Time Reborn: From the Crisis in Physics to the Future of the Universe,

Houghton Mifflin Harcourt
, 2013. $28.00 (352 pp.). ISBN 978-0-547-51172-6

Lee Smolin is a very good popularizer of science, a provocative thinker, and a skilled polemicist. His latest popularization, Time Reborn: From the Crisis in Physics to the Future of the Universe, displays an uneasy mix of those abilities. It is a clear, well-written description of the physics of time and timelessness, an introduction to some intriguing new ideas, and a carefully crafted argument for a highly controversial viewpoint. Unfortunately, that last element is presented in a way that will confuse lay readers who lack the background to distinguish among standard physics, open questions, and the highly speculative notions that are largely Smolin’s own.

Smolin’s first thesis is that physics has fallen into a trap in which time and novelty have been systematically devalued. He describes a series of steps in this “expulsion of time” from physics—from the description of dynamical processes as timeless curves in configuration space to the discovery of the relativity of simultaneity to the notorious frozen formalism of quantum cosmology. I was impressed. I know of no other popular physics book that succeeds in explaining such diverse concepts as configuration space, the Wheeler–DeWitt equation, effective field theory, and the block universe. I was also intrigued by the connections Smolin draws among the disparate roles of time in physics.

The author’s second thesis is that this expulsion of time arises from a basic fallacy: the mistaken attempt to extend to the universe as a whole an approach that works for subsystems—the “Newtonian paradigm.” Here, I find the argument far less persuasive. Certainly the universe is a unique system, and we cannot control initial conditions, so tests of physical laws must be different. But the far more sweeping claim upon which Smolin’s thesis rests seems to be largely a statement of philosophical prejudice.

Smolin is ambitious, indeed. He wants a theory in which the laws of physics are themselves determined, and argues that this is only possible if the laws evolve in time, presumably along with the meta laws that govern their evolution. He wants to replace quantum mechanics with a highly nonlocal hidden-variable theory, and to replace the standard probabilistic interpretation with one in which probabilities are determined by actual outcomes of past measurements. His notion of time requires that we abandon the relativity of simultaneity, and for that, he likes shape dynamics, a recent reformulation of general relativity in which time reparameterization invariance is traded for local scale invariance.

I like shape dynamics, too. I’m working on a paper on the subject right now. But it is far too early to consider it anything more than an interesting new avenue to explore. Smolin’s treatment of this topic is, unfortunately, typical: His book does a poor job of distinguishing established results, speculative ideas, and personal opinions. Expert readers will have no trouble seeing the broader perspective. But few of the intended readers of Time Reborn will know that its claim that quantum mechanics requires an isolated system with an outside observer is not the only interpretation. Fewer still will know that the book’s depiction of timelessness in the Wheeler–DeWitt equation is just one interpretation among many, or that Poincaré recurrence occurs only for quantum systems with discrete energy spectra, or that “quantum graphity” has yet to reproduce any known physics.

My biggest reservation about this book, though, is its lack of humility. Smolin, a critic of the more extravagant pronouncements of string theorists, now declares that his view of time is “the key to the meaning of quantum theory and its eventual unification with space, time, gravity, and cosmology.” He claims to show—not argue or contend, but show—that “no extension of the Newtonian paradigm can yield an acceptable theory of the universe as a whole.’’ He asserts that “the only way to avoid the fallacy and the paradox of an improbable universe is to base our explanation … on time-asymmetric physics.” In the book’s last chapter, he finally steps back and confesses that he is merely “try[ing] to construct reasoned arguments.” But that’s too little, too late.

Again, experts will discount the hyperbole. But lay readers are liable to conclude that Smolin is clearly right and that other physicists are just being pigheaded in not following his lead, or, worse, that correctness in physics is essentially a matter of opinion.

Would I recommend the book? To a colleague, yes: While portions are irritating, it is valuable to have to think about the issues it raises, even if only to clarify one’s disagreements. To a lay reader, only hesitantly: perhaps as part of a reading list that could also include Sean Carroll’s From Eternity to Here: The Quest for the Ultimate Theory of Time (Dutton, 2010; reviewed in Physics Today, April 2010, page 54); Julian Barbour’s more difficult The End of Time: The Next Revolution in Physics (Oxford University Press, 1999); and, for an ambitious reader, Robert Geroch’s wonderful General Relativity from A to B (The University of Chicago Press, 1978; reviewed in Physics Today, May 1979, page 71).

Steven Carlip is a professor of physics at the University of California, Davis. He specializes in quantum gravity and has conducted research in quantum cosmology, emergent spacetime, and shape dynamics.