Visible Rays

A Historical Essay on the Nature and Meaning of Light
By David Park
Princeton University Press, 377 pp.

Like all good histories of science, David Park’s sweeping account of human attempts to explain the physics of light is a history of gobbledegook. Which is not to say the book is confused or confusing, or that what the ancients had to say on the subject was so ignorant it’s not worth revisiting. To write a history of science is to coax modern minds to think like past cultures. Since from the perspective of the present what people used to believe about the natural world is inevitably wrong, as an intellectual discipline the history of science is a mind game of the highest order.

The U.S. philosopher C. S. Peirce declared it impossible to believe something one knows to be false. But that is precisely the aim of histories of science. Euclid was convinced that vision was the result of rays emanating from the eyes and somehow sensing whatever they were looking at. In order to understand why he thought so, you have to be able to see through the eyes of a Greek who’s been dead for 1,600 years. You have to be able to accept something you know is patent nonsense.

What is light, anyway? Even today, how many of us would be capable of providing an explanation that is not purely rote? (“The visible portion of the electromagnetic spectrum . . . and, um . . .”) In some ways, the question is more interesting than the answer. Imagine yourself in the Mediterranean at the birth of rationalism. What would you make of this mysterious aspect that makes vision possible but that comes and goes? Light issues from the sun, fire illuminates. But when the sun goes down or the embers die, darkness closes in. Is light a property? A force? A substance? Stare into a bright light and then close your eyes. Why can you still “see” the light? Place a pencil at an angle in a glass of water. The evidence of your eyes says the pencil bends, even though you know it doesn’t.

The explanations offered by our intellectual ancestors were ingenious and internally consistent. In recounting them – in making them make sense – Park, emeritus professor of physics at Williams College, excavates the underpinnings of our contemporary answers to the same questions. One cannot fully understand the present if one fails to appreciate the circuitous route by which we got here.

Though the theories proffered by early investigators may seem risible today, they were not without utility. Before the birth of Christ, in an era before matches, every corner apothecary in Greece sold “burning lenses,” used to ignite fires by focusing the rays of the sun. Legend has it that in 212 B.C., when the Romans besieged the Greek city of Syracuse, Archimedes set fire to the invading fleet with an array of mirrors. In 1973, at a naval base near Athens, a Greek engineer named Ioannes Sakkis attempted to determine whether this was possible using 50 to 60 bronze mirrors pointed at a rowboat more than 50 metres out in the bay. In less than two minutes, the target was ablaze.

The book is dotted with such tidbits, and laced with capsule histories of the personalities who contributed to the puzzle of light. Nor is The Fire Within the Eye just a catalogue of shifting theories of light. It is also a history of how inquiry in natural philosophy itself changed – of how different ages demanded different arguments, and how what was admissible as evidence changed accordingly. It was Galileo who declared that the universe “is written in the language of mathematics,” and certainly from his time on an explanation not grounded in mathematics hardly counted as such. By the later chapters, dealing with Michael Faraday, James Clerk Maxwell, Max Planck, Einstein and the rest, there is almost nothing but mathematics. The equations are the explanations.

Even some of the greatest thinkers have found this unsatisfying. Park recounts that when Leibniz read Newton’s Principia, he objected to the introduction of a force of gravity without an accompanying explanation of where it came from and what it was. He told anyone who would listen that, beyond some mathematics, Newton had accomplished nothing at all.

More unsettling is the fact that, if one doesn’t have the math, one can’t fully understand the physical explanations of one’s own age. Quantum mechanics is as foreign and strange as the eidola theory of Leucippes of Miletus. In a word,

  • Globe and Mail October 18, 1997