“ The experiments I am about to relate . . . may be repeated with great ease, whenever the sun shines,” Young told the audience.
Whenever the sun shines. Young wasn’t overstating the simplicity of his experiment. “ I made a small hole in a window-shutter, and covered it with a piece of thick paper, which I perforated with a fine needle,” he said. The pinhole let through a ray of light, a sunbeam. “ I brought into the sunbeam a slip of card, about one-thirtieth of an inch in breadth, and observed its shadow, either on the wall, or on other cards held at different distances.”
If light is made of particles, Young’s “slip of card” would have cast a sharp shadow on the wall in front, because the card would have blocked some of the particles. And if so, Newton would have been proved right.
If, however, light is made of waves, as Huygens claimed, then the card would have merely impeded the waves, like a rock impedes flowing water, and the wave would have gone around the card, taking two paths, one on either side of the card. The two paths of light would eventually recombine at the wall opposite the window shutter to create a characteristic pattern: a row of alternating bright and dark stripes. Such stripes, also known as interference fringes, are created when two waves overlap. Crucially, the central fringe would be bright, exactly where you’d expect a dark shadow if light were made of particles.
We know about interference from our everyday experience of waves of water. Think of an ocean wave hitting two openings in a coastal breakwall. New waves emerge from each opening (a process called diffraction) and travel onward, where they overlap and interfere with each other. In regions where the crests of both waves arrive at the same time, there’s constructive interference and the water is at its highest (analogous to bright fringes of light); and in regions where the crest from one wave arrives at the same time as the trough of the other, the waves cancel each other out and there’s destructive interference (corresponding to dark fringes).
Young saw such optical interference fringes. Specifically, since he was working with sunlight, which contains light of all colors, he saw a central region that was flanked by fringes of colors. The central region, upon closer inspection, was seen to be made of light and dark fringes. The numbers of these fringes and their widths depended on how far away the pinhole in the window shutter was from the screen or wall. And the middle of the central region was always white (a bright fringe). He had shown that light is wavelike.
There must have been disbelief in the audience, for Young was going against Newton’s ideas. Even before Young’s lecture, articles written anonymously in the Edinburgh Review had been heavily critical of his work. The author, who turned out to be a barrister named Henry Brougham (he became Lord Chancellor of England in 1830), was scathing, calling Young’s work “ destitute of every species of merit” and “the unmanly and unfruitful pleasure of a boyish and prurient imagination.”
It was anything but. Soon enough, Young’s ideas got further support from other physicists. His experiment led to what’s now called the double-slit experiment and was in fact the first formulation of it—the very same experiment whose virtues Feynman extolled during his lecture at Cornell. In the more standard double-slit experiment, Young’s sunbeam is replaced by a source of light. And instead of a “slip of card” placed in the sunbeam’s path to create two paths for the light, the double-slit experiment creates two paths of light by letting the light fall on an opaque barrier with two narrow slits or openings through which the light can pass. And on the screen on the far side, you see an interference pattern, essentially fringes similar to what Young saw on the wall opposite the window shutter (if the screen is a photographic plate, or a piece of glass coated with photosensitive material, then the image can be thought of as a film negative: dark regions will form where the film is being exposed to light). You don’t see just two strips tapering away, which you’d expect if light behaved as if it came in lumps. It’s behaving like a wave.
So, well before quantum physics was even a gleam in anyone’s eyes, Young had seemingly settled the debate between Newton and Huygens (despite skeptics who continued to favor Newton). Young came down in favor of Huygens’s light-is-a-wave idea. And so things stood until the quantum revolution.
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The revolution began with bewildering discoveries in the early 1900s, including Albert Einstein’s 1905 assertion that light should be thought of as being made of particles, because it was the only way to explain a phenomenon known as the photoelectric effect (which helps us convert sunlight into electricity, giving us the technology of solar panels). These particles of light came to be called photons. For any given frequency or color of light, a photon of light is the smallest unit of energy, and it cannot be divided any further: the light cannot come with any less energy than contained in one photon. Einstein’s argument is somewhat involved, but for now, if we accept the idea that there are certain situations in physics where you have to treat light as made of particles, then the double-slit experiment starts challenging our intuitive sense of reality.
Feynman spoke of the double-slit experiment as embodying the “central mystery” of quantum mechanics. To show why, he replaced the gun shooting bullets (or, in our case, grains of sand) with a source of electrons. Everyone in the 1960s knew that electrons came in lumps. They are one of the many types of elementary particles that make up the subatomic world, including photons. We’ll use photons instead of electrons. The fact that the experiment, its results, and its implications don’t change whether we are using photons, which are particles of light without any mass, or electrons, which are particles of matter with some mass, leads to its own set of mystifying questions. As Feynman said, both are screwy in the same way.
