After 100 years, scientists capture first images of ‘second sound’ — What is it?

Scientists capture rare ‘second sound’ phenomenon (Image: Canva)

Ever wondered about heat rebounding like sound within a stagnant fluid? Researchers at MIT have now witnessed it happen — in real life — within a supercooled gas. It's referred to as 'second sound', and it behaves like nothing most individuals study in school.

A brief wave of heat in ultracold gas

In a new paper in Science, MIT researchers actually imaged heat travelling like sound waves. This long-suspected phenomenon, second sound, was observed in a peculiar state of matter. The authors employed lithium-6 atoms chilled to within a hair of absolute zero, where ordinary laws of physics start to warp.

Lithium atoms pair up and glide without friction at such temperatures. This frictionless gliding transforms the gas into a superfluid, which lets heat flow as a wave — rather than slowly spreading out. The outcome? Heat sloshes back and forth, even though the fluid looks otherwise peaceful.

Co-author Richard Fletcher likened the effect to boiling water. One side warms up first, but rather than bubbling, the heat radiates back and forth unseen. "It's as if the water appears calm, but the heat is jumping around," he said in a statement.

Cracking the code with radio signals

Second sound was originally suggested by physicist László Tisza in 1938. But up till now, it had only been intimated by weak ripples in density. It could not be seen directly because conventional heat-mapping devices don't function at these temperatures.

At such ultracold temperatures, materials do not radiate infrared radiation on which regular thermography relies. So the MIT researchers adopted a new strategy. They monitored the way lithium atoms vibrated at various radio frequencies as a function of their temperature. The hotter atoms moved more rapidly, enabling scientists to track them as they moved, frame by frame.

Lead author Martin Zwierlein described this as a milestone. "We can now take sharp images of the superfluid as it hits the critical temperature," he said. "It reveals to us precisely how heat ceases to act normally and starts to propagate in waves."

Clues to the cosmos and future tech

The results bring more than scientific interest. Knowledge of second sound might enable scientists to learn about the flow of heat in neutron stars and within high-temperature superconductors. Such materials, which conduct energy with virtually no loss, hold the potential for efficient power systems.

Zwierlein further added that their ultracold gas bears significant characteristics similar to electrons in superconductors and even the interior matter of neutron stars. "This is a million times thinner than air," he said, "but it allows us to probe things that we can't access easily."

By observing heat travel like sound, researchers now have a better avenue to investigate the universe's most intense conditions — and maybe unlock new means of harnessing energy right on Earth.