Physicists confirm the fascinating existence of "second sound"

Heat usually spreads until it fades away. In everyday life, a warm spot in liquid quickly blends with cooler areas, and everything settles at a single temperature.

MIT researchers, after exploring a superfluid quantum gas, have shown that heat can travel in a wavelike manner called second sound, instead of spreading out and calming down.

Pantxo Diribarne from the Atomic Energy and Alternative Energies Commission and the University of Grenoble Alpes in France, sees this as a chance to unravel more mysteries about peculiar states of matter.

Understanding “second sound”

The strange and incredible phenomenon known as “second sound” refers to a state where heat moves like a wave, not by diffusion like we’re used to.

Instead of slowly spreading out, thermal energy pulses through a material in much the same way sound travels through air.

It’s not something you’d experience in everyday life, but in ultra-cold or highly ordered systems – like certain crystals or quantum fluids – second sound reveals a completely different side of how energy can move.

This wave is different from how temperature typically flows. Instead of dissipating steadily until it is fully spread out, the heat pulses like ripples on a pond. It’s like heat is speaking a language we rarely get to hear.

The phenomenon known as quantum turbulence comes into play when normal and superfluid components move together at large scales, then lose lockstep at smaller scales.

Superfluids and extreme physics

A superfluid is a special liquid that moves without viscosity. In helium-4, this behavior appears at temperatures below about −456 °F (-271°C).

When the fluid is both superfluid and normal, friction between the two forms can still appear. This friction can produce swirling structures in the superfluid, but it also allows temperature pulses (second sound) to zip through.

Scientists are keen to study high-temperature superconductors, which carry current with little power loss. Some say that second sound might shed light on thermal transport in these systems.

Neutron stars, those incredibly dense objects in space, may also carry clues. A quantum fluid could occupy their interiors and possibly channel heat in ways that match second sound patterns.

Why second sound matters

Researchers tested second sound in helium to see if the same wave idea appears in other exotic materials. Discovering a pattern in superfluid helium might help interpret signals in advanced physics experiments.

With second sound, the puzzle of how energy flows becomes more precise. This clarity supports efforts to design technologies that harness quantum effects, like sensitive sensors or more efficient cooling systems.

The team used new imaging approaches to watch heat pulses bounce through the fluid. By capturing that movement, they separated normal heat spread from the heat wave that never truly mellowed.

Data analysis indicated that the speed of these waves is roughly 49 feet/s (15 meters/s) for helium at 1.6 K, though slight changes in temperature and pressure can shift that speed.

The wave eventually diminishes, but it travels long enough to confirm a distinct second sound.

Quantum tools and second sound

To measure second sound accurately, researchers used a resonant cavity filled with superfluid helium.

This setup allowed them to create and track standing temperature waves that offered a direct glimpse into the behavior of vortex lines and the space between them.

They paired this with particle-tracking techniques using hollow glass microspheres.

These tiny tracers helped capture the motion of the fluid itself, and showed how heat pulses affected surrounding particles – without disturbing the second sound signal.

Insights from turbulence

Past studies tried to explain second sound by focusing on vortex lines, which are small, swirling cores in the superfluid.

Recent work suggests these lines set a key spacing level where wave-like temperature movement can dominate.

The surprising outcome is that friction does not single-handedly decide how heat flows.

Instead, large-scale circulation and vortex tangles form a cascade that shapes when ordinary heat conduction switches to a traveling wave.

Research might push second sound concepts into higher temperatures. That would bridge a gap between helium superfluids and solid systems that show wave-like temperature travel.

Critics note that temperature swings and mechanical vibrations sometimes mask delicate signals. To address this, scientists plan stricter temperature control and more refined imaging in the next generation of tests.

Temperature independence

One of the most surprising findings is that the behavior of second sound remained nearly unchanged across different temperatures.

Researchers expected the friction between fluid components to vary more significantly, but the measurements showed very little temperature dependence.

This suggests that something else, possibly the structure of the fluid’s internal turbulence, plays a larger role than previously thought.

That discovery opens the door to rethinking how energy is lost in quantum fluids, especially in systems where traditional viscosity doesn’t apply.

Second sound and future technology

If second sound ideas link to superconductors, we might improve next-gen energy lines. Some also dream of applying wave-based cooling in labs.

On cosmic scales, linking superfluid features to neutron star interiors could hint at how these stars shed energy.

Tracking those waves might lead to fresh insights into the behavior of matter under crushing forces of gravity.

Even though heat normally spreads until it dies down, the phenomenon of second sound defies that notion.

Scientists are now exploring how temperature pulses might drive new physics in quantum fluids and even in cosmic bodies.

The study is published in arXiv.

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