Here’s what happens if you use photons. Unlike what we got with particles of sand, you don’t get two bands of light on the screen. Instead, you get fringes, similar to the interference pattern that Young observed, suggesting that photons are behaving like waves. To get a sharply defined set of fringes, it’s best to use light of one color. So the source can be, say, streaming out an intense beam of photons of red light that pass through the double slit.
When both slits are open, you get the interference pattern, suggesting that light (which we know now is made of particles) is going through both slits. But if you close one of the two slits (doesn’t matter which one), the interference pattern disappears, clearly suggesting that light is going through only one slit and there’s nothing for it to interfere with.
The experiment, however, really starts messing with our minds when we consider a source that emits one photon at a time. We’ll come to the ways in which physicists invented sources to do that. It wasn’t possible in 1964, when Feynman was giving his lecture. For now, let’s assume we have such a source in hand. If so, each photon goes through the apparatus, and we make sure there’s only one photon passing through the setup at a time. The photon hits the photographic plate on the far side and creates a spot. If we let enough spots accumulate, our intuition says that these photons should act like grains of sand and line up behind each slit. There should be no interference pattern.
We’d be wrong. As it happens, even though each photon seems to be landing at some random position, fringes emerge when enough photons have made their mark on the photographic plate. Each photon makes a dark spot on the plate; places where the photons mostly land become dark stripes, and fringes build up over time.
This is somewhat curious. It’s clear that we can get an interference pattern when one wave interferes with another. But our photons are going through the apparatus one by one. There’s no interference between one photon and the next, or the first photon and the tenth, and so on. Each photon is on its own. Nonetheless, each photon is mostly landing on the photographic plate at those positions that eventually become regions of constructive interference and mostly avoiding those places that become regions of destructive interference. We get interference fringes. It’s as if each photon is exhibiting wavelike behavior, as if it’s int
erfering with itself.
This is happening even though we create each photon as a particle, and detect it on a photographic plate as a particle: the results seem to suggest that between the creation and detection, each particle acts like a wave, and somehow goes through both slits simultaneously. How else do you explain the interference pattern?
If that’s not mysterious enough, consider what happens if we try to find out which slit a photon goes through (our intuition, after all, says that it surely went through just one slit, not both). Say you have a mechanism for detecting the passage of a photon through one or the other slit without destroying the photon. If you do that, the interference pattern goes away (meaning the photon stops behaving like a wave and starts acting like a particle)—and you get a pattern that’s simply the sum of the “lumps” going through each slit. Stop trying to sneak a peek at the photon’s path and it goes back to behaving like a wave—the interference pattern reemerges.
There’s yet another way to appreciate this mystery. When you are not looking at the photons’ paths, individual photons almost never go to certain places on the photographic plate—the places that eventually become regions of destructive interference. But if you start monitoring their paths, they will go to the very locations that they otherwise shun. What’s going on?
The curious behavior continues. If you were to fire grains of sand at the double slit, and if you knew everything about the initial conditions of each grain of sand (its initial velocity, the angle at which it leaves the sand gun, etc.), you can predict using Newton’s laws exactly where the grain of sand will end up on the screen opposite the double slit, taking into account any deflections due to the interaction with the slits. This is how physics is supposed to work. But you can’t do that with photons (or electrons, or anything quantum mechanical for that matter).
Even if you have all the information about a single photon as it leaves the source and goes toward the double slit, you can only calculate the probability of the photon landing on a certain part of the photographic plate. For example, the photon could land at any one of the many regions of constructive interference—but there’s no way to tell exactly where any particular photon will go. Nature, at its deepest, seems inherently nondeterministic. Or is it merely hiding its secrets, and we haven’t dug deep enough yet?
The questions pile up. Between the production of the photon and its eventual detection, both proofs of its particle nature, the photon ostensibly behaves like a wave if we choose not to look at which path it takes, and as a particle otherwise. Does the photon “know” we are looking at its wave nature or particle nature? If so, how? And can we fool the photon, say, by not revealing our hand until it has crossed the double slit as a wave, and only then choosing to see which slit it went through, thus examining its particle-like behavior?
Maybe there is a simpler answer: that the photon is always a particle and always goes through one or the other slit. And something else, something that our standard theories don’t account for, goes through both slits to produce the wavelike behavior. In that case, what is that something?
If it crossed your mind that human consciousness is somehow involved in causing the photon to behave one way or the other, you wouldn’t be alone in thinking so. As often happens when confronted with two mysteries (in this case the odd behavior of the quantum world and the inexplicable nature of consciousness), it’s almost human nature to want to conflate the two.
It’d be twenty years on from Feynman’s lecture at Cornell that the double-slit experiment would be done using single photons. It was an example of how, from Young’s efforts in the early 1800s to modern versions, physicists continue to use the double-slit experiment to understand the nature of reality. The experiment hasn’t changed in its conceptual simplicity for more than two hundred years, but it has become technologically more and more sophisticated, as experimenters keep thinking of clever ways to trick nature into revealing its profoundest secrets.
