What Is Light?
No one really has the faintest idea.
The Fire Within the Eye: A Historical Essay on the Nature and Meaning of Light
By David Park
Princeton University Press; 550 pages; $29.95
Light, without which we can see nothing, is itself invisible. It seems to move from place to place instantaneously, passing ghostlike through solid materials such as glass. If it is part of the material world, it is certainly, by dint of its surpassing subtlety, the part that is closest to spirit: So thought the astronomer Johannes Kepler. It is identical to God, mystics like Mani and John the Evangelist declared. Plus, it can give you a tan.
"We all know what light is, but it is not easy to tell what it is," Dr. Johnson once said with nice understatement. Small wonder it has set great minds at odds. Take the simplest of all questions: Is light a thing? Those inclined to say "yes" have tended to conceive of light as a rush of particles--little tennis balls, to borrow an image from René Descartes. The French philosopher went on to conjecture, quite groundlessly, that it was the spin on these tennis balls that accounted for color. Isaac Newton deemed this nonsense, since spinning balls curve in flight, whereas colored light beams don't. Yet Newton too was a particle man, and by applying his laws of mechanics to the tiny tennis balls of light, he was able to explain all sorts of optical phenomena.
Plato and Leibniz were among those who agreed with Descartes and Newton that light was a thing. An equally formidable roster--Aristotle and Aquinas, Newton's scientific contemporaries Christiän Huygens and Robert Hooke, the 18th-century mathematician Leonhard Euler--opposed them, maintaining that light was rather an activity of something else, the way sound was a wave in the air. Off on the sidelines, making an ass of himself, was Goethe, who hated the very idea of thinking about light analytically. (In coming up with his own cockamamie theory of light and color Goethe trusted "sympathetic perception" over experiment.) The ongoing quarrel among these and lesser parties was one of the greatest in the history of thought, and it is recounted with brio by David Park, a fearfully erudite professor emeritus of physics at Williams College, in The Fire Within the Eye. Park has done a heroic job of assembling the best that has been said and thought about light by poets and religious visionaries, as well as by scientists.
It's the scientists, though, who have the best story. (Light has always been a cheap metaphor for poetic visionaries, and the things they make it stand for--truth, beauty, goodness--are so tediously respectable.) The dilemma they faced was a neat one. Every item of experience had to be either a thing or the property of a thing--what else could there be? Yet suppose light were a stream of particles. Wouldn't the particles collide when one beam of light crossed another? In particular, it would be impossible for two people to look directly into each other's eyes--there'd be tennis balls all over the place. Waves, by contrast, do not bump: Ripples on a pond pass through one another peaceably.
B ut suppose light were a wave. Waves, by nature, spread out; they propagate through their medium in every direction from a disturbance. That is why I can hear you around the corner. I cannot see you around the corner. So light must travel in a straight line, unlike sound or any other known wave.
The particle view was ascendant during the 18th century for two reasons, observes Professor Park: "One was the simple fact that sound waves turn a corner but light does not; the other was the authority of Isaac Newton." The whole time, though, the Newtonians were close to being massacred by the laziest of experiments. Dip an oar into the water. It appears bent. The effect is called refraction, and it results from the fact that light travels at a different speed through water than it does through air. Both the wave-theorists and the particle-theorists knew that. They also knew that particles and waves are refracted in precisely opposite directions when they move from a faster to a slower medium. (I remember my reaction on seeing this minor profundity proved in baby physics--Cool!) They would certainly see which way the oar in the water bent. But there was one small yet critical thing they didn't know: whether light traveled slower in air or in water. The Newtonians looked at the angle the oar made and hoped the answer was air, for that would vindicate their particle theory. The anti-Newtonians hoped it was water, for that would vindicate their wave theory.
And the anti-Newtonians were right, as the ingenious Léon Foucault--who later mounted a huge pendulum in the Pantheon to demonstrate the rotation of the earth--showed in 1850 when he contrived to time a flash of light as it traveled down a 10-foot tube of water and back again. Yet this proved to be an anticlimax. The Newtonians had long since been undone, and by even more delicate effects: shadows cast by a horsehair, bands of light and dark produced by a pair of tiny slits. By such means, a London doctor, Thomas Young, and a French civil engineer, Augustin Jean Fresnel, had independently shown that adding one beam of light to another could yield darkness--mutual annihilation. This couldn't happen if light consisted of particles: Tennis balls plus tennis balls equals more tennis balls. But when waves get added, one plus one can be zero, if the peaks of one combine with the troughs of the other.
It was a feat of pure thought, not experimental finesse, that finally disclosed what these waves were. In 1865 the Scotsman James Clerk Maxwell wrote down a half-dozen lapidary equations that unified electricity and magnetism. He noticed an arresting implication. Because a dying magnetic field gives rise to an electric field and vice versa, the two ought mutually to propagate through space as a wavelike pulse. It was the work of a moment to deduce the speed of this pulse. That turned out to be ... the empirically measured speed of light. Maxwell leapt to the irresistible conclusion that he had unified not two but three phenomena.
But what was the damn stuff that was waving? Pump the air out of a bell jar, and you can't hear the bell inside anymore. But what do you pump out so that you cannot see the bell? "Luminiferous ether" was the fanciful answer--the vast expanses of space had to be filled with it. But it proved ethereal to the point of nonbeing--a grave drawback for an Aristotelian substance. The subtlest experiments failed to detect it, and in 1905 Albert Einstein realized why: Its existence would make the laws of physics extremely ugly. It would privilege a certain frame of reference--its own. If you were standing still in the ether, Maxwell's equations would work fine. If you were moving through it, they'd be distorted. But, Einstein reasoned, who's to say who's moving and who's stationary? So he invented relativity, turning ether into just another social construct.
Thus at the end of the classical era of physics, light was a wave without a medium--a disagreeably Zenlike state of affairs. Then things got even worse. In the laboratories of early 20th-century physicists, light started behaving like a thing again. It was observed knocking electrons out of atoms in the manner of quantized packages of energy, which were dubbed "photons." Sometimes it showed itself as a particle, sometimes as a wave. The new quantum physics said that light was both--but then so was everything else in the universe, including the Empire State Building, including you. Though quantum physics has since become the best-corroborated theory in the history of science, no one has the slightest idea what it means.
Jim Holt is a longtime contributor to The New Yorker—where he has written on string theory, time, infinity, numbers, truth, and bullshit, among other subjects—and the author of Stop Me If You’ve Heard This. He is also a frequent contributor to the New York Times. He lives in Greenwich Village, New York City.