Physicists Capture First Ever Images Of “Second Sound” In Superfluid

A team of physicists at MIT have captured the phenomenon of “second sound” in direct images for the first time.

In usual materials, heat prefers to spread out from a localized source until it dissipates into its surroundings. But in certain materials, this is not the case. This includes superfluids, a state of matter caused by cooling atoms to extremely low temperatures. In this state, the superfluid can flow infinitely with no loss of energy or viscosity.


In superfluids, which have plenty of other strange properties to boast of, heat does not move in the same way. Instead, in this friction-free state, physicists predicted heat would propagate as a wave, known as “second sound”.

“It’s as if you had a tank of water and made one half nearly boiling,” Assistant Professor Richard Fletcher explained in a press release. “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.” 

Second sound in action.
Image credit: MIT

Capturing the movement of heat in such fluids is tricky, as they give off no infrared radiation. However, the team found that lithium-6 fermions resonate at different frequencies depending on their temperature. This allowed them to track the movement of resonating fermions, revealing the heat was moving like sound waves.

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

The team plans to continue to map the behavior of heat in other ultracold gases, and believe their findings could be applied to other exotic materials, such as the conditions found inside neutron stars.

The paper is published in Science.

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