Once Before Time: A Whole Story of the Universe

Once Before Time: A Whole Story of the Universe, Martin Bojowald Alfred A. Knopf, New York, 2010. $27.95 (320 pp.). ISBN 978-0-307-27285-0

Of the four known forces of nature, gravity was the first to be described and remains the least understood. In contrast to the trio of gauge interactions—the electromagnetic and the strong and weak nuclear forces—gravity has resisted all efforts to render it in a quantized form. Indeed, quantizing gravity is a challenge that has kept some of the brightest minds in theoretical physics busy for several decades.

Most theorists hope that general relativity and quantum mechanics will be unified, as special relativity and quantum mechanics were unified by Paul Dirac. Conceptually, however, nothing could be harder. After all, special relativity is a force-free theory dealing with artificially constructed inertial motions. General relativity describes a force enmeshed in the very structure of space and time. The stage becomes one of the actors.

In Once Before Time: A Whole Story of the Universe by Martin Bojowald, an associate professor at the Pennsylvania State University, we learn that quantum gravity—in particular, the theory of loop quantum gravity (LQG)—has much in common with atomic theory. Just as the quantization of energy levels explained how electrons attracted to atomic protons can resist collapsing, so too can the discrete nature of spacetime in LQG prevent the collapse of spacetime into the dreaded singularity of classical general relativity.

In inspired but not overstated prose, Bojowald, a leading LQG theorist, does an excellent job in presenting some of the formidable and complex concepts of LQG with the addition of meaningful literary and philosophical quotations and meditations. The book would be of interest to science enthusiasts academic and not, and would make a fine addition to public libraries. Some explanations may go on for too long (does the nonspecialist need eleven pages on Penrose diagrams?); nonetheless, the many analogies and metaphors are used in a clear and pedagogical way. Bojowald’s emphasis on what can or can’t be observed is refreshing; one particularly nice discussion considers the possibility that light propagation from distant gamma-ray bursts might feel a slight, measurable dispersion due to the atomic nature of spacetime. He leaves out the outrageous possibility that gravity is simply not quantizable—after all, we can only find out by trying.

Many theorists have attempted to connect LQG with string theory, both of which are leading contenders for a theory of quantum gravity. But as Lee Smolin explains in Three Roads to Quantum Gravity (Basic Books, 2001), the two approaches are very different in spirit. Broadly speaking, LQG derives from the geometric approach to spacetime, whereas string theory derives from particle physics. Using original ideas of Smolin and Carlo Rovelli from the early 1990s, LQG theorists attempt to quantize spacetime by deconstructing it into geometric atoms woven by loopy threads that vibrate and heave in ways reminiscent of John Wheeler’s spacetime foam. In string theory, the many excitations of the fundamental string correspond to force quanta and particles of matter; the graviton, which is the force quantum of gravity, is just one of those. Somewhat joyfully, Bojowald writes that unlike string theory, LQG does not need extra spatial dimensions or supersymmetry, both of which remain undetected. (Excitement is mounting as the Large Hadron Collider collects more data.)

The discrete LQG approach promises to do more than protect spacetime from collapse into a singularity. As the title suggests, Bojowald explains how it might be possible to patch right through the cosmological singularity—the Big Bang—and consider what was happening before it. And black holes, which also exhibit spacetime singularities, may be rescued by LQG from their ultimate demise—evaporation into oblivion. More important, LQG may protect against exposing a naked singularity, a point where the very fabric of spacetime breaks down. For both the Big Bang and black holes, LQG poses that there is time and space beyond the singularity. However, Bojowald is careful to remind us that those possibilities are far from being realized. “We cannot be sure about the exact state whence the universe as we know it came,” he says, adding, “although quantum cosmology can clarify questions about the universe in unprecedented ways, it does leave modest freedom for the myths.”

Bojowald’s honesty is to be commended. Of the many valuable lessons we learn in Once Before Time, a most relevant one is that physics is an empirical science. As Bojowald himself puts it, “It is important to remember what the aim of science is: to describe Nature; and Nature has her own sense of beauty.” In science so far removed from experimental guidance, it is wise to be humble.