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Quantum physics: Packet man

Graham Farmelo delights in a study of Albert Einstein's under-appreciated contributions to quantum theory.

Einstein and the Quantum: The Quest of the Valiant Swabian

Princeton University Press: 2013. 9780691139685 | ISBN: 978-0-6911-3968-5

In 1941, US physicist John Wheeler visited Albert Einstein, the arch quantum sceptic, at his home in Princeton, New Jersey. Wheeler was hoping that the beauty of the new version of quantum theory developed by his brilliant student Richard Feynman would persuade Einstein to accept that the theory was simply a natural development of well-founded classical ideas. The sage of Princeton listened in silence as Wheeler set out his case, but afterwards was no more enthusiastic. “Of course, I may be wrong,” he said, “but perhaps I have earned the right to make my mistakes.”

Einstein was by that time a semi-detached member of the physics community, admired much less for his current work than for his achievements. Many of his colleagues thought his views on quantum theory cranky — Robert Oppenheimer dismissed them as “cuckoo”. That opinion is sometimes echoed today in popular books, many of which underestimate his contributions to the theory.

Albert Einstein at his home in Berlin. Credit: UNDERWOOD & UNDERWOOD/CORBIS

In Einstein and the Quantum, Douglas Stone attempts to put that right. He describes Einstein's work on the theory using few equations, combining scientific and biographical accuracy with wide accessibility. Stone, a distinguished condensed-matter physicist at Yale University in New Haven, Connecticut, brings a wealth of physical insight and — less predictably — an impressive familiarity with the work of leading Einstein scholars.

In 1900, Max Planck introduced the revolutionary idea of energy quantization in the interaction between matter and radiation in black bodies. But, as Stone explains, it was Einstein who first understood the implications. In 1905, the 26-year-old physics wizard radically suggested that the energy of electromagnetic radiation is transferred in the discrete amounts that Planck called quanta. For physicists of the day, long familiar with James Clerk Maxwell's wave description of light, Einstein's notion was beyond heretical. Few leading theoreticians took it seriously, least of all Planck.

Even Einstein wavered. He strove for years to understand radiation quanta, for example by tinkering with Maxwell's equations of electromagnetism. Eventually he abandoned this approach, having introduced the useful but murky concept of wave–particle duality. Yet, more than any other scientist, Einstein ran with the quantum idea. Applying it to the vibrational energies of atoms, he used it to predict that the specific heats of solids should vanish as the temperature is lowered towards absolute zero. Quoting an early statement of Einstein's about atomic energy, Stone adds with characteristic pith that energy quantization “is not a mathematical trick; it is the way of the atomic world. Get used to it.”

Each of the 29 chapters in Einstein and the Quantum is brief, pacey and lucid (although some titles are perhaps too clever: for example, 'Stalking the Planck'). The breadth and depth of Einstein's contribution in this area becomes overwhelmingly clear. Eleven years after his first great paper on the subject, he delivered a theory of transitions that introduced into quantum theory the idea of probabilities, which he came to despise. Finally, in 1924, he built on the thinking of Indian physicist Satyendra Bose about quantum gases and predicted that, under some conditions, a high proportion of particles could occupy the lowest quantum state, enabling quantum effects to appear in the everyday world. This was later called Bose–Einstein condensation and was first observed experimentally in 1995.

Stone covers all this with clarity and even tackles Einstein's little-known 1917 paper on the quantization of chaotic systems. This chapter will probably leave non-specialists scratching their heads, but it is worth a read because it demonstrates that there is more to Einstein's oeuvre than even most quantum physicists know. Stone concludes that Einstein's work was worthy of four Nobel prizes, and it is a measure of the book's achievement that his claim sounds quite reasonable.

It was left to Werner Heisenberg, Erwin Schrödinger and Paul Dirac to set out the full-blown quantum theory of matter in the mid-1920s. Einstein was a formidable critic of the theory, although he was always outwitted in argument by his friend Niels Bohr — a topic treated only briefly in the book, probably because this ground is so well-trodden. Yet all the originators were indebted to Einstein's thinking. As Max Born later said, he was “clearly involved in the foundation of wave mechanics and no alibi can disprove it”.

In old age, Einstein seemed indifferent to his reputation as a fuddy-duddy, but the criticisms may have hurt more than he let on. I have often wondered how he felt when he saw the Princeton University Players' production of William Shakespeare's The Tempest in July 1953, especially when Prospero contemplates the fleeting nature of existence that leaves “not a rack behind”. Einstein died less than two years later. He was proud to have built the great edifice of relativity, but still profoundly dissatisfied with quantum theory, which he was confident would be superseded.

Was he wrong? Some theoretical physicists are now speculating that space and time might in some sense emerge from the more fundamental quantum, so it may be that scientists will one day regard Einstein's greatest achievement as pioneering a theory he believed was terribly flawed. In the meantime, Stone's rewarding book helps us to appreciate the remarkable extent of that feat.

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Farmelo, G. Quantum physics: Packet man. Nature 502, 300–301 (2013). https://doi.org/10.1038/502300a

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