Question: What is spin-spin coupling?

Philip Dolan answered on 13 Jun 2011:

We say that particles have an intrinsic angular momentum. You could imagine this to be like having a football and even when you’ve stopped it moving around in circles it still has some rotational energy. I.e it’s spinning. In classical physics you can stop the football spinning, but sub-atomic particles can never be stopped!

It’s worth saying you’re only imagining that it’s spinning. Electrons are pretty tiny, and if you do the maths, in order to have the angular momentum that it does, it’s surface would have to spinning at least as quick as the speed of light.

Anyway when we do spectroscopy on atoms we realise that the colours they emit are not quite what they should be, unless we take into account the spin on the electron coupling to the spins of the protons and neutrons in the nucleus. If we do take into account that the fact that them spinning (you could imagine if they span in the same direction they’d have slightly more energy, but if the electron span in the opposite direction they’d have less) then you get the right answer.