The relationship between the processes of photon decay into an electron-positron pair and photon emission from vacuum with pair production in an external electromagnetic field is investigated. It is known that in the case when the external field is not capable of producing particles from vacuum within the zeroth order in radiative interaction, the contribution of radiation is also zero, and the probability of photon decay in accordance with the optical theorem can be associated with the imaginary part of the second-order Feynman diagram containing a fermion loop. In this paper, the main attention is paid to the problem with unstable vacuum. It is shown that in this case the statement of the optical theorem is modified since a nonzero probability of emission with pair production must be added to the probability of photon decay. In our numerical calculations, both of these probabilities are obtained nonperturbatively with respect to the interaction with an external alternating electric field for various photon polarizations. The results of calculating the imaginary part of the one-loop diagram turned out to be in complete agreement with the optical theorem. It is also shown that the locally-constant field approximation is inapplicable in the region of low photon energies and can give a significant error in the high-energy region. The paper also analyzes the phenomenon of vacuum dichroism, i.e., the dependence of the above-described contributions on the photon polarization. Keywords: quantum electrodynamics, strong fields, nonlinear effects, dichroism, polarization tensor, birefringence.

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Дата начала обработки статистических данных - 27 января 2016 г.