SEPTEMBER 17, 2016
“IMAGINE A BOX OF LIGHT,” Albert Einstein told Niels Bohr in 1930, continuing their argument of 20-odd years. If we let a single photon — a particle of light — escape from that box and we clock when it left, then we’ll know the time it was emitted. If we weigh the box before and after, we’ll know the photon’s energy, because E=mc^2. Knowing both time and energy definitively, however, was allegedly impossible, according to the theory Einstein himself proposed but never adopted: quantum mechanics. Einstein was particularly annoyed by the Heisenberg uncertainty principle, with its view of the world as a set of probabilities instead of definite things: “God does not play dice with the universe,” he famously quipped. His box-of-light thought experiment was intended to disprove the theory once and for all.
“I will never forget the image of the two antagonists as they left the club,” Léon Rosenfeld, the Belgian physicist, later wrote of the public debate. “Einstein, with his tall and commanding figure, who walked tranquilly, with a mildly ironic smile, and Bohr who trotted along beside him, full of excitement.”
But Bohr soon saw the flaw in Einstein’s reasoning: as the photon escapes, the box recoils to conserve momentum, making the positions of both box and photon uncertain. According to Einstein’s general theory of relativity, the time of emission would suffer the same ambiguity. Relativity, the theory that Einstein loved, provided evidence for the theory he eschewed. This exchange was the final round in their argument. Although Einstein never liked quantum mechanics, he could not disprove it. Bohr was the theory’s greatest champion and spent decades developing it, but he also wrote of the “deep and lasting impression” made by those meetings with Einstein. He went to his death with a box of light drawn on his chalkboard, testimony to his abiding respect for Einstein’s doubt.
Einstein outlined both relativity and quantum mechanics in separate papers in the journal Annalen der Physik in 1905 — the physicist’s annus mirabilis. These theories are also the first two subjects of Carlo Rovelli’s Seven Brief Lessons on Physics, a spare, poetic, and thoughtful look at the major revolutions in the field since that extraordinary year. General relativity posits a warped spacetime within which objects move, a gravitational field that “is not diffused through space [but] is that space itself.” Quantum mechanics says that matter comes in discrete, “finished packages,” which only become real when they interact with something else: “It’s as if God had not designed reality with a line that was heavily scored but just dotted it with a faint outline.” The third and fourth lessons explain these theories’ implications for the very large and the very small, respectively, and prepare the reader for “quantum loop gravity” in the fifth lesson. That theory, which Rovelli himself helped establish, attempts to reconcile general relativity and quantum mechanics by proposing that not only matter but space itself is quantized:
that space is not continuous, that it is not infinitely divisible but made up of grains, or “atoms of space.” […] They are called “loops,” or rings, because they are linked to one another, forming a network of relations that weaves the texture of space, like the rings of a finely woven, immense chain mail.
In addition to his substantial contributions to quantum gravity theory, Rovelli has also published three popular science books, but it is his fourth, Seven Brief Lessons, that became a best seller in Italy. What sets it apart is both its breadth and concision: over a hundred years of scientific innovation surveyed in fewer than 90 pages. Rovelli often describes the science in beautifully clear lay terms. Consider the fourth lesson’s sublime ending:
A handful of types of elementary particles, which vibrate and fluctuate constantly between existence and nonexistence and swarm in space, even when it seems that there is nothing there, combine together to infinity like the letters of a cosmic alphabet to tell the immense history of galaxies; of the innumerable stars; of sunlight; of mountains, woods, and fields of grain; of the smiling faces of the young at parties; and of the night studded with stars.
The story of the universe unfolds on Rovelli’s pages, and his metaphors bring physics to life — we can see the chain mail and read the cosmic alphabet. But elsewhere there is simply not enough on the page for a lay reader to grasp the physics. For instance, while Rovelli makes much of the personal conflict between Bohr and Einstein, setting up a fundamental tension in his narrative, he never explains the thought experiment itself. He quotes Einstein as saying, “Imagine a box filled with light, from which we allow a single photon to escape for an instant” — but that’s it for the science behind this pivotal moment. A few extra sentences would have allowed the reader to stand next to Bohr at the chalkboard and contemplate the box of light for herself.
