Negative temperature is REAL (Purcell & Pound experiment)

Hi everyone,

Jonathon Riddell here. Here we explain the Purcell and Pound experiment which showcases a real, physical system with negative temperature.

Edit: In this video I refer to "low temperatures." It would be more proper to refer to them as "sufficiently low" but actually in the case of this exact experiment, the experiments were carried out at 300 Kelvin!

What is entropy? https://youtu.be/YbT__-H5QjA

Jaynes' principle of maximum entropy: https://youtu.be/l-akiVsT2mI

Article Purcell and Pound: https://journals.aps.org/pr/abstract/10.1103/PhysRev.81.279

Recent article for Potassium-39 at negative temperature: https://science.sciencemag.org/content/339/6115/52

Many lasers example: https://journals.aps.org/prb/abstract/10.1103/PhysRevB.46.6760

To view my publication history: https://scholar.google.com/citations?user=V2UZXZMAAAAJ&hl=en ... https://www.youtube.com/watch?v=dOdc7Qco258