Physicists don’t tend to use universal language freely, so since Lord Kelvin dubbed the base measure of his temperature scale “absolute zero,” that should be a sign that there is reason for the “absolute” nature of the measure to be taken seriously.
Unlike the centigrade scale, where 0 degrees marks the freezing point of fresh water, or the Fahrenheit scale, where 0 is the stable temperature of a mix of water, ice, ammonium chloride and sea salt, 0 Kelvin—absolute zero—is, theoretically, the coldest thing imaginable. This is the temperature when all motion comes to a crawl, when molecules stop quivering and atoms stop moving.
In Nature, Zeeya Merali reports on the work of Stefan Braun and colleagues, a group of physicists that managed to overthrow Kelvin’s absolutism by creating “an atomic gas with a sub-absolute-zero temperature for the first time.”
Through an elaborate approach—harnessing lasers and rapidly-flipping magnetic fields—the scientists figured out how to push atoms to temporarily take on energies below absolute zero. The scientists manipulated a cloud of potassium atoms trapped by lasers, says Merali:
At positive temperatures, the atoms repel, making the configuration stable. The team then quickly adjusted the magnetic fields, causing the atoms to attract rather than repel each other. “This suddenly shifts the atoms from their most stable, lowest-energy state to the highest possible energy state, before they can react,” says Schneider. “It’s like walking through a valley, then instantly finding yourself on the mountain peak.”
At positive temperatures, such a reversal would be unstable and the atoms would collapse inwards. But the team also adjusted the trapping laser field to make it more energetically favourable for the atoms to stick in their positions. This result, described today in Science, marks the gas’s transition from just above absolute zero to a few billionths of a Kelvin below absolute zero.
Researchers imagine that if new materials could somehow be made out of below-freezing atoms, then they would probably act in some mighty strange ways.
For instance, Rosch and his colleagues have calculated that whereas clouds of atoms would normally be pulled downwards by gravity, if part of the cloud is at a negative absolute temperature, some atoms will move upwards, apparently defying gravity.
Even in physics, it seems, some rules are made to be broken.