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The ground state of hydrogen is $^{2}S_{\frac{1}{2}}$ with nuclear spin $I=\frac{1}{2}$. The levels are split by $F=1,F=0$. By the rules of electric dipole transition, $\Delta l = \pm 1$. But in this case it is going from $l=0$ to $l=0$.

Is this an allowed electric dipole transition?

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In the real world transitions are rarely (never?) forbidden because the assumptions we make rarely hold exactly. You are quite correct that the 21 cm transition has $\Delta\ell = 0$ and is therefore forbidden. However it can occur (very slowly) as a magnetic dipole transition.

The lifetime of the excited state is around $10^{15}$ seconds, and we can observe the radiation from the transition only because (a) in interstellar space atoms can go this long between collisions, and (b) there's a hell of a lot of hydrogen out there.

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    $\begingroup$ Also to point out that it is not generally visible in general lab conditions, as the doppler width is about $~\sim 10^{-4}eV$ $\endgroup$ – user44840 Jun 12 '15 at 16:22

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