TO MUCH fanfare, Italy celebrated 150 years since its unification two weeks ago. Less exuberantly, America is commemorating the 150th anniversary of the outbreak of the civil war, a failed attempt to undo its union. Amid this flurry of historical fissions and fusions it is easy to overlook another, arguably more significant unification set in motion in spring 1861. In March of that year James Clerk Maxwell, a Scottish physicist (pictured above), published the first piece of a four-part paper entitled "On physical lines of force". Sprinkled amid the prose in the Philosophical Magazine were equations which revealed electricity, magnetism and light to be different manifestations of the same phenomenon.
By the mid-19th century scientists had a fair understanding of each of the three components of electromagnetism, as the phenomenon has come to be called. They knew, for instance, that the distribution of electric charges was linked to the pattern of electric fields and that magnetic poles cannot exist in isolation, in other words that there were no single magnetic charges. They also knew that a moving magnet generates an electric current in a wire coil, as demonstrated by Michael Faraday several decades earlier at the Royal Institution (a short walk from The Economist's offices in London). However, no one could explain precisely why that was.
Maxwell's aim was initially to forge a mathematical link between electricity and magnetism that would capture these experimental results. (The issue was a burning one for the Victorians who had just been spectacularly stymied in their efforts to get the trans-Atlantic telegraph connection to work. Understanding how electricity and magnetism interacted, it was thought, would help to overcome the problem of the delay and deterioration experienced by the signal as it travelled along the underwater cable.)
He also realised that varying the strength of an electric field would generate a changing magnetic field, even in empty space with no moving electric charges to speak of. A changing magnetic field, of course, gives rise to an electric field, as had been established by Faraday. Might the two fields nudge each other along in a self-perpetuating, wave-like manner? Maxwell's calculations made it clear—they could. And the speed at which such an electromagnetic wave would propagate through a medium was inherently linked to the medium's electrical and magnetic properties. When Maxwell plugged the relevant values, which had been obtained recently by experimenters in Germany, into his equations, out popped Fizeau's figure for the speed of light. Convinced that this was no accident, Maxwell went on to suggest that light is, in fact, an electromagnetic wave. Physics had got its first unified theory.
2011 is awash with anniversaries of notable events from the annals of the physical sciences. Chemists will be celebrating 350 years since the publication of Robert Boyle's "Sceptical Chymist", a tract which marked the birth of their science, at least in its modern guise. One hundred years ago in April, meanwhile, Heike Kamerlingh Onnes, a Dutch physicist, discovered that some materials are superconductors—as they are cooled towards absolute zero they allow electric charge to flow with no resistance. In May of the same year Ernest Rutherford, a New Zealand-born British boffin, put forward (also in the Philosophical Magazine) the familiar model of the atom as composed of a dense nucleus orbited by tiny electrons. Although physicists have since come up with more elaborate projections of the subatomic reality, the Rutherford model is, unlike the earlier plum-pudding version, basically right—which is why it continues to be taught to schoolchildren the world over. And it has been 30 years since Alan Guth, an American particle physicist, published a paper suggesting that instants after the Big Bang the universe underwent a phase of rapid expansion; the inflationary theory has since become cosmological received wisdom and forced astrophysicists to take particle physics seriously.
Worthy intellectual accomplishments, all. Yet they pale in comparison with Maxwell's. This is not just because, unlike a lot of subsequent theoretical advances, his insight has already yielded a century's worth of tangible results, from radio to mobile phones. (Only a century because it took scientists several decades before they grasped the theory's full significance and put it into practice.) Nor is it because he championed the abstract idea of fields, a fecund notion that underpins much of modern physics. No, Maxwell's greatness lies elsewhere still. He showed that nature ought not to be taken at face value, and that she can be cajoled into revealing her hidden charms so long as the entreaties are whispered in mathematical verse. In doing so he paved the way for the pursuit of physicists' holy grail: the grand unified theory, a set of equations which would explain all there is to know about physical reality. As tends to be the case with grails, this one, too, may prove unattainable. Unless there are inherent limits on human understanding—itself an unfathomable premise—there will always be more apparently disparate phenomena to explain at one fell swoop.
Maxwell remains the great unsung hero of human progress, the physicists' physicist whose name means little to those without a scientific bent. His life's work, which also includes remarkable contributions to thermodynamics (not to mention taking the world's first colour photograph, also 150 years ago) is among the most enduring scientific legacies of all time, on a par with those of his more widely acclaimed peers, Isaac Newton and Albert Einstein. It deserves to be trumpeted.
