Three Roads to Quantum Gravity

by Lee Smolin
232 pages,
ISBN: 0465078354

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Quantum Theory of Gravity Within Reach
by David Colterjohn

Most of us were probably in the upper grades of elementary school when we first saw that picture of a stable, sun-like nucleus with electrons revolving around it like planets. When we think of the word 'atom' that's probably still the image that springs to mind. But according to Lee Smolin, Professor of Physics at Pennsylvania State University, the universe isn't made up of things at all. It is a constantly evolving network of relationships and processes better likened to the flow of information in a computer than to outdated notions about large, stable things like atoms. That's not all: the 'big bang' itself was actually something more like the 'big freeze'; space and time are not continuous, infinitely divisible properties, but come in discrete, countable units; and as for empty space, well, that's better described as a "color electric superconductor."

"We are in the midst of a revolution," writes Smolin, whose latest book, Three Roads to Quantum Gravity, was written for us civilians as a dispatch from "the front." He starts out reassuringly enough, saying that he wants to use language that will allow "any interested reader" to understand recent developments at the cutting edge of physics. A few pages later, just as one's comfortable, three-dimensional armchair starts to detach itself from Newtonian terra firma, readers who accept his challenge will learn that the book is actually aimed at the intelligent layperson. While some might find this a flattering promotion, the lay reviewer trembles in fear.

The best place to start is by pointing out that a coherent theory of quantum gravity is the elusive grail of modern physics. In Newton's universe, space and time were taken to be absolutes, a kind of fixed background against which the celestial dance of heavenly bodies could take place. Einstein's theory of general relativity toppled this stately edifice by proving that space and time are merely aspects of the relationships between objects. General relativity introduced a sort of variable geometry to space-time and is a truly fundamental background independent theory, which works very well at describing large-scale events. The problem is that general relativity's predictions about gravity break down when it tries to describe events at the atomic level.

Quantum theory was invented by the likes of Neils Bohr, Werner Heisenberg, and Erwin Schr÷dinger to explain why atoms are stable and don't just fall apart, a task quite beyond Newtonian physics. By asserting that sub-atomic particles can be described as discrete quantities of energy it (apparently) does a very good job at describing the rules that govern the sub-atomic world, but still takes as a given Newton's obsolete fixed background of space and time. Quantum theory then, is a background-dependent theory.

Both general relativity and quantum theory have been challenging our everyday sensory perceptions for almost a hundred years. Both theories have proven very fruitful in answering certain questions about the world we inhabit, but at the same time each flatly contradicts the other. To reconcile these two great strands of physics someone will have to come up with a single unifying, background independent theoryłthe quantum theory of gravity.

After a hundred years wandering in the wilderness, Smolin optimistically believes that physicists are within a decade of outlining exactly such a theory. Over the last twenty years, three groups of physicists and mathematicians have been converging on this goal. At last, he enthusiastically exclaims, "we are closing in on the beast." Three Roads to Quantum Gravity is an attempt to provide the rest of us with a topographical map so we can better understandłand hopefully supportłthis altogether outlandish quest.

The first of the three roads to this objective starts from the assumptions of quantum theory and is known as "string theory." String theory replaces the old idea of "particles" with tiny, vibrating "loops of string." These strings are very, very tiny and require that an extra six (sometimes more) spatial dimensions be coiled up tightly or "compactified," safely out of the reach of our three-dimensional perceptions. There are many consistent string theories, which may in fact describe many possible universes. String theorists hope to come up with a single unifying theory known as "M Theory" that will finally bring quantum theory and general relativity together.

Smolin himself has been driving some of the mathematical heavy equipment needed to construct the second road, which takes general relativity as its starting point and is known as "loop quantum gravity." In this view space is built up of discrete, constantly changing "spin networks" of loops that intersect dynamically. This theory has been waggishly described as the "study of knots, links and kinks." These loops are even smaller than strings and might possibly form the background that quantum theorists need so badly.

The third road Smolin describes is really more like a secret goat path through the mountains. It outlines the contributions of a handful of lonely geniuses who "discarded both relativity and quantum theory" to introduce new concepts and new mathematical tools that can be purloined at need by workers in the other fields.

Significant progress has recently been made along all three of these paths but puzzles, conundrums and other roadblocks remain. Smolin believes that the string theorists and the quantum loop gravity folks may actually be looking at the same thing from slightly different perspectives. What remains is to synthesize these new insights into an all-encompassing quantum theory of gravity.

Smolin is a genial guide who knows full well that these ideas are hard to grasp. He helps his readers over some of the rough patches by agreeing that it may be "hard to visualize" one or another of the wild ideas he's talking about. Those of us who are not mathematically inclined can take comfort when he argues that for many years general relativity was misunderstood, even by the physicists who specialized in it.

Human beings are large mammals whose senses have evolved over hundreds of millions of years to survive in a three-dimensional Newtonian universe. It's almost impossible to wrap one's mind around some of the ideas, describing the world on the sub-atomic "Planck scales", that Smolin discusses. When speaking of Heisenberg's uncertainty principle (that we cannot know both the position and the motion of a particle) Smolin admits that "it is hard to work one's way through to the logical consequences. . . when one's first response is simply to disbelieve it. I myself do not really believe it, and I do not think I am the only physicist who feels this way." But since the principle is an integral part of a theory that explains important facts about the world we live in, the author cheerfully continues to use it in his work.

There is more here of course. Using analogies, anecdotes, diagrams and an extraordinarily large number of possible cats, the author tries to explain things like the superposition principle, hidden regions, the holographic principle, and the notion that our universe is but one 'phase' in a much more complex 'multiverse.' His prose is so deceptively simple that a casual reader might forget that he is translating the most fearsome mathematics known to humankind into ordinary English sentences.

Some of the most interesting and easily accessible passages are those in which the author describes the turning points that define the trajectory of his own career. Smolin's biggest break came when he dropped one of his own papers into a mailbox for one person only to find that it was accidentally read by another. He was offered one of his first jobs as a result. One of his most important insights came when he was waiting in a garage for his car to be fixed. Even the most mathematically-challenged reader ought to be able to feel the breathtaking excitement Smolin experienced when he and a couple of colleagues suddenly realized that the formulas they had just scribbled on the blackboard represented "exact solutions to the full equations of the quantum theory of gravity."

Does Smolin succeed in explaining these concepts to "any interested reader?" It's probably safer to say that he paints a series of tantalizing, mirage-like visions that shift kaleidoscopically in and out of focus. Nevertheless, he literally inflames a reader's sense of curiosity and leaves one determined to try to learn more. ņ


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