American Scientist

Three Roads to Quantum Gravity. Lee Smolin. viii + 232 pp. Basic Books, 2001. $24.00.

The two greatest achievements of 20th-century physics are quantum theory—a theory of the very small, which attempts to elucidate the nature and behavior of subatomic particles—and Einstein's general theory of relativity—a theory of the very large, whose purview is the properties of stars, galaxies and the universe itself. These two theories together with Newton's laws enable physicists to describe and predict the behavior of physical systems ranging in size from 10^-15 to 10²⁵ meters.

But for some physicists, including Lee Smolin, this is not enough. They seek to develop nothing less than a complete description of the entire physical universe, a "theory of everything." This requires pushing back the boundaries of our knowledge into realms far removed from the reach of the most discerning scientific instruments, using only logic, mathematics and deep physical insight into the nature of reality.

For his entire scientific career Smolin has pursued a quantum theory of gravity, which would unite quantum theory and the theory of general relativity. Such a theory would improve our understanding of many features of our universe that are currently not well explained, such as the nature of black holes, the evolution of the universe and quantum theory itself.

But this would be only the beginning. Smolin and others hope that a quantum theory of gravity would answer some of the most fundamental questions of all: What exactly are space and time? Is our universe all there is, or are there uncountably many parallel universes? Are the laws of physics contingent or necessary? This is a pretty tall order. There is no guarantee that a theory of quantum gravity would provide the answer to these questions. Indeed, it often happens that the advent of a fundamentally new theory modifies the discourse altogether: Questions that were formerly of primary importance may come to be seen as the wrong questions, or even as nonsensical. Smolin is not only aware of this, he is counting on it; this is part of the allure of a radically new theory.

In Three Roads to Quantum Gravity, Smolin discusses what we have already learned about the answers to some of these fundamental questions and offers an overview of promising avenues of investigation. He does an excellent job of conveying the scope of current frontiers of knowledge and the excitement they have engendered. Using clear, accessible language, he takes the reader on a mind-boggling tour of three possible roads to a final theory: loop quantum gravity, string theory and more esoteric approaches to quantum gravity. Much of the discussion is based on his own work, as he was directly or indirectly responsible for many of the spectacular successes of the loop approach to quantum gravity, including its prediction of the discreteness of space itself (which, it should be said, has not been verified experimentally).

What makes this book absolutely compelling, though, is Smolin's choice of topics. We are the beneficiaries of his decades of reflection, analysis and profound thinking about the nature of reality itself. Smolin has isolated those few principles that he believes will stand the test of time to emerge in some form or other in a final theory.

The first of these principles is that "there is nothing outside the universe." This necessitates the view that the laws of nature are background-independent—that is, they do not depend on a fictitious universe of fixed points in time and space; rather, they are inherently relative.

The second is that "in the future, we shall know more." This requires us to adopt an entirely new logic to describe the physics of the universe—namely, a logic in which the truth or falsity of any statement depends on the relationship between the observer and the object of the statement. Although (as Smolin points out) this idea is not new, recognizing its implications for cosmology may necessitate an entirely different mathematics for describing the universe.

Third, there are "many observers, not many worlds." The problem of measurement in quantum theory—exemplified by the conceptual confusions of quantum cosmology and by the Schrodinger cat paradox as applied to the universe as a whole—can be circumvented if one relativizes the universe, so that there is one universe with many different mathematical descriptions, each arising from a division of the universe into observer and observed. This obviates the need for the Everett interpretation of quantum mechanics, which posits multiple universes.

The fourth principle is that "the universe is made of processes, not things." Thus reality consists of an evolving network of relationships, each part of which is defined only in reference to other parts.

Amazingly, Smolin makes these four principles seem almost self-evident, when they are anything but. Some of them are, in fact, highly contentious.

