The story of quantum physics, it’s worth reflecting on the importance of Einstein’s “heuristic principle.” He explained, and experiments later verified, that light travels in the form of compact, localized, and indivisible chunks. But you’ll recall that there are a few instances, like interference and diffraction, where a wave theory does the trick. It sure must have been tempting, therefore, for the early quantum physicists to say that light is not just a particle nor just a wave, it is both.
QUANTUM QUOTE
Physicists use the wave theory on Mondays, Wednesdays and Fridays and the particle theory on Tuesdays, Thursdays and Saturdays.
—British physicist Sir William Henry Bragg
Of course, this can’t possibly be true, can it? If light were a concentrated particle, then how in the world could we “define” its wavelength and frequency? If it were a diffuse wave, then how could it possibly gather enough wallop to knock an electron from a polished metal? It has to be one or the other, so which is it? Fortunately, with a century of hindsight, we can now state more definitively that light is not just a particle nor just a wave, it is both!
More precisely, light turns out to be either a particle or a wave depending on how we choose to measure it. If we measure the effect of light passing through two slits, it acts like a wave. If we measure the effect of light scattering off of a carbon sample, it acts like a particle. This schizophrenic nature of light is now accepted as a fundamental tenet of quantum physics. It’s so well accepted that physicists have given it a fancy name, wave-particle duality.
The only problem is that we can’t really explain why it’s this way in the first place. Nor can we say for sure what mood a light ray is in when we are not making any measurements at all. The best we can really say is that, between measurements, light is both particle and wave simultaneously. It will turn into one or the other the next time we make a measurement.
From a purely pragmatic standpoint, however, it becomes evident pretty quickly that duality is a necessary oddity. Without a wave theory, physicist couldn’t explain Young’s double-slit interference pattern. And without a particle nature, physicist couldn’t explain Compton’s subatomic billiards. Given that both are needed to have the full picture, this logically challenging concept is what we call complementarity.
DEFINITION
Wave-particle duality refers to the notion that light exhibits both wave and particle properties, depending on how it is being measured.
Complementarity takes this a half step further, suggesting that not only are the two viewpoints correct, they are both necessary to provide a complete description of light. Both aspects cannot be observed with the same measurement, only one or the other.
ALBERT EINSTEIN
Albert Einstein, or “Johnnie” as his wife would call him, was a rather poor pupil, distasteful of the rigid “coercion” imposed by formal pedagogy. Because of his lackluster academic record, he was originally turned down from university posts in Germany, the Netherlands, and Switzerland. Eventually he settled into his first job as “technical expert, third class” in the Swiss patent office in Bern. Where better to develop a knack for distilling out the essential points from a complex problem and for separating the wheat from the chaff?
Life as a patent examiner insulated Einstein from the conservative pressures of traditional academia. He was free to pursue problems in an unconventional way. He did so with gusto and developed a problem-solving style all his own. His trademark approach was to latch onto a bold hypothesis and then see where it led. For example, his work on relatively theory began by imagining, simply, how the universe would appear if you were to climb aboard a ray of light.
Although a naturally born physicist, Einstein cared deeply about more than the lifeless, physical world. He was a highly engaged citizen who actively resisted the violence of the two World Wars. As a Jew, he was driven from his homeland to America, where he sent a series of letters to President Roosevelt urging a cautious approach to the military dimension of atomic energy. In 1952 he was even offered the presidency of Israel. In the end he respectfully declined; his vocation was physics.
He was a founding father of quantum physics and, as we’ll see, made many contributions over decades to its development. Yet in the end, true to his maverick nature, he distanced himself from the mainstream interpretation. He preferred to pose thought-provoking questions from the opposition, and our modern understanding of quantum theory is all the richer because of these. Remarkably, Einstein’s original papers on the quantum theory of light and special relativity—not to mention three other seminal but unrelated works—were all published in 1905. In recognition of his outstanding efforts, the patent office promoted him in 1906 to “technical expert, second class.”
One reason it’s so difficult for us to accept the dual wave-particle nature of light is that it challenges our ability to picture what light is supposed to look like. But at the end of the day, does it really matter? Recall that the fundamental utility of physics is its ability to predict physical phenomenon. Once we accept light’s dual nature, and then select the right interpretation for the measurement at hand, we can make perfectly reasonable, verifiable, and useful predictions.
We will table this discussion for now, but don’t worry; we’re only getting started. Things are going to get lots weirder yet.
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