You could say that the idea of a magnetic monopole started in 1269. In that year, the scholar, soldier and monk, Pierre de Maricourt, was part of the crusading army of Charles Duke of Anjou, laying siege to the city of Lucera in Italy. During the siege he wrote a document, the Epistola de Magnete, that identified for the first time that a magnet had a north and a south pole. This begs the question: can there be a single pole – a magnetic monopole?
By the nineteenth century Gauss's law for magnetism, enshrined as one of Maxwell's equations, stated mathematically that magnetic monopoles do not exist. Maxwell's equations – which posit that only electric charges occur in nature – can be made fully symmetric under the interchange of the electric and magnetic fields if magnetic charges also exist. A courageous Pierre Curie was the first to suggest that magnetic monopoles could conceivably be present in nature in a paper published in 1894.
Paul Dirac took up Curie’s challenge. In 1931, he hypothesized that a magnetic monopole could exist within the framework of quantum mechanics. He saw the monopole as the end of an infinitely thin infinitely long solenoid called a Dirac string. The string does not pose a problem as long as it cannot be detected. The mathematics that ensured this gave rise to a quantum relation between the electric and magnetic charges that explains the quantization of electric charge, as long as at least one monopole exists.
Although experimental evidence for the monopole is still lacking, some modern theories, such as Grand Unified Theories, String Theory, M-theory all require magnetic monopoles. Leading string theorist Joseph Polchinski described the existence of monopoles as "one of the safest bets that one can make about physics not yet seen".
Since 1931 physicists have been searching for magnetic monopoles in cosmic rays; trapped in bulk matter, in lunar dust, meteors and on Earth; and at accelerators where they would be produced in high-energy particle interactions. MoEDAL – the Monopole and Exotics Detector at the Large Hadron Collider (LHC) – has been custom-made to continue the search, along with the general-purpose LHC detectors, to the multi-TeV realm.
In 2008 physicists awoke to headlines such as “Magnetic monopoles discovered” (“Magnetic Monopoles in Spin Ice” Nature 451: 42-45, 2008) in a low energy condensed matter system. Since then there have been time a number of results in the field of condensed matter concerning magnetic monopoles. But the experiments are observing emergent phenomena – due to the collective behaviour of the system’s electrons and ions – that look and act like monopoles, but are not.
“Observation of Dirac monopoles in a synthetic magnetic field”, published in Nature on January 2014 by a team led by David Hall at Amherst College in Massachusetts, is another much-heralded example of an emergent-monopole result. The paper is an experimental tour de force in quantum simulation, where a simulacrum of a Dirac monopole was engineered in a quantum system for the first time.
How is this so? In 1931 Dirac showed mathematically that when a monopole passes through the quantum cloud of an electron it leaves in its wake a vortex in which there is zero electron density – like water swirling as it drains from a sink.
Hall’s group ingeniously constructed a quantum analogue of Dirac’s mathematics. They made a Bose-Einstein condensate – a small nebula of ultracold rubidium atoms. The condensate acts as a single-matter wave that plays the role of the electron in Dirac’s conception. Applying an external magnetic field to the condensate orients the magnetic spin of the atoms to create a ‘vortex’ – the ‘monopole’ is formed at its endpoint.
In the experiment the magnetic field of the monopole is represented by a property of the way the spins are arranged, called their vorticity – where the vortex line plays the part of the Dirac string. Amazingly, Hall’s team managed to create an image of their synthetic monopole and string with a ‘shadowgraph’ created by shining a laser beam through the condensate.
Steven Bramwell, a pioneer of the study of emergent 'monopoles' in spin ices, reminds us “There’s a mathematical analogy here, a neat and beautiful one. But they’re not magnetic monopoles.” Indeed, Hall is aware of the limits of his group's work. "Our monopoles wouldn't be registered by a compass," he says. "We haven't been able to reproduce properties such as the mass of the particle in our experiment, but we have created an analogue of the magnetic part. That might provide some insight into natural monopoles."
Certainly the search for a real magnetic monopole has only just begun at the LHC. In 2015 we shall have a rejuvenated, higher luminosity, higher energy, LHC with the MoEDAL experiment officially joining the quest for the monopole for the first time.