The night sky above Earth blazes with the distant fierce fires of countless stars, and when we stare up at this magnificent spectacle of stellar fireworks, we cannot help but wonder how this show came to be. What scientists know now, or at least what they think they now know, is that the Universe was born about 13,800,000,000 years ago in the Big Bang, when it began as an exquisitely small Patch, much smaller than an elementary particle, and then–in the tiniest fraction of a second–expanded exponentially to reach macroscopic size. Something–we do not know what–made that tiny Patch experience this bizarre runaway inflation. Mysteries are enticing, singing a haunting sirens’ song to those who care to listen to its captivating melody. One of the best-kept secrets of the Cosmos involves a weird hypothetical elementary particle called a magnetic monopole. According to theory, these exotic magnetic monopoles should exist somewhere in the Universe–and yet not one solitary magnetic monopole has ever been found lurking anywhere in Spacetime.
If a bar magnet is cut in half, the outcome is a duo of smaller bar magnets–and each magnet sports its own south pole and north pole. But hypothetical magnetic monopoles–if they really are out there somewhere–travel to the beat of a different drummer. These exotic elementary particles that clearly “do their own thing” can have either a south pole, or a north pole, but not both.
Alas, for the past 70 years, physicists have hunted for these exotic particles that should have been born in abundance in the Big Bang, only to come up empty-handed. A monopole is defined as a magnetic version of a charged particle, such as a negatively charged electron, or a positively charged proton. Because in particle physics a monopole is an isolated magnet with only one magnetic pole (a north without a south pole, or vice versa), a magnetic monopole would have a net magnetic charge.
Electric monopoles exist as particles that sport either a positive or negative electric charge. Magnetism, of course, seems somewhat analogous to electricity. This is because there exists in nature a magnetic field that possesses a direction that is defined as running from north to south. However, the analogy breaks down in scientific attempts to detect the magnetic counterpart of the electric charge. Even though we can find electric monopoles in the form of charged particles, scientists have never been able to observe a magnetic monopole.
The only magnets that we know of are all dipoles–with north and south ends. When a bar magnet is split into two pieces, you do not get either a north or south pole–both separated pieces still possess both poles. The two new dipole magnets are simply identical, smaller versions of the original dipole magnet. No matter how many times the magnets are split into individual particles, all that will emerge are increasingly more numerous, smaller dipole progeny.
When we study the way magnetism works in the world that we are familiar with, what we see is consistent with Maxwell’s equations. Maxwell’s equations describe the unification of electric and magnetic field theory in respect to one of the four known fundamental forces of nature: the electromagnetic force. The other three known forces of nature are the strong nuclear force, weak nuclear force, and gravity.
Maxwell’s equations were first published by the Scottish mathematical physicist James Clerk Maxwell (1831-1879) between 1861 and 1862, and they demonstrate that we could swap electric for magnetic fields and not observe any appreciable difference. This means that the two are symmetrical. Even today Maxwell’s equations are still used on a practical level in telecommunications, engineering, and medical applications–to list only a few. However, one of these equations–Gauss’s law for magnetism–indicates that there are no magnetic monopoles in the Universe. Nevertheless, many physicists think that there is good reason to suspect that these elusive elementary particles are really there. This is because their existence in nature would explain why the electric charge is quantized–that is, why it always appears to come in integer multiples of the charge of an electron, rather than in a continuous array of values. Indeed, the French physicist Pierre Curie (1859-1906), as far back as 1894, pointed out–in contrast to Maxwell’s Gauss’s law–that magnetic monopoles could really exist in nature, despite the fact that none had been detected.
The quantum theory of magnetic charge began with a paper by the English theoretical physicist Paul A.M. Dirac (1902-1984) in 1931. In this paper, Dirac demonstrated that if any magnetic monopoles exist in the Cosmos, then all electric charge in the Cosmos must be quantized. Since Dirac’s paper, several systematic hunts for the elusive magnetic monopoles have been conducted. Alas, not one has found a single magnetic monopole anywhere in the Universe.
Historically, many researchers attributed the magnetism of lodestones to two different “magnetic fluids” (“effluvia”). These early scientists proposed that there existed a north-pole “fluid” at one end and a south-pole fluid at the other, which attracted and repelled each other in a way similar to positive and negative electric charges.
However, an improved understanding of electromagnetism in the 19th-century indicated that the magnetism of lodestones was better explained by Ampere’s circuital law, rather than “fluids”. Andre-Marie Ampere (1775-1836) was a French physicist and mathematician who was one of the founders of classical electromagnetism. Ampere’s circuital law relates the integrated magnetic field around a closed loop to the electric current flowing through the loop. However, it was actually James Clerk Maxwell (not Ampere) who derived it using hydrodynamics in his 1861 paper.
The magnetism that we see today can be attributed entirely to the movement of electric charges. Indeed, the equations describing electricity and magnetism are “mirror images” of one another. However, there is one important difference between the two. Protons and electrons carry electric charges, but there is no known particle that carries a magnetic charge. A magnetic monopole would be the first to carry a charge, and if one were ever detected, electricity and magnetism would finally be equal. If even one solitary magnetic monopole were found inhabiting the Universe, this important discovery would profoundly effect the foundations of physics.