Editor's Note: This article was first published in the May/June 2011 issue of Books & Culture. The author, Karl W. Giberson, is now based at Stonehill College, where he teaches writing and Science-and-Religion.

Dear Father in heaven, I'm not a praying man, but if you're up there and you can hear me show me the way … show me the way.
—George Bailey in It's a Wonderful Life

In the classic movie It's a Wonderful Life, George Bailey is treated to what historians call a "counterfactual"—he gets to look at a parallel universe that differs from his only in that he is not in it. The alternate reality, provided by his guardian angel, helps the discouraged and suicidal George appreciate the great value of his life.

Few things stimulate the imagination like a good counterfactual. What if that attempt on Hitler's life had succeeded? What if John Kennedy had lived? What if that asteroid had not struck the earth millions of years ago, driving the dinosaurs to extinction? What if, as Stephen Jay Gould speculates at the end of his book Wonderful Life, we "Wind back the tape of life, and let it play again. Would the replay ever yield anything like the history that we know?"

Ambitious counterfactuals ask us to imagine a world where we can "boldly go where no one has gone" at speeds faster than light. Or back in time. Or where alien life forms are attacking Earth. Counterfactuals have long been the stuff of fiction or speculation. They were imaginative exercises, intended to entertain or instruct. But physicists are now telling us that they may be real. Max Tegmark, a leading physicist who wrote a cover story on parallel universes for Scientific American and heads up the Foundational Questions Institute at MIT, believes there really is a world where Hitler was assassinated and Kennedy was not. There is a world where the dinosaurs did not go extinct, thus denying mammals the ecological room they needed to develop. In fact, Tegmark believes there is a world—a truly wonderful world—where George Bailey was a real character living in a real town named Bedford Falls with a real wife named Mary Hatch.

Such provocative claims are entirely compatible with leading physical theories today. Like so many of the scientific revolutions of the past century—relativity, quantum mechanics, Big Bang Cosmology—the latest speculations from our species' most fertile brains are preposterous and strange.

Curiouser and Curiouser

I thought of a maze of mazes, of a sinuous, ever growing maze which would take in both past and future and would somehow involve the stars.
—Dr. Yu Tsun in Jorge Luis Borges'
"The Garden of Forking Paths"

Brian Greene's The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos makes the case for all manner of alternate worlds, produced in every way imaginable. If we define "reality" as "all there is," there are physical theories that imply the existence of realities that include other universes with laws of nature slightly different from ours; there are implied realities with the same laws as this universe, but different histories; and there are implied realities that bear no resemblance to ours. The only thing common to all these "realities" is that they cannot be empirically detected from our reality.

Greene's resume makes him hard to dismiss. An undergraduate at Harvard and a Rhodes Scholar at Oxford, he is a professor of physics and mathematics at Columbia, and an important science popularizer. His first book, The Elegant Universe, sold more than a million copies. His second, The Fabric of the Cosmos, spent 25 weeks on the New York Times bestseller list.

In The Hidden Reality, Greene surveys nine independent theoretical possibilities for parallel universes or multiverses, as they are now called. He labels them quilted, inflationary, brane, cyclic, landscape, quantum, holographic, simulated and ultimate. They arise—sometimes quite naturally—from mathematical theories that range from challenging to impossible. With one possible exception that I will comment on later, these alternate realities are not hypothesized to explain empirical observations in our universe. The universes—and this includes the one we know and love—arise as solutions to equations.

Physicists are used to equations with many solutions. Even the simple quadratic equation we mastered in introductory algebra has two solutions. When it appears in an empirical context, we have to examine the two solutions and see if either of them or both correspond to the real world. The quadratic equation for the path of a baseball, for example, has a solution matching the path of an actual ball and another solution that does not match. Freshmen physics majors learn to throw away those solutions that don't match empirical reality.