Consider also the slightly exotic presentation of the book’s one equation, which is actually compact notation for what are known as Einstein’s field equations. These are fundamental to general relativity; they describe both the gravitational field itself — that is, the warping of spacetime — and the way things move within that spacetime:
A few pages earlier, Rovelli defines “R” as the “mathematical object” that captures “the properties of a curved space.” “R” describes the warping. But the author could also have explained that the “g” is the field itself and the “T” describes the energy and momentum of an object in the field. Instead, he says that he “cannot resist giving [the equation], even though you will almost certainly not be able to decipher it.” This is a bit like placing Chinese characters in an English-language book without a translation. Math becomes Warhol-esque visual art; the equations are not practical tools of the trade but indecipherable decorations. Mathematics is the language of scientific advancement, and the book’s suggestion that math is incomprehensible undermines its aim of illuminating such advancements.
What remains after Rovelli’s omissions is a set of carefully curated explanations that the uninitiated must take largely on faith. Seven Brief Lessons is, in other words, an uncluttered book with perhaps a little too much breathing room. The reader is presented the science, but doesn’t always have an opportunity to participate in its unfolding — ironic, since the primacy of interaction is a key consequence of quantum mechanics theory. Rovelli points to this consequence throughout his book, asking in the second lesson whether “we must accept the idea that reality is only interaction,” and finding in the fifth that, “Once again, the world seems to be less about objects than about interactive relationships.”
This implication of quantum mechanics — that only interactions, not individual things, are real — mystified Einstein and led him to conclude not that the theory was incorrect, but that it was not the final answer; to him, it was only capable of describing the statistical behavior of large groups of systems, not the exact behavior of an individual system. Rovelli sums up Einstein’s view from the vantage point of the quantum physicist: “The same Einstein who had shown that time is not universal and that space is curved was now saying that the world cannot be this strange.”
Einstein’s doubt wafts through the prose like perfume, reinforcing one of the book’s themes: “Genius hesitates.” If Rovelli’s primary goal is to describe the revolutionary advancements in physics in the last century, his secondary goal seems to be to pay homage to “genius,” a term the author uses time and again. Fewer than 20 scientists are named in the book, including Anaximander and Aristotle, and most are theorists. This selective veneration glosses over the work of thousands of men and women who helped turn nascent ideas into verified facts.
A little credit goes a long way. Instead of ending his discussion of Hawking radiation (the “heat” of black holes) with “No one has ever observed this heat […] but Hawking’s calculation is convincing,” Rovelli could have mentioned the efforts of Jeff Steinhauer in Israel, Bill Unruh in Canada, or Franco Belgiorno in Italy to produce Hawking radiation in the laboratory. When he addresses the Nobel-winning work of Joseph Taylor and Russell Hulse — who provided the first indisputable observational evidence for general relativity (“The effects of these ‘gravitational waves’ are observed in the sky on binary stars”) — he neglects to name the two scientists. Elsewhere, Rovelli writes of the bending of light predicted by relativity, “In 1919 this deviance was measured and the prediction verified.” A phrase crediting Sir Arthur Eddington’s photographs of the solar eclipse showing the bent light would have brought to life the observational journey that made the theoretical one possible.
The sheer number of hours, data points, and calculations necessary to develop and prove the modern theories of physics are evidenced only obliquely in Rovelli’s history. Yet we understand the world not simply because a genius comes along once or twice a century with an “elementary intuition,” as Rovelli describes Einstein’s, but because thousands of us observe the world tirelessly, with every instrument at our disposal. The “intuition” that results is only the tip of the iceberg. Rovelli’s prose reads in places like gossip overheard while eavesdropping on an exclusive club of geniuses — few of whom are observers and all of whom are male; there is a “they” capable of such work, witnessed by an “us” who are not.
