Copenhagen was snow covered when Bohr received a warning from Arnold Sommerfeld that Compton’s bullet was on the way. Einstein was lingering in the warm zone, sailing toward Palestine, taking the pleasures of sunning on deck, quite oblivious to the teaser sent to Bohr. Sommerfeld had managed to escape Germany’s inflation by accepting a yearlong visiting professorship in Madison, Wisconsin, and on January 21, 1923, he sent Bohr a heads-up that was a marvel of high excitement and low information, “The most interesting thing that I have experienced scientifically in America … is a work of Arthur Compton in St. Louis. After it, the wave theory of Rontgen-rays [X-rays] will become invalid … I do not know if I should mention his results [which had not yet appeared in print]. I only want to call your attention to the fact that eventually we may expect a completely fundamental and new lesson.”
That should have kept Bohr pacing. The wave theory of X-rays dead? But then the wave theory of light, too, would be invalid and Bohr’s whole contribution to physics might come under question. His picture of the atom depended on waves. During the 10 years since he first proposed that electrons change orbits while emitting light waves, physicists had found many subtleties in atomic spectra, but always the discoveries were interpreted as alterations in a light wave. Sommerfeld
had been among the most fertile of these interpreters, and now he was reporting that the jig was up! How? Why?
Compton’s idea was not published for several more months, so Bohr could only stew. In the spring when Compton’s idea finally appeared, in an oddity of history, it was signed by the Dutch physicist, Peter Debye. Einstein was back in Berlin by then. Debye’s paper hailed him as a prophet and his light quanta as real. The data cited, however, were not Debye’s but Compton’s.
The unknown Arthur Holly Compton was an American Mid-westerner with a striking, great head, broader even than Einstein’s, that sat on a tall, wide-shouldered body. He looked like a bull, although temperamentally he was a Ferdinand, more apt to smell the flowers than charge a red cape. Scientifically, he was an experimentalist, like Millikan, out to uncover facts rather than joining the theoreticians who look for underlying principles. Also like Millikan, he had dismissed Einstein’s light quanta out of hand.
Compton spent years studying X-rays and the photoelectric effect without ever considering a role for light quanta in his experiments. He took Maxwell’s wave theory to be as settled as the Copernican view that the earth orbits the sun. In 1919 he arrived in Cambridge to do a year of research at the Cavendish laboratory. A year’s work in Europe was a standard way to round out an American scientist’s education. Rutherford, godfather to Bohr’s quantum atom, directed work at Cambridge, yet when Compton left he was still fully committed to classical theories of electric fields.
Compton studied X-rays, a new kind of radiation that had been discovered only at the end of the previous century. X-rays had seized the public’s imagination because they could travel through closed drawers and sealed envelopes. The rays fogged photographic plates that lay hidden in such closures and the ghostly images they created provided a visible sign of an invisible world. Throughout the years before the Great War physicists disputed just what X-rays might be. Some thought they were small particles, others argued that they were electromagnetic waves, but by the time Compton received his doctorate in 1916 the answer was in: X-rays are akin to light, but they have a much higher frequency. They were part of the same electromagnetic
spectrum that includes infrared, ultraviolet, radio, and all those other forms of invisible radiation that secretly flood our universe. This point had been settled by Einstein’s friend, Max von Laue, and Sir Lawrence Bragg. Working independently, both showed that crystals could produce interference patterns in the same way that more coarse objects can interfere with light waves. So when Bohr read Sommerfeld’s news that “the wave theory of Rontgen rays will become invalid,” he knew that more than X-rays had come into question. The wave theory of light rested on exactly the same evidence—interference producing darkness in light—as did X-rays.
For years Compton had fired X-rays against a metal surface and checked the results. More than Einstein’s meditating ever could, Compton’s lab work matched the scientist’s popular image. While Einstein sat in his study jotting ideas in a notebook, Compton bent over a big, noisy machine that produced zapping sounds and sparks. When X-rays hit human bodies they go right through the skin, but they scatter in all directions when they hit metal. When Compton began his studies, very little was known about X-ray scattering, and he was “merely” trying to learn as much as he could about the topic. What happens when X-rays cannot penetrate something? Where do they go? How does the metal react? Compton had no theories or hypotheses to test. He later wrote, “The mistaken notion is to get some idea and then try to prove it…. The real thing a scientist tries to do when he is faced with a phenomenon is to attempt to understand it. To do that he tries all the possible answers he can think of to see which one of them works best.” This is science in the spirit of Galileo, who looked through a telescope simply to see what he could see.
