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We鈥檝e never seen a magnetic north pole without its opposite number, but theory demands that these strange monopoles exist. So why don鈥檛 we make one instead?

JIM PINFOLD hurries me through the low-lit corridors of the theory department at the near Geneva in Switzerland. Posters announcing conferences of yesteryear plaster the walls. On one door, an A4 poster adorned with a Nike swoosh announces 鈥淧hysics: Just Do It鈥�. Through another door, a group of physicists stand in pensive silence around a blackboard covered in chalk hieroglyphics as they sip sparkling wine from plastic cups. It is barely midday. 鈥淲alk quickly through here,鈥� he says. 鈥淚n case you get any funny ideas.鈥�

For Pinfold, theory has always been a means to an end; in this case, a shortcut to the car park from which he will drive me over the Swiss-French border to his latest experiment. The ultimate end, he hopes, will be proving an idea that has been burning in the minds of theorists for decades: that magnetic monopoles exist.

A monopole is a magnetic north pole without its accompanying south, or a south without its north. No one has ever seen one. And if our present understanding of particles and forces is correct, we don鈥檛 have the remotest chance of snaring one.

The stuff of legend (Image: Kolchoz)

Pinfold, a silver-haired, fast-talking Londoner who works at the University of Alberta in Edmonton, Canada, begs to differ. 鈥淚鈥檇 give us about the same chance of finding a monopole as we would have given of finding the Higgs boson 10 years ago,鈥� he says. 鈥淚t鈥檚 a bullish answer, but it鈥檚 my answer.鈥�

Magnetic monopoles鈥� no-show has long been a bugbear to the sort of physicist that sees truth in the beauty of mathematical formulae, and among the most beautiful are the four equations collated by James Clerk Maxwell in the 1860s. These encapsulate the idea that electricity and magnetism are two manifestations of the same thing: the fundamental force of electromagnetism. Maxwell鈥檚 equations predicted the existence of individual, freely moving electric charges 鈥� things nature supplies in abundance in the form of particles such as electrons and protons. To achieve an aesthetically pleasing symmetry in the equations, similar freewheeling magnetic charges had to exist, too.

This was less obvious. North and south magnetic poles that attract and repel each other do exist, just as positive and negative electric charges do. But from the humblest bar magnet to Earth鈥檚 mighty interior dynamo, magnetic poles only ever crop up tied together in pairs. Chop a magnet in half and, like Walt Disney鈥檚 sorcerer鈥檚 apprentice with his magic broom, you forge two new complete magnets, each with a north and south pole. Faced with a brute fact of nature, Maxwell eschewed beauty and wrote the freely moving monopole out of his equations 鈥� and out of history.

However, monopoles made a comeback thanks to Paul Dirac, a British theorist who was notoriously word-shy and obsessed with mathematical beauty. Quantum theory was all the rage in 1931, and Dirac began applying its ideas to Maxwell鈥檚 classical electromagnetism.

Dirac鈥檚 calculations showed that even if there was just one magnetic monopole in the entire universe, its existence would explain why all the electric charge we see comes in the same bite-sized chunks of +1 or -1.

This time, the idea stuck. Forty years on, physicists discovered that the electromagnetic force and the weak nuclear force that controls radioactive decay could be rolled into one, and were seeking ways to unify this force with a third, the strong nuclear force. and independently showed that monopoles were essential if such a 鈥済rand unified theory鈥� were to make predictions in accordance with reality.

鈥淲ithout monopoles, you would have to expect electrically charged particles to have all sorts of charges,鈥� says 鈥榯 Hooft, who is at Utrecht University in the Netherlands and who .

Particle zoo

Fellow theorist of the University of California, Santa Barbara, also has no doubt about the reality of monopoles. 鈥淥f all the new things we have predicted 鈥� supersymmetry, strings and so on 鈥� I would still put the very highest confidence in monopoles,鈥� he says.

So where are they? Researchers have looked for monopoles everywhere, from Antarctic ice to moon rocks. Things acting like monopoles have even been created in various supercooled materials in the lab (see 鈥�Mission impossible?鈥�).

But the closest we鈥檝e come to the real thing was on Valentine鈥檚 night in 1982. That was when physicist saw a promising event in a monopole detector he had set up in a basement at Stanford University in California. It proved to be a one-night stand, prompting some wag to post Cabrera this loving note exactly a year later: 鈥淩oses are red/Violets are blue/The time has now come/For monopole two鈥�.

These days Cabrera鈥檚 monopole and a couple of other even more ambiguous sightings are regarded as experimental blips. But Dirac鈥檚 calculations provide a ready-made excuse for the monopole鈥檚 absence. They show that the smaller the unit of electric charge is, the larger the unit of magnetic charge must be. Because the basic electric charge is incredibly small 鈥� you need around 1000 billion electrons to power a hearing aid for just 1 second 鈥� the basic magnetic charge is so large that it would take an implausible amount of energy to make a particle carrying it.

Precisely how much energy depends on the variant of force unification that you choose 鈥� theorists have come up with several ways of performing this feat over the years. If you follow the variant that underlies our current best theory of particles and forces, the standard model, then the only time that enough energy was around to make monopoles in any abundance was in the first infinitesimal fraction of a second after the big bang. About the same time, a period of rip-roaring cosmic expansion known as inflation is thought to have occurred that would have scattered monopoles to the four winds. 鈥淨uite plausibly what was an overabundance of monopoles would be diluted to one in the entire volume of the visible universe,鈥� says Polchinski.

