Researchers discover the fastest possible speed of sound

Scientists from 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. The upper limit for the speed of sound is 36 kilometers per second. That is about twice as fast as the speed of sound in diamond, which is the hardest known material in the world.Sound waves can travel through different mediums, including air and water, but they move at different speeds depending on the material they travel through. Sound waves travel through solids much faster than they travel through liquids or gases. Researchers note that is why you can hear the sound of an approaching train if you listen to the tracks much earlier than you can just listening to the air.

Einstein's theory of special relativity predicts an absolute speed limit the wave can travel, known as the speed of light, equal to about 300,000 kilometers per second. What wasn't known, until now, was if sound waves have an upper speed limit when traveling through solids or liquids. The speed of sound is dependent on a pair of dimensionless fundamental constants, the fine structure constant, and the proton-to-electron mass ratio.

Those numbers play a role in understanding the universe and relate to a range of topics, including nuclear reactions such as proton decay and nuclear synthesis in stars. In this research, the team theorized the speed of sound should decrease with the mass of the atom. The prediction implies that the speed of sound is the fastest and solid atomic hydrogen.

The theory would imply the speed of sound is fastest in solid atomic hydrogen, which is only a solid at very high pressure above 1 million atmospheres. That would be pressure comparable to the core of a gas giant planet like Jupiter, where hydrogen is a metallic solid. Predictions are that metallic hydrogen would be an electrical conductor and room-temperature superconductor. Using quantum mechanical calculations to test the prediction, the team found that the speed of sound in solid atomic hydrogen is close to the theoretical fundamental limit.