Hello! I am reading about Time Dependent Perturbation Theory from Griffiths book on QM. He derives the fact that for a system with 2 levels with energies ##E_b > E_a##, if you send light to it (or a sinusoidal perturbation) the probability of an electron going from state a to state b is the same as going from state b to state a. I can't say it is very intuitive but the math is clear. Then he talks about spontaneous emission, when the electron goes from b to a without shining any light due to the fact that the ground state is never zero in QED. He then says that stimulated and spontaneous emissions are basically both stimulated emissions, just that in the first case it is a filed put there on purpose (not existing there without any external intervention). Based this explanation I am a bit confused. If spontaneous emission is the same as stimulated emission, why don't we have spontaneous absorption, too (i.e. an electron to go from a to b with no external intervention)? If for an external photon with the energy (##E_b-E_a##) the electron is equally likely to go from a to b and from b to a, why when the photon comes from the ground state of the vacuum, it doesn't behave the same way? Does this have to do with QED arguments, or am I missing something here?