Physicists look for equations with solutions that model or describe real-world phenomena. But there is an art to interpreting solutions. In one famous case, the great French physicist Paul Dirac, who won the Nobel Prize in 1933, found an equation with a solution he was sure described an electron. But it had a second solution describing some other particle. Dirac had enormous confidence in mathematics and was sure the second solution must describe something real that had not yet been discovered. Inspired by his confidence, experimental physicists went looking for the reality behind this second solution and discovered anti-matter—an unimaginably odd form of matter. In some strange way, the mathematics knew about anti-matter before the mathematician.

Steven Weinberg, who won the Nobel Prize in 1979, agrees with Dirac: "Our mistake is not that we take our theories too seriously, but that we do not take them seriously enough," he wrote in his classic The First Three Minutes. "It is always hard to realize that these numbers and equations we play with at our desks have something to do with the real world."

Greene, who quotes Weinberg with approval, admits that multiverse theorists are skating on ice of uncertain thickness, looking for elusive evidence to confirm theories that may or may not relate to anything real: "In the absence of compelling experimental or observational results," he cautions near the end of the book, "deciding which mathematics should be taken seriously is as much art as it is science."

Nevertheless, as Greene's ambitious survey makes clear, multiverse speculation is really not that much different from the sort of speculation that, in the past, has led haltingly from suggestive mathematics to physical reality.

What Exactly Is a Multiverse?

And new philosophy calls all in doubt,
The element of fire is quite put out
The sun is lost, and the earth
And no man's wit can well direct him where to look for it.
—John Donne, "An Anatomy of the World"

All multiverses share one property—they provide a possibly infinite set of variations on the concept of a universe. The simplest example is the one with which Greene opens his survey: the Quilted Multiverse. In this variation, space stretches to infinity in all directions, and what we think of as "our" universe is just the small patch visible from our location. Light from more distant patches has not reached us so we don't know they are out there. But there is no good reason to deny the reality beyond what we can see. Because the Quilted Multiverse is infinite, all sorts of things have to happen.

To keep this simple, let's consider only our Milky Way Galaxy. Although the Milky Way is huge, it is not infinite. It contains a finite number of protons, electrons and neutrons, arranged at any given moment in a particular configuration. If space goes on forever, there must be other regions of space the size of the Milky Way with the exact same number of particles. In fact, infinity being what it is, there must be an infinite number of such regions.

Since the Milky Way is finite in all respects, it cannot have an infinite set of different configurations, just as the letters on a Scrabble board cannot be arranged in an infinite number of ways. Some of these regions must have their particles arranged exactly as they are in our Milky Way. The particles in some regions must have the same history as the particles in our galaxy. And there must be regions that are exact or almost exact duplicates of our Milky Way.

Moreover, some of these "duplicates" will have an Earth-like planet with a history matching ours. Others will have a history where Hitler got assassinated and Kennedy did not. Some will have George Bailey as a real character. Some will have a family called the Simpsons, living in a town called Springfield. There are only a finite number of ways to arrange the particles, and only a finite number of possible histories, so many of the arrangements will copy or almost copy our situation here in the Milky Way.

If we grant that space is infinite, these various regions exist with mathematical certainty, at least from a scientific perspective. And each of Greene's multiverse models produces an infinity of universes in some distinctive way. The Inflationary Multiverse generates them by inflating spacetime bubbles, of which our universe is one. The Quantum Multiverse generates them by splitting the universe every time a quantum measurement occurs (which rescues Schrödinger's infamous cat). The Simulated Universes are produced by an advanced civilization of clever programmers, who have created Matrix-like programs, similar to the one you are embedded in right now. The Ultimate Multiverse is so crazy that even Greene can't buy it (unlike Max Tegmark). Elevating the "principle of fecundity" to the status of creator, this ensemble is created just because there is no basis to set aside any legitimate mathematical model. Anything not forbidden by the mathematics is real, no matter how odd.

