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A physicist explains the ‘greatest remaining mystery’: The nature of time

Joseph Peschel is a freelance writer and critic in South Dakota.

No one writes about the cosmos like theoretical physicist Carlo Rovelli. He may not be as well known in the United States as the late Stephen Hawking, Neil deGrasse Tyson or Alan Lightman. But in his books, “Seven Brief Lessons on Physics” and “Reality Is Not What It Seems,” Rovelli explains physics with reverence and exuberance, in ways that even a book reviewer without a PhD can understand. In his newest work, “The Order of Time,” Rovelli shares his enthusiasm as he discusses scientifically and philosophically the “greatest remaining mystery”: the nature of time. This book, like his previous works, is translated by Erica Segre and Simon Carnell, and their poetic interpretation of his words, I surmise, derives strongly from the original Italian.

In the first part of this book, Rovelli describes the state of modern physics; in the second he depicts a world without time. The final and most difficult part of the book “becomes a fiery magma of ideas” devoted to the discovery of the essential components of time.

Some ideas in “The Order of Time” are a bit confusing and require a re-reading, but Rovelli includes only one equation in his new book, and he even apologizes for its appearance. This time it’s: S ≥ 0 (the delta of S is always greater than or equal to zero). You don’t have to know exactly what the equation means, other than that it’s the second law of thermodynamics: Heat passes from hot bodies to cold, never the reverse. Rovelli calls the equation “time’s arrow,” the only equation of “fundamental physics that knows any difference between past and future.”

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Rovelli talks about time as a complex collection of layers and then strips away those layers, revealing “how the temporal structure of the world is different from our perception of it.” Rovelli is the sort of scientist who compares poet Rainer Maria Rilke’s “eternal current” to the idea of the “intrinsic difference between past and future.” He’s certainly not the only physicist to refer to Rilke’s “Duino Elegies.” But as you might have guessed, Rovelli, a loop quantum gravity theorist, does not believe in the Einsteinian notion of a “block universe,” in which “the distinction between past, present and future is only a stubbornly persistent illusion.” He’s not even certain Einstein believed it, since Einstein wrote the famous words in a letter upon the death of a friend. Rovelli’s opinion is certain to raise a few eyebrows, but that’s expected when you disagree with Einstein. (Rovelli does devote a few touching pages to demonstrate that the anguished Einstein didn’t actually believe what he’d written in the letter.)

Some of Rovelli’s critics, like physicist Lisa Randall, who also quotes Rilke, have accused Rovelli of romanticism, but I find an elegant grandeur in how he relates the worlds of science, philosophy and art. What’s more, scientists and philosophers, going back to ancient times, have weaved a connection between poetry and physics, or, as it was once called, “natural science.” Even today, Tyson and others remind us that celestial objects are often named for characters like Jupiter, Umbriel and Oberon from mythology, poems and plays. But I can think of no one other than Rovelli who would begin all but one chapter of his nonfiction book with excerpts from the “Odes” of Horace. That single aberrant chapter’s epigraph quotes Shakespeare’s “Henry IV, Part I.” Rovelli doesn’t limit himself to literary references, either. He alludes to Beethoven’s “Missa Solemnis,” and he quotes from Lewis Carroll’s “Alice In Wonderland,” the Grateful Dead’s “Walk in the Sunshine” and Paul McCartney’s “Fool on the Hill.”

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Just about any time you write about a technical subject for a lay reader, you open yourself to criticism that you are making things so simple that they are inaccurate, and, occasionally, statements intended as interpretation can be mistaken as fact. Randall’s criticism of Rovelli’s “Reality Is Not What It Seems” in the New York Times faulted Rovelli for presenting interpretation as fact. Her beef was Rovelli’s explanation of quantum mechanics, in which he presents the idea that electrons exist only when they interact. Randall claimed Rovelli presented the idea as fact when it was only interpretation. Apparently, his view differed from her own so much that she missed the part a couple dozen pages later where Rovelli clearly said that it is his “least unreasonable” interpretation. Rovelli even replied with a letter to the Times and on Facebook. I suspect that Rovelli organized his new book to make clear what is fact, what is interpretation and what is speculation. Some technical sections require a closer reading or re-reading — Rovelli even suggests that you might want to skip Chapters 9 and 10 — but the distinction between fact and opinion is clear.

Rovelli’s new story of time is elegant and lucidly told, whether he is revealing facts or indulging in romantic-philosophic speculation about the nature of time. Who can resist reading a physicist who writes, “We can go back to serenely immersing ourselves in time — in our finite time — to savoring the clear intensity of every fleeting and cherished moment of the brief circle of our existence”?

By Carlo Rovelli, translated by Erica Segre and Simon Carnell

Riverhead. 240 pp. $20