limit of the Universe – that of light in a vacuum. But the absolute top speed of sound, through any medium, has been somewhat trickier to constrain. 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 about 300,000 km per second. However, until now it was not known whether sound waves also have an upper-speed limit when traveling through solids or liquids

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.

Researchers tested sound waves with a range of materials in order to find the answer to a specific theory of their own: the speed of sound should decrease with the mass of the atom. According to their prediction, the sound should be fastest in solid atomic hydrogen. 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.

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.

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.