In a Blaze of Brilliance — How Light’s Speed was Finally Clocked

Late in his career, when he had won the Nobel Prize for Physics and had clocked light’s speed with an accuracy no one had thought possible, the American physicist Albert Michelson was asked why he studied light.  Michelson did not hesitate.  “Because it’s so much fun,” he said.

In 1879, the American physicist Albert Michelson clocked light's speed to within 99.9998 percent of accuracy. Credit: Wikimedia Commons.

In 1879, the American physicist Albert Michelson clocked light’s speed to within 99.9998 percent of accuracy. Credit: Wikimedia Commons.

Ever since Euclid published his “Optics” in 300 BCE, many had had fun with light.  Bouncing rays off mirrors, bending them through lenses, light’s early students marveled at ordinary beams.  But it took 20 centuries of study for anyone to measure light’s blazing speed.  Light’s earliest students wondered whether light even had a speed.  The Greek philosopher Empedocles thought so, but Aristotle argued that light was “not a movement.” Islamic scientists such as Alhacen and al-Biruni disagreed with Aristotle — light’s speed was finite — but Kepler and Descartes considered light to be instantaneous.  During the Scientific Revolution, Galileo proposed the first light speed test.  Set two men on hillsides kilometers apart, Galileo suggested.  Have one flash a lantern, and the other reply with his own flash as soon as he saw the first.  Galileo never conducted his speed test, but in 1667, Florence’s Academia del Cimento did.  Alas, even from the hillsides kilometers apart, the time lapse between flashes was far too quick to measure.  If light had a speed, how could anyone hope to clock it?  Then in 1676, a Dutch astronomer working in Paris used the clockwork of the planets to finally time a beam of light.

In the half-century since Galileo had first seen Jupiter’s moons, their pinpoints of light  had been charted on timetables. Comparing these tables to his own observations, the astronomer Ole Roemer spotted a discrepancy. When the earth and Jupiter were on opposite sides of the solar system, the Jovian moon Io emerged from behind the giant planet several minutes late. In August 1676, Roemer made a bold prediction. On November 9, with Jupiter at its farthest from earth, Io’s reappearance would be 11 minutes behind schedule. When Roemer’s prediction proved precise, the Dutch astronomer Christiaan Huygens triangulated the distance to Jupiter, divided distance by time, and announced a speed of light that astounded even Isaac Newton — 231,745,536 meters per second. Though nearly 25 percent slow, it was a start.

A half century later, British astronomer James Bradley used “stellar aberration,” the  apparent shift of the stars due to the earth’s orbital velocity around the sun, to come closer. Sunlight, Bradley announced, reaches the earth in eight minutes and twelve seconds. He was just  eight seconds off. Then in the mid-1800s, two French physicists gave light a more precise speed.

In 1849, Hippolyte Fizeau sent a beam of limelight through a spinning cogwheel. Chopped into pulses, the beam sped across Paris to the hills of Montmartre and back. The whole effect was like spinning a spoked wheel in front of a TV screen, where the TV’s stroboscopic light sometimes syncs its pulses through the spokes, sometimes goes slower, making the wheel appear to spin backwards.  Timing the pulses, the shaggy-bearded Fizeau measured light at 316,197,472 meters per second. Closer, closer. . . .

In 1849, using mirrors and a cogwheel, the French physicist Hippolyte Fizeau obtained the first near estimate of light's speed. Credit: Bruce Watson.

In 1849, using mirrors and a cogwheel, the French physicist Hippolyte Fizeau obtained the first near estimate of light’s speed. Credit: Bruce Watson.

Thirteen years after Fizeau’s test, Leon Foucault, already famous for his pendulum proving the earth’s rotation, came up with the most ingenious idea yet.  Foucault equipped his lab with mirrors, one fixed, one whirling at 800 r.p.m.  He then bounced light off the spinning mirror, reflecting it to the stationary mirror 20 meters away. By the time the beam returned, even at the speed of light, the spinning mirror had moved the tiniest fraction of a degree. Measuring the angle defined by the beam coming and going, computing the mirror’s angular motion into time, Foucault clocked light at 298,050,509 meters per second. Foucault’s measure lasted until the late 1870s when a young American ensign turned his attention to light.