Smolin next turns to a survey of "What We Have Learned," providing a fascinating look at the thermodynamics and quantum mechanics of black holes, as formulated primarily by Jacob Bekenstein, William Unruh and Stephen Hawking. Smolin justly emphasizes the ideas themselves rather than their chronological development. (For a more precise history, see Kip Thorne's Black Holes and Time Warps.) He discusses Bekenstein's idea that black holes have an entropy proportional to the area of their horizon in so-called Planck units, Unruh's idea that observers accelerating through the vacuum will perceive themselves as being embedded in a gas of hot photons at a temperature proportional to their acceleration, and Hawking's discovery that black holes radiate with a temperature inversely proportional to their mass. As Smolin observes, these profound ideas—which derive from considerations of thermodynamics, statistical mechanics, relativity and quantum mechanics—will almost certainly play a central role in any future theory of quantum gravity, regardless of its final form.

Next, Smolin draws on these ideas to argue that space itself must have a discrete structure. This, in turn, carries with it the implication that the amount of information contained in any particular volume must be finite, a surprising result to those trained to look at the world in a continuous manner. The result that the information of any region of space is proportional to the area of its boundary in Planck units establishes a fundamental limitation on the nature of physical systems, called the Bekenstein bound. The power of this principle lies in its universality—any viable theory of quantum gravity must explain why it holds.

It has generated much excitement in the field that the current leading candidates for a theory of quantum gravity (the loop approach and string theory) both predict that the Bekenstein bound should obtain. Smolin devotes a few chapters to explaining the two theories. The loop approach predicts that spacetime consists of evolving systems of relationships known as spin networks, and these spin networks predict that space is fundamentally discrete. Thus loop quantum gravity embodies many of the general principles for which Smolin argued at the outset. His description of the advances in this area made by himself and others provides an intimate look at the process of scientific research. String theory also makes the prediction that space is discrete, although in a rather different fashion.

Smolin explains how both theories, despite their manifest differences, lead to the Bekenstein bound. He also points out that practitioners of loop quantum gravity and string theory tend to be divided into sometimes opposing camps, each side somewhat suspicious of the methods and results of the other. He and a few other physicists are working hard to overcome the sociological barriers separating the two groups. Indeed, one of his aims in this book is to synthesize the best of both worlds in order to reach a higher level of understanding.

The search for some kind of common ground between the two camps is the subject of a chapter in which Smolin discusses the so-called holographic principle, first introduced by Gerard 't Hooft (recipient of the 1999 Nobel Prize in Physics) and later developed by many other physicists. The strong holographic principle asserts that, although there are three-dimensional things, all the information about those things is contained in the two-dimensional boundary that surrounds them, just as a three-dimensional image of an object is contained in a two-dimensional hologram. The weak holographic principle goes further in asserting that there are no things, only processes, and that these processes are merely the exchange of data across two-dimensional screens. According to this theory, the three-dimensional world is the flow of information. Making sense of this idea certainly poses a challenge, but as Smolin points out, making sense of seemingly wild and crazy ideas is exactly how physics progresses to the next level of understanding.

In the last chapter Smolin spends a little time discussing M theory, which is a current candidate for a theory of everything. He also offers a brief summary of an argument he makes in much more detail in The Life of the Cosmos, concerning one possible scientific explanation of why the laws of nature are what they are. He ends the chapter with a challenge for any future theory: If space really is discrete, then why does it appear to be continuous?particularly since calculations indicate the overwhelming improbability of it being both discrete microscopically and smooth macroscopically.

Smolin says in his opening chapter that he is an optimist. This is brought home most clearly by the epilogue, in which he makes some rather bold predictions for the future of physics. In particular, he predicts that we will have a working theory of quantum gravity by 2010, or 2015 at the latest, and that within 10 years after that we will have experimental evidence in its favor. One has to admire his intrepid spirit and utopian vision.

Roger Penrose, one of the world's foremost physicists, has called Lee Smolin a "brilliant, original thinker," to which I would add the adjective "deep." He is one of a small handful of physicists around the world who have been able to make genuine progress on an extraordinarily difficult problem that has stumped some of the world's finest minds for 80 years. In Three Roads to Quantum Gravity he has brought together many profound insights, gleaned from his own research and the work of others, regarding what is surely the most difficult problem in physics today. Only the future will tell whether or not his predictions will come to pass, but this book is required reading for anyone interested in the problem of quantum gravity and the future of theoretical physics.