Physicists capture 'second sound' for the first time — after nearly 100 years of searching

An artist's illustration of particles moving inside a box.

Scientists have captured direct images of heat behaving like sound — an elusive phenomenon called 'second sound' — for the very first time.

Imaged within an exotic superfluid state of cold lithium-6 atoms by a new heat-mapping technique, the phenomenon shows heat moving as a wave, bouncing like sound around its container.