Physicists capture 'second sound' for the first time — after nearly 100 years of searching

Scientists have captured direct images of heat behaving like sound — an elusive phenomenon called 'second sound' — for the very first time.

Imaged within an exotic superfluid state of cold lithium-6 atoms by a new heat-mapping technique, the phenomenon shows heat moving as a wave, bouncing like sound around its container.

Understanding the way that second sound moves could help scientists predict how heat flows inside ultradense neutron stars and high-temperature superconductors — one of the "holy grails" of physics whose development would enable near-lossless energy transmission. The researchers published their findings in the journal Science.

"It's as if you had a tank of water and made one half nearly boiling," study co-author Richard Fletcher, an assistant professor of physics at Massachusetts Institute of Technology (MIT), said in a statement. "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."

Typically heat spreads from a localized source, slowly dissipating across an entire material as it raises the temperature across it.

But exotic materials called superfluids needn't play by these rules. Created when clouds of fermions (which include protons, neutrons and electrons) are cooled to temperatures approaching absolute zero, atoms inside superfluids pair up and travel frictionlessly throughout the material.

Related: Physicists make record-breaking 'quantum vortex' to study the mysteries of black holes

As a result, heat flows differently through the material: instead of spreading through the movements of particles within the fluid, as it typically flows, heat sloshes back and forth within superfluids like a sound wave. This second sound was first predicted by the physicist László Tisza in 1938, but heat-mapping techniques have, until now, proven unable to observe it directly.

"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," study senior-author Martin Zwierlein, a professor of physics at MIT, said in the statement. "The character of the heat wave could not be proven before."

To capture second sound, the researchers had to solve a daunting problem in tracking the flow of heat inside ultracold gases. These gases are so cold that they do not give off infrared radiation, upon which typical heat-mapping, or thermography, techniques rely.

Instead, the physicists developed a method to track the fermion pairs through their resonant frequencies. Lithium-6 atoms resonate at different radio frequencies as their temperatures change, with warmer atoms vibrating at higher frequencies.

By applying resonant radio frequencies corresponding to warmer atoms, the scientists made these atoms ring in response, enabling them to track the particles’ flow frame by frame.

"For the first time, we can take pictures of this substance as we cool it through the critical temperature of superfluidity, and directly see how it transitions from being a normal fluid, where heat equilibrates boringly, to a superfluid where heat sloshes back and forth," Zwierlein said.

The physicists say that their groundbreaking technique will enable them to better study the behaviors of some of the universe's most extreme objects, such as neutron stars, and measure the conductivity of high-temperature superconductors to make even better designs.

"There are strong connections between our puff of gas, which is a million times thinner than air, and the behavior of electrons in high-temperature superconductors, and even neutrons in ultradense neutron stars," Zwierlein said. "Now we can probe pristinely the temperature response of our system, which teaches us about things that are very difficult to understand or even reach."