In mid-2025, a team of physicists at the Massachusetts Institute of Technology achieved something that had eluded researchers for nearly a century: a direct view of the elusive “second sound.” This is the rare regime where heat no longer merely diffuses but travels as a wave, echoing like sound through a quantum fluid. The finding, published in the journal Science, turns a long-standing prediction into a measured reality and opens doors to both fundamental insights and transformative technologies.
“Second sound” finally filmed: a 90-year-old quantum phenomenon revealed. © Maximillian-cabinet, iStock
From theory to a first direct glimpse
In 1938, physicist László Tisza proposed that in a superfluid, heat could propagate like a collective vibration rather than by slow diffusion. Instead of smearing out, thermal energy forms a standing or traveling wave, while the fluid’s overall mass stays eerily still. This counterintuitive behavior is the hallmark of superfluidity, a macroscopic quantum state with frictionless flow.
Until now, scientists saw only faint traces of this effect, usually via subtle density ripples riding alongside the thermal wave. At MIT, researchers have now captured the phenomenon head-on, mapping temperature oscillations directly inside an ultracold gas. “The second sound is the signature of superfluidity,” says Martin Zwierlein, “and seeing it directly confirms decades of theory.”
An ingenious way to photograph temperature
The challenge was formidable: how do you “see” heat in a system so cold it emits virtually no infrared light? The team turned to lithium‑6 atoms, whose internal resonance frequencies shift subtly with temperature. By tuning radio-frequency signals to match warmer atoms, they could make those atoms respond, lighting up the thermal landscape like a topographic map.
This let the researchers record temperature patterns frame by frame, capturing the rise and rebound of the thermal wave as it coursed through the cloud. Crucially, the technique works even below the critical temperature where the gas becomes superfluid, resolving the moment order takes hold. As coauthor Richard Fletcher noted, “For the first time, we can take images across the transition and watch a normal fluid become a superfluid.”
Why a wave of heat matters
Second sound isn’t a parlor trick; it’s a powerful diagnostic of quantum order and interaction strength. The speed and shape of the wave reveal how particles exchange momentum, how entropy flows, and where the border between normal and superfluid components lies. That information constrains theories of strongly interacting matter and points to regimes where collective behavior dominates.
The system studied—a dilute, ultracold Fermi gas—acts as a pristine analog for other complex materials. By dialing interactions in a clean, controllable environment, the team can test model Hamiltonians that also describe high‑temperature superconductors. Insights from one can sharpen our understanding of the other, bridging atomic physics and condensed matter.
From neutron stars to next‑gen devices
In astrophysics, neutron stars likely host superfluid neutrons in their crusts and cores. How heat pulses move through that interior influences cooling rates, starquakes, and rotational glitches. Direct measurements of second sound on Earth help calibrate models of such extreme environments, bringing distant stellar dynamics into sharper focus.
Closer to home, the same physics underpins candidate mechanisms for high‑Tc superconductivity, where electrons form pairs and march in lockstep. Mapping thermal waves and dissipation could guide the design of materials with lower losses and higher critical temperatures. That, in turn, would reshape energy transmission, sensing, and quantum electronics.
A technique built to travel
Because it tags temperature through resonance, the MIT approach avoids fragile infrared thermography and works where light-based methods fail. It can be adapted to other atoms, lattice geometries, and interaction regimes, providing a unified toolkit for quantum materials. The payoff is a direct window on entropy flow—long the missing piece in many-body experiments.
Key advantages include:
- Real-time tracking of thermal waves with high spatial resolution.
- Operation at ultralow temperatures where standard probes struggle.
- Compatibility with tunable interactions, traps, and lattice potentials.
- Applicability to diverse quantum systems beyond lithium‑6 gases.
What the next wave could reveal
With the method established, researchers can now map how second sound couples to vortices, quasiparticles, and collective modes. They can test universality across bosonic and fermionic superfluids, chart non‑equilibrium dynamics, and probe disorder‑driven transitions. Systematic scans should pin down transport coefficients and refine predictive theories.
The ambition goes further: apply similar ideas to strongly correlated electron materials, where imaging entropy could expose the scaffolding of unconventional superconductivity. Each new measurement tightens the feedback loop between experiment and theory, compressing timelines from discovery to design.
A new lens on order in motion
Second sound has stepped from theory into view, carrying heat as a coherent signal through quantum matter. By turning temperature into an image, the MIT team transformed an invisible quantity into a dynamic observable. The result is a sharper, more granular grasp of superfluid order—and a pathway to decoding other enigmatic phases where entropy doesn’t just spread; it sings.
