Magnetic monopole
A hypothetical elementary particle carrying a single, isolated magnetic pole — the magnetic counterpart to electric charge.
Overview
A magnetic monopole is a hypothetical elementary particle that carries a net magnetic charge[1]. In classical electromagnetism, magnetic field lines always form closed loops, implying that magnetic charges only ever appear as inseparable north–south pairs. The existence of an isolated magnetic charge would be the first known violation of this symmetry and would force a deep restructuring of Maxwell's equations into a fully self-dual form[2].
Although no confirmed monopole has ever been observed in vacuum, several theoretical frameworks — including grand unified theories (GUTs), superstring compactifications, and electroweak extensions — predict their existence at energy scales ranging from 103 GeV to 1016 GeV[3]. The article below summarizes the present state of theoretical and experimental knowledge on the topic.
History of the concept
The first systematic discussion of magnetic monopoles in modern physics dates to 1931, when Paul Dirac demonstrated that the existence of a single magnetic charge anywhere in the universe would, through quantum-mechanical consistency, explain the observed quantization of electric charge[4]. The argument was a turning point: it elevated the monopole from a curiosity to a structural element that could complete the symmetry of electromagnetism.
Dirac quantization
Dirac's relation,
e g = n ℏ c / 2, with n ∈ ℤ,
links the elementary electric charge e with the elementary magnetic charge g. Even one monopole — anywhere — suffices to demand discrete electric charges throughout the cosmos[5].
't Hooft–Polyakov monopole
In 1974, Gerard 't Hooft and Alexander Polyakov independently showed that any non-Abelian gauge theory in which a simple group is spontaneously broken to a subgroup containing a U(1) factor automatically contains stable, finite-energy soliton solutions that behave as magnetic monopoles[6]. This result tied the existence of monopoles to the same mathematical machinery that successfully described the weak and strong forces.
Properties and predicted parameters
The simplest GUT monopole is expected to be extremely massive and to carry a magnetic charge that is an integer multiple of the Dirac unit. Its size is set by the inverse mass of the heavy gauge bosons of the unified theory, while its core contains regions where the full symmetry of the theory is restored[7].
| Property | Predicted value | Notes |
|---|---|---|
| Mass | 1016 GeV/c2 | Set by GUT breaking scale |
| Magnetic charge | g = ℏc / 2e | Dirac unit |
| Core radius | ~10−30 m | Inverse heavy-boson mass |
| Stability | Topological | Cannot decay perturbatively |
| Spin | 0 or 1/2 | Model dependent |
Detection efforts
Experimental searches for magnetic monopoles span more than five decades and exploit the unique signatures predicted by the theory: enormous ionization, characteristic induced currents in superconducting loops, and unusual track morphology in track-etch detectors.
Cosmic-ray searches
Large-area arrays such as MACRO, IceCube, and ANTARES have placed stringent limits on the flux of relativistic monopoles arriving from astrophysical sources[8]. The current 90% confidence-level bound on the flux of GUT-scale monopoles is roughly Φ < 1.4 × 10−18 cm−2 s−1 sr−1.
Accelerator experiments
The MoEDAL collaboration at the LHC uses arrays of nuclear-track detectors and a dedicated magnetic-monopole trapping array to search for monopole pair production in proton–proton collisions[9]. No candidate has yet survived analysis, leading to mass limits in the 800–3500 GeV range depending on the assumed production mechanism.
Condensed-matter analogues
In spin-ice materials such as Dy2Ti2O7, low-energy excitations behave mathematically like emergent magnetic monopoles, although they are confined within the lattice and are not fundamental particles[10]. These analogues nevertheless allow controlled study of magnetic-charge dynamics in a laboratory.
Cosmological implications
Standard hot Big Bang cosmology, when combined with grand unified theories, predicts an overproduction of magnetic monopoles in the early universe — a discrepancy known as the monopole problem[11]. The proposed resolution, cosmic inflation, dilutes any pre-existing monopole density to negligible levels and was one of the earliest motivations for inflationary cosmology.
Despite the dilution, a small relic abundance might persist and could be detectable today; the Parker bound, derived from the survival of the galactic magnetic field, currently provides one of the tightest astrophysical limits on this relic flux[12].
See also
References
- 1. Jackson, J. D. Classical Electrodynamics, 3rd ed., Wiley, 1998, §6.11.
- 2. Goddard, P.; Olive, D. I. "Magnetic monopoles in gauge field theories", Reports on Progress in Physics 41, 1357 (1978).
- 3. Preskill, J. "Magnetic monopoles", Annual Review of Nuclear and Particle Science 34, 461 (1984).
- 4. Dirac, P. A. M. "Quantised singularities in the electromagnetic field", Proc. R. Soc. A 133, 60 (1931).
- 5. Wu, T. T.; Yang, C. N. "Concept of nonintegrable phase factors and global formulation of gauge fields", Physical Review D 12, 3845 (1975).
- 6. 't Hooft, G. "Magnetic monopoles in unified gauge theories", Nuclear Physics B 79, 276 (1974); Polyakov, A. M. JETP Lett. 20, 194 (1974).
- 7. Bogomolny, E. B. Sov. J. Nucl. Phys. 24, 449 (1976).
- 8. IceCube Collaboration, "Search for relativistic magnetic monopoles", European Physical Journal C 82, 146 (2022).
- 9. MoEDAL Collaboration, "Magnetic monopole search at the LHC", Nature 602, 63 (2022).
- 10. Castelnovo, C.; Moessner, R.; Sondhi, S. L. "Magnetic monopoles in spin ice", Nature 451, 42 (2008).
- 11. Zel'dovich, Ya. B.; Khlopov, M. Yu. Phys. Lett. B 79, 239 (1978).
- 12. Parker, E. N. Astrophysical Journal 160, 383 (1970).