Philosophers like to think science works in an orderly way, in which scientists propose hypotheses, conduct experiments to validate their ideas, then emerge triumphantly from the laboratory to publish their results. Sometimes that happens.
But some very important breakthroughs have come after scientists stared in slack-jawed dismay at entirely unexpected experimental results. Their predictions had flopped, and it wasn’t merely Murphy’s Law taking its toll. Their experiments had stumbled upon something unexpected that turned out to have a much larger impact than what they had been looking for.
The Lingering Death of the Ether
Consider the quest of nineteenth century physicists to measure the Earth’s absolute motion through space. Aristotle and the ancients had thought something must fill the space between the Earth and the stars. Nineteenth century physicists gave the stuff a name—the ether—and built mathematical theories describing its mechanics. James Clerk Maxwell built his famed theory of electromagnetism by assuming that electric and magnetic forces traveled through the ether, which led him to realize, correctly, that light was an electromagnetic wave.
If the ether was the fabric of space, as classical physicists believed, it must be stationary, so measuring the speed of light going in different directions should reveal how fast the Earth was moving through space. Albert A. Michelson invented a dandy little optical system that seemed just fine for the job. It split a beam of light into two beams at right angles to each other, then reflected them back to the source for comparison. If the Earth was moving through the ether, there would be an “ether wind” that would affect the two beams differently, because of their different directions, and would make the two pulses arrive at slightly different times. He figured that this experiment would reveal the Earth’s velocity through the ether, but his 1887 experiment failed to show any sign of the expected ether wind.
Michelson and other physicists went off scratching their heads. Their results didn’t fit with two centuries of classical physics, and they couldn’t find any mistakes. George FitzGerald and Hendrik Lorentz, two prominent physicists, separately suggested that the Earth’s movement through the ether might somehow conceal the ether wind. But that didn’t solve the problem; they were only adding fudge factors to save the ether. To see reality, scientists had to look past the ether, and that required the genius of Albert Einstein. His theory of relativity cracked the case by showing that the speed of light was constant for all observers—except for Michelson, who remained a firm believer in the ether. Relativity didn’t just dispose of the ether or the idea of stationary space. It transformed people’s view of the universe to reveal matter in a continuum of space and time. A quest for ether that yielded no detectible effects led to a revelation.
Let There Be Light: Whatever That Is
Undetectable effects are far from the only experimental problem that can lead to a new understanding of the world. Different measurements of the same thing can yield disconcertingly different results. That was the case with light. Isaac Newton thought light consisted of a series of tiny particles, but others thought light was made of waves. The wave versus particle debate raged for a century until Thomas Young scored what seemed a decisive victory for waves in 1807.
Young passed light through two closely spaced slits, producing alternating light and dark bands on a screen behind them. The only way to explain that pattern, he said, was if light was waves which rose and fell in strength like ripples on a pond. Whether the waves produced a bright band or a dark one depended on how the waves from the two slits overlapped. If the peaks of one wave matched the peaks of the other, the two peaks formed a bright band, but if the peaks of one matched the valleys of the other, they cancelled, leaving a dark band. Newton’s little particles clearly couldn’t do that.
For decades, the wave theory held sway; but nothing is ever completely settled in physics. Scientists keep trying new experiments to probe the limits of nature, or just to look for something interesting to explore. And in 1887, while Michelson was trying in vain to find the ether wind, Heinrich Hertz noted something odd while fiddling around with electrically charged metal objects. Illuminating them with ultraviolet light caused their charges to drain away. Inserting a glass window to block the ultraviolet light stopped the leakage.
Hertz didn’t stop to explore the effect in much detail. He was too busy proving Maxwell’s theory of electromagnetism and demonstrating radio transmission. But others were intrigued, and tried to make sense out of what is now called the “photoelectric effect.” They were puzzled that the wavelength of the light was crucial. Bright visible light produced no electrons, but faint ultraviolet light did produce some. Cranking up the ultraviolet produced more electrons. Yet the wave theory predicted that the number of electrons produced should depend on the light intensity, not the wavelength. The two-slit experiment had showed that light was a wave, but the photoelectric effect showed light doing something that no wave should do.
Again, Einstein came to the rescue. He explained the photoelectric effect, and it earned him the Nobel Prize. Essentially, light has a split personality. In Young’s experiment, it acted like waves. In Hertz’s, it acted like little chunks of energy called photons. Ultraviolet photons had enough energy to blast electrons free from the metal; visible photons didn’t have enough. In other words, when asked if light is made of waves or particles, Einstein said “both.” And again, an experiment that defied previous theories led the way to a new understanding of one of the universe’s most important phenomena.
The Next Mystery
The photoelectric effect was only the beginning. Judging by light’s behavior, it seemed to want to frustrate physicists. When they tried an experiment designed to look for light waves, they saw waves. When they were looking for photons, they saw photons. When they tried something that might show both, they saw one or the other, but never both. This cosmic frustration has not gone away. It seems like an inherently perverse law invented by some cosmic Murphy.
Quantum mechanics is one rather large impossible-to-believe thing. In fact, Einstein himself complained that some parts of quantum mechanics just didn’t make sense. One notion he found particularly troubling was that the quantum-mechanical properties of pairs of particles could remain linked even when they were far apart. He conducted one of his famous thought experiments to show the problem. Suppose someone created a pair of particles so their spins were equal and opposite, and they sped away in opposite direction. Quantum mechanics says no one can know the spin of either particle until they measure the spin of one of them—and making that measurement will instantly give the other particle the opposite spin, no matter how far apart they are.
Nonsense, thought Einstein, and in 1935 he and his colleagues Boris Podolsky and Nathan Rosen concluded that the quantum entanglement thought experiment revealed the logical fallacy of quantum mechanics. Einstein later scoffed at linked spin as “spooky action at a distance.” Yet even Einstein’s thought experiments can go wrong. Three decades later, Irish physicist John Stewart Bell showed Einstein’s had its own logical flaw—it assumed that objects could be affected only by nearby objects, something not inherent in quantum mechanics. In 1972, physicists Stuart Freedman and John Clauser devised a physical experiment, which showed Bell was right. The quantum-mechanical spins of particles could remain entangled when far apart. Spooky action at a distance might not make sense, but it works.
So now we have quantum entanglement, a way to generate pairs of photons which remain entangled as they go their separate ways through the universe until one is called upon to reveal its secret property—thereby determining the property of the other. No one can fiddle with one photon without affecting its partner. Or so we think, until someone else has their own surprising moment in a laboratory.
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