Magnetic monopoles
Do isolated magnetic north or south poles (magnetic monopoles) exist as fundamental particles, as predicted by some grand unified theories, or is magnetic charge always found in inseparable north-south pairs as observed so far?
History and references
Still open in the literature as of 2026.
Dirac showed in 1931 that if even one magnetic monopole existed anywhere in the universe, it would explain why electric charge comes in discrete quantized units. Grand unified theories similarly predict monopoles should have been created in the early universe, though cosmic inflation may have diluted them to unobservable rarity. Extensive searches (including at the LHC's MoEDAL experiment) have never found a fundamental magnetic monopole, though monopole-like quasiparticle excitations have been engineered in spin-ice materials since 2009 — a different, emergent phenomenon.
A minus sign in Maxwell's equations conspicuously left unbalanced hints that somewhere in the universe, a single free north pole might be waiting to be found.
Explanations that circulated
- That 'magnetic monopole quasiparticles' observed in spin-ice condensed-matter systems since 2009 are the same as the fundamental elementary particle Dirac predicted — these are emergent collective excitations in a material, not free fundamental particles
References
- Review of Particle Physics: Magnetic Monopoles, Particle Data Group, Particle Data Group Review, 2024
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