Today's the first day it's actually hot in New York. And now, we wanna be cold again. Or eat cold things. Like the coldest popsicle on Earth, from the coldest freezer on earth. But it turns out the coldest freezer isn't a freezer, in the traditional sense, at all. It's a series of magnets.
Back in 2003, scientists at MIT cooled a sodium gas to below one nanokelvin. Pause. In other words, the gas was one-half-billionth of a degree above absolute zero (-460 degrees Fahrenheit). That is cold.
To get temperatures that low, a traditional freezer (or any typical container, for that matter) is useless. Atoms that cold will stick to the walls, keeping them from getting cold enough. To keep the atoms away from the walls of the container they used lasers and developed a "gravito-magnetic trap," which keeps the atoms in place without touching them. One researcher said it's just like a pressure cooker, but in reverse. Allow the gas cloud to expand (using, say, a gravito-magnetic trap), and the atoms cool down.
The process was interesting. First they had to capture a few million sodium atoms using an "optical tweezer" (i.e., a really focused laser beam). They reduce the laser's power so the more energetic atoms escape while the less excited atoms to stay behind and cool down (a process called "evaporative cooling"). The team transfers the atoms (at a temperature of around 30 nanokelvins) to the all-important magnetic trap where they undergo a separate process called "adiabatic decompression." The magnets condense the atoms into a cloud and lower its temperature from 30 nanokelvin to three. One more cycle of "evaporative cooling" occurs, and you're left with 30,000 sodium atoms at a temperature of 450 picokelvins—the lowest temperature ever recorded. Cold, and impossible without this monster of amachine. [MITnews, NASA, physicsworld]
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