Walter Isaacson - Einstein - His Life and Universe

Niels Bohr, Lasers, and “Chance”

By far the most important manifestation of Einstein’s midlife transition from a revolutionary to a conservative was his hardening attitude toward quantum theory, which in the mid-1920s produced a radical new system of mechanics. His qualms about this new quantum mechanics, and his search for a unifying theory that would reconcile it with relativity and restore certainty to nature, would dominate—and to some extent diminish—the second half of his scientific career.

He had once been a fearless quantum pioneer. Together with Max Planck, he launched the revolution at the beginning of the century; unlike Planck, he had been one of the few scientists who truly believed in the physical reality of quanta—that light actually came in packets of energy. These quanta behaved at times like particles. They were indivisible units, not part of a continuum.

In his 1909 Salzburg address, he had predicted that physics would have to reconcile itself to a duality in which light could be regarded as both wave and particle. And at the first Solvay Conference in 1911, he had declared that “these discontinuities, which we find so distasteful in Planck’s theory, seem really to exist in nature.” 28

This caused Planck, who resisted the notion that his quanta actually had a physical reality, to say of Einstein, in his recommendation that he be elected to the Prussian Academy, “His hypothesis of light quanta may have gone overboard.” Other scientists likewise resisted Einstein’s quantum hypothesis. Walther Nernst called it “probably the strangest thing ever thought up,” and Robert Millikan called it “wholly untenable,” even after confirming its predictive power in his lab. 29

A new phase of the quantum revolution was launched in 1913, when Niels Bohr came up with a revised model for the structure of the atom. Six years younger than Einstein, brilliant yet rather shy and inarticulate, Bohr was Danish and thus able to draw from the work on quantum theory being done by Germans such as Planck and Einstein and also from the work on the structure of the atom being done by the Englishmen J. J. Thomson and Ernest Rutherford. “At the time, quantum theory was a German invention which had scarcely penetrated to England at all,” recalled Arthur Eddington. 30

Bohr had gone to study with Thomson in Cambridge. But the mumbling Dane and brusque Brit had trouble communicating. So Bohr migrated up to Manchester to work with the more gregarious Rutherford, who had devised a model of the atom that featured a positively charged nucleus around which tiny negatively charged electrons orbited. 31

Bohr made a refinement based on the fact that these electrons did not collapse into the nucleus and emit a continuous spectrum of radiation, as classical physics would suggest. In Bohr’s new model, which was based on studying the hydrogen atom, an electron circled a nucleus at certain permitted orbits in states with discrete energies. The atom could absorb energy from radiation (such as light) only in increments that would kick the electron up a notch to another permitted orbit. Likewise, the atom could emit radiation only in increments that would drop the electron down to another permitted orbit.

When an electron moved from one orbit to the next, it was a quantum leap. In other words, it was a disconnected and discontinuous shift from one level to another, with no meandering in between. Bohr went on to show how this model accounted for the lines in the spectrum of light emitted by the hydrogen atom.

Einstein was both impressed and a little jealous when he heard of Bohr’s theory. As one scientist reported to Rutherford, “He told me that he had once similar ideas but he did not dare to publish them.” Einstein later declared of Bohr’s discovery, “This is the highest form of musicality in the sphere of thought.” 32

Einstein used Bohr’s model as the foundation for a series of papers in 1916, the most important of which, “On the Quantum Theory of Radiation,” was also formally published in a journal in 1917. 33

Einstein began with a thought experiment in which a chamber is filled with a cloud of atoms. They are being bathed by light (or any form of electromagnetic radiation). Einstein then combined Bohr’s model of the atom with Max Planck’s theory of the quanta. If each change in an electron orbit corresponded to the absorption or emission of one light quantum, then—presto!—it resulted in a new and better way to derive Planck’s formula for explaining blackbody radiation. As Einstein boasted to Michele Besso, “A brilliant idea dawned on me about radiation absorption and emission. It will interest you. An astonishingly simple derivation, I should say the derivation of Planck’s formula. A thoroughly quantized affair.” 34

Atoms emit radiation in a spontaneous fashion, but Einstein theorized that this process could also be stimulated. A roughly simplified way to picture this is to suppose that an atom is already in a high-energy state from having absorbed a photon. If another photon with a particular wavelength is then fired into it, two photons of the same wavelength and direction can be emitted.

What Einstein discovered was slightly more complex. Suppose there is a gas of atoms with energy being pumped into it, say by pulses of electricity or light. Many of the atoms will absorb energy and go into a higher energy state, and they will begin to emit photons. Einstein argued that the presence of this cloud of photons made it even more likely that a photon of the same wavelength and direction as the other photons in the cloud would be emitted. 35This process of stimulated emission would, almost forty years later, be the basis for the invention of the laser, an acronym for “light amplification by the stimulated emission of radiation.”

There was one part of Einstein’s quantum theory of radiation that had strange ramifications. “It can be demonstrated convincingly,” he told Besso, “that the elementary processes of emission and absorption are directed processes.” 36In other words, when a photon pulses out of an atom, it does not do so (as the classical wave theory would have it) in all directions at once. Instead, a photon has momentum. In other words, the equations work only if each quantum of radiation is emitted in some particular direction.

That was not necessarily a problem. But here was the rub: there was no way to determine which direction an emitted photon might go. In addition, there was no way to determine when it would happen. If an atom was in a state of higher energy, it was possible to calculate the probability that it would emit a photon at any specific moment. But it was not possible to determine the moment of emission precisely. Nor was it possible to determine the direction. No matter how much information you had. It was all a matter of chance, like the roll of dice.

That was a problem. It threatened the strict determinism of Newton’s mechanics. It undermined the certainty of classical physics and the faith that if you knew all the positions and velocities in a system you could determine its future. Relativity may have seemed like a radical idea, but at least it preserved rigid cause-and-effect rules. The quirky and unpredictable behavior of pesky quanta, however, was messing with this causality.

“It is a weakness of the theory,” Einstein conceded, “that it leaves the time and direction of the elementary process to ‘chance.’ ” The whole concept of chance— “Zufall” was the word he used—was so disconcerting to him, so odd, that he put the word in quotation marks, as if to distance himself from it. 37

For Einstein, and indeed for most classical physicists, the idea that there could be a fundamental randomness in the universe—that events could just happen without a cause—was not only a cause of discomfort, it undermined the entire program of physics. Indeed, he never would become reconciled to it. “The thing about causality plagues me very much,” he wrote Max Born in 1920. “Is the quantumlike absorption and emission of light ever conceivable in terms of complete causality?” 38