When a cold snap hits and the temperature drops, there’s nothing to stop it from falling below zero, whether Celsius or Fahrenheit. Either zero is just a mark on a thermometer. But drive a temperature lower and lower, beyond the coldest realms in the Arctic and past those in the most distant reaches of outer space, and eventually you hit an ultimate limit: absolute zero.
It’s a barrier enforced by the laws of physics below which temperatures supposedly cannot possibly go. At minus 459.67 degrees Fahrenheit (or minus 273.15 Celsius), all the heat is gone. Atomic and molecular motion ceases. Trying to create a temperature below absolute zero would be like looking for a location south of the South Pole.
Of course, scientists perceive such barriers as challenges. And now some lab trickery has enabled researchers to manipulate atoms into an arrangement that appears to cross the forbidden border. With magnets and lasers, a team at Ludwig-Maximilians University Munich in Germany has coaxed a cloud of 100,000 potassium atoms into a state with a negative temperature on the absolute scale.
“It forces us to reconsider what we believe to know about temperature,” says Ulrich Schneider, one of the leaders of the research team.
As a bonus, the weird configuration of matter might provide clues to some deep mysteries about the universe.
Schneider and his colleagues relied on laser beams to trap the atoms in a grid, kind of like the dimples in an egg carton. By tuning the lasers and applying magnetic fields, the team could control the energy of the atoms, key to manipulating temperature.
Ordinarily, not all the atoms in a sample possess the same amount of energy; some are slow-moving, low-energy sluggards, while others zip about like speed demons. A higher proportion of zippy atoms corresponds to a higher temperature. But most of the atoms are always slower than the very fastest—when the temperature is positive.
With their magnet-and-laser legerdemain, the German scientists pushed the majority of the potassium atoms to higher energies, the opposite of the usual situation. Though that may not seem like a big deal, the switch messed with the mathematics that determines the gas’s temperature, leading to a negative value. Technically, physicists define temperature as a relationship between changes in entropy (a measure of disorder) and energy. Usually more energy increases a system’s entropy. But in the inverted case, entropy decreases as energy increases, flipping the sign of the relationship from positive to negative. The atoms had a temperature of minus a few billionths of a kelvin, the standard unit on the absolute scale.
The catch is that scientists reached temperatures “below” absolute zero in a mathematical sense only. While the negative temperatures were numerically lower than absolute zero, they weren’t colder. In fact, the gas was superhot, hotter than anything with a positive temperature could ever be.
Besides achieving a weird temperature state, the new work replicates a peculiar feature of the universe. Negative temperature systems also possess negative pressure, which on cosmic scales is causing the universe to expand faster and faster. Physicists call the universe’s negative pressure field “dark energy,” but they haven’t been able to figure out exactly what it is. Perhaps negative pressure in a lab could offer insights.