Particle Vs. Wave – Light’s Timeless Question

Since light was first studied in the sixth century B.C., arguments had raged about its essence. Some Ancient Greeks considered light to flow in particles, like the atoms that made up all matter. Others thought it to be shimmering waves called eidola.  Particle or wave?  The debate summoned all of light’s mysteries.

Thomas Young's sketch of two-slit interference based on observations of water waves. Credit: Wikimedia Commons.

Thomas Young’s sketch of two-slit interference based on observations of water waves. Credit: Wikimedia Commons.

Everyone knew that sound traveled in waves, but light was different.  You could hear someone around a corner shouting but you could not see him.  A bell in a glass jar fell silent when the air was pumped out, yet it could still be seen.  The debate continued.  By the 1600s, Rene Descartes was comparing light to tennis balls bouncing and spinning.  Then in 1678, the Dutch astronomer Christian Huygens calculated precisely how light — “an infinity of waves” — bent, bounced, and flowed through the ether.  Huygens’ wave theory convinced many, but when Isaac Newton held forth in 1703, the debate seemed settled.

Light, Newton admitted, sometimes behaved “with motions like that of an Eel,” yet only particles could travel in such straight lines. “Sounds are propagated as readily through crooked Pipes as through streight ones,” Newton wrote in The Opticks, “but Light is never shown to follow Crooked Passages nor to bend into the Shadow.”  Hence light was most likely “corpuscular.”

Throughout the 1700s, Newton’s particle theory reigned supreme.  Then in 1801, British polymath Thomas Young conducted an experiment so original, so unsettling, so perfect that it has been called “the single most influential experiment in modern physics.”  In what is now known as “Young’s experiment,” the simple overlapping of light beams rekindled the essential debate about light — particle or wave?

Suppose light were waves, posited Young, who had made extensive studies of sound.  When light waves cris-crossed, they should “interfere” with each other, like ripples in a pond.  Where two waves were in sync, their crests boosting each other, the overlapping light should be brighter.  But where the crest of one wave met the trough of another, they should cancel each other, just as waves in a pond overlap to still the water.  The result would be darkness, or at least dimness.

Young was soon projecting parallel beams of a single color through adjacent slits.  There on his wall where the beams overlapped, Young saw the pattern he expected — vertical bars, bright and dark. Light waves, Young concluded, are “capable of neutralizing or destroying each other, and of extinguishing the light where they happened to be united.”  This he called “the general law of the interference of light.”  But Young’s theory met widespread derision.  One critic called interference “one of the most incomprehensible suppositions that we remember to have met with in the history of human hypotheses.” Unable to convince anyone, Young left light behind, taking up linguistics where his discoveries helped decipher the Rosetta Stone.  Definitive proof of light waves would have to wait another decade.  And, given Newton’s cache in England, such proof would have to come from across the English Channel.

Budding scientists at the Ecole Polytechnique in Paris all learned particle theory, yet by 1814, one alumnus was beginning to doubt.   “I tell you I am strongly tempted to believe in the vibrations of a particular fluid for the transmission of light and heat,” Augustin-Jean Fresnel wrote his brother. “One would explain the uniformity of the speed of light as one explains that of sound; and. . . why the sun has for so long shined upon us without diminishing its volume, etc.”

Although doubting Newton, Fresnel was much like him. Newton was known as “fearful, cautious, and suspicious.” Fresnel’s colleagues considered him un homme froid (a cold man). Both Fresnel and Newton were plagued by bad health.  And like Newton, Fresnel had what he called “a taste for exactitude” that sent his equations sprawling across page after page.

While working as a civil engineer in rural France, Fresnel began studying light.  Within a year of taking up his studies, he presented a paper to the Academie des Frances suggesting that light might be made of waves.  Newton’s French disciples martialed their defenses. The aging scientists knew Newton to be right on everything — on gravity, on motion, on the calculus, the prism, and the rainbow. Now they were asked to believe that an unknown civil engineer, not yet thirty, who held no teaching or research position, had a better grasp of light than their “philosophic sun.” Such nonsense might go on indefinitely unless put to a contest.

