As revealed in a new Nature Physics study, a team from Imperial College London has set an almost record-breaking "low bar" for the coldest molecules on Earth, and perhaps the universe, by chilling them to 50 millionths of a degree above absolute zero. It doesn't appear to be the coldest molecular temperature ever achieved – that's retained by a team from MIT – but the strange two-step method used this time around opens up a world of possibilities for future endeavors.
When atoms move or vibrate, it means they are energetically excited, and by default are “hotter.” In order to cool something down then, you need to stop it moving. Normally, this involves cooling the environment, but although this is a good way to make some ice cubes, it’s not effective enough at driving the temperature of an atom down to absolute zero.
The coldest atoms on Earth have ever got is 0.00000000005 degrees Kelvin, which is obviously incredibly close to absolute zero – zero degrees Kelvin. This was achieved by using the Doppler Effect, the very same phenomenon that makes sirens sound weird when you drive past them thanks to sound wave compression.
The Doppler Effect also applies to light. If an atom is emitting light, and it’s moving towards the viewer, the frequency it experiences will move up into the blue part of the spectrum. If it moves further from the viewer, it will shift into the lower frequency red part of the spectrum.
This also means that, with the right frequency, an atom moving into a beam of light will get blue-shifted, get energetically excited and will emit a photon of light in a random direction. As a result of this effect, it will ultimately lose momentum, slow down, and cool.
As mentioned, that’s enough to get a single atom down to 50 trillionths of a degree, something known as the Doppler Limit. The team from Imperial College London wanted to know how cold individual molecules, not just individual atoms, could get – a far harder task.
The team got to work at the Centre for Cold Matter, and chose molecules of calcium monofluoride to work their ice magic on. They started out using the Doppler Effect to significantly cool them, and then went one step further.
King Sisyphus tells the tale of a monarch who was forced to push a boulder up a hill, only to watch it fall down again, for all eternity. It just so happens that there’s a process known as Sisyphus cooling, which uses several lasers to create a type of wave that essentially drains atoms of energy by constantly pushing them up an "energy hill."
With this additional effect, the calcium monofluoride atoms were cooled to a temperature of 50 millionths of a degree above absolute zero. As far as we can tell, that’s one of the lowest temperatures of any molecule ever recorded, even if it’s nowhere close to being as cold as those individual atoms.
Scientists rarely back down from a challenge, and the quest to reach absolute zero is one of them. There have been many, many attempts to get there, but after a century of debate, academia has declared that such a feat is impossible.
That, however, doesn’t mean that scientists have stopped trying, and this study represents another step towards that chilly nadir; another wonderful example of scientists trying to find out where the boundaries of nature lie.
Atomic winter is coming, make no mistake about it.