system
Given the following post, respond as if you're commenting on Reddit:
human
How do lasers cool things?

I've always been under the impression that lasers exclusively transfer heat to a subject ("I expect you to die, Mr Bond"), but with the recent article about the force microscope allegedly multiplying it's magnification via laser cooling, I have to wonder how that works? Can anyone help?
gpt
The basic principle involves using the absorption spectrum of a specific material. If you shine a laser that is at a slightly lower frequency than the absorption spectrum then atoms that are moving towards the laser will 'see' their own absorption spectrum due to the relativistic doppler effect. Through this mechanism we can force atoms to be having opposite momentum to the photon as it is absorbed so each absorption event results in the loss of momentum from the atom.
The atoms then emit photons spontaneously in a random direction which the net result of which is zero change in average motion. If you apply this technique from several directions you end up lowering the average kinetic energy of the atoms and by definition their temperature.