On March 17, 1817, the Academie des Frances announced the challenge. Entrants were  to calculate, using the most refined math, how light traveled around an obstruction.  Contestants were given eighteen months. Fresnel took a leave of absence from his engineering post.

For wave theory to be plausible, the math had to work. Could a light wave, like water, swirl around an object and advance in a straight line? Everyone knew that water striking a barrier surged backwards. Could anyone imagine light forming such eddies? If wave theory itself were to advance, these patterns had to be proven on paper. Huddled in his mother’s house near the English Channel, Fresnel set out to try.

He began by attacking particle theory. It could not account for all of light’s motions.  Arrows or tennis balls bounced and bent as predicted, but neither would interfere with other light to cause dark stripes. Those who clung to particles, Fresnel believed, were succumbing to a longing as old as humanity itself — for simplicity. But nature, he recognized, “does not dread difficulties of analysis.” The difficulties came quickly.

Fresnel labored for months over the math. Christiaan Huygens’ wave theory, devised before Newton wrote his Principia, did not use calculus.  Now Fresnel did, using Newton to disprove Newton.  The key tool was the integral. An integral, denoted in an equation by an elegant S (∫), measures curves and the areas they sweep out. Integrals calculate a missile’s trajectory, the graceful shapes of seashells, and as Fresnel saw, the motion of a wave. Applying calculus to light waves, Fresnel crafted what are now called the “Fresnel Integrals.”  To the novice they look like modern-day hieroglyphics, a lattice of the highest math that sets the mind spinning. To stare at Fresnel’s equations is to feel yourself drawn towards the infinite complexity of the universe. Symbols and signs are stacked on each other like layers of a cake. Then come the Greek letters, not just the familiar pi (π) but lambda (λ — wave length), sigma (∑ — defining a summation of numbers), and kappa (κ — curvature). All are crowded inside parentheses, crowned by exponents, huddled in brackets within brackets. The whole is far greater than the sum of the parts, and if you stare long enough, Fresnel’s concerto of calculus is both hypnotic and inspiring. To think that this is how light behaves, to know that a single man opened this door to the eternal is to grasp light’s complexity and marvel at human discovery.

On April 20, 1818, the battle of Particle vs. Wave approached its barricades. Using a Latin epigram Natura simplex et fecunda, “Nature simple and fertile,” Fresnel submitted his contest entry.  Only one other was received.  A panel of judges convened.  The committee, made chiefly of “emissionists” supporting Newton, deliberated throughout summer, fall, and into the new year. One emissionist, mathematician Simeon Poisson, did his own calculations. If light beamed at a disc behaved as Fresnel asserted, Poisson argued, it would leave a bright spot in the dead center of its shadow. Cest absurd!  Emissionists thought they had trumped Fresnel, but the committee chairman molded a freckle-sized disc and aimed a beam at it. On the wall behind, there at the center of its shadow – the dead center — was a perfect pinpoint of light. Poisson refused to budge, but the committee was convinced.

Eleven months after submitting his entry, Fresnel was declared the winner. He cared little for the honor, later telling Thomas Young that acclaim did not compare with the thrill of discovery. But from the moment Fresnel won the Academie contest, particle theory was in retreat.   Light, it seemed, was a wave, one that could be calculated with a precision that explained all its mysteries.  At least until quantum theory muddled the waters.


Author photoBruce Watson is the author of Light: A Radiant History from Creation to the Quantum Age (Bloomsbury, February 2016).  The book traces humanity’s evolving understanding and control of light, starting with creation myths, then moving into scripture, philosophy, architecture, Islamic science, art history, poetry, physics, and quantum physics.

Watson’s previous books include Freedom Summer, Sacco and Vanzetti, and Bread and Roses.  Watson’s work has appeared in the Boston Globe, the Los Angeles Times, American Heritage, the Wall Street Journal, the Washington Post, Yankee, Reader’s Digest, and Best American Science and Nature Writing 2003.

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