# Probability of a spin flip transition in neutral hydrogen

1. Nov 29, 2013

### Stengah

1. The problem statement, all variables and given/known data

I'd like to calculate the probability of a spin flip transition occurring in the ground state of neutral hydrogen. That is, the transition from parallel to anti-parallel spin configuration of the electron and proton. This transition produces the 21-cm line.

2. Relevant equations

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3. The attempt at a solution

The answer is well known to be $$2.9\times10^{-15} s^{-1}$$
but I'm not sure how to get there. Right now I'm mostly just looking for a rough outline of what it would take.