What are pop physics books for? I ask as somebody who has spent a fair amount of cash on them, but who is uncertain of his own motivations. Read enough of these volumes, and you cannot help absorbing some intuition about the twin Copernican leaps of the twentieth century: relativity (there are no absolute or privileged frames of reference), and quantum physics (at small scales, the world is inherently blurry and probabilistic). But relativity and quantum-ness are not information any ordinary person will be able to use, except as metaphors. Men were sent to the moon without anybody involved needing to know, at any point, that either relativity or quantum physics existed.
Ever since Stephen Hawking produced the unexpected blockbuster A Brief History of Time (1988), it has been accepted that these books are mostly for erudite display, even though being seen with one only proves one’s tourist status. This publishing niche, after all, follows an iron law: thou shalt not belabour the reader with math. Hawking, although already a celebrity, was browbeaten by his editor into eliminating all but one equation, E = mc², from his Brief History.
Carlo Rovelli has obeyed the same discipline in writing The Order of Time, freshly translated from a successful 2017 Italian original. He gives us the Second Law of Thermodynamics as a simplified equation (∆S ≥ 0) on page 27, and immediately pleads, “Forgive me for the equation—it’s the only one in the book.” Sabine Hossenfelder’s Lost in Math offers a similar disclaimer, half-disavowing its own title. It’s a little discouraging: just once, even as a layman, you’d like to have one of these books shriek “Get ready to taste the pain of Greek letters.” But these are both subtle, original, even delightful books. There is no character of unpalatable nutrition to either, as there is with Hawking’s mathless, mirthless bestseller.
Rovelli’s goal is to offer a personal view of the phenomenon of time, specifically as a top physicist at the forefront of the trade. The relevant question here is not only whether time exists, but whether someone situated as close as possible to the inner secrets of the universe thinks time exists. Mercifully, Rovelli’s authority is as lightly worn as his classical learning.
Rovelli’s book is physics as poetry. Perhaps the reader may take it as an unintended warning if I call the book a “meditation,” but that is the rough idea. In other hands, endless detours into Rilke and Aristotle and Augustine of Hippo might be unbearable. Here, the presence of past saints and sages and litterateurs is not merely justified; Rovelli makes them active, living participants in an important discussion.
Theoretical physics has fragmented into rival camps or tendencies since about 1975, and Rovelli does not necessarily dwell in the mainstream, despite enjoying high personal distinction. He is a developer of “loop quantum gravity” theory, which is one of several candidate approaches for solving the preoccupying problem of theoretical physics: that relativity and quantum physics are hard to reconcile mathematically, and the approaches to sorting this out are difficult to test. This puts Rovelli, given the social environment of contemporary physics, in a particular religious denomination.
His idiosyncratic answer to whether time exists turns out to lean startlingly far toward “No.” Even after we learn about relativity, we all naturally retain the suspicion that there must be a clock lying on a table somewhere outside the universe, measuring a single, objective, universally binding flow of time. It takes an experienced tourist to surrender this illusion, although Rovelli has one bravura trick that helps: rather than telling the familiar story of how Einstein conceived of relativistic time dilation, and then mentioning the later measurements that confirmed it, he describes relativity as a simple measurable fact (“Time passes faster in the mountains than it does at sea level,” since it is further from the centre of the local gravitational field) and only then marvels at how Einstein was able to guess this without measuring. Rovelli has turned the usual order of time in a physics book on its head.
But he goes further in his attack on intuitions. The physics tourist might still be confident—since most of his prior tour guides will have said so—that even if time is purely local and eludes non-relativistic measurement, events still have an objective order. In the classic account, the ultimate anchor is entropy: you should always be able tell whether a picture of the universe is running forwards or backwards by watching for whether heat flows in the “correct” direction, from hot to cool.
Causes and effects thus remain safely distinguishable, and you can’t squeeze time out of foundational physics altogether. But Rovelli is not convinced this is so. To say more about this audacious and startling argument would be to spoil the book, and possibly disclose the reviewer’s ignorance. The point is that these ideas will be new to the lay reader, and are presented entertainingly.