Albert Michelson, son of Prussian immigrants, grew up in Sierra Nevada mining camps during the Gold Rush. Like many landlocked boys, Michelson dreamed of the sea. Too young to serve in the Civil War, he earned an appointment to the U.S. Naval Academy. As an ensign, Michelson measured his ship’s speed by taking readings of wind and water. Something in the process of calculating against the wind, across the water, stayed with him. A few years later, while teaching physics at the Naval Academy, Michelson read of Foucault’s light speed and thought he could come closer.

Michelson’s equipment — a lens, a steam boiler, a tuning fork, and two mirrors — cost him ten dollars. For such a price, he would measure light with the precision of the stars. Each of Michelson’s many tests began an hour before sunrise or sunset, when he found light “sufficiently quiet to get a distinct image.” In a storage shed perched on the seawall of Annapolis’ Severn River, Michelson fired up the boiler. Within minutes, its steam spun one mirror, slowly at first, then so fast it sometimes flew off its mooring. When the mirror was whirling at 257 revolutions per second — yes, per second in 1879 —  Michelson was ready.  Aiming his lens, he caught sunlight and bounced it off the whirling mirror. The beam split the leafy campus. (A line of inlaid discs on the Annapolis campus now charts light’s path in this historic test.) Striking the fixed mirror precisely 605.05 meters away, Michelson’s beam scorched back to the glass that had since spun a fraction of a fraction of a revolution. Michelson filled his log with data – Date of Test, Distinctness of Image, Speed of Mirror, Displacement of Image… but only one number really mattered. It varied with each test, but on average, Albert Michelson’s measurement of light — 299,851,365 meters per second — was 99.9998 percent of its actual speed.  The news made headlines.  “It would seem that the scientific world of America is destined to be adorned with a new and brilliant name,” the New York Times wrote.  “Ensign A.A. Michelson, a graduate of the Annapolis Naval Academy, and not yet 27 years of age, has distinguished himself by studies in the science of optics which promise a method for the discovery of the velocity of light with almost as much accuracy as the measurement of the velocity of a projectile.”

Across the US Naval Academy campus at Annapolis, Maryland, discs embedded in pavement trace the path of Michelson's light speed test. Credit: Wikimedia Commons.

Across the US Naval Academy campus at Annapolis, Maryland, discs embedded in pavement trace the path of Michelson’s light speed test. Credit: Wikimedia Commons.

Though Michelson studied light “because it’s so much fun,” he lived in awe of it. In lectures, he urged budding physicists to notice “the exquisite gradations of light and shade, and the intricate wonders of symmetrical forms and combinations of forms which are encountered at every turn.” Yet aesthetics never interfered with his determination to pinpoint light’s speed. During the 1920s, when he was in his seventies, Michelson beamed light across mountaintops in Southern California, bouncing it off a whirling 8-sided prism and fixing a speed — 299,796,647 meters per second — that stood until the age of lasers.

Today, using lasers and oscilloscopes, we know that light travels at EXACTLY 299,792,458 meters per second (in a vacuum).  Small wonder that the universal mathematical symbol for light’s speed is “c,” short for the Latin celeritas, or “swiftness.”


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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2 thoughts on “In a Blaze of Brilliance — How Light’s Speed was Finally Clocked

  1. nicely written… i have read the story of ether before…. but i got a lot of more information.
    and i always wondered why we use c for light’s speed… finally i got my answer… thanks 🙂

  2. I love the answer “Because it is so much fun” Is that documented somewhere? I have really been struck how SPIE surveys of people working in our field, working with light, consistently brought out, a sense that it wasn’t really work, it was fun. This came especially from top people. I wonder if and how the sense of joy contributed to their success. It certainly allowed them to put in many hours and likely helped them attract and work with bright people. I’d like to see how this compares across professions. I suspect that economics is called the dismal science for good reason, though there was a recent debate in the Financial Times about why, despite Keynes’ predictions, people were still working so many hours. As I read it, the conclusion was that many people fundamentally enjoyed work

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