Outer space may be cool, but not cool enough for physicists at the Massachusetts Institute of Technology who have chilled a gas to the coldest temperature ever recorded.
The coldest known place in nature is in deep space where gases are 3 degrees above absolute zero, or about -454 degrees Fahrenheit. The physicists bested that frigidity by more than 2 degrees.
In a microscopic cavity created by the repelling walls of magnetic fields, the researchers cooled a sodium gas to only a half-billionth of a degree above absolute zero. Absolute zero, or -460 degrees Fahrenheit, is the temperature where all atoms stop moving.
This is the first time that a gas has been cooled below 1 nanokelvin, which is one-billionth of a degree Kelvin.
"To go below 1 nanokelvin is a little like running a mile under four minutes for the first time," said Nobel laureate Wolfgang Ketterle, co-leader of the MIT team, which also included David Pritchard, a physicist at the university.
Cold Atoms Create New Matter
At room temperatures, atoms move at the speed of a jet airplane. At less than 1 nanokelvin, the atoms screech to a crawl, moving only one inch every 30 seconds. Atomic energy corresponds to heat since atoms' motion generates heat. So very slow-moving atoms are also very cold.
Creating a very, very cold gas is not just a cool achievement, it also helps physicists better understand a new form of matter that forms when gases get extremely cold. And since atoms in the frigid material behave in a much more predictable way than warmer ones, scientists believe it could someday be used as a tool to take extremely precise measurements.
Ketterle and a group at the University of Colorado at Boulder were the first to get a glimpse of this new matter in 1995 when they cooled a gas to 1 microkelvin — one-millionth of a degree Kelvin. Two years later, Eric Cornell and Carl Wieman documented the peculiar form when they cooled sodium gas to a few billionths of a degree above absolute zero. At this temperature, the atoms lose their separate identities and begin to behave as one.
"They start marching in step," said Aaron Leanhardt, an MIT graduate student who was part of the team. "Rather than millions of atoms moving independently at random, they move in step and become one."
The matter is known as Bose-Einstein condensate, named after an Indian scientist, Satyendra Nath Bose, and Albert Einstein, who predicted in 1924 how matter might behave near absolute zero.
Cold Atom Tools
To cool a sodium gas to a temperature about 1,000 times cooler than the previous tries, the MIT team borrowed old techniques and added one new one.
The scientists first used lasers to push against the atoms to slow them. Then they removed hotter atoms by selecting out ones that bounced near the edges of the magnetic box.
"It's like if you roll a ball up a hill, the more energy it has, the higher it goes," explained Leanhardt. "So if I throw away all the balls at the top of the hill, I leave behind the low-energy balls."
Finally, they lowered the pressure within the magnetic box to allow the gas to expand. Since gas warms when it's under pressure and cools as it expands, the gas cooled even more.
"It's a very important result," said Neil Sullivan, a physicist at the University of Florida in Gainesville who works with extremely low temperatures. "By understanding physics at such a simple, pure level, we can apply this knowledge in a whole range of applications."