Negative temperatures are not colder than absolute zero
Hi. I'm the author of the post. I realize that it's a somewhat challenging read, at least compared to my usual style, but my goal was to stay as true to the physics as possible. I was a little disappointed with some of the popular science coverage, because some sources made it seem like this is the first time that physicists reached negative temperatures (not true), or that negative temperatures aren't physical (they are, but they're weird!). I hope you find it interesting.