To this point, the nucleus has been assumed to interact with the electron only through its electric field. However, every field line outside the loop must return inside the loop, as shown in figure 6 .
All field lines cross the plane of the dipole going up inside the loop and down outside the loop.
The electron has additional energy due to the interaction of its spin dipole with this field, and hence the spin-spin Hamiltonian is The operator does not commute with this Hamiltonian. Since the electron has no orbital angular momentum, there is no nuclear spin-orbit effect. It can be shown that because the wavefunction has spherical symmetry, only the delta function term contributes from the spin-spin Hamiltonian. The transition is exceedingly slow, but the huge amounts of interstellar hydrogen make it readily observable. It is too slow to be seen in a terrestrial laboratory by spontaneous emission, but the frequency can be measured to very high accuracy by using stimulated emission, and this frequency is in fact one of the best-known numbers in all of physics.