Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

The 21 cm Hydrogen line is from the transition between the hyperfine levels of the ground state of the hydrogen atom.

So, what interaction is coupling the hyperfine levels?

I suspect that it is not the usual EM interaction, which causes the 1s-2p transition.

share|cite|improve this question
up vote 6 down vote accepted

The 1s energy level is split into two levels, one where the electron spin and nuclear spin are parallel, and the other where the electron spin and nuclear spin are antiparallel.

The 21 cm line is the transition between these two 1s levels.

The transition is a magnetic dipole transition.

share|cite|improve this answer
Then it is simply EM between the electron and the nucleus? – peterh Jun 23 '14 at 16:34
it is a magnetic dipole transition (M1) as opposed to the more common electric dipole transition (E1). It is still an electro-magnetic transition between two states of the hydrogen atom. The lower energy state is the state with the electron spin and nuclear spin antiparallel, meaning their magnetic dipole moments are parallel. – DavePhD Jun 23 '14 at 17:00

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.