Sound moves faster through some materials (solid or liquid) than others. Physicists have had difficulty finding the maximum speed of sound because it’s next to impossible to measure every material on Earth to find the answer. But now, a team of scientists has calculated the speed limit using fundamental constants – numbers in physics that govern interactions and properties of subatomic particles. They found that soundwaves can go no faster than 36 kilometers (22 miles) per second under conditions found naturally on Earth. That’s double the speed of sound traveling through a diamond.
Physicist Kostya Trachenko of Queen Mary University, who was involved in the study, said:
The common wisdom was that diamond has the highest speed of sound because it is the hardest material, but we didn’t know whether there was a theoretical fundamental limit to it. The theoretical bound is about twice the speed of sound in a diamond.
The limit only applies to liquids and solids at pressures naturally found on Earth. Sound waves could move faster and surpass this speed limit at pressures millions of times that of Earth’s atmosphere.
Trachenko said:
We have a pretty good idea of these values because if they were changed even a bit, the Universe wouldn’t look at all like it does. If you change these constants by a few percent, then the proton might not be stable anymore, and you might not even have the processes in stars resulting in the synthesis of heavy elements, so there would be no carbon, no life.
We believe the findings of this study could have further scientific applications by helping us to find and understand the limits of different properties such as viscosity and thermal conductivity relevant for high-temperature superconductivity, quark-gluon plasma, and even black hole physics.
The team – from the University of Cambridge in the UK, Queen Mary University of London, and Russia’s Institute for High-Pressure Physics – used the following two well-known physical constants: the fine structure constant plus the ratio of proton mass to electron mass.
The fine structure constant characterizes the strength of interactions between charged particles. Soundwaves are merely disturbances that move energy from one place to another. They’re atoms in movement. Atoms can only move so fast. That limits the speed.
Soundwaves move through liquids or gases much slower than they would through solids. For example, you can hear a train coming if you put your ear to the rail long before you can listen to it in the air when you’re standing there.
Soundwave knowledge is useful to study the inside of Earth and earthquakes. The physicists say they can even use them to understand the interior of stars.
Chris Pickard, a Professor of Materials Science at the University of Cambridge, who was also involved in the study, said:
Soundwaves in solids are already hugely important across many scientific fields. For example, seismologists use sound waves initiated by earthquakes deep in the Earth interior to understand the nature of seismic events and Earth composition properties. They’re also of interest to materials scientists because sound waves are related to important elastic properties, including the ability to resist stress.
The equation the team developed to find the speed of sound could prove to be a valuable tool (so long as the results remain consistent) for understanding not only materials but the broader universe as well.
