Physicists at MIT have achieved a major feat, capturing a bizarre quantum effect called second sound on film, where heat ripples through matter like a wave instead of spreading gradually. First predicted in 1938, the phenomenon had eluded direct observation for more than 90 years – until now.
The findings provide a new method to study extreme states of matter. Researchers have said the breakthrough could have far-reaching implications – from refining models of neutron stars to accelerating the quest for practical high-temperature superconductors.
What is second sound?
Under normal conditions, heat spreads simply. However, in quantum states such as superfluids, it behaves differently. Instead of diffusing it pulses like a sound wave, surging back and forth while the surrounding fluid remains still.
“It’s the hallmark of superfluidity, but until now, we could only detect it indirectly,” Professor Martin Zwierlein, the developer of this method, explained.
Cracking a 90-year puzzle
The primary challenge in this experiment was in measurement. Experiments of this kind require gases cooled to absolute zero temperature. The temperature has to be so low that atoms emit no infrared radiation, rendering traditional thermal imaging ineffective.
To overcome this obstacle, the MIT team employed lithium-6 atoms, a rare isotope whose resonance frequency shifts depending on temperature. By applying carefully tuned radio waves, the researchers managed to cause warmer atoms to resonate, effectively allowing the movement of heat to be tracked across the gas.
This innovative method produced the first instance of second sound in action, revealing heat waves rippling back and forth like acoustic vibrations.
“For the first time, we can actually take pictures of this substance as it cools into a superfluid. We can literally watch it transition from a normal fluid to a quantum one,” said co-author Richard Fletcher.
The new method has clear advantages, including real-time visualization of heat propagation, unprecedented temperature precision, and reliability in extreme conditions where traditional thermography fails.
Scientific and technological implications
Beyond solving a decades-old mystery, the ability to observe second sound offers powerful tools for multiple disciplines.
In astrophysics, neutron stars are known to have vast superfluid layers. Understanding how heat propagates in such states may improve models of their evolution and internal structure.
On earth, the findings may inform research into superconductivity. High-temperature superconductors have long been viewed as the ‘Holy Grail’ of energy technology, with potential applications in lossless power transmission, magnetic levitation, and advanced computing.
“Our gas cloud is a million times less dense than air, yet its behavior closely resembles how electrons act in superconductors,” said Zwierlein. “That makes it an extraordinary system for study.”
Future directions
The MIT researchers plan to expand their approach to study other quantum materials under even more extreme conditions. Future work will explore how second sound interacts with additional quantum effects, with the goal of developing predictive models for new superconducting compounds.
The direct capture of second sound shows that even long-standing theoretical predictions in physics may eventually yield to persistence and ingenuity.
This discovery gives scientists a new way to study hidden effects in nature, helping us better understand the universe and develop future energy technologies.