Credit: ZME Science.A cloud of gas normally spreads out quickly. But aboard China’s Tiangong space station, scientists made a cloud of atoms disperse so slowly that they could still detect it seven seconds after letting it go.
They had pushed the atoms astonishingly close to the ultimate limit of absolute coldness. When something is really hot or really cold, it means that the atoms are vibrating extremely fast or extremely slowly, respectively. By measuring how slowly the cloud expanded, the researchers calculated an equivalent temperature of just 9.7 picokelvins — or 0.0000000000097 kelvins above absolute zero. That’s nearly four times lower than the previous record, and about 280 billion times below the temperature of the faint radiation that fills the universe.
The achievement, reported in the journal Science, is not simply another extreme-temperature stunt meant for the record books and then forgotten. A cloud that spreads this slowly could serve as an exceptionally sensitive instrument for measuring gravity and testing the laws of physics.
One major caveat before reading on about this quite amazing physics experiment: the space station itself wasn’t cooled to 9.7 picokelvins. Nor is that the cloud’s ordinary thermodynamic temperature. Equivalent temperature expresses the energy associated with the atoms’ expansion after their trap is released. The less energy it has, the more slowly it spreads.
When Atoms Start Behaving Like One
Absolute zero, in the ballpark of minus 273.15 degrees Celsius, is the lower limit of temperature. It can’t get any colder than that ever. Meanwhile, the maximum theoretical temperature that the laws of physics allow for is ridiculous in comparison: 1.417 × 10³² °C (142 nonillion degrees Celsius; 1.4 followed by 32 zeroes).
To put this latest achievement into perspective, even the radiation left over from the Big Bang bathes space in a glow of about 2.7 kelvins. The new cloud’s equivalent temperature is numerically about 280 billion times smaller.
At extremely low temperatures, atoms can behave in ways that seem impossible in everyday life. Rather than acting as individual particles, many settle into the same quantum state and behave collectively, like one enormous matter wave.
Physicists call this a Bose-Einstein condensate. Satyendra Nath Bose and Albert Einstein predicted it in the 1920s, but scientists didn’t create one in a laboratory until 1995. The achievement later earned a Nobel Prize in Physics.
Producing condensates is challenging enough on Earth. Gravity makes it harder to hold atoms in the extremely weak traps needed for further cooling. Researchers have therefore turned to brief moments of weightlessness in drop towers, and eventually to orbit.
NASA’s Cold Atom Laboratory created the first Bose-Einstein condensate in Earth orbit in 2018, and later experiments aboard the International Space Station reached picokelvin energy scales.
Next, in 2021, a German-led experiment achieved the previous 38-picokelvin record in a drop tower, but could observe the freely expanding atoms for only about two seconds.
They Escaped Gravity, Then Had to Imitate It
The new experiment began with roughly two million atoms of rubidium-87 inside the Cold Atom Physics Rack, installed in Tiangong’s Mengtian laboratory module.
Crossed laser beams held the atoms in a tiny trap. The researchers gradually weakened it, letting the most energetic atoms escape. The principle resembles hot coffee cooling as energetic molecules evaporate from its surface: what remains has less energy.
But microgravity, which helped with other challenges related to performing such an experiment on Earth, created an unexpected problem.
On Earth, gravity helps pull escaping atoms away, making this cooling process more efficient. In orbit, this assistance disappears.
So, the team used a magnetic field gradient to mimic gravity’s useful effect. Ultimately, they managed to make a condensate of about 23,000 atoms at a conventional temperature of 3.1 nanokelvins. Nearly 90 percent of the atoms occupied the condensate.
To push further, the researchers moved the cloud into a second, wider laser trap and loosened its grip over 1.175 seconds. That reduced the energy the atoms would release as they spread apart.
“Only the interaction energy changes,” coauthor Xuzong Chen of Peking University explained to Science.
When the team finally released the cloud, its expansion speeds were only about 70 to 88 micrometers per second, depending on direction, and remained visibile for seven seconds.
Why Seven Seconds Matters More Than the Record
This artist’s conception shows how China’s Tiangong space station will look when it’s fully assembled. Credit: China Manned Space Engineering Office.“The 7 seconds is an even more important figure of merit than the 10 picokelvin,” Holger Mueller, an atomic physicist at the University of California, Berkeley, told Science.
One promising use is atom interferometry, in which scientists split and recombine matter waves to reveal extraordinarily small changes in motion and forces. The longer the atoms remain available for measurement, the greater the potential sensitivity.
Such instruments could test Einstein’s equivalence principle, which states that different objects fall at the same rate in a gravitational field, regardless of their composition. Just as a heavy bowling ball and a light feather drop at the exact same speed in a vacuum, Einstein realized that gravity pulls on everything equally—meaning your body can’t tell the difference between the pull of Earth’s gravity and the push of an accelerating rocket. Any convincing deviation from the equivalence principle would be a major clue to physics beyond our current theories.
What’s more, the Tiangong team hopes to create arrays of laser light that mimic the structure of crystals and help explore phenomena such as superconductivity.
