Magnetic monopole search, past and present

Rajantie replies: Alfred Goldhaber raises an interesting point that the radius of a magnetic monopole has to be larger than its Compton wavelength. As with the ’t Hooft–Polyakov monopole, the nonzero size could be due to some new particles whose mass would have to be around 10–100 GeV for a TeV-scale monopole. So far the Large Hadron Collider (LHC) has produced no evidence of new particles beyond the standard model, but that does not necessarily rule out the existence of magnetic monopoles because we currently do not have a good enough theoretical understanding of the properties those particles would need to have.

The requirement for a finite monopole size is a consequence of the monopole’s strong magnetic charge, and the electromagnetic duality means that the same conclusion would also apply to particles that have a strong electric charge. The strong charge means that the classical picture of a field around a static source may not apply, and hence the nonzero size could also be due to quantum mechanical effects without any new particles.

Our theoretical understanding of strongly coupled quantum field theories is limited, but lattice field theory simulations ¹ show that in its simplest form, quantum electrodynamics allows relatively strong charges, although not as strong as the Dirac charge of a magnetic monopole. The maximum charge allowed for a magnetic monopole in the standard model without any new particles is an interesting and still open theoretical question.

Either way, the argument implies that if magnetic monopoles exist, they would have a nontrivial size and shape, which could be studied in future experiments.

Because of space limitations, I could not do justice to the wide range of fascinating ways people have been trying to find magnetic monopoles. Christopher Harrison and Ken Frankel highlight some of the pioneering attempts. Although those searches did not produce positive results, they paved the way for future experiments, and their method of using a SQUID (superconducting quantum interference device) to search for monopoles is still being used in the MoEDAL experiment at the LHC.