Too Much of a Good Thing

There was a young man from Trinity,
Who solved the square root of infinity.
While counting the digits,
He was seized by the fidgets,
Dropped science, and took up divinity.
—Anon.

Greene is convinced, although far from dogmatic, about the reality of the multiverse. He is impressed that so many "parallel-universe proposals … emerge unbidden from the mathematics of theories developed to explain conventional data and observations."

The unbidden character of the multiverse is a most interesting development. Quantum mechanics, for example, was developed in the early part of the 20th century to explain the structure and behavior of matter and its interactions with light. A half-century later, some physicists became convinced that the theory predicted the multiverse. Inflation theory was developed to explain some odd features in the expansion of this universe after the Big Bang. A decade later it seemed to suggest that universes were constantly inflating. With so many different ways to make a multiverse, how can we not get onboard?

I am not so sure I agree with Greene on this point. In fact, it seems to me that the "unbidden emergence" of the multiverse from so many different theories might more reasonably be viewed as an artifact of complex mathematics. Perhaps any mathematical system capable of describing a universe will somehow imply the existence of other universes. If inflation and quantum mechanics, for example, separately suggest, via two unrelated mechanisms, that there are many universes, one of them is probably wrong. And if there are nine unrelated ways to produce a multiverse, then eight of them are probably wrong. And if eight separate mathematical systems can all spuriously imply the reality of a multiverse, what real confidence can we place in number nine? We might compare this to a prosecutor who has a really solid embezzlement case against nine unrelated people, all of whom would have committed the crime in very different ways. Far from seeming like a compelling argument that the case is solved, this might suggest that our prosecutor is a bit too creative.

So Why Believe?

Maybe there is only one universe, and it is the way it is because it is not any old world; it is a creation that is endowed by its Creator with precisely the finely tuned laws and circumstances which have enabled it to have a fruitful history.
—John Polkinghorne, biologos.org/blog/john-polkinghorne-on-natural-theology-part-iv/CP3/

I mentioned above that there is one possible exception to the non-empirical character of the multiverse: that is the anthropic principle. Our universe is provocatively fine-tuned for life. The agnostic Fred Hoyle famously wrote, in 1981:

A common sense interpretation of the facts suggests that a superintellect has monkeyed with physics, as well as with chemistry and biology, and that there are no blind forces worth speaking about in nature. The numbers one calculates from the facts seem to me so overwhelming as to put this conclusion almost beyond question.

Hoyle's "common sense interpretation" is widely, although not universally, accepted. Our universe does seem unusually fine-tuned for life. This is an empirical claim, based on observations, not a faith claim based on theology. Cosmic fine-tuning implies the multiverse in the same way that the motion of galaxies implies the existence of dark matter. An infinity of different universes solves the mystery about why one universe is finely tuned for life. In this sense the multiverse is simply a good theory—hypothesis would be a better term—explaining some puzzling observations.

Such a claim, however, makes sense only with the assumption of naturalism. If everything must be explained in an exclusively scientific way, then this may be the best option. But what if naturalism is not an all-encompassing explanatory constraint? Is it possible that our universe might be a creation with its properties determined by a creator? In that case, there is no mystery as to why the universe is finely tuned: A super-intellect has monkeyed with the physics.

Unfortunately Greene passes silently over this option, despite its ubiquity in the contemporary discussion and, most likely, in the minds of many of his readers. The theistic option, of course, intrudes from outside science and is something of a "god of the gaps," but the belief that God created our wonderful universe is hardly an indefensible claim and should be a part of such discussions.

Karl W. Giberson is professor of physics at Eastern Nazarene College and director of the Forum on Faith and Science at Gordon College. With Francis S. Collins, he is the author of The Language of Science and Faith (InterVarsity). His biography of John Polkinghorne is forthcoming. And with Randall J. Stephens, he has written The Anointed: Evangelical Truth in a Secular Age, to be published by Harvard University Press this fall.

Copyright © 2011 by the author or Christianity Today/Books & Culture magazine.
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