What is light? The Dutch mathematician and physicist Christiaan Huygens publishes his Treatise on Light, written twelve years earlier in Paris. Light, he argues, is a wave: a disturbance passed along through an invisible medium, the ether, spreading out in ever-widening spheres, the way sound spreads through air. Every point the wave reaches sends out little waves of its own, and together they make the next wavefront. From this, Huygens explains why light bends as it enters glass or water: it slows down there, so the wavefront turns, like a line of marchers stepping from pavement into mud. He even explains why a strange crystal from Iceland shows everything seen through it twice.

The most famous scientist alive disagrees. In the Latin edition of his Opticks, the English physicist and mathematician Isaac Newton adds questions at the end, phrased as doubts but meant as arguments. “Are not the Rays of Light very small Bodies emitted from shining Substances?” he asks, as the English editions that follow put it. Particles fly in straight lines, which would explain why light casts sharp shadows. A wave, Newton points out, would bend around an obstacle into its shadow, just as we hear sounds around a corner. As for the bending of light at the surface of glass, he puts it down to the glass pulling the particles in, which would make them speed up inside it. For the next hundred years, most physicists side with him.

Two beams of light can cancel each other out. The English physician and physicist Thomas Young holds a slip of card, a thirtieth of an inch wide, in a narrow beam of sunlight. Light passes on both sides of it, and in its shadow appear stripes, bright and dark. Block the light on one side of the card, and the stripes vanish. Young explains them as waves: where the crests from the two sides meet, they add up, and where a crest meets a trough, they cancel. Light’s waves must be tiny: those of red light are less than a thousandth of a millimeter long.

He presents this finding to the Royal Society on November 24. The critic Henry Brougham, writing anonymously in the Edinburgh Review, has already called an earlier paper of Young’s “destitute of every species of merit.”

A prediction meant to sink the wave theory saves it instead. The French Academy of Sciences offers a prize for the best account of how light bends around obstacles. The French engineer and physicist Augustin Fresnel, who builds roads for a living, enters a wave theory worked out in exact mathematics. Light’s waves are so short, he finds, that they bend only slightly into shadows, making the fine fringes seen along their edges.

Most of the judges favor particles. One of them, the French mathematician and physicist Siméon Denis Poisson, works out a consequence of Fresnel’s mathematics that seems absurd: in the shadow of a small round disk, there should be a bright spot, right at the center. Another judge, the French astronomer and physicist François Arago, tries it. The spot is there. Fresnel wins the prize on March 15. Two astronomers had seen the spot a century earlier, but nobody remembered.

A race settles a question as old as the two theories. Newton’s particles should speed up inside water, Huygens’s waves should slow down. Arago proposed the test in 1838: send light through air and through water, and see which arrives first. Light is far too fast to time with a clock, but the French physicist Léon Foucault uses a mirror spinning hundreds of times a second. In the moment light takes to travel to a distant mirror and back, the spinning mirror turns a little, so the returning beam lands slightly off. Through a tube of water, it lands farther off: light took longer. Foucault reports it on May 6, six weeks ahead of the French physicist Hippolyte Fizeau, who finds the same.

One theory explains light, electricity, and magnetism together. The Scottish physicist James Clerk Maxwell writes down equations for electric and magnetic forces, building on the English scientist Michael Faraday’s experiments. They predict waves of electric and magnetic force rippling through space, and they say how fast: about 310,000 kilometers a second, close to the speeds measured for light. So light, Maxwell concludes, is “an electromagnetic disturbance in the form of waves.” Huygens’s waves are waves of something after all.

Twenty-three years later, the German physicist Heinrich Hertz makes such waves with sparks in his laboratory – waves far too long to see. Today we call them radio waves.

Waves can’t explain everything light does. Light shining on a metal knocks electrons out of it, which physicists call the photoelectric effect, and the Hungarian-born German physicist Philipp Lenard found that brighter light knocks out more of them, but not faster ones. A brighter wave carries more energy, so why don’t the electrons fly off faster?

