Quantum electrodynamics based upon Euler-Heisenberg or Born-Infeld Lagrangians predict photons to move according to an effective metric, which is dependent on the background electromagnetic field in a given space domain. In other words, photon trajectories are curved in presence of electromagnetic fields, meaning that an effective gravity is acting upon the photons. Charged fermions in the same space domain are obviously electromagnetically interacting; one thus wonder whether the interaction mediating virtual photons are also affected by this effective gravity. If the answer is yes, fermions will likely be affected, according to extended charge models, so that electromagnetic fields will show as another source of gravity for any kind of particles.
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