Of course, the idea that only few are capable of such radical “intuitions” is hardly incorrect. Einstein and Hawking and Aristotle are men whose ideas were so far beyond those of their contemporaries that the “genius” label certainly fits. But if Rovelli’s goal is “a rapid overview of the most fascinating aspects of the great revolution that has occurred in physics […] and of the questions and mysteries that this revolution has opened up,” then surely the reader should be initiated into some of those mysteries. To be sure, Rovelli’s poetic and markedly untechnical language is engaging, and, in this way, the book succeeds. But, at the same time, the flagrant omissions, both historical and scientific, erect a barrier between the world of the scientist and that of the reader. The club of physicists feels small and exclusive, when in fact it is hundreds of thousands strong, with rolling, open admission.
This feeling of exclusivity is antithetical to Rovelli’s own enthusiasm for the autodidact. “You don’t get anywhere by not ‘wasting’ time,” he asserts in the opening paragraph, promoting Einstein’s pre-1905 year of “loafing aimlessly”: “It is thus that serious scientists are made.” He further describes his own first exposure to relativity while lounging by the Mediterranean Sea and reading a textbook: “Undistracted by schooling, one studies best during vacations.” In a lovely and compelling description of that summer, he writes that the theory became real to him “as if by magic: as if a friend were whispering into my ear an extraordinary hidden truth, suddenly raising the veil of reality to disclose a simpler, deeper order.” Wonderful! Except … what book? What if I, too, wanted to have my veil lifted? If I did find myself near an ocean with sufficient leisure time to take on this transformative book, I couldn’t; I’m not invited to be a fellow autodidact. A bibliography would remedy this omission without cluttering the text.
Readers who do bother to verify Rovelli’s facts for themselves will find that Einstein published four groundbreaking papers in 1905, not three; that there are approximately three thousand observed extrasolar planets, not “thousands of billions of billions of billions”; and that we have barely been able to get a satellite out of the solar system, much less a hundred million times farther, clear of the Milky Way, to take the photograph Rovelli labels “the Galaxy” (which is most likely a picture of our neighbor Andromeda). These latter two gaffes are both in the third lesson, the weakest of the seven. Unsurprising for a quantum physicist, Rovelli seems to have a preference for the small. Compare the inspired close of the fourth lesson, about the “cosmic alphabet” telling the history of “mountains, woods, and fields of grain,” with the rather pedestrian ending to the third: “The universe began as a small ball and then exploded to its present cosmic dimensions. This is our current image of the universe, on the grandest scale that we know […] Do other similar universes exist, or different ones? We do not know.”
Rovelli is at his best when discussing the implications of quantum mechanics, and it’s perhaps no accident that the most radical and fascinating aspect of the book is what his own theory implies about time. The problem with time, which emerges in the sixth lesson, is that “[t]he difference between past and future exists only when there is heat.” Here, so that the punch line can be appreciated, Rovelli takes enough space to develop this long-standing conundrum within thermodynamics: the “flow of time” is an illusion, the result of our inability to observe the world closely enough. Having dismantled everything common sense tells us about the physical world, the book culminates in this final coup, the death of time.
In the end, Rovelli’s eloquent descriptions of the irresistibly bizarre universe will likely win over most readers. And the respect shown in the final lesson for disparate worldviews — scientific, philosophical, and religious — shows an impressive degree of intellectual maturity:
The world is complex, and we capture it with different languages, each appropriate to the process that we are describing. Every complex process can be addressed and understood in different languages and at different levels. These diverse languages intersect, intertwine, and reciprocally enhance one another, like the processes themselves.
Such reverence for mystery and complexity, coupled with Rovelli’s clear scientific mastery, is rare indeed — neither the simplistic tone of books that equate science with mysticism, nor the strident atheism of many scientists’ writings. Rovelli’s love of and reverence for this mysterious world, “where space is granular, time does not exist, and things are nowhere,” is clear on every page. His Seven Brief Lessons is full of tantalizing revelations about the wonderful strangeness of the universe that, like the universe itself, sometimes keeps us at arm’s length.
Jennifer Carson holds a SB in physics from MIT and a PhD in astrophysics from UCLA. She worked on the Fermi Gamma-Ray Space Telescope at Stanford and currently teaches college physics. She recently finished her first novel.