Random Acts of Science (Published 2010)

Quantum mechanics is the most revolutionary scientific theory to appear in the past 150 years. In the atomic domain, it superseded laws first set out by Isaac Newton a quarter of a millennium earlier and has since had an unbroken string of successes. Today, it continues to give an utterly reliable account of the behavior of the subatomic world, yet there are nagging doubts that there is something rotten at its core.

In his lively new book, “Quantum,” the science writer Manjit Kumar cites a poll about the interpretation of quantum mechanics, taken among physicists at a conference in 1999. Of the 90 respondents, only four said they accepted the standard interpretation taught in every undergraduate physics course in the world. Thirty favored a modern interpretation, laid out in 1957 by the Princeton theoretician Hugh Everett III, while 50 ticked the box labeled “none of the above or undecided.” Almost a century after a few physicists first set out the basic theory, quantum mechanics is still a work in progress.

It was the German theoretician Max Planck who first presented the idea that energy is fundamentally granular. In a lecture given in the closing weeks of 1900, he described his bizarre proto-theory that when light and matter interact, energy cannot be transferred in arbitrary amounts, as would be expected on the basis of Newton’s account. Rather, Planck suggested, energy transfers take place only in discrete chunks, which he called “quanta.” A deeply conservative thinker, he was never comfortable with this notion, which he saw as a “purely formal assumption,” and was unconvinced when the young Albert Einstein suggested — in what he considered to be his only revolutionary contribution to science — that it was possible to think of light in terms of particles, later called photons. Planck died, almost 50 years later, unwilling to believe the picture of light that he himself had introduced. This is a classic example of the adage that physics progresses through a succession of funerals — of the pioneers who could not live with the consequences of their most radical work.

In resisting the photon concept, Planck was in good company. Another influential skeptic was the Danish physicist Niels Bohr, a remarkably profound thinker and inveterate mumbler who continually struggled to find coherent expressions of his ideas. (“You should never express more clearly than you can think,” he would whisper to often-baffled colleagues.) Bohr at first refused to believe in the reality of photons, even after the American experimenter Arthur Compton first found compelling evidence for them in 1922. For a short time, Einstein was in the vanguard of quantum theory, while Bohr lagged ­behind.

For many of the physicists who forged the first comprehensive quantum theory in the second half of the 1920s, Bohr was a kind of intellectual godfather. Through cajoling and persistent tactful criticism, he helped them to do their best work and produce the components of the theory, whose coherence and unity emerged only gradually. One of its creators, the taciturn English physicist Paul Dirac, liked to point out that quantum mechanics was the first mathematical theory in science in which the discoverers did not fully understand the meaning of the terms in their own equations.

“Quantum” is a wide-ranging account, written for readers who are curious about the theory but want to sidestep its mathematical complexities. It’s full of a surprising amount of detail, perhaps rather more than most readers will want. The story is chock-full of colorful characters, including the two physicists who independently set out the first two versions of the theory, which initially appeared to be quite different. The first was the young Werner Heisenberg, not two years past his doctorate, fun-seeking and intensely competitive, not least at the Ping-Pong table. The other was the older Erwin Schrödinger, an Austrian polymath who scandalized his conservative colleagues by showing up at conferences in his climbing gear, sometimes accompanied by an adolescent lover.

Kumar will not win prizes for historical originality. This is an unapologetically orthodox account, largely derived from the standard sources and without the benefit of some of the latest scholarship. Occasionally, the narrative appears to be driven by a wish to thread together every amusing story, anecdote and famous quotation. There is, however, no doubt about the author’s skill in making accessible the philosophical controversies in his story, especially the debates between Bohr and Einstein. For Bohr, physics was not about finding out what nature is, but about what can be said about it. Quantum mechanics was a complete theory of the behavior of matter and light, and we just have to come to terms with the limitations it places on what can be known, for example as illustrated by the Heisenberg uncertainty principle. Einstein was having none of it. He believed that there is an objective world out there and that it is the job of scientists to describe it. The appearance of probabilities in the theory was, for him, evidence of its incompleteness.

In 1964, after both Einstein and Bohr had died, the Irish physicist John Bell did something they had failed to do; he found a way of testing experimentally which of their opposing viewpoints most accurately described nature, by laying out a mathematical theorem. In the denouement of “Quantum,” Kumar describes the result of the experiment, which I shall not reveal, though I think it fair to say it leaves us feeling that the story of quantum mechanics is not yet over.

In the late 1970s, I had the pleasure of talking with John Bell about the Bohr-­Einstein debates during a train journey from Oxford to London. Every seat was taken, so we had to stand. Pressed against me by sullen commuters, Bell summarized his apparently reluctant conclusion as we pulled into Paddington station: “Bohr was inconsistent, unclear, willfully obscure and right. Einstein was consistent, clear, down-to-earth and wrong.”

Einstein, always his own man, never cared whether his colleagues regarded him as wrongheaded. While the public all over the world regarded him as a kind of sage, he knew that his fellow physicists — especially younger ones — saw him as eccentric or even senile. In the early 1940s, the theorist John Wheeler visited him at his home in Princeton to brief him on a new development in quantum theory and to ask if he would now accept it. “I still can’t believe that the good Lord plays dice,” Einstein replied. After a pause, he added, “Maybe I have earned the right to make my mistakes.” Yet Einstein never publicly accepted that he was mistaken; nothing was going to persuade him to change his way of looking at the world. A few years later, he told a friend that he believed “in a world that objectively exists, and which I, in a wildly speculative way, am trying to capture.” It seemed he could not live with the consequences of his most revolutionary idea.

Kumar ends his fascinating book with the verdicts of some of today’s leading physicists on Bohr’s and Einstein’s contrasting views on quantum mechanics. It is clear from this that quite a few of Einstein’s most distinguished successors believe he was right to say that the theory is fundamentally unsatisfactory and that we need a deeper account of reality. The sage of Princeton may yet have the last chuckle.