*The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos*

**Brian Greene**

Knopf, 2011

384 pp., $29.95

## Karl W. Giberson

### George Bailey's Wonderful Universes

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.

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Charles Jackson

The need for mutiverses comes from the need of infinity to explain the power of chance, for all reality points to too little time and too little space for human existence without God. You are going to see more and more books on the multiverses as more and more scientists need chance and not God. with the tie that binds

Jon

Thanks for the review! Does multiverse theory (in general, no particular one) have any support in the philosophical community? After all, they're the ones always discussing "possible worlds" lol.

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