At first glance, Compton’s research project looked straightforward. X-rays were expected to hit metal in much the same way that waves from the Pacific Ocean hit a California cliff. They splash, eroding the wall and sending water droplets flying in all directions. There is also an invisible energy transfer from wave to rock, heating the rock a bit, as though the ocean waves were a dim form of sunlight. Most of the variation in all this splashing and erosion depends on the nature of the cliff rock, but clear as this analogy is, it was only partly helpful in understanding Compton’s observations. He learned quickly that it did
not matter what metal he used, the scattering patterns looked alike. This surprise was like discovering that the Pacific Ocean splashes off sandstone and granite in exactly the same way. Then there were secondary effects to sort through. For one thing, when X-rays hit metal there is a notable photoelectric effect. You don’t get electricity when the Pacific meets California.
Another challenge for Compton was figuring out what were X-ray scatterings and what were other effects. It was like trying to understand Niagara Falls without knowing that light can form a rainbow. You see the Falls and the cloud of spray. You also see a perpetual rainbow deep in the spray. Where does that come from? From the water? The cloud? The rock? Compton’s lab work, in essence, tried to sort secondary, rainbow-type effects from the primary, splashing effects.
X-rays were thought to be like light, so in many ways Compton’s studies were similar to watching what happens when light hits a metal surface. If the metal is dull, the light scatters in all directions. If it is highly polished, the metal becomes a mirror and reveals the image of the light source. Sometimes too, light striking metal can produce a “fluorescent” effect—the metal gives off its own light. Obviously, if you are trying to account for light scattering, you need to know which light is scattered and which is added through fluorescent effects.
Compton patiently worked through all these complications, determining what was scatter, what was fluorescence, and what was simple illusion. He studied X-ray effects as he worked on his doctorate at Princeton and kept it up through work in Minneapolis, Pittsburgh, Cambridge, and then in St. Louis, where he taught physics at Washington University. Washington was a small and remote school, but it had the X-ray equipment Compton needed and did not have the established physicists whose thought would dominate their department. In St. Louis, Compton said, he was free “to develop what I had conceived of as my own contribution.”
Neither Bohr nor Einstein had much patience for lab work. Experiments demand an ability to tinker with equipment and refine setups as the moment requires. It was more than asking questions and checking for answers. It meant sweat in the eye and grit on the thumb,
whereas—if they had wanted—Einstein and Bohr could have done their physics while dressed in dinner jackets. One surprise Compton observed while wrestling with his X-ray equipment was that the scattered X-ray changes its frequency. He had explored the effect very deeply to make sure it really resulted from the scattering and not from one of the many secondary effects. Finally he had obtained a clear picture: X-rays approach the metal at one frequency and, when they scatter, they leave the metal at a different frequency. This change was like discovering that a violet flower’s mirror image looks red. Compton, of course, could not know that this was exactly what poor Hendrik Kramers had wanted to look for in his proposed experimentum crucis and that Bohr had argued against. So Compton continued to puzzle over an explanation for his odd observation.
Compton eventually wrote a report for the National Research Council in which he set forth his X-ray data and observations. Although he still had no explanation for the frequency change, he included it and many other findings in his data, establishing numbers and equations that had long eluded physicists. He also showed a new experimental way of using X-rays to calculate the number of electrons in an atom. This technique, at last, gave physicists an independent technique to confirm quantum assumptions about atomic structure. Compton had moved to the first rank among experimental physicists, but he still wondered about that frequency change. Why does a scattered X-ray change frequency but light bouncing off a mirror does not?
Compton had moved into Einstein’s sort of physics, the kind that calls for obsessive thinking in shower stalls and during walks along Washington University’s tree-lined paths. Finally, as a new school year was getting under way and Einstein was setting off for Japan, Compton took his great step. Although Maxwell’s wave theory offered no explanation for the change in X-ray frequencies, the light-quanta hypothesis did. Kramers’ lost idea was back. If light quanta exist, they will inevitably lose some energy during a collision with an atom. In wave theory, frequency and energy are not tied together, but in light quanta they are bound together in mathematically tight handcuffs. Lose energy, lower frequency. The math even explains why the change
is detectable in X-rays but not mirrors. Visible-light frequencies are already low, so a small drop in energy will produce a very small change in its frequency. Red light, that is, stays red. X-rays, however, with their high frequency will produce a greater frequency shift per unit of energy lost.
In November, Compton gave his breakthrough lecture, telling his students that the theory of light quanta was indispensable for explaining his X-ray scattering experiments. Centuries earlier Isaac Newton had presented his discoveries about light to bored and bewildered students who had no feeling for the revolution being set before them. The same story applied to Compton. He had a bullet aimed for the heart of classical physics, but few appreciated it. In December he reported his discovery to the American Physical Society’s annual meeting, and two weeks later he sent the report to The Physical Review. The journal, however, was not in the habit of receiving revolutionary papers and did not schedule its publication until May 1923.
