Consider the magnetic field generated by a long thin cylindrical coil, called a solenoid. Lines of magnetic field draw together in a region of intense field at one end of the solenoid; they extend through the interior of the solenoid; then they emerge and fan out at the other end. A true north pole would be a spot from which magnetic lines of field emerge, a source of field. It would be analogous to a positive charge, which literally creates lines of electric field emerging from itself. Looked at not too closely, one end of the long thin solenoid appears to be a north pole. Magnetic field lines emerge from it and spread out. In fact, however, they do not all emerge from a single point, nor are they created at that end of the solenoid. It is only an approximate north pole. Similarly, the other end of the solenoid behaves somewhat like a south pole, a point where lines of field coalesce at a point and terminate. In this sense it is analogous to a negatively charged particle, a terminus of lines of electric field, or a “sink” of electric field. Again, however, this end of the solenoid is only approximately a pole.
The difference between the electric and magnetic fields found in nature can be expressed in this way. Lines of electric field can begin and end. Lines of magnetic field never begin or end.2 An electric charge is a primitive source or sink of electric field. From a positive charge emerge lines of electric field and into a negative charge disappear lines of electric field, regardless of the state of motion of the charges. As we now know, charge is carried by electrons, protons, and other fundamental particles. No such primitive sources and sinks of magnetic field have ever been discovered, neither as fundamental particles nor in any other form. Instead, it seems that all of nature’s magnetism arises from the motion of charge. Although this fact simplifies the theory of electromagnetism, accounting for all phenomena in terms of electric charge alone, it does introduce into electromagnetism a puzzling asymmetry that has so far escaped a basis of theoretical understanding.
If a rod had a true north pole at one end, and a true south pole at the other end, it would create a magnetic field scarcely different from that created by the thin solenoid. Yet at the microscopic level there would be important differences. At the ends of the rod, field lines would begin and end. At the ends of the solenoid, field lines only seem to begin and end. In fact, the lines of field thread their way through the core of the solenoid unbroken. Likewise, in an ordinary magnet, field lines disappearing into one end pass continuously through the interior of the magnet and appear at the other end. Ampère had no way of knowing the exact nature of the internal currents within a magnet that produce its magnetic field. Since no effort to measure a current within a magnet succeeded, he could only assume that the magnetism was produced by tiny microscopic current loops within the material, not by any bulk flow of charge over a measurable distance.
Nearly a century had to elapse before the correctness of Ampère’s hypothesis of microscopic current loops could be verified. According to modern atomic theory, electrons within every atom whirl in orbits of radius 10–10 m or less. In addition, every electron spins about its own axis, a further and even smaller-scale manifestation of moving charge. Because of these rotational motions of charge within every atom, no material is free of some magnetic properties. However, for most materials, the individual spins and current loops are oriented in all directions at random, and the magnetic fields they create cancel each other out, leaving no large-scale magnetic effect. In a few materials, called ferromagnets, the current loops become aligned and remain so. Then their fields reinforce each other and a large-scale magnetic field results.3 Iron, the best-known ferromagnetic, is commonly employed for magnets, although alloys of iron and other metals are superior for producing strong permanent magnets. Probably the most remarkable fact about ferromagnetism is that it arises from the spin motion of electrons, not the orbital motion. Not even the vision of Ampère could have foreseen this. Each spinning electron is truly an elementary magnet. For those few materials whose energy is lowered when the axes of electron spin array themselves in parallel, there occurs a natural tendency for self-reinforcement of the atomic magnetism, resulting in the large-scale magnetism discovered more than 2,000 years ago by the Greeks.
In spite of the beautiful explanation of magnetism afforded by modern atomic theory, many physicists remain troubled by the asymmetry of electromagnetism. If elementary poles as well as elementary charges existed, electric and magnetic phenomena would be in perfect balance. Both electric and magnetic fields would have their own primitive sources. As moving charges produce magnetic field and react to magnetic field, so moving poles would produce electric field and react to electric field. For every electric phenomenon there would be a precisely equivalent magnetic phenomenon. However, it seems that nature has chosen to be economical rather than symmetric. Charge alone is sufficient to produce both electric and magnetic phenomena. Only charge has been found.
In 1931 the eminent British physicist Paul Dirac postulated that perhaps isolated magnetic poles (monopoles) do exist but had until then escaped detection. He even worked out a property of such poles, their minimum pole strength. Interest in the possible existence of such poles has not died. Indeed, modern developments in particle theory have rekindled interest in the possibility of an as yet undiscovered fundamental particle carrying pole strength instead of charge. Physicists have looked for such a particle in the debris from high-energy nuclear collisions at the largest accelerators, and in the still higher-energy cosmic radiation arriving from outer space.4 The results: no poles.
As techniques are refined, the search will go on, for the physicists’ faith in the symmetry of nature is hard to shake. There can be no doubt that if magnetic poles exist at all, they are exceedingly rare, at least in our part of the universe. Antiparticles are also rare (fortunately), yet their existence confirms another symmetry of nature. If poles do not exist, the physicist must seek to answer the question: Why not? The hope is that the apparent asymmetry of electromagnetism is a reflection of some deeper still-to-be-discovered symmetry.
1 Not all of Ampère’s contemporaries accepted his idea that microscopic currents within magnets account for their magnetism. Indeed the full verification of Ampère’s theory had to await the twentieth-century developments of atomic theory.
2 In 1957 John Wheeler made the intriguing suggestion that perhaps electric field lines also never begin or end. What if, he asked, the thing that appears to us to be a charged particle (such as an electron) is really only an entry point to a four-dimensional spacetime tunnel (he called it a “wormhole”) that emerged at some distant point at what appeared to be another charged particle of the opposite sign?
3 A ferromagnetic material need not be a magnet. Within one magnetic domain of microscopic size (but containing billions of atoms) all of the atomic magnets align to create a large field. But unless the domains also line up to reinforce one another, a large piece of the material will not act as a single coherent magnet.
4 In 1963 I participated in one such search.