Previously, we have been considering only conventional electrodynamics, according to which magnetic charges are excluded. This is in accordance with experiments, where electric fields are always generated by electrically charged particles, whereas magnetic fields are generated from electric currents. Nevertheless, there is a priori no reason to exclude magnetic charges, and as shown by Dirac (1931) their existence would have interesting theoretical consequences. Especially, the magnetic charge will necessarily be quantized due to quantum-mechanical effects. Current ideas about the fundamental forces and the origin of the universe also strongly suggest that magnetic monopoles were in fact created in the early history of the universe. Most of these monopoles would have annihilated each other again (in the same way as matter and antimatter annihilate each other), but a small fraction may have survived, although they will be extremely difficult to detect, mainly due to their large mass. Even if magnetic monopoles do not exist, the underlying mathematical model is very interesting, and it has had a great impact on our understanding of gauge theories.
KeywordsAngular Momentum Electromagnetic Field Magnetic Flux Unitary Transformation Magnetic Charge
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