A team of researchers has directly observed a thermal phenomenon never before seen in action, confirming the existence of what they are now calling second sound—a highly unusual way in which heat moves through certain ultra-cold materials. Unlike in typical substances where heat dissipates outward, in superfluid quantum gases, heat propagates in rhythmic waves, bouncing back and forth like sound waves traveling through air.
This breakthrough, achieved by scientists at the Massachusetts Institute of Technology (MIT) and published in Science, represents a major step in understanding quantum materials, with potential applications in superconductivity, advanced energy transfer, and even astrophysics.
A New Way Heat Moves Through Matter
In conventional materials, thermal energy spreads gradually from a hotter region to a cooler one, much like how a cup of coffee cools as heat escapes into the air. However, in superfluid quantum gases, the standard rules of thermal conduction do not apply. Instead of dispersing randomly, heat moves in coherent waves, a phenomenon known as second sound.
Richard Fletcher, an assistant professor of physics at MIT, explained the concept using an analogy:
“It’s as if you had a tank of water and made one half nearly boiling. 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.”
This behavior occurs in superfluid states, where gases are cooled to temperatures approaching absolute zero (-273.15°C). In this extreme environment, atoms behave collectively, forming a frictionless fluid in which heat does not simply spread—it moves in organized waves.
Overcoming the Challenge of Seeing the Invisible
Although physicists have theorized and indirectly measured second sound in the past, no one had ever visualized it directly. One of the primary challenges was that ultra-cold quantum gases do not emit infrared radiation, making traditional heat-mapping techniques ineffective.
To solve this, MIT researchers developed an entirely new approach using radio-frequency tracking of lithium-6 fermions—subatomic particles whose frequency response changes depending on temperature. By carefully measuring these shifts, the team was able to track heat propagation in real-time, effectively capturing the motion of second sound for the first time.
Implications for Quantum Science and Beyond
While this phenomenon may seem distant from everyday applications, its implications are far-reaching. Second sound plays a crucial role in:
- Superconductivity: Understanding how heat moves in exotic materials could advance the development of zero-resistance electrical conductors.
- Quantum Computing: Improved thermal management techniques could help create more efficient cooling systems for quantum processors.
- Astrophysics: Neutron stars exhibit extreme conditions similar to superfluid states, meaning insights from this research could help explain their internal dynamics.
Lead researcher Martin Zwierlein emphasized the significance of this discovery, stating:
“Second sound is the hallmark of superfluidity, but in ultracold gases so far you could only see it in this faint reflection of the density ripples that go along with it. The character of the heat wave could not be proven before.”
With this breakthrough, scientists now aim to study how second sound behaves in different materials and conditions. Future research will focus on applying this knowledge to practical technologies, particularly in the fields of quantum materials, superconductors, and space exploration.
By capturing second sound for the first time, researchers have opened the door to a new understanding of thermal physics—one that could redefine how heat is managed in some of the most advanced technologies of the future.