Five years earlier, the German physicist Max Planck explained the light hot objects give off by assuming that matter gives off and takes up energy only in packets, though he took that to be a feature of matter, not of light. In a paper he calls “very revolutionary,” the German-born physicist Albert Einstein, who works at the patent office in Bern, proposes that light itself comes in such packets, quanta, each with an energy set by the light’s color: the bluer the light, the more energy each quantum carries. An electron that takes up a quantum gets all its energy at once, so it flies off faster for bluer light, and brighter light only means more quanta. He predicts exactly how fast, for every color. For years, hardly any physicist accepts it. When Einstein receives the Nobel Prize for 1921, it’s for his law of the photoelectric effect, and the prize says nothing about the quanta behind it.

Einstein’s equation passes a hard test, and still isn’t believed. The American physicist Robert Millikan, who doesn’t believe in quanta, spends years measuring the electrons that light knocks out of metals, cutting the metals’ surfaces clean inside a vacuum. Einstein’s equation holds for every color he tries, and from it he measures Planck’s constant, the number that sets each quantum’s energy, with a precision of about half a percent. Yet Millikan still can’t accept quanta. Light interferes, as Young showed, and bends into shadows, as Fresnel showed. How could lumps of energy do that? Einstein’s equation, he writes, can’t yet be seen as resting on “any sort of a satisfactory theoretical foundation.”

Light can collide. The American physicist Arthur Compton shines X-rays, a kind of light with very short waves, at a block of carbon, and measures the X-rays that bounce off. They come back with longer waves than they went in with, and the more sharply they’re deflected, the longer. A wave of light shaking an electron would send out waves just as long as its own. But Compton can explain the shift exactly: each quantum hits a single electron, like one billiard ball hitting another, and loses some of its energy, which lengthens its wave. Light’s quanta carry not just energy but momentum, a push. In 1926, the American chemist Gilbert Lewis gives them the name we still use: photons.

So is light a wave or a stream of particles? The English theoretical physicist Paul Dirac, working in Copenhagen, applies the new quantum mechanics to light itself. In his theory, light is a quantum field. Its energy comes in photons, each taken up whole, in one place, as Einstein said. But the chance of finding a photon here or there spreads and interferes like a wave, so light still makes Young’s stripes and Fresnel’s bright spot.

Each photon, as Dirac puts it in his textbook three years later, “interferes only with itself.” Physics, he writes there, needn’t say what a photon does on its way, only predict where it will land. Refined over the next two decades into quantum electrodynamics, his theory becomes one of the most precisely tested in all of physics.

The American physicist Hugh Everett, a graduate student at Princeton, asks what follows if quantum theory applies to everything, including the instruments and people observing an experiment. Its equations never make a photon end up in just one place. The textbooks add a rule for that: whenever someone observes an outcome, every possibility but one vanishes. Everett drops the rule. Then every outcome the equations allow happens. An observer who sees a photon land here and one who sees it land there are both real, in branches of reality that no longer affect each other.

Hardly anyone takes him seriously. When he visits the Danish physicist Niels Bohr in Copenhagen in 1959, the meeting goes nowhere. Everett, already working on defense research for the Pentagon, never works on physics again. In the 1970s, the American physicist Bryce DeWitt brings the theory back, calling it the many-worlds interpretation. Those many worlds are now called the multiverse. Everett dies in 1982, at 51.

Back to the two slits of Young’s day, with one change: since Dirac’s, experiments have sent photons through them one at a time. Each lands as a single spot, yet after thousands, the spots build up Young’s stripes all the same. In his book The Fabric of Reality, British physicist David Deutsch, at Oxford, asks what that means.

Open two more slits, interleaved with the first two, and the stripes spread farther apart; some places that photons used to reach now get none at all. Something coming through the other slits must be stopping each photon from going there – something we can’t see, which shows up only in these interference patterns. Deutsch calls these “shadow photons,” and the ones we see “tangible photons.” For every photon we see, there must be at least a trillion we don’t see.

Photons aren’t special. Electrons sent one at a time build up the same stripes, as the Japanese physicist Akira Tonomura’s team shows in 1989, and in 2019, molecules of some 2,000 atoms interfere too. So every kind of particle has its shadow counterparts, and they act on each other as the ones we see do: they make up a huge number of universes like ours, which affect each other only weakly, through interference. Nor is any of them special: each photon is tangible in its own universe and a shadow in all the others. Reality is a multiverse, as Everett’s theory says.

Today, quantum theory has passed every test put to it, yet the multiverse is still a minority view among physicists. Many hold that the theory only predicts what we’ll observe. That leaves them Deutsch’s question: if not shadow photons, what keeps the photons from those places?