If you could turn your eyes inward so as to examine the busy neurons within your skull, you would have graphic evidence of how important light is to the human organism. Nearly one-third of the billions of nerve cells in your brain do nothing but interpret the light entering your eyes to present a world rich in color, detail and meaning. That striking neurophysiological fact demonstrates how central to humanity is our highly developed vision.
Many centuries before neurons were dreamed of, however, or the structure of the eye was known, "light" was a compelling idea because in its speed and intangibility, it seemed ungraspable. And we have not yet penetrated to the deepest heart of light. Although we understand it better than we once did, it still carries an apparently insoluble quantum puzzle, that it is both wave and particle. Although we use it for ordinary purposes like playing the latest compact disc or telephoning a friend, it carries spiritual weight. Like space, time and other vast concepts, the idea of light has always drawn us; we have never stopped thinking about it.
In "The Fire Within the Eye," David Park shows us how that thinking has progressed. An emeritus professor of physics at Williams College, Park could undoubtedly explain in great detail how a laser works or how light produces sound from a CD. But he is after different game than the modern gadgetry of light or even its profound quantum discrepancy. His book, he writes, is "a history of thought about light, rather than the story of a gradual climb toward truth."
It is good to hear from a physical scientist steeped in history and philosophy who knows that the latest scientific knowledge is not the only truth that has accrued around light. Of course that scientific knowledge carries special weight because the physical nature of light is extraordinary. (Light acts in some ways like a stream of tiny billiard balls and, in other ways, like an undulating ocean wave. This enigma emerged after Einstein discovered those tiny particles, which came to be called photons, in 1905. It has baffled the finest scientific minds since, from Einstein to Richard Feynman who, with two colleagues, won a Nobel Prize in physics in 1965 for the modern theory of light, called quantum electrodynamics, or QED.)
"The Fire Within the Eye," however, concentrates on the period dating from before the time of Socrates to the 19th century. Only the book's last chapter is devoted to the recent intense activity in the science of light, which is characteristic of this century. "Ideas ripen slowly," says Park, who believes that the 20th century has not been more important for the scientific understanding of light than the three preceding centuries. He adds, "We who live in this century do not need to have its leading intellectual assumptions explained to us." I do not fully agree with either position. The uncovering of the photon and QED theory are unique and historic achievements, as is the vast progress in the technology of light (which may finally explain the wave-particle paradox). And many teachers of science, like myself, find they can hardly do enough to explain to their students the "leading intellectual assumptions" of the day.
But for the story of light before this century, Park is an excellent guide. He begins with the early Greek philosophers, who had no way to sense and analyze light other than through their eyes and minds. For them, the objective properties of light were entwined with the subjective nature of vision, whereas we moderns know that the light we see is electromagnetic radiation of certain wavelengths, radiation that is emitted at other wavelengths as well, such as those of invisible ultraviolet light.
The first philosophers lacked this physical knowledge and found it difficult to answer even the very first question about vision: How is the image of an object conveyed to the viewer? Now we know that light travels to the eye, either directly from a source or after modification by the material world. To the viewer, however, the act of seeing requires a directed focusing of the eye and one's attention. That subjective reality may explain why the philosopher Empedocles (who gave us the four elements) wrote in 450 BC about a "visual ray" that left the eye in order to seek out and sense the thing that was seen. Plato also accepted this idea and introduced a "fire within the eye" as the origin of the visual ray. I know from my own experience that some people still find the notion of a ray from the eye eminently reasonable. To them, the contemporary scientific view is not obvious; it takes some explanation to present the modern version of things.
Park shows how the idea of light entered into philosophy and religion, often symbolically but sometimes explicitly. This was the case with Manichaeism, the dualist doctrine, originating in AD 3, that equated God with light and Satan with its absence. The historical tale continues through time and space, as thinkers from Alexandria, the Near East and Europe contributed to the ongoing investigation and as the philosophical and the religious views of the world were slowly influenced by science. In the 13th century, Robert Grosseteste, bishop of Lincoln, wrote about light in a way that mixed an appreciation of its role in the creation with hints of a physical theory. In the early 17th century, Galileo Galilei used the newly invented telescope to examine cosmic light and drew startling conclusions about the universe that conflicted with religious views. Later in the same century, Rene Descartes wrote "Dioptrique"; this work about optics contains a famous image (one of many pictures illuminating "The Fire Within the Eye") showing how light rays enter the eye, as determined by experiments with an ox's eye.
The analytical and experimental approach continued, and in 1666, Isaac Newton dismembered white light into colors with a simple glass prism. After deep consideration and with some ambiguity, he decided that light was made of particles. Then, as Park shows, theories of light fell into two scientific camps: one viewing it as a particulate and one viewing it as a wave. Not until the late 19th century, with the work of James Clerk Maxwell and Heinrich Hertz, was light definitively shown to be a traveling electromagnetic wave. But that seemingly final result was soon upset by new evidence, and as this century began, Einstein found that he must reintroduce the theory of light particles. Today both notions collide in the wave-particle duality.
Park narrates this long and complex story with transparent prose. Such writing gives a sense of careful thinking behind the clear writing. The style, nearly always nontechnical (it strays occasionally, as in the long discussion of how rainbows are created), is suitable for nonscientists and nonphilosophers, and there is a helpful glossary. In fact, the book combines the scholarly with the approachable. Those who want to delve seriously will find an index, a bibliography and extensive footnotes, but these do not necessarily make the book inaccessible to casual readers.
The only thing lacking here is that Park does not discuss our emotional response to light, either directly or through the medium of art. Some might consider such a discussion inappropriate in a book that is more or less scholarly history, but to my mind that is one reason light has gripped us so tightly. Park does, however, insert enlivening asides that explore light in various ways. There is, for instance, a fascinating discussion of how color was perceived in ancient times. The colors of blood, stars, the sea and the sky, as given by Homer, Euripides and others do not always match what we now see: "For whatever reason," writes the author, "the world may not have looked to them as it looks to us."
In sections called "How to Paint Space" and "How to Draw Distance," Park describes how the struggle to put three-dimensional reality onto a flat surface was resolved during the Renaissance, and in his brief discussion of contemporary light, Park touches on the Hubble Space Telescope and other 20th century wonders of light.
While the deeper meanings of these wonders may not be completely understood today, with time they undoubtedly will. Just as the significance of the discovery of the electron (1897), the atomic nucleus (1911) and the invention of the transistor (1947) was not apparent until years later, the meaning of the photon, light's building block and Einstein's stunning revelation in 1905, will become apparent with the dawn of the 21st century. Then, perhaps, we will be able to add another chapter in the history of light that David Park has told so well.