= Below-cutoff electric-dipole lifetime
A radiative lifetime depends on the available electromagnetic modes as well as on the atomic transition. In the ideal parallel-<perfect conductor> geometry of <polarization-selective spontaneous emission between conducting plates>, an in-plane transition dipole has no coupled propagating <electric dipole radiation> mode below the relevant <waveguide cutoff frequency>.
If that idealized <electric dipole transition> is the only decay channel, its rate is zero and the corresponding lifetime diverges. This is a statement about a restricted channel and geometry, not a general prediction of an immortal excited atom. Finite plates, absorbing surfaces, dipole misalignment and other allowed transitions can restore decay. A normal transition dipole can couple to the surviving <transverse electromagnetic mode>.
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