A research collaboration between Queen Mary University of London, the
University of Cambridge and the Institute for High Pressure Physics in
Troitsk has discovered the fastest possible speed of sound.
The result- about 36 km per second—is around twice as fast as the speed of
sound in diamond, the hardest known material in the world.
Waves, such as sound or light waves, are disturbances that move energy from
one place to another. Sound waves can travel through different mediums, such
as air or water, and move at different speeds depending on what they're
travelling through. For example, they move through solids much faster than
they would through liquids or gases, which is why you're able to hear an
approaching train much faster if you listen to the sound propagating in the
rail track rather than through the air.
Einstein's theory of special relativity sets the absolute speed limit at
which a wave can travel which is the speed of light, and is equal to about
300,000 km per second. However until now it was not known whether sound
waves also have an upper speed limit when travelling through solids or
liquids.
The study, published in the journal Science Advances, shows that predicting
the upper limit of the speed of sound is dependent on two dimensionless
fundamental constants: the fine structure constant and the
proton-to-electron mass ratio.
These two numbers are already known to play an important role in
understanding our Universe. Their 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 life-supporting molecular structures can
emerge. However, the new findings suggest that these two fundamental
constants can also influence other scientific fields, such as materials
science and condensed matter physics, by setting limits to specific material
properties such as the speed of sound.
The scientists tested their theoretical prediction on a wide range of
materials and addressed one specific prediction of their theory that the
speed of sound should decrease with the mass of the atom. This prediction
implies that the sound is the fastest in solid atomic hydrogen. However,
hydrogen is an atomic solid at very high pressure above 1 million
atmospheres only, pressure comparable to those in the core of gas giants
like Jupiter. At those pressures, hydrogen becomes a fascinating metallic
solid conducting electricity just like copper and is predicted to be a room
temperature superconductor. Therefore, researchers performed
state-of-the-art quantum mechanical calculations to test this prediction and
found that the speed of sound in solid atomic hydrogen is close to the
theoretical fundamental limit.
Professor Chris Pickard, Professor of Materials Science at the University of
Cambridge, 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 the properties of Earth composition. They're also of
interest to materials scientists because sound waves are related to
important elastic properties including the ability to resist stress."
More information:
"Speed of sound from fundamental physical constants" Science Advances (2020).
DOI:
Tags:
Physics