The gyromagnetic factor in electrodynamics, quantum theory and general relativity

Starting with some basic facts about the gyromagnetic factor g in Maxwellʹs theory, we review the special role played by a g factor g = 2 in quantum mechanics and elementary particle physics, and we draw attention to the same value g = 2 for the black holes and many other (electro-)vacuum solutions of general relativity. We strengthen and extend this special role of g = 2 in general relativity by considering a class of slowly rotating, charged mass shells, showing that the black-hole value g = 2 is extremely robust. Therefrom, we advance the hypothesis that the coincidence between these preferred g values signifies a deep common root of quantum theory and general relativity.