And there is something else that charge and mass have in common. Both, when accelerated, can emit radiation. The radiation emitted by accelerated charge is commonplace. It is light (or infrared or X rays or cell-phone signals). Such radiation was predicted by James Clerk Maxwell in the 1860s and confirmed for radio waves by Heinrich Hertz in the 1880s. The radiation emitted by accelerated mass is called gravitational radiation. It was predicted by Albert Einstein in 1916 and confirmed a hundred years later when researchers at an installation called LIGO1 detected gravity waves sent on their way by coalescing black holes 1.3 billion years ago.
It is electric charge that puts and end to the periodic table. Within the atomic nucleus there are positively charged particles—protons—but no negatively charged particles.2 The protons repel each other, and would blow a nucleus apart but for the but the overpowering nuclear force (acting on neutrons as well as protons) that holds the nucleus together. Holds it together up to a point. Eventually, for very heavy nuclei, the electric repulsion becomes more than the nuclear forces can counteract, and the nucleus does fly apart. It is for this reason that no nuclei heavier than uranium exist in nature.
Which charge is called positive and which negative is entirely arbitrary, and is the result of historical accident. The definition that led to electrons being negative and protons positive probably stems from a guess made by Benjamin Franklin about the middle of the eighteenth century. His choice of nomenclature was based on the erroneous supposition that it is positive electricity that flows most readily from one object to another. We now know that it is the negative electrons that are mobile and account for the flow of electric current in metals.
Charge is still very mysterious to the physicist. Why is it quantized? Why does the electron charge (the smallest quantum unit) have the magnitude that it does? And if we think of a particle as a small structure spread out over a tiny region of space, why do the various bits of charge making it up not repel each other and cause the whole particle to disintegrate and fly apart? No one knows satisfactory answers to these questions. It is also perplexing that almost all particles have exactly the same magnitude of charge. If the charge of the electron is called –e (it is negative) then the charge of every other “long-lived” particle is either –e or +e or zero. No other possibilities are realized in nature. We have no understanding of this fact. The true nature of charge and the reason it comes only in lumps of a certain size remain important unsolved problems in particle physics.
Some history: Early studies of electricity involved not only electrical force, but also—indeed to a greater extent—the phenomenon of electrification itself. In 1600, William Gilbert proposed that electrical effects arise from an electrical fluid. With some substantial modifications, the doctrine of the electrical fluid held sway until late in the nineteenth century. Even our present concept of charge does not differ very drastically from old ideas about the electrical fluid.
Basically the idea of the electrical fluid is very simple. Matter was supposed to contain, besides its material constituents, a rather ethereal fluid. According to Gilbert, friction could release a part of the fluid into the space surrounding an object, and the gradual flow of the fluid back to its parent object accounted for electrical attraction. According to this view, an inflated balloon, under normal conditions, contains its assigned quota of electrical fluid. After being rubbed with wool, it suffers a deficiency of the fluid, the lost fluid being distributed in the space around the balloon. A bit of paper drawn to the balloon is being borne on the tide of returning fluid. The eventual neutralization of the balloon (that is, the elimination of any electric effects) occurs when all the fluid has returned to the balloon. That was an appealing theory for the then-known facts, but it was a theory that could not survive new experiments of the early eighteenth century. Gilbert was unaware of electrical repulsion.
When Stephen Gray discovered the conduction of electricity through metals in 1729, Gilbert’s idea of a separate electrical fluid attached to each material object gave way to the idea of a single electrical fluid that could flow from one body to another. Then the discovery of electrical repulsion led to the introduction of a two-fluid theory to rival the one-fluid theory. The two fluids corresponded to what we now call positive and negative charge. Benjamin Franklin held to the one-fluid theory, and in 1747 made the inspired suggestion that the total amount of electrical fluid remained forever constant, any loss of fluid by one body being exactly compensated by an equal gain of fluid by another body. In Franklin’s view, electric neutrality represented a “normal” amount of fluid, what we now call positive charge represented an excess of fluid, and what we now call negative charge, a deficiency of fluid. Although Franklin correctly guessed the law of charge conservation, he and his contemporaries had not yet abandoned Gilbert’s original idea that the electrical fluid exists in the space surrounding an electrified object as well as in the object itself. Experiments of Franz Aepinus reported in 1759 showed that there was, in fact, no evidence for any electrical fluid occupying the space outside an electrified body. At this point, the fluid, having retreated to the interior of material objects (except in certain phenomena such as sparks), scarcely differed from our modern idea of charge. Gradually toward the end of the eighteenth century the two-fluid theory reemerged as the dominant theory of electricity, in part because it proved to be more convenient in calculations of electrical force, in part because it was championed by influential scientists such as Charles Coulomb. The two fluids came to be called charge, and for a century the source of all electrical (and magnetic) phenomena was successfully described as a pair of fluids that could flow through some materials, be held fast on other materials, that acted as the source of electrical force, and that, when present in equal quantities, canceled each other’s effects.
The idea that electric charge was not a continuous fluid, but that instead definite fixed amounts of charge were associated with each atom, was suggested by Michael Faraday as early as 1840. That charge is indeed concentrated in quantized lumps on individual particles was finally verified beyond doubt by J. J. Thomson’s discovery of the electron in 1897. Obviously charge in tiny discrete bundles appears to be a very different picture from charge as a fluid spread throughout a substance. Most important, it renders irrelevant the idea of a separate fluid, distinct from the material basis of the substance. Charge becomes a property of matter, not an additional substance added to matter. Yet, viewed more carefully, our modern idea of charge is not so significantly different from Gilbert’s original idea of an electrical fluid. We do not really have any deeper understanding of the nature of charge. In answer to the question “What is charge?” we can’t say much more than Gilbert could have said. Charge is that certain something possessed by material objects that makes it possible for them to exert electrical force and to respond to electrical force. Although we speak of charge as an intrinsic property of a particle, it is still a property distinct from purely mechanical properties such as mass and size. It is true that we cannot change the charge of a particle without destroying the particle altogether, yet our mental picture of charge is still a picture of something extra, a “substance” carried by charged particles, a substance that neutral particles lack. The concept of charge has undergone a steady evolution, and several revolutions, since the electrical fluid of Gilbert. Yet it remains a mystery at the deepest level. The fact that we “understand” electric and magnetic phenomena—that we can support a vast range of experimental results on a simple and powerful theoretical framework and can successfully predict new phenomena—should not delude us into thinking that we really understand electric charge.
1 Laser Interferometer Gravitational-Wave Observatory. It consists of two hyper-sensitive detectors located in the states of Louisiana and Washington.