Second sound

Usually, when something gets warmed up, heat tends to spread outward before eventually dissipating. But things are a little different in the world of superfluid quantum gas. For the first time, MIT scientists have successfully imaged how heat actually travels in a wave, known as a “second sound,” through this exotic fluid. Understanding this dynamic could help answer questions about high-temperature superconductors and neutron stars. In the world of average, everyday materials, heat tends to spread out from a localized source. Drop a burning coal into a pot of water, and that liquid will slowly rise in temperature before its heat eventually dissipates. But the world is full of rare, exotic materials that don’t exactly play by these thermal rules. Instead of spreading out as one would expect, these superfluid quantum gasses “slosh” heat side to side—it essentially propagates as a wave . Scientists call this behavior a material’s “second sound” (the first being ordinary sound via a density wave). Although this phenomenon has been observed before, it’s never been imaged. 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 (a.k.a. heat-mapping). The results of this study were published in the journal Science , and in an 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. “It’s as if you had a tank of water and made one half nearly boiling,” Fletcher said. “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.” These superfluids are created when a cloud of atoms is subjected to ultra-cold temperatures approaching absolute zero (−459.67 °F). In this rare state, atoms behave differently, as they create an essentially friction-free fluid. It’s in this frictionless state that heat has been theorized to propagate like a wave. This might feel like a big “so what?” After all, when’s the last time you had a close encounter with a superfluid quantum gas? But ask a materials scientist or astronomer, and you’ll get an entirely different answer. While exotic superfluids may not fill up our lives (yet), understanding the properties of second wave movement could help questions regarding high-temperature superconductors (again, still at very low temperatures) or the messy physics that lie at the heart of neutron stars . Darren lives in Portland, has a cat, and writes/edits about sci-fi and how our world works. You can find his previous stuff at Gizmodo and Paste if you look hard enough.