Physics & Math

THEY seem magical: magnets, every child's favourite science toy. Two otherwise ordinary lumps of metal draw inexorably closer, finally locking together with a satisfying snap. Yet turn one of them round and they show an entirely different, repulsive face: try as you might to make them, never the twain shall meet.

If magnets seem rather bipolar, that's because they are. Every magnet has two poles, a north and a south. Like poles repel, unlike poles attract. No magnet breaks the two-pole rule - not the humblest bar magnet, not the huge dynamo at the heart of our planet. Split a magnet in two, and each half sprouts the pole it lost. It seems that poles without their twins - magnetic "monopoles" - simply do not exist.

That hasn't stopped physicists hunting. For decades they have ransacked everything from moon rock and cosmic rays to ocean-floor sludge to find them. There is a simple reason for this quixotic quest. Our best explanations of how the universe hangs together demand that magnetic monopoles exist. If they are not plain to see, they must be hiding.

Now, at last, we have might have spied them out. The first convincing evidence for their existence has popped up in an unexpected quarter. They are not exactly the monopoles of physics lore, but they could provide us with essential clues as to how those legendary beasts behave.

So what attracts physicists to monopoles? Several things. First, there's symmetry - a purely aesthetic consideration, true, but one that for many physicists reveals a theory's true worth. For over a century, we have known that magnetism and electricity are two faces of one force: electromagnetism. Electric fields beget magnetic fields and vice versa.

Accordingly, the classical picture of electromagnetism, formulated in the late 19th century, is pretty much symmetrical in its treatment of electricity and magnetism. But although positive and negative electric charges can separate and move freely in electric fields, magnetic "charge" remains bound up in pairs of north and south poles that cancel each other out. "No monopoles" is another way of saying that there is no such thing as a freely moving magnetic charge.

In 1931, this puzzling asymmetry caught the attention of the pioneering quantum physicist Paul Dirac. He pointed out that quantum theory did not deny the possibility of monopoles; on the contrary, they could be quite useful. His calculations showed that monopoles existing anywhere in the universe would explain why electric charge always comes in the same bite-size chunks, or quanta.

Even so, monopoles were little more than a curiosity, and the lack of any obvious examples nearby dampened the enthusiasm for the chase. That all changed in the 1960s with the wide acceptance of the big bang theory - the idea that the universe began in a fireball governed by a single force that has since splintered into the fundamental forces we see today. The great ambition of physics became to construct a theory that would reunite these forces.

There are many different approaches to this goal, and almost all have an odd feature in common: they say that chunks of magnetic charge must have been created in the very first fraction of a nanosecond of the universe's existence. Some theories, like Dirac's original idea, suggest these monopoles are very massive, with a mass around 10¹⁶ times that of a proton. Other approaches suggest more modest beasts with a mass only a few thousand times the mass of the proton. But all predict they should be there.

Shady characters

Suddenly monopoles assumed a new significance. Not only would the detection of magnetic monopoles be a major boost for "grand unified" theories of how the universe began, but finding the mass of a monopole would help distinguish which of those theories were on the right track. "The search has a low chance of paying off, but a very high importance if it did," says Steven Weinberg of the University of Texas at Austin, who won the Nobel prize for physics in 1979 for his work on force unification.

Sheldon Glashow of Harvard University, who also took a share of the 1979 prize, took the monopole idea a stage further. That same year, he suggested that beefy, Dirac-type monopoles might also be the answer to one of cosmology's most important unsolved problems: they might be the identity of the unseen dark matter that is thought to make up most of the universe and to have formed the structures that led to galaxies.

Physicists thus had a wealth of reasons to believe that these "cosmic" monopoles must exist somewhere. But where? Besides the odd tantalising glimpse, no experiment has yet produced convincing evidence of their existence (see "Race for the pole").

There are reasons to believe they never shall. According to the inflationary theory of the universe's origin, which has gained wide currency since the 1980s, the cosmos expanded enormously fast just after the big bang. This expansion should have carried most, if not all, of the monopoles created in the first instants of the universe to a patch of the cosmos so distant that they, and information about them, will probably never reach us. Game over?

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