Helium's liquidity at low temperatures allows it to carry out a transformation called Bose–Einstein condensation, in which individual particles overlap until they behave like one big particle. Hence, they never settle into the solid state. But helium atoms are so light and weakly drawn to one another that even when ordinary atomic motions have quieted, the atoms jiggle with zero-point motion, a slight momentum imparted by the quantum uncertainty principle. When most liquids are cooled, the slight attraction between atoms in the fluid finally begins to overcome heat vibrations, and the particles settle into a regular order, namely a solid. Key to the effect is helium's unique ability to remain liquid down to absolute zero (–459.67 degrees F, or –273.15 degrees C), the temperature at which atoms theoretically stop moving. The viscosity was so low that Kapitsa, who won his own Nobel Prize for the work, coined the term "superfluid" to describe it-after "superconductor," the term for a material that conducts very high electric currents without resistance. It wasn't until 1938 that the Russian physicist Pyotr Kapitsa and, independently, the British duo of John Allen and Don Misener measured the flow rate of helium below that temperature through a pair of glass disks attached to a plunger and a long, thin glass tube, respectively. Onnes found that helium (technically, the helium 4 isotope) began to readily conduct heat below –455.67 degrees F (–270.92 degrees C), also known as the lambda point. But unlike other quantum stuff, superfluid helium's weird behavior is visible to the naked eye.Īn early sign of helium's odd behavior was observed back in 1911 by the Dutch physicist and 1913 Nobel physics laureate Heike Kamerlingh Onnes, a master of refrigeration who was the first to liquefy helium. Like plenty of other physics experiments that make you go-"Huh?"-superfluidity flows from the counterintuitive rules of quantum mechanics. "But if you did that with helium at low temperature and came back a million years later," he says, "it would still be moving." Atoms in the liquid will collide with one another and slow down. "If you set a cup with a liquid circulating around and you come back 10 minutes later, of course it's stopped moving," says John Beamish, an experimental physicist at the University of Alberta in Edmonton. No longer a mere liquid, the helium has become a superfluid-a liquid that flows without friction. Researchers have known for decades that if you cool liquid helium just a few degrees below its boiling point of –452 degrees Fahrenheit (–269 degrees Celsius) it will suddenly be able to do things that other fluids can't, like dribble through molecule-thin cracks, climb up and over the sides of a dish, and remain motionless when its container is spun. That's because soft drinks are nothing like the superfluid helium shown in this video. You don't have to worry about a soft drink spontaneously overflowing its rim or shooting up and out of the straw from which you're trying to drink.
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