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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’s 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 media, including air and water, but they move at different speeds depending on the material they are traveling through. Sound waves travel through solids much faster than they travel through liquids or gases. The researchers note that this is why you can hear the sound of an approaching train if you hear the tracks much earlier than you can simply hear the air.
Einstein’s special theory of relativity predicts an absolute limit on how fast the wave can travel, known as the speed of light, equivalent to about 300,000 kilometers per second. What was not known, until now, was whether sound waves have an upper speed limit when traveling through solids or liquids. The speed of sound depends on a pair of dimensionless fundamental constants, the fine structure constant and the proton-electron mass ratio.
Those numbers play a role in understanding the universe and relate to a variety of topics, including nuclear reactions like proton decay and nuclear synthesis in stars. In this research, the team theorized that the speed of sound should decrease with the mass of the atom. The prediction implies that the speed of sound is the fastest and most solid atomic hydrogen.
The theory would imply that the speed of sound is faster in solid atomic hydrogen, which is just a solid at a very high pressure above 1 million atmospheres. That would be a pressure comparable to the core of a gas giant planet like Jupiter, where hydrogen is a metallic solid. The predictions are that metallic hydrogen would be an electrical conductor and a superconductor at room temperature. 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.
