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. In the world of average, everyday materials, heat tends to spread out from a localized source. Scientists call this behavior a material’s “second sound” (the first being ordinary sound via a density wave). “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,” lead author Martin Zwierlein said in a press statement. This novel technique allowed the researchers to essentially zero in on the “hotter” frequencies (which were still very much cold) and track the resulting second wave over time.