Heat is not supposed to behave like this. In everyday life, warmth seeps and diffuses, spreading from hot to cold in a slow, smearing process that never looks anything like a crisp sound wave. Yet physicists have now confirmed that under extreme quantum conditions, heat can travel as a coherent wave, a phenomenon known as “second sound” that turns temperature itself into something that sloshes, echoes, and rings.
After decades of theory and indirect hints, researchers have finally captured direct images and measurements of this effect in superfluid helium, showing that second sound is not just a mathematical curiosity but a real, trackable motion of energy. The result opens a new window on quantum matter, from ultra cold lab experiments to the violent interiors of neutron stars, and it hints at technologies that might one day steer heat with the same precision we now reserve for light and radio.
What “second sound” actually is
When I talk about second sound, I am really talking about a radical rewrite of how heat can move. In familiar materials, temperature spreads by diffusion, like a drop of dye dispersing in water, which is why a hot pan slowly warms the air around it instead of sending out sharp thermal pulses. In a quantum fluid cooled to extreme lows, however, the atoms can move in lockstep, and under those conditions, theory predicts that heat does not just smear out, it essentially propagates as a wave, with packets of thermal energy marching together the way pressure variations do in ordinary sound.
That is the core of what recent experiments have now confirmed, turning the long standing idea of a “second” kind of sound into a directly observed reality. In these systems, the usual acoustic vibrations are still there, but layered on top is a second channel in which entropy and temperature oscillate, so that a region of fluid can be alternately hotter and cooler as the wave passes. As one report on Scientists Confirm the Incredible Existence of this effect explains, the strange part is not that heat moves, but that it does so in a disciplined, wave like fashion instead of the usual random shuffle.
From abstract theory to visible waves
The idea that heat might behave like sound is not new, but for most of its history it lived in equations rather than in images. The concept emerged from quantum descriptions of superfluid helium in the late 1930s, when theorists realized that a liquid with almost no viscosity could support two distinct kinds of collective motion, one tied to density and one tied to entropy. That prediction lingered for generations as a kind of exotic footnote, a 90-year-old puzzle that seemed almost impossible to probe directly because it required both extreme cold and exquisitely sensitive measurements.
Over time, experimentalists chipped away at the problem, building ever more refined cryogenic setups and detection schemes to look for subtle temperature oscillations. The recent work that finally made second sound visible did not come out of nowhere, it stands on decades of incremental progress in cooling helium, controlling its flow, and tracking tiny changes in its state. One account describes how a 90-year-old quantum prediction has now been filled in by direct observation, turning what had been an invisible heat wave into something researchers can actually see and measure.
MIT’s “sloshing heat” images
The turning point came when a team at MIT developed a way to watch heat itself slosh back and forth inside a superfluid. Instead of inferring second sound from indirect signatures, they engineered a setup where they could inject a localized burst of heat into helium cooled to the superfluid state, then track how that disturbance moved. For the first time, they captured direct images of this motion, revealing a standing wave pattern in which temperature rose and fell rhythmically, like water in a shaken glass, rather than simply diffusing away.
In their description of the work, the researchers emphasize that they were not just listening for a subtle acoustic signal, they were effectively filming the “first sounds of heat sloshing in a superfluid” as the thermal wave bounced between boundaries. The experiment, carried out in a carefully controlled cryogenic apparatus associated with the Research Laboratory of Electronics, showed that the oscillations matched the predicted behavior of second sound and that the wave speed and pattern lined up with theory. As one summary of the MIT work notes, this was the first time anyone had literally seen heat move like this, rather than just deducing its presence from bulk measurements.
Physicists finally “hear” second sound
Once those images were in hand, other teams moved quickly to confirm and extend the result, turning second sound from a niche curiosity into a robustly observed phenomenon. In one widely shared demonstration, researchers showed that when they pulsed heat into superfluid helium, the resulting temperature wave traveled across the fluid and reflected from boundaries in a way that could be picked up as a distinct signal. The effect was striking enough that a short video explaining how physicists have confirmed the existence of second sound circulated widely, highlighting how strange it is to see heat behave like a ripple instead of a blur.Other reports framed the achievement as the culmination of a nearly century long search, noting that second sound had been theorized in 193 and that it took about 100 years of advances in low temperature physics and imaging to finally capture it cleanly. One detailed account describes how Physicists used sensitive thermography techniques to pick up the hallmark wave pattern, stressing that conventional temperature measurements, which average over space and time, would have completely missed the effect. By treating heat as something that could be imaged frame by frame, they were able to “hear” second sound in the data as a clear, periodic signal.
Taking the temperature of a quantum wave
Confirming that second sound exists is one thing, but characterizing it with precision is another, and that is where more recent work has focused. To turn this phenomenon into a tool, researchers need to know how fast the thermal wave travels, how strongly it interacts with the fluid, and how its properties change with temperature and pressure. That means measuring not just the presence of oscillations, but their detailed temperature profile, so that each crest and trough can be mapped onto the underlying quantum state of the liquid.
