monopole

A monopole is a hypothetical particle or excitation that carries a single magnetic pole—north or south—unlike ordinary magnets, which are dipoles. If magnetic monopoles exist, they would possess magnetic charge. Electric monopoles, by contrast, are ordinary point charges already described by standard electromagnetism; magnetic monopoles would be a qualitatively different object.

In classical electromagnetism, Maxwell's equations imply that magnetic fields arise from dipoles and that magnetic charge is not isolated. A true magnetic monopole would require modifying the equations to include a magnetic charge density ρm and a magnetic current jm, yielding ∇·B = μ0 ρm. The theory is more symmetric under duality, and Dirac showed that the presence of even a single monopole would impose a quantization condition on electric charge: eg = nħc/2.

Theorists have constructed non-singular monopole solutions in grand unified theories; the 't Hooft–Polyakov monopole is a finite-energy soliton arising from non-Abelian gauge fields after symmetry breaking. Magnetic charge in these settings is topological in nature and does not require a Dirac string.

Experimental searches for elementary monopoles have not produced a confirmed discovery. Experiments in cosmic rays and at particle colliders, including dedicated detectors, have set limits on monopole flux and production cross sections that depend on mass and charge. No unambiguous signal has been observed to date.

In condensed matter physics, monopole-like excitations appear as emergent quasiparticles. In spin ice materials, for example, effective magnetic charges reside on a lattice and are constrained by Dirac-string-like connections, providing a laboratory realization of monopole physics without elementary particles.