For nearly a century, physicists have suspected that heat, under the strangest conditions, could behave like sound. Now, MIT scientists have finally captured it on camera: a phenomenon called “second sound,” where heat doesn’t just diffuse outward but sloshes back and forth like a wave.⁠ ⁠ The effect emerges inside a superfluid, a frictionless state of matter created when clouds of atoms are cooled to temperatures near absolute zero. In this extreme environment, lithium-6 atoms pair up and move without resistance. Instead of fading evenly, a burst of heat oscillates through the superfluid, invisible to the eye but strikingly similar to ripples of sound.⁠ ⁠ To reveal it, researchers developed an ingenious heat-mapping method. Normally, thermography relies on infrared radiation, but ultracold gases emit none. The team instead tuned into the atoms’ radio-frequency “voices,” which shift with temperature. By making the “hotter” fermions resonate, they filmed the pure motion of heat, watchin...