Scientists at the Massachusetts Institute of Technology (MIT) have achieved a breakthrough by directly observing the phenomenon known as “second sound” in an ultra-textual substance. This phenomenon involves the transmission of heat through a special state of matter, resembling the propagation of sound waves.
Typically, thermal energy is dispersed in the environment, with a hotter object cooling down and the surrounding materials heating up until thermal equilibrium is reached. However, certain uncommon materials exhibit unique properties that allow physicists to conduct experiments. Superfluids are an example of such materials, characterized by their zero viscosity, enabling the flow of substances without any resistance or friction. It has long been hypothesized that heat can be transmitted through superfluids, similar to sound waves, hence the name “second sound.” However, direct observation of this phenomenon has not been possible until now.
Richard Fletcher, Assistant Professor and one of the authors of the study, provides an analogy to illustrate the phenomenon: “Imagine you have a tank of water, and one half is almost boiling. If you were to observe it, the water would appear completely calm, but suddenly the other half starts heating up, then the warmth oscillates back and forth, while the water appears motionless.”
To visualize and study this phenomenon, researchers had to develop a novel approach to detecting heat. While infrared sensors are typically employed for this purpose, the ultra-textual substance created by the researchers cools the quantum gas to almost absolute zero. At such low temperatures, infrared radiation is not emitted, necessitating the use of radio waves instead of infrared sensors.
The researchers used a quantum gas composed of lithium fermions-6. They discovered that the higher the temperature of these fermions, the higher the frequency of their resonance. The team transmitted a higher radio frequency to the gas, causing the hotter fermions to resonate in response. By tracking the different fermions that resonated at various times, the scientists were able to visualize the “second sound” as thermal waves fluctuated back and forth.
Martin Zwierlein, the lead author of the study, explains the significance of their findings: “For the first time, we can capture an image of this substance as it cools past the critical temperature of the superfluid, and directly observe its transition from a regular liquid state with evenly distributed heat to a superwearing state where heat fluctuates back and forth.”
This observation of the peculiar “second sound” phenomenon can contribute to a better understanding of thermal conductivity in rare states of matter, such as superconductors and neutron stars. In turn, this understanding may facilitate the development of more advanced systems.
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