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Nevertheless, some [[theoretical physics]] models predict the existence of these [[magnetic monopoles]]. [[Paul Dirac]] observed in 1931 that, because [[electricity]] and magnetism show a certain [[symmetry]], just as [[Quantum electrodynamics|quantum theory]] predicts that individual [[negative and non-negative numbers|positive]] or negative electric charges can be observed without the opposing charge, isolated South or North magnetic poles should be observable. Using quantum theory Dirac showed that if magnetic monopoles exist, then one could explain the quantization of electric charge---that is, why the observed [[elementary particles]] carry charges that are multiples of the charge of the electron.
Certain [[grand unified theories]] predict the existence of monopoles which, unlike elementary particles, are [[solitons]] (localized [[energy]] packets). Using these models to estimate the number of monopoles created in the [[big bang]], the initial results that contradicted cosmological observations---the monopoles would have been so plentiful and massive that they would have long since halted the expansion of the universe. However, the idea of [[Cosmic inflation|inflation]] (for which this problem served as a partial motivation) was successful in solving this problem, creating models in which monopoles existed but were rare enough to be consistent with current observations.<ref>{{cite book |first=Alan|last=Guth|authorlink=Alan Guth|title=The Inflationary Universe: The Quest for a New Theory of Cosmic Origins|id=ISBN 0-201-32840-2|publisher=Perseus|year=1997}}.</ref>
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== Tipos de materiales magnéticos ==
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