Cooling usually pushes matter through a simple sequence. A gas condenses into a liquid, and with further cooling the liquid locks into a solid. Helium helped reveal that the quantum world can take a very different route. In the early 20th century, researchers found that helium, when chilled to extreme temperatures, can enter a superfluid state. In that form, it can move without dissipating energy and shows other counterintuitive behaviors, including creeping up and out of containers.
That discovery left physicists with an even more intriguing puzzle: if a superfluid is cooled further still, does it settle into a new phase, or does “frictionless motion” remain the end of the story? Scientists have been chasing that answer for about fifty years.
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