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WHAT DOES IT MEAN “TO BE”?
The Road to Reality, from Copenhagen to Brussels
The idea of an objective real world whose smallest parts exist objectively in the same sense as stones or trees exist, independently of whether or not we observe them . . . is impossible.
—Werner Heisenberg
Q uantum physics has been with us for about a century. But for almost two centuries before the birth of quantum physics, our ideas of how nature works were governed by laws discovered by Isaac Newton. He elucidated his laws in the Principia , an astonishing treatise published in 1687. Crucial to the Newtonian conception of nature was that it was made of particles of matter whose dynamics were governed by the forces acting upon them, including the mutually attractive force of gravity. Light too was regarded as having particle nature, though this was debated. Huygens, Young, and others challenged this, arguing for light’s wave nature. So, while the Newtonian universe was one of particles of matter, light stood apart, its place in the categories of things that make up the world—the ontology of the world—somewhat unclear.
A French prince and physicist, Louis de Broglie, centuries later, would recount this time in the history of physics rather eloquently: “ When Light reaches us from the sun or the stars it comes to the eye after a journey across vast spaces void of Matter. It follows from this that Light can cross empty space without difficulty . . . it is not bound up with any motion of Matter. Hence a description of the physical world would remain incomplete unless we were to add to Matter another reality independent of it. This entity is Light. Now what is Light? What is its structure?”
As de Broglie wrote, such questions were looming large in the 1860s, when Scottish scientist James Clerk Maxwell developed the mathematical foundation for physicists to start thinking of light as a wave.
Maxwell’s work first involved unifying electricity and magnetism, which until then had been viewed as separate forces, into one force. Building on earlier work by the English physicist and chemist Michael Faraday, Maxwell came up with a theory combining electricity and magnetism, and predicted that they move as one electromagnetic wave. He presented these ideas on December 8, 1864, to the Royal Society of London. The ontology of nature had changed. In addition to particles, it now included electromagnetic fields—oscillations of energy—that moved at the speed of light. Particles were localized, but fields were diffuse and could spread and exert an influence far, far from where they originated.
Maxwell argued that light too is an electromagnetic wave. But his ideas met with some resistance. While physicists could imagine electromagnetic waves moving through a medium, such as a wire, they had trouble envisaging light as an electromagnetic wave moving through the vacuum of space, as it would have to.
But even before questions about the nature of light could be answered, Maxwell’s hypothesis about electromagnetism had to be proved. In 1879, the Prussian Academy of Sciences (in Berlin) put out a call for what came to be called the Berlin Prize problem. The prize was for experimentally verifying Maxwell’s ideas. Entries were due by March 1, 1882, with the winner to be awarded 100 ducats (a ducat was either a gold or a silver coin used in Europe during the Middle Ages, and even into the nineteenth and early twentieth centuries). One of the scientists thought most likely to win the prize was the prodigiously talented German physicist Heinrich Hertz. That year, Hertz considered the problem but gave up on it, for he could see no clear experimental way forward. “ But in spite of having abandoned the solution at that time, I still felt ambitious to discover it by some other method,” he later wrote.
No one won the prize in 1882.
Hertz, however, in just a few years solved the puzzle. He designed an experiment that proved Maxwell correct. The experiment involved building a transmitter of electromagnetic waves, and a receiver—and showing that these invisible waves did indeed exist and could propagate through air. Hertz had inadvertently discovered radio waves.
When asked about the usefulness of such waves, Hertz reportedly said, “
It is of no use whatsoever. This is just an experiment that proves Maestro Maxwell was right. We just have these mysterious electromagnetic waves that we cannot see with the naked eye. But they are there.”
Hertz’s experiments validated Maxwell’s theory of electromagnetism. Eventually, it would become clear that light too is an electromagnetic wave. It consists of an electric field and a magnetic field, which each vibrate in mutually perpendicular planes. And light itself travels in a direction that is perpendicular to both the constituent electric and magnetic fields. The frequency of vibration, or the frequency of the electromagnetic wave (ν ), turns out to be equal to the velocity of light (c) divided by its wavelength (λ).
But while doing this experiment, Hertz stumbled upon another curious phenomenon that would, within a decade, challenge the light-is-a-wave argument. The phenomenon is now called the photoelectric effect. When light falls on certain metals, it can eject electrons. Most important, for a given metal, the electrons are ejected only when the light is above a threshold frequency unique to that metal. Below that frequency, regardless of how much light falls on the metal, no electrons are ejected. Above the threshold frequency, two things happen. One is that the number of electrons ejected increases as the intensity of the incident light increases. The other is that increasing the frequency of the light increases the energy of the ejected electrons.
Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality Page 2