The Vision Thing
By Dick Teresi
Published: September 28, 1997
The Fire Within
A Historical Essay on the Nature
and Meaning of Light.
By David Park.
Illustrated. 377 pp. Princeton, N.J.:
Princeton University Press. $29.95.
In 1611, on a trip to Rome, Galileo brought, in addition to his telescope, a little box of fragments of a rock recently discovered by alchemists in Bologna. The mineral, barium sulfide, in those colorful times went by the name ''solar sponge.'' To impress (dismay?) his Aristotelian colleagues, Galileo took them into a dark room, opened the box and showed them the cold light of the solar sponge. His message: Aristotle was wrong; light is not simply what he called an ''accident'' (symbebekos) or ''quality of a transparent medium,'' nor merely a concomitant of heat. It is a thing, a substance existing separately from an illuminated environment. One can soak it up in a rock in Florence, transport it to Rome and unleash it at will. That should have put an end to that argument once and for all. Only the details needed to be worked out.
One message of ''The Fire Within the Eye,'' a graceful book by David Park, is that we're still working on the details; Aristotle may yet have the last laugh. An emeritus professor of physics at Williams College, Park covers a bizarre amount of material, from theories of ancient Greek atomists to the invention of the laser. He is as coherent as a ruby laser, never straying from the question: what is light? So many times in this essay we think we have it. Then, click, the room goes dark as Park eases us back into shadows. In the first sentence he walks out of the dark night into his lighted house. In the final sentence he returns to the shadows, and we find ourselves still in darkness, but somehow illumined by the experience.
We begin with the Greek philosopher Democritus, the first man in the West to propose atomic theory, when he smelled bread being baked by his sisters and concluded that particles from the baking bread were sloughed off and wafted to his nose. It was a theory with legs. Democritus and Leucippus proposed a similar hypothesis for light: visible objects slough off eidola, veils of matter one atom thick that retain their shape as they fly in every direction. It is the eidola our eyes perceive. If this were true, however, wouldn't we see better downwind from an object?
Another Greek, Empedocles, was the first to suggest that the eye projects a ray that feels out objects, then returns to the eye to create an image in the mind. This theory was still being taught in the 17th century despite obvious drawbacks. It cannot explain ''how we can lock onto a star so quickly,'' Park points out, when stars are light-years away. Park warns us not to laugh at the ancient theories. Most were, at least, attempts to explain light and vision in physical rather than metaphysical terms. Is light a thing? The Roman general Marcellus, besieging Syracuse in 212 B.C., got a clue. Archimedes arranged a series of mirrors so that the sun's rays were focused on Marcellus' fleet, reducing his barges to ashes. If light is not a substance, what burned Marcellus' boats?
Park refuses to ridicule anyone. He shuns the stance of the omniscient modern, looking kindly but condescendingly on our foolish predecessors. But with Aristotle, he almost loses his patience; when Aristotle tells us light is ''the activity of what is transparent,'' Park says, ''We feel like someone who expected dinner and has been given a salted peanut.'' Even so, old, apparently silly ideas have a way of resurfacing.
Abu Ali al-Hasan, born in Iraq in the 10th century, destroyed the ray theory merely by looking into the sun. It hurt. Clearly the rays are going into the eye, not out. He proposed that objects we see in our minds are reproductions of real objects: a mountain is reproduced point for point on the eye's lens, only in tiny form. Thus he fomented a mathematical crisis. How could a teensy mountain in the eye have all the points of a real mountain? The answer is that both have an infinite number of points, points being dimensionless. (Georg Cantor, 900 years later, made infinite sets a part of respectable mathematics.)
Galileo said light emerges from the hearts of atoms. In fact, we now know each atomic element emits distinctive colors because of the jiggle of its electrons. Isaac Newton said light is corpuscles, particles. But the emissionists, his followers, were crushed by wave theorists, first by Christian Huygens, logically, and then experimentally by Thomas Young, who showed that light shining through two tiny slits creates an interference pattern on a screen behind them. Only waves do that. But Newton returned when Max Planck, Albert Einstein and others discovered that light does act like chunks, particles, in addition to waves. ''There is more similarity than difference between a ray of light and the smallest particles that make up a brick,'' Park writes.
So is light a wave or a particle? Park says these are just words that ''refer to mental images that help us understand the calculations and the experiments.'' It is one thing to say light behaves like a wave or a particle, another to say it is a wave or a particle: ''A wave is an accidental quality, a wiggle in some medium, whereas a particle is more like a substance.'' We must think in terms of fields. When you flick off a switch, you disturb the field and the light goes out. What happens here affects there. Quantum theory, dominated by fields, is ''a science of qualities, not of things,'' Park writes. ''Nature has revealed to us the precious secret that light is an attribute of a field -- an accident, as people used to say.'' Aristotle!
''The Fire Within the Eye'' was my only companion on a series of bicycle trips this summer through western Massachusetts. As the book, strapped to my rear rack, faded under the pounding of the sunlight, it became clear to me that light is a thing. But, stopping to read ''Fire'' at a farm stand in Sunderland, I learned from Park why the air shimmers over the superheated tobacco fields. Light is a quality, a wiggle. When a rainbow sprang from the mists of a waterfall in Montague, I flipped pages for an explanation. I found six different theories, with lovely diagrams, from Aristotle to Descartes and Newton. Particles, bouncing about mathematically, explain rainbows better than waves do. But then I read why I could hear trucks rumbling toward me from around the bend, but could not see them. Waves again. Sound waves, being longer, take corners better than light waves.
In the end, Park returns to the shadows: ''Only a few of us want to pitch our tents in a plain bathed in the sun's vertical rays. Most of us move naturally toward shadow. . . . I have tried not to write this book as a chronicle of light driving out darkness, of good methodology supplanting an old one that was bad, of enlightened effort bringing answers to stubborn old questions. To some extent that has happened, but mostly the questions themselves have changed.''
At one point, the Arab philosopher al-Kindi is credited with al-Hazan's point-for-point image theory. Other than that, I have but one complaint. Park, a theorist, admits he distrusts experiment and treats science as if it were merely a history of ideas. Experiments are often not well explained. A case in point is Ernest Rutherford's 1911 discovery of the nucleus. Park ignores this breathtaking experiment, devoting his time entirely to Niels Bohr's interpretation of it. It was an experiment made for Park. Rutherford's assistants sat for weeks in a dark room, watching sparks made by alpha particles as they ricocheted off gold atoms and struck zinc sulfide detector screens. It was the first proof that matter consists mostly of empty space.
This is, admittedly, an arcane objection. Most readers accept that science is invariably popularized by theorists, not experimenters. This was not always so. Galileo often wrote in vernacular Italian rather than in Latin to reach a broader audience. And the finest experimenter of all, Michael Faraday, lectured to children. This impoverished, uneducated 19th-century English bookbinder, who learned science by reading the books he bound, invented the dynamo (electrical generator) and countless other devices for studying electricity. He also invented the near-mystical concept of the field, never taken seriously in his time but later embraced by Einstein and quantum physicists. Faraday started the ''Christmas Lectures'' for children in 1826, and in his first talk he argued that all known scientific processes were illustrated by a burning candle. I can see him now, his rough bookbinder's hands, fashioners of a thousand experiments, gesturing in the candlelight before the children. Perhaps someday another experimenter will emerge from the shadows of the lab to explain the light. Until that day arrives, no one can hold a candle to David Park.