ELi5: How does a laser have “negative temperature” if it’s not cold?
In: Physics
The ELI5 explanation is that it comes down to how we define “temperature” in a physics context.
We have an intuitive idea of what temperature means from every-day life. But for science we want more precise, more technical definitions, that are useful, interesting and consistent with all our other terms.
And it turns out that when we use that definition we can get some weird results where you can have a “negative temperature” in a system (although intuitively that system will be “hotter” in the intuitive sense). It sounds kind of silly, but it works out consistently with a bunch of other areas of physics, so we go with it.
If you want a more detailed explanation there is a decent askscience thread on this [over here](https://np.reddit.com/r/askscience/comments/12wj25/how_does_negative_temperature_work_ex_in_lasers/), which gets into the more technical stuff.
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