A research collaboration between Queen Mary University of London and the University of Cambridge, in the United Kingdom, and the Institute of High Pressure Physics, in Russia, has discovered the fastest possible speed of sound. The result, some 36 kilometers per second, is about twice as fast as the speed of sound in diamond, the hardest known material in the world, they reveal in the magazine Science Advances.

Waves, like sound or light waves, are disturbances that move energy from one place to another. Sound waves can travel through different media, like air or water, and they move at different speeds depending on what they go through.

For example, move through solids much faster than they would through liquids or gases, so you can hear a train approaching much faster if you listen to the sound that travels down the train tracks rather than through the air.

Einstein’s theory of special relativity establishes the absolute speed limit at which a wave can travel, which is the speed of light, and is equal at about 300,000 kilometers per second. However, until now it was not known whether sound waves also have an upper speed limit when traveling through solids or liquids.

The study shows that the prediction of the upper limit of the speed of sound depends on two dimensionless fundamental constants: the fine structure constant and the proton-electron mass ratio.

These two numbers are already known to play an important role in understanding our Universe. Its 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 molecular structures that support life can emerge.

However, the new findings suggest that these two fundamental constants also can influence other scientific fields, such as materials science and the physics of condensed matter, by setting limits to specific properties of materials, such as the speed of sound.

The scientists tested their theoretical prediction on a wide range of materials and tackled a specific prediction from their theory that the speed of sound should decrease with the mass of the atom. This prediction implies that the sound it is the fastest in solid atomic hydrogen.

However, hydrogen is an atomic solid at very high pressure above 1 million atmospheres only, a pressure comparable to that of the nucleus of gas giants like Jupiter. At those pressures, hydrogen turns into a fascinating metallic solid that conducts electricity like copper and is predicted to be a superconductor at room temperature.

Therefore, the researchers conducted quantum mechanics calculations generation to test this prediction and found that the speed of sound in solid atomic hydrogen is close to the theoretical fundamental limit.

The teacher Chris Pickard, professor of materials science at Cambridge University, explains that “sound waves in solids are already very important in many scientific fields. For example, seismologists use sound waves initiated by earthquakes deep inside the Earth. to understand the nature of seismic events and the properties of the Earth’s composition. They are also of interest to materials scientists because sound waves are associated with important elastic properties, including the ability to resist stress. “