FORCES AND FIELDS
ALTHOUGH GRAVITY IS INTRINSICALLY the weakest force of nature by far, it is the force that dominates events at astronomical scales of distance and even on terrestrial scales. The reason is that gravitational forces are always attractive, so that the gravitational forces exerted on a terrestrial object (you, for instance) by the vast number of atoms in the earth all add together. By contrast, electromagnetic forces can be attractive or repulsive, and the contributions of the negatively and positively charged particles in matter tend to cancel each other out almost exactly. Thus, electromagnetic forces do not make their presence felt in daily life in an obvious way, as gravity does, even though they actually play the central role in most of the phenomena that we can directly observe. Because electromagnetic forces are more elusive and also mathematically much more complicated than Newtonian gravity, it took a long time and the work of many scientists to unravel their secrets. The most important of these were Charles Augustin de Coulomb (1736–1806), Edward Cavendish (1731–1810), Alessandro Volta (1745–1827), André Marie Ampère (1775–1836), Hans Oersted (1775–1851), Georg Simon Ohm (1789–1854), and Michael Faraday (1791–1867). The discoveries of these men were ordered, extended, and developed into a unified and coherent theory by James Clerk Maxwell (1831–79), probably the greatest physicist of the nineteenth century.
One of the crucial steps on the way to Maxwell’s theory was the idea of “fields.” (Interestingly, this enormously important theoretical concept was proposed not by a theorist, but by one of history’s great experimentalists, Faraday.) Newton’s theory of gravity was based on the idea of “action at a distance.” That is, one body exerted a gravitational force directly upon another across the intervening space without any intermediary. Faraday, by contrast, conceived of there being electric and magnetic force fields filling all of space. Electric charges and electric currents produce these fields and also are acted upon by them. One can think of electric fields as being made up of “lines of force” that stretch from positive charges to negative ones and pull them together in a manner not unlike elastic bands.
As it turns out, these fields have lives of their own. They contain energy and act not only upon electrically charged particles of matter but upon each other as well. Indeed, these fields are just as real as material particles. When Maxwell completed his theory, he discovered that its equations implied that waves can propagate in these fields, and that these waves travel at the same speed as light.
Indeed, subsequent experiments showed that light actually consists of such electromagnetic waves. Thus, Maxwell’s theory achieved a unification of three realms of phenomena that for a long time had been thought to be quite distinct: electricity, magnetism, and optics. In fact, a far larger unification is involved, because electromagnetic forces are responsible for the interactions among and within atoms. Thus, the physical properties of matter (such as heat conductivity, elasticity, opacity, viscosity, and so on), which are studied in “Condensed Matter Physics,” as well as the chemical properties of matter, are all based upon the electromagnetic interactions of particles.
LAVOISIER, ANTOINE (1743–94), “the Father of Chemistry,” was born in Paris to a wealthy family. After studying law, his interests turned to science. Although he performed many important experiments, his greatest contribution was to bring order to the theoretical chaos of chemistry. Chemists at that time labored under an upside-down theory according to which substances were thought to burn by releasing something into the air called “phlogiston,” rather than by combining with something in the air, namely oxygen. Joseph Priestley, who discovered oxygen, thought it was “dephlogisticated air”; and Henry Cavendish, who discovered hydrogen, thought it was water with extra phlogiston, while oxygen was water lacking phlogiston. Lavoisier showed that combustion was really a process of oxidation, and that the whole idea of phlogiston was mistaken. At a time when many chemists still believed in four fundamental elements—air, earth, fire, and water—Lavoisier made a remarkably accurate table of thirty-three elements (only three of which turned out later to be compounds). He brought rational order also to chemical terminology, which hitherto had been totally confused. (Zinc oxide was called “flowers of zinc”; iron oxide was “astringent Mars saffron”; lead oxide was “red lead” in England and “minium” in France; sulfuric acid was “oil of vitriol,” etc.) Lavoisier clarified the distinctions between, and relations among, salts, acids, oxides, and so on, and invented the modern system of chemical nomenclature. During the totalitarian madness of the Terror, charges were trumped up against Lavoisier and he was guillotined. Lagrange observed, “It required only a moment to sever that head, and perhaps a century will not suffice to produce another like it.”
While Maxwell’s theory involved new forces, phenomena, and concepts, it is at heart a Newtonian theory. Like Newtonian mechanics it is based on a set of equations (indeed, like Newton’s, they are “second-order differential equations”) that deterministically govern the evolution in time of a set of coordinates and momenta. True, the notion of a coordinate must be broadened to include not only the positions of particles (as considered by Newton) but also the magnitudes of the fields that exist at every place in space. Still, Maxwell’s theory involved an extension, not an abandonment, of Newtonian concepts and principles.
MAXWELL, JAMES CLERK (1831–79) was born in Edinburgh. He graduated from Trinity College, Cambridge, in 1854, held professorships at Marischal College in Aberdeen and King’s College in London, and in 1871 was named the Cavendish Professor of Physics at Cambridge University. While at Aberdeen he wrote a sixty-eight-page prizewinning paper on the nature of Saturn’s rings, demonstrating that they could be stable only if made up of disconnected particles. The Astronomer Royal, Sir George Airy, described it as one of the most remarkable applications of mathematics he had ever seen. This led Maxwell to think about the motions of molecules in gases and to apply statistical methods to understanding them. He discovered, independently of Boltzmann, the “Maxwell-Boltzmann distribution” of the velocities of gas particles, and he made other fundamental contributions to statistical mechanics and thermodynamics. Inspired by Faraday’s ideas, he next began to work on electricity and magnetism. He developed a theory of the dynamics of “fields” or “lines of force” that reached mathematical completion in four differential equations now called Maxwell’s equations, the greatest achievement of nineteenth-century physics. Maxwell was a deeply pious man whose personality was marked by gentleness and modesty; at the same time, he was called “the most genial and amusing of companions.” In his last years he selflessly nursed his ailing wife until he was rendered incapable of doing so by the cancer that killed him at the age of forty-eight.