The elegant equations of classical electromagnetism written by James Clark Maxwell in 1861 display a remarkable symmetry between electric and magnetic fields except for their sources. We know about electric charges but we have not found magnetic charges. Bar magnets are dipoles with two poles, north and south, for the magnetic field, resembling the configuration of an electric field sourced by a pair of positive and negative electric charges. However, we had never seen experimental evidence for a magnetic monopole, namely a magnetic charge with only one magnetic pole, a net north or south, from where magnetic field lines emanate, just like the electric field sourced by an electric charge. In a symmetric theory of electromagnetism, magnetic monopoles should exist.
The existence of monopoles with a net magnetic charge was proposed by Paul Dirac in 1931 to explain the quantized (discrete) values of electric charges. Dirac found that magnetic charges should be an integer multiple of a fundamental unit, g_D, equal to the electron charge, e, divided by twice the fine-structure constant, or about 68.5e.
In classical physics, the existence of magnetic monopoles restores symmetry to Maxwell’s equations. But in the broader context of quantum mechanics, Gerard `t Hooft and Alexander Polyakov showed in 1974 that magnetic monopoles are required in Grand Unified Theories of the strong, weak and electromagnetic interactions. Since the electric charge is quantized, magnetic charges are unavoidable in these theories. Magnetic charges with the lowest mass must be stable because magnetic charge is conserved and they cannot decay into lower-mass particles.
So far, extensive experimental searches were only able to place lower limits on the possible mass of magnetic monopoles. Recent searches involve collider experiments such as MoEDAL, TRISTAN, PETRA, CDF, D0, HERA, and cosmic-ray observatories such as MACRO, Baikal, Baskan-2, Soudan-2, Ohya, KGF, AMANDA, ANTARES and IceCube.
This week, CERN reported that the latest MoEDAL experiment at the Large Hadron Collider placed the tightest collider limits yet. Collisions between protons could produce a single virtual photon or fusion of two photons that produce pairs of magnetic monopoles. In addition, pairs of monopoles with opposite magnetic charges could be pulled out of the vacuum by the enormous magnetic fields created in near-miss heavy-ion collisions. The latter mechanism was first calculated by Julian Schwinger in 1951, for the production of electron-positron pairs out of a sufficiently strong electric field. At a strength above 10^{18} Volts per meter, an electric field can accelerate a virtual electron-positron pair up to its rest mass over the spatial scale of its wave function and create pairs of opposite electric charges out of the vacuum. By symmetry, the same mechanism could apply to magnetic charges for a sufficiently strong magnetic field.
In its proton-proton collider, the MoEDAL team found no magnetic monopoles and set a lower limit of 3,900 times the proton mass (3.9 TeV) on magnetic monopoles with 1–10 times the Dirac charge, g_D. In the Schwinger effect experiment, the team set the strongest-to-date lower limit of 80 proton masses on monopoles with a magnetic charge of 2–45 g_D.
Based on these limits, magnetic monopoles must be very massive and weakly interacting. Cosmological data indicates that 84% of the matter in the Universe is invisible, made of a substance never detected in laboratory experiments. In fact, the Large Hadron Collider also failed to detect candidate particles for dark matter. Could magnetic monopoles be the dark matter?
In a paper that I wrote in 2017 with my brilliant collaborator, Misha Medvedev, we showed that a cosmic population of magnetic monopoles would screen magnetic fields that are observed in galaxy clusters. The analogous screening effect for an electric field in a medium of electric charges was identified by Peter Debye and Erich Hückel in 1923. Misha and I showed that the observed magnetic fields in galaxy clusters imply that these systems are smaller than the Debye-Hückel screening length for the magnetic monopoles in them, thus setting an upper limit on the cosmological density of magnetic monopoles. The limit is a thousand times smaller than the density needed to account for dark matter, precluding monopoles as candidates for dark matter.
But the searches must go on. A future detection of magnetic monopoles or dark matter will shed new light on physics beyond the Standard Model and could help develop future technologies. Extraordinary evidence requires extraordinary funding.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. His new book, titled “Interstellar”, was published in August 2023.
