Physicists clock the fastest possible speed of sound
Interstellar mysteries solved.
Scientists have discovered the fastest possible speed of sound: about 36 kilometres per second. Sound waves move at different speeds in solids, liquids and gases. Within those states of matter they travel faster in warmer liquids compared with colder ones. Physicist Kostya Trachenko of Queen Mary University of London and his colleagues wanted to figure out the upper limits of how fast sound could travel.
This exercise was largely theoretical. The researchers found that the answer, which is about twice as fast as sound moves through solid diamond, depends on some fundamental numbers in the universe. The first is the fine structure constant, which is a number that describes the electromagnetic force that holds together elementary particles such as electrons and protons. The second is the proton-toelectron mass ratio of a material, which, as it sounds, is the ratio of mass from protons and mass from electrons within the atomic structure of the material.
It’s not possible to test this theoretical top speed in the real world, because the maths predicts that sound moves at its top speed in the lowest mass atoms. The lowest mass atom is hydrogen, but hydrogen isn’t solid, unless it’s under super-high pressure that’s a million times stronger than that of Earth’s atmosphere. That might happen at the core of a gas giant like Jupiter, but it doesn’t happen anywhere nearby where scientific testing is possible. Instead Trachenko and his colleagues turned to quantum mechanics and maths to calculate what would happen to sound zipping through a solid atom of hydrogen. They found that sound could travel close to the theoretical limit of 127,460 kilometres per hour, confirming their initial calculations. In contrast, the speed of sound in air is roughly 1,235 kilometres per hour.
The movement of sound in such extreme and specific environments may seem unimportant, but because sound waves are travelling vibrations of molecules, the speed of sound is related to many other properties of materials, such as the ability to resist stress. Understanding the fundamentals of sound could help illuminate other fundamental properties of materials in extreme circumstances.
For instance, previous research has suggested that solid atomic hydrogen could be a superconductor. Knowing its fundamental properties could be important for future superconductivity research. Sound could also reveal more about the hot mix of quarks and gluons that made up the universe an instant after the Big Bang, and could be applied to the strange physics around the gravity wells that are black holes. Other researchers have studied ‘sonic black holes’ to gather insight into these cosmic objects.
“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 hightemperature superconductivity, quarkgluon plasma and even black hole physics,” Trachenko said. STEPHANIE PAPPAS