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.