Meanwhile, in Zürich, Peter Debye read Compton’s report to the National Research Council. Like Kramers, Debye had already considered the possibility of change in frequency, but had never published it. He might not have thought that the experiment was worth the trouble because it would only prove what everybody, except Einstein, already thought: light quanta do not exist. But after he read Compton’s paper, Debye knew what the data meant. In March, while Compton’s paper was undergoing a leisurely preparation for publication, Debye’s paper arrived at the office of the Zeitschrift für Physik, the world’s leading journal for news of the quantum revolution. In April, still a month before Compton’s article appeared, Debye’s article was published. Einstein, Planck, Bohr, Heisenberg—every physicist interested in quanta—now knew that the wave theory of electromagnetism was down, possibly (even probably) out.
Usually when two scientists discover the same law, the more celebrated of the two gets the larger slice of fame pie. Newton was credited as the father of the calculus, even though Leibnitz published first and used the better notation system. As the father of gravity, Newton already well outdistanced Leibnitz in public acclaim. Similarly, Darwin was also well-established as a scientist and popular author when Alfred
Wallace submitted his essay on natural selection. Darwin’s fame grew spectacularly, even though Wallace’s paper still stands as a model for a clear and direct statement of how natural selection works to produce new species.
In April, Arthur Compton looked doomed to a similar fate. Debye had gotten into print first and was known to all of Europe’s important physicists. He had been at Göttingen and was now in Zürich. Compton was off in wild-west country. It was Sommerfeld who saved Compton’s fame. Months before the Debye article appeared, Sommerfeld had already begun lecturing in America about the importance of Compton’s work. Furthermore, Sommerfeld was the author of the leading textbook on quantum theory and, in the summer of 1923, as he revised his book for its latest edition he described a “Compton effect.” So Debye was consigned to the role of Alfred Wallace. To his credit, Debye took his fate without bitterness. Many years later a historian asked him if the discovery should be called the Compton-Debye effect. No, Debye insisted, the person who did most of the work should get the name.
Another puzzle is why Einstein never anticipated the Compton effect. If, using theoretical principles alone, Kramers and Debye could predict the frequency change, why didn’t Einstein think of it too? Perhaps he did. Einstein’s letters often included obscure references to ideas that he had chased down as full of promise and later rejected as temptations from the devil. Einstein had a tough definition of understanding. It would not have been enough for him to say, Gee, if the energy decreases, then υ will have to decrease too. Before proposing any such idea he would have insisted on knowing how much the frequency decreases. He had provided exact numbers for the way gravity would bend starlight. A prediction of the Compton effect would have had to be as precise. Compton’s equation linked energy change and frequency change precisely with the angle of scattering. Different angles led to different numbers. Einstein would have demanded just that precise an equation before he reported his idea, and perhaps fame had become just too distracting for him to do the math.
There was also a dog that did not bark. Johannes Stark was silent through all this, even though back in 1909 when he and Einstein
stood alone, Stark had called for experiments with X-rays and angular variation that, if successful, would prove light quanta were real. Stark had never quite predicted the Compton Effect, but he came close, close enough to tempt even a milder man to say “I told you so.” Yet Stark kept silent. Apparently he so resented Einstein’s success that he no longer cared to remind anyone of his good, ground-laying work in quantum theory.
Compton’s stride settled most doubts. Scientists are not like judges who can make rulings and even allow a person’s execution, when a law points one way and a fact points the other. Scientists must go with the facts. When Einstein had only logic on his side, his would always be a minority position, an embarrassment for the facts of light’s interference but not their refutation. The Compton Effect put a new fact on the table. Electromagnetic reality can behave like a billiard ball bouncing off other billiard balls. Compton’s step took him and all who followed him into the world that Einstein alone had inhabited for many years. From there they could see not only the meaning of hυ as light quanta but could also grasp the problem that Einstein had been posing since 1909—light has the properties of both particles and waves, so what is it?
Physics had reached a crisis it had not known since the days when Johannes Kepler showed that the earth is one more planet making elliptical orbits around the sun. That put an end to Ptolemy’s system, yet there was still a mass of Aristotelian theory that explained motion very well on a resting earth but made no sense if the earth itself was moving. It required Galileo’s and Newton’s combined efforts to create a new physics that overcame those contradictions. Now, in hυ, physics had a mathematical idea as successful as the moving earth, but which made everything known from before seem as false as wartime news. On the bright side, this revolution was too important for Einstein to continue wasting his time being famous. Compton’s bullet served rather like the ghost of Hamlet’s father, who returned to whet the prince’s “almost blunted purpose,” and Einstein resumed thinking full-time about the mysteries of physics.