In Search Of The Elusive Magnetic Monopole: A Curious Asymmetry

Michael Faraday's experiments and James Clerk Maxwell's laws of electromagnetism showed us the beautiful symmetry between the phenomena of electricity and magnetism, and how one is simply the manifestation of the other, how one cannot be understood without the other.

We know about the similarities between the two forces when considered as separate entities, as well as the striking differences, the most prominent one being the non-existence of magnetic monopoles.

Claimed to be the holy grail of physics, the magnetic monopole is a hypothetical particle with a magnetic charge, analogous to an electric charge.

Speaking in technical terms, the smallest electric entity to exist independently is the electron, with a charge equal to one unit electron (=1.609 x 10^-19 Coulomb), while the smallest magnetic entity known to exist is the magnetic dipole (consisting of two poles: north and south, inseparable).

Experimental observation of the fact that breaking up a magnet gives two weaker magnets and not isolated north and south monopoles was believed to be due to the non-existence of magnetic monopoles, something which shows up in Maxwell's second law — the area integral of the magnetic field over a surface is equal to zero.

However, there is strong evidence, theoretical and now even experimental, to believe in their existence.

The year was 1931, and a quiet physicist had just published a paper that would soon put the monopole under everyone's radar.

Paul Dirac's 1931 paper, titled ‘Quantised Singularities in the Electromagnetic Field’, shows, in his own words, "a symmetry between electricity and magnetism quite foreign to current views."

Before Dirac, it was assumed that both Maxwell's classical theory and the quantum theory prohibited the existence of isolated magnetic monopoles. Dirac carefully examined the mathematical arguments underlying this belief and discovered a subtlety that had been missed.

As was his tradition, Dirac skillfully made use of his mathematical skills and the existing quantum theories to prove that the existing theory did not preclude the existence of magnetic monopoles, but only placed a subtle restriction on a significant property, which had been assumed to imply monopole inexistence.

Building up logically from mathematical arguments, Dirac gives us a relation between the electric charge and magnetic charge, which shows how one is quantised in terms of the other.

His theory showed that, contrary to assumptions, existence of magnetic monopoles would not disrupt the existing theories, but would, in fact, provide a greater symmetry to the theory of electromagnetism, related as the value of the electric and magnetic charges were in Dirac's final expression.

The physical consequences and predictions of a monopole's existence were many and varied, and theorists started looking for their existence in their own books.

Nature never stopped revealing its varied facets, and as the years passed by, physics, contrary to the opinion of Albert Michelson, had gone much beyond just finding constants to the eighth decimal.

As energies in our accelerators increased, new physics emerged almost every day. Neatly drawn boundaries were quickly blurred, and search for ever more general theories intensified.

The fundamental forces of nature were identified and, above everything else, the need for a universal theory of everything, accommodating all our basics, was wreaking havoc in the world of physics.

Different groups of scientists, using different approaches, were putting in all they had in their papers, in search of the holy grail.

Dirac's paper didn’t just make the idea of an almost perfect symmetry between electricity and magnetism seem like a possibility, but also formed the impetus for monopole searches in all the upcoming theories, whether they be focused on electromagnetic fields or theories of everything.

As it turned out, many of these theories also had evidence of the monopole in them, possessing the same basic symmetries as Dirac's original child. Despite these successes, a strong experimental verification of the theory was lacking, leading Dirac himself to doubt their existence.

In his paper, Dirac found that the attractive force between two one-quantum poles was only a little less than 5,000 times that between the electron and proton, citing that as a possible reason for the monopole invisibility.

Later theories also predict confinement of a monopole-antimonopole pair, due to their strong magnetic forces, into a state known as the monopolium. This state must, by its very nature, be neutral, presenting difficulties in detection.

But it immediately leads one to wonder about the monopolium decay into photons, analogous to what happens when an electron and its antiparticle, the positron, run into each other, naturally forming the idea behind one of the ways in which physicists are currently looking for the monopole.

One of the most popular and significant searches for the monopole has been going on at the European Organization of Nuclear Research (CERN), as part of the MoEDAL (Monopole and Exotics Detector at the LHC) experiment, since 2015.

Proton-proton collisions of the highest energy orders in the world are carried out and analysed as a part of this dedicated experiment, which consists of, firstly, more than a hundred NTD (Nuclear Track Detector) sheets. A monopole, if produced in a collision, would leave a track on travelling through these sheets.

The other half of the experiment consists of tracking detectors containing tonnes of aluminium (Al) bars, used because of their unusually high magnetic moment. On striking these bars, the monopole would lose its energy and bind to an Al nucleus, making further detection and study possible.

Despite the various searches going on for it, in the insides of giant colliders and in the folds of extremely complicated theories of everything, the magnetic monopole continues to elude us, presenting one of the most interesting mysteries of the modern age.

If and when we find it, would there be a way of assimilating it in the existing theory of classical electrodynamics?

Along with providing many explanations and lost links, the monopole's discovery would open several new horizons in physics, and possibly beyond it.

Every discovery in science has propelled us forward, either by supporting existing theories or by breaking them. We believe the monopole possesses the same strength.

As we make our particle colliders bigger and better, and as our theorists fill pages upon pages with virtually unsolvable maths, we look forward to a day when we'll tell the next generation about the hidden solution, about the monopole.