Existence of "second sound" confirmed by Physicists

Fascinating existence of "second sound" captured for the first time

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. First theorized in 1938, heat's wave-like flow through superfluid's, known as "second sound", has proven difficult to directly observe. Now, a new technique has finally done it, and could be used to study neutron stars and high-temperature superconductors.  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. Scientists have captured direct images of heat behaving like sound, an elusive phenomenon called 'second sound', for the very first time. Imaged within an exotic superfluid state of cold lithium-6 atoms by a new heat-mapping technique, the phenomenon shows heat moving as a wave, bouncing like sound around its container.

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. Understanding the way that second sound moves could help scientists predict how heat flows inside ultra dense neutron stars and high-temperature superconductors, one of the "holy grails" of physics whose development would enable near-lossless energy transmission. 

A superfluid is a special liquid that moves without viscosity. In helium-4, this behaviour 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. "It's as if you had a tank of water and made one half nearly boiling," study co-author Richard Fletcher, an assistant professor of physics at Massachusetts Institute of Technology (MIT), said. "If you then watched, the water itself might look totally calm, but suddenly the other side is hot, and then the other side is hot, and the heat goes back and forth, while the water looks totally still." Typically heat spreads from a localized source, slowly dissipating across an entire material as it raises the temperature across it. But exotic materials called superfluid's needn't play by these rules. Created when clouds of fermions (which include protons, neutrons and electrons) are cooled to temperatures approaching absolute zero, atoms inside superfluid's pair up and travel frictionlessly throughout the material.

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 which 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 which 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. Heat flows differently through the material: instead of spreading through the movements of particles within the fluid, as it typically flows, heat sloshes back and forth within superfluid's like a sound wave. This second sound was first predicted by the physicist László Tisza in 1938, but heat-mapping techniques have, until now, proven unable to observe it directly.

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 which offered a direct glimpse into the behaviour 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. "Second sound is the hallmark of super fluidity, but in ultra cold gases so far you could only see it in this faint reflection of the density ripples that go along with it," study senior-author Martin Zwierlein, a professor of physics at MIT, said. "The character of the heat wave could not be proven before." To capture second sound, the researchers had to solve a daunting problem in tracking the flow of heat inside ultra cold gases. These gases are so cold that they do not give off infrared radiation, upon which typical heat-mapping, or thermography, techniques rely. Instead, the physicists developed a method to track the fermion pairs through their resonant frequencies. Lithium-6 atoms resonate at different radio frequencies as their temperatures change, with warmer atoms vibrating at higher frequencies.

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 superfluid's and solid systems which 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. By applying resonant radio frequencies corresponding to warmer atoms, the scientists made these atoms ring in response, enabling them to track the particles’ flow frame by frame. "For the first time, we can take pictures of this substance as we cool it through the critical temperature of super fluidity, and directly see how it transitions from being a normal fluid, where heat equilibrates boringly, to a superfluid where heat sloshes back and forth," Zwierlein said.

One of the most surprising findings is that the behaviour 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. This discovery opens the door to rethinking how energy is lost in quantum fluids, especially in systems where traditional viscosity doesn’t apply. The physicists say that their ground breaking technique will enable them to better study the behaviours of some of the universe's most extreme objects, such as neutron stars, and measure the conductivity of high-temperature superconductors to make even better designs. "There are strong connections between our puff of gas, which is a million times thinner than air, and the behaviour of electrons in high-temperature superconductors, and even neutrons in ultra dense neutron stars," Zwierlein said. "Now we can probe pristinely the temperature response of our system, which teaches us about things that are very difficult to understand or even reach." 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 behaviour 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.