Hossenfelder’s tour de force serves a less familiar function for a physics book: reportage. Hossenfelder is a serious German particle physicist who works professionally in the same approximate area as Rovelli. But where Rovelli’s book is a literary café chat that guides the reader to the brink of religious matters—decay and death and inevitability—Hossenfelder’s Lost in Math is an almost Lutheran social-philosophical attack on her own tribe. I am trying not to appeal to unfashionable stereotypes of the genial Latin and the stern Teuton here, but if you read both books, this is bound to strike you.
Hossenfelder feels she is part of a failed generation of physicists. Particle physics, the inquiry into the basic building blocks of the universe, arrived at a fairly neat “standard model” of the world in the 1970s. The task before theoretical physics at that time was to reconcile relativity and quantum mechanics—the latter scheme has trouble making gravity fit comfortably—and hopefully to do it in a way that accounts for cosmological anomalies such as “dark matter.” The solution was expected to involve the discovery of funky new unobserved particles alongside those in the accepted standard model, particles that must be hard to observe at the energy levels achievable with earthbound experimental equipment.
The republic of science set about building better, increasingly expensive equipment, culminating in CERN’s Large Hadron Collider (LHC), beneath the suburbs of Geneva. The LHC was designed specifically to reach energy levels thought likely to point the way beyond the standard model. It was supposed to reveal fissures in the model, clues about its presumptive insufficiency.
But the result was a physics tragedy, one Hossenfelder has documented by bumming around the world to the labs and offices of various star physicists and sticking a tape recorder under their noses. The LHC experiments have done nothing but confirm the standard model to more and more decimal places. Newspaper readers saw what looked like rejoicing in 2013 when the LHC confirmed the existence and the predicted properties of the “Higgs boson,” the remaining keystone of the standard model. But those predictions were already nearly fifty years old when the Higgs boson was “found,” and the old instinctive way of doing theoretical physics has yielded no further progress. No new beasts have been added to the “particle zoo.” No telltale cracks in the foundations were revealed.
Everyone to whom Hossenfelder speaks seems intellectually paralyzed. She plays Socrates, pretending to expect some older, wiser figure to give her an answer to the question “Now what?” and she meets with shrugging everywhere. She does not like the proliferation of sects in theoretical physics, and she thinks the field has been led down a forking path of blind alleys by the persistence of old ideas of mathematical intuition. In her account physics is lost, not so much in math, but in para-mathematical or pre-mathematical prejudices.
Physics arrived at the standard model with continual help from the notions—closely related but not interchangeable—of “beauty” and “elegance” and “symmetry.” Physics seeks out sensible order and chases apparent numerical coincidences. Think of the way William Prout spotted that atomic weights tended to be integer multiples of the mass of hydrogen, or of Dmitri Mendeleev inferring the existence of unknown chemical elements by spotting holes in what we now call the periodic table.
Clues like these bore fruit steadily for a long time, even though their philosophical basis is murky. There is no “reason” for the universe to favour integers and simple ratios and uniform probability distributions. The physicists’ dependence upon mathematical elegance, according to Hossenfelder, may now have metastasized into a harmful addiction.
The experimental apparatus needed for particle-physics investigations will soon reach a scale of absolute unbuildability, especially if funding agencies get tired of building bigger and bigger colliders that merely confirm the already known. Investigators have begun to build complex theoretical frameworks, elaborate cathedrals of string theory and multiverses, that are not testable even in principle. Advanced physics seems to be in danger of becoming scholastic, in the historical sense—a mass exercise in Platonist navel-gazing.
Readers of Lee Smolin’s The Trouble With Physics (2006) are familiar with this fear, but the Large Hadron Collider triumph-disappointment has emphasized its urgency. (Smolin, like Rovelli, is a loop quantum gravity hierophant.) For all that the LHC did to advance the frontier of physics, it might almost be considered an objet d’art. As a particle physicist, Hossenfelder takes this personally. Smolin’s taunting of string theorists seemed shocking and gauche a decade ago, but it looks like jocular indulgence in contrast with Hossenfelder’s forensic rage.
Hossenfelder’s interrogations of colleagues are fun, and it is true that the general reader will need very little math to understand what she is attempting socially. It is important to Hossenfelder that she be heard by non-physicists. She is painfully aware that we laymen pay the bills, and is almost ashamed of the lingering social prestige of physics, which is what encourages us to pay those bills. At the end of Lost in Math one is almost surprised that she has not denounced other pop-physics books as a snare for talented youth who might be doing better things with their lives. Maybe that part got edited out.