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.