Scientists confirm the incredible existence of ‘second sound’

Here’s what you’ll learn as you read this story:

• Generally, when something warms up, the heat dissipates before it finally dissipates. But things are a little different in the world of superfluid quantum gases.

• For the first time, MIT scientists have successfully imaged how heat travels through this exotic fluid in waves known as “second sound.”

• Understanding this dynamic can help answer questions about high-temperature superconductors and neutron stars.

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In the world of average, everyday materials, heat radiates from local sources. Drop a burning coal into a pot of water, and the liquid will slowly rise in temperature before its heat finally dissipates. But the world is full of rare, exotic materials that don’t exist Exactly Play by these thermal rules.

Instead of spreading as one might expect, these superfluid quantum gases “slosh” at the hot end—it essentially propagates as a wave. Scientists call this behavior the material’s “second sound” (normal sound through the first density wave). Although this phenomenon has been observed before, it has never been illustrated. But recently, scientists at the Massachusetts Institute of Technology (MIT) were finally able to capture this movement of pure heat by developing a new method of thermography (aka heat-mapping).

The results of this study have been made public In the journal ScienceAnd in a university press release highlighting the achievement, MIT assistant professor and co-author Richard Fletcher continued the boiling pot analogy to describe the inherent strangeness of “second sound” in these exotic superfluids.

“You have a tank of water and one half is like boiling,” Fletcher said. “If you look, the water itself may look perfectly still, but suddenly one side is hot, and then the other side is hot, and the heat goes back and forth, while the water looks perfectly still.”

These superfluids are formed when clouds of atoms are brought to ultra-cold temperatures close to absolute zero (−459.67 °F). In this rare case, the atoms behave differently, as they essentially create a frictionless fluid. It is in this frictionless state that heat is theorized to propagate like a wave.

“The second sound is a characteristic of superfluidity, but in ultracold gases you can only see it in this faint reflection of the accompanying density waves,” lead author Martin Zwierlein said in a press release. “The character of heat waves has not been proven before.”

To finally capture this second sound in action, Zweierlein and his team had to think outside the usual thermal box, because trying to track the heat of an ultracold object has a big problem—it doesn’t emit normal infrared radiation. So, MIT scientists designed a way to leverage radio frequencies to track certain subatomic particles known as “lithium-6 fermions,” which can be captured through different frequencies in relation to their temperature (ie hotter temperatures mean higher frequencies, and vice versa). This novel technique allowed the researchers to essentially zero in on the “hot” frequencies (which were still too cold) and track the resulting second wave over time.

This may sound like a big “so what?” After all, when was the last time you had a close encounter with a superfluid quantum gas? But ask a materials scientist or an astronomer, and you’ll get a completely different answer.

While exotic superfluids may not fill our lives (yet), understanding the properties of second wave motion may help answer questions about high-temperature superconductors (again, yet). a lot low temperatures) or the messy physics at the heart of neutron stars.

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