One group tackled this by building an experiment specifically designed to “take the temperature” of second sound, using ultra sensitive thermometers and carefully timed heat pulses to reconstruct the wave. Their results showed that the speed and attenuation of the wave matched long standing theoretical predictions, and they traced the effect back to the two fluid model of superfluid helium first developed in the mid twentieth century. A report on how Wave like heat moves through a superfluid notes that this work builds on earlier experiments by Vasilii Peshkov, but pushes the precision to a new level, turning a qualitative observation into a quantitative benchmark for quantum hydrodynamics.
Imaging vortices and the structure of superfluids
Second sound is not just a curiosity about how heat moves, it is also a powerful probe of the internal structure of superfluids. When helium enters the superfluid state, it can form quantized vortices, tiny swirling cores where the otherwise frictionless flow twists into microscopic whirlpools. These vortices are central to how the fluid responds to rotation and external forces, but they are notoriously hard to see directly, which is where wave like heat becomes a surprisingly useful ally.
By sending second sound through a superfluid and watching how the wave scatters and distorts, researchers can infer the presence and arrangement of these vortices, effectively using temperature oscillations as a kind of sonar. One detailed account explains how Physicists used new imaging techniques to map the swirling cores in the superfluid, showing that second sound interacts strongly with these structures. In that work, the team framed their results under the banner of “Understanding” the strange and incredible behavior of superfluids, arguing that wave based heat transport is a key to decoding how these quantum liquids organize themselves at the smallest scales.
Why superfluids make heat behave so strangely
To make sense of why second sound appears in superfluids but not in your kitchen sink, I find it helpful to think about how order changes the rules of motion. In a normal liquid, atoms jostle randomly, and any local burst of heat quickly gets scrambled by collisions, which is why temperature gradients smooth out. In a superfluid, by contrast, a large fraction of the atoms occupy the same quantum state, moving coherently in a way that suppresses friction and allows the fluid to flow without resistance through narrow channels and around obstacles.
This coherence means that disturbances can propagate without being immediately damped, and that includes disturbances in entropy and temperature. When a heat pulse enters such a fluid, it can set up a collective oscillation in which the “normal” and “superfluid” components move relative to each other, carrying thermal energy as a wave. One overview of these experiments notes that this behavior is a hallmark of the superfluid state, which arises when atoms are cooled to extremely low temperatures so that they all move in the same way. A report on how Feb experiments captured the first ever images of second sound in a superfluid emphasizes that this state of matter, created by cooling atoms to extremely low temperatures, is what allows heat to move in such an organized fashion.
From lab tanks to neutron stars
Although the most vivid demonstrations of second sound happen in tabletop cryogenic setups, the physics behind it reaches far beyond the lab. Superfluidity is expected to occur in the dense interiors of neutron stars, where matter is compressed to extreme densities and cooled by rapid neutrino emission. In that environment, the same two fluid dynamics that govern helium in a dewar could shape how heat and angular momentum move through a star, influencing everything from its cooling rate to the sudden “glitches” seen in pulsar rotation.
One recent explainer on neutron star behavior describes How Tiny Vortices Inside a Neutron Star Create Giant Pulsar Glitches, highlighting how quantized vortices in a superfluid core can store and suddenly release angular momentum. The same vortices that twist the flow in helium can, at stellar scales, help trigger abrupt changes in a pulsar’s spin, and second sound like heat waves may play a role in redistributing energy during those events. The fact that we can now watch similar vortices and thermal waves in the lab gives astrophysicists a concrete model for processes that unfold deep inside a Neutron star, far beyond the reach of direct observation.
Why controlling heat waves could matter on Earth
Back on Earth, the practical stakes of second sound lie in the promise of controlling heat with far more finesse than diffusion allows. Modern electronics, from gaming laptops to data center servers, are limited as much by thermal management as by raw processing power, and engineers are constantly searching for ways to move heat away from sensitive components more efficiently. If materials could be engineered to support wave like heat transport at higher temperatures, it might become possible to route thermal energy along specific paths, reflect it from boundaries, or even focus it, much as we now do with light in fiber optics.
Some researchers have already begun to sketch out how this might work, arguing that understanding the dynamics of heat energy in materials under extreme conditions could lead to better models for improving energy technologies. One analysis of these experiments notes that Understanding the bizarre second sound waves in superfluids could inform designs for advanced cooling systems and even shed light on the behavior of particles deep within stars. Another report points out that by observing heat travel like sound, researchers now have a better avenue to investigate the universe’s most intense cosmic environments and, potentially, new means of harnessing energy right on Earth, as described in a piece on after 100 years of chasing this effect.
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