鈥淨uite plausibly, there is just one monopole in the entire visible universe鈥�

Or, says Pinfold, we might just find one down a tunnel in Geneva.

A narrow, rattly lift just vacated by a technician and his wheelbarrow carries us down 100 or so metres to a cavernous underground hall that is home to one of the giant experiments of the Large Hadron Collider (LHC) 鈥� and to Pinfold鈥檚 venture. Now is a good time to visit the bowels of the LHC: the accelerator is switched off until early 2015 while it undergoes an upgrade.

We climb a series of metal stairways back up from the cavern floor and reach a platform where a thin metal pipe emerges from the middle of a whitewashed door. Next year protons will once again zip along this pipe at 99.9999991 per cent of the speed of light and collide head-on up to 600 million times a second with other protons travelling at the same speed in the opposite direction. The innards of these high-energy protons provide the raw material for the LHC鈥檚 investigations of material reality, as all manner of exotic and unfamiliar particles are fleetingly created in the resulting mini fireballs.

Pinfold鈥檚 experiment is small by LHC standards, and its feel seems to be almost a rebuke of the LHC鈥檚 expensive cutting-edge technology. It is called MoEDAL (short for the Monopole and Exotics Detector at the LHC), and its principal component is a series of metal cases attached to the walls around the platform. Neatly outlined in yellow masking tape, each trails a curl of wires to the floor.

By the time protons start colliding again, these cases will entirely surround the crash site. 鈥淢ind out,鈥� says Pinfold from beneath his hard hat as he invites me to crane my neck into a pit immediately below the beam pipe. 鈥淚f you bang your head there, it鈥檚 hard.鈥�

Within each of the metal boxes are detectors consisting of a series of stacked layers of plastic that act as a form of photographic plate. Because a monopole must carry a huge magnetic charge, it would rip through the polymer bonds of the plastic, etching a trail whose size, shape and alignment would reveal the particle鈥檚 character. Once produced, monopoles are expected to be highly stable 鈥� so the experimental set-up also includes trapping detectors in which any passing monopole might be bottled and kept for further tests: a true exotic from the particle zoo.

There is an improvisatory spirit about MoEDAL. Although the powers that be at CERN gave the go-ahead for the experiment in 2010, the 60-strong collaboration must raise its own funding. With money from official sources tight, the team is looking to crowdsource some of it. Meanwhile, part of the monitoring of the experiment鈥檚 output will be done by pupils at a school in the UK (see 鈥�Hello, Mrs Chips鈥�). The uncertainty about what, if anything, the experiment will see is part of its charm, says Richard Soluk, MoEDAL鈥檚 technical coordinator. 鈥淵ou don鈥檛 make a breakthrough looking for things people expect you to find,鈥� he adds.

Even though the mini fireballs produced at the LHC are the most energetic ever made in a particle accelerator, they fall far short of what is needed to make a monopole if you take the standard model at face value.

But Pinfold鈥檚 experiment is far from a fool鈥檚 errand. Fundamental physics is in a funk. While the discovery of the Higgs boson slotted the final piece into the jigsaw of the standard model, the theory is clearly incomplete. It manages to describe electromagnetism and the weak and strong forces in a broadly satisfying way. Yet it remains silent on the fourth, gravity, and it doesn鈥檛 give any clues as to the identity of dark matter and dark energy, which seem to make up 96 per cent of what is out there.

These are far from the only question marks hanging over the standard model, and scepticism about it is further fuelled by that nagging question of aesthetics. Although its predictions, as far as they go, agree peerlessly well with experiments, mathematically the standard model is a bit of a kludge, a pasting together of different bits of theory with the occasional safety pin and sticking plaster to cover the gaps. 鈥淚t would be very, very surprising if it were the end of things,鈥� says Polchinski.

Many attempts to improve on the standard model have emerged in recent years. Some of these exotic theories, such as ones that predict the existence of extra dimensions, predict a significantly lighter monopole too, says 鈥榯 Hooft. This dramatically boosts the LHC鈥檚 chances of producing a monopole and MoEDAL鈥檚 chances of bagging one. 鈥淭heir energy would be much closer to where the LHC or its future descendants can reach, so the prospects look brighter,鈥� adds 鈥榯 Hooft.

鈥淚f monopoles are light, it boosts the chances of producing one at the LHC鈥�

All that means Polchinski is crossing his fingers for an upset. 鈥淎ny kind of new physics would turn the field upside down,鈥� he says. 鈥淔or monopoles to be within the reach of the LHC will require us to be lucky, but I think we have to look everywhere.鈥�

At the end of a long day, the CERN cafeteria is a good place to muse over monopoles. Stickers by the buttons on the water fountain offer three choices, 鈥淥rder鈥�, 鈥淐haos鈥� and 鈥淪elf-destruct鈥�. I opt for a beer instead, and rejoin Pinfold in the bustle of physicists and technicians. He is also a member of the ATLAS collaboration, one of the teams that found the Higgs boson, but it is clear that his heart beats for monopoles 鈥� as perhaps do those of many a soul in the yellowing corridors of CERN鈥檚 theory department.

鈥淎ll we need is one,鈥� says Pinfold. 鈥淓veryone was expecting the Higgs. But no one is expecting this.鈥�

See the quest in pictures: 鈥�Race for the monopole: where we鈥檙e looking for it鈥�

This article appeared in print under the headline 鈥淭he stuff of legend鈥