The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos Hardcover – January 25, 2011

Brian Greene's previous books are exemplars of what science writing should be: clear, wide-ranging in discussion and respectful of the intelligence of his audience. The Elegant Universe and The Fabric of the Cosmos are two of my three favorite popular science books. The third, Kip Thorne's Black Holes and Time Warps, is another superb example of science writing at its best. Now Brian Greene has added another masterpiece to the list. Everything that distinguishes Greene's writing style is in evidence in The Hidden Reality. His elegant prose is enjoyable to read. His brilliant ability to explain difficult abstract ideas in everyday language using easily understood examples still amazes me. And his use of vivid word pictures that always seem perfectly matched to the topic he's discussing propels his narrative forward so that the reader is never bored.

Yes The Hidden Reality is more accessible than his previous books. This book seems easier to read and is readily understandable. In his earlier books, I often read a paragraph several times in order to fully comprehend what Greene was attempting to communicate. That is something science and math majors are used to doing when reading textbooks but difficult for those not as scientifically adept. Greene's first two books dealt with Quantum Mechanics, String Theory and Einstein's Special and General Theories of Relativity: vast math-intensive topics that he was able to distill masterfully. The Hidden Reality inhabits a more abstract world, a conceptually challenging world. I quickly found Greene's more casual approach extremely helpful, even comforting, when I felt slightly adrift. The topics he discusses begin with the geometry of the universe: whether it is spherical (or positively curved), flat (with zero curvature) like a tabletop, or negatively curved like a Pringle.

The book devotes considerable time to the critical question of whether the universe is finite or infinite in size, something which has profound scientific and philosophical implications. It is a statistical certainty that in an infinite universe, regions of local space like ours will be endlessly repeated. In other words, assuming an infinite spatial universe with an expanding big-bang beginning, there are only a finite number of possible matter and energy configurations because the amount of energy and matter is finite. But there is an infinite amount of space within which those configurations will play out. Greene uses the example of a friend named Imelda whose passion for clothing has her purchasing 1000 pairs of shoes and 500 dresses. If Imelda is blessed with an infinitely long lifespan then, despite her vast wardrobe, if she changes outfits daily, within 1400 years she will have exhausted all possible new combinations. Imelda will be forced to repeat her sartorial choices. Philosophically, of course, all of that repetition of stars, planets and life's building blocks suggests that there are an infinite number of doppelgangers of each and every one of us. These infinite duplicates of ourselves would inhabit similar worlds that are forever hidden from mutual observation because the speed of light is finite. As Einstein showed in his Special Theory of Relativity, light-speed (300,000 km/sec) is the fastest rate by which information can be communicated. The bottom line: in an infinite universe the overwhelming bulk of reality remains hidden from its inhabitants by vast distances or by parallel dimensions harboring realities of every possible configuration.

In a finite spherical universe, on the other hand, the light from distant objects should ultimately traverse it several times, leading to multiple images of galaxies, for example. This hasn't been observed as yet, suggesting that the universe is either finite but huge or actually infinite in size. Though the size and shape of the universe remain undetermined, scientists when cornered tend to believe its size is infinite. Recent data also suggests that the universe is flat like a tabletop in shape.

Greene discusses all of the current hot topics in cosmology: brane-worlds, the multiverse, the holographic universe, unseen parallel worlds in dimensions separated by millimeters, our universe as a super-advanced computer program, the essentially hidden nature of reality. These are topics that have been discussed in other books but seldom with the passion for communication and clarity of thought that Greene exhibits in this one. The topics here are abstract concepts that exist at the very boundaries of human thought but Greene somehow manages to bring them down to earth. Even if you don't understand everything, the scientific vistas that Greene offers in this superb book are breathtaking in their intellectual beauty. You will find your personal horizons exponentially expanded. The Hidden Reality is replete with excellent illustrations that illuminate the material and are fun to look at. If this kind of science intrigues you then you will love this book. Brian Greene has written another masterpiece in a difficult genre.

Beginning in the 16th Century, physics started to change from a purely scholastic mode of inquiry, in which questions were answered by argument from first principles and ancient authority, into a scientific one, in which observation and mathematical law predominated. With the introduction of Newton's work and his (and Leibniz') invention of the calculus, physics became a modern science, in which mathematics played a key role not only in testing theories, but in predicting phenomena as well. Even so, it was still possible for the non-scientist to understand much of the work of physicists, as it still dealt (for the most part) with laws and phenomena that could be observed, experienced, or at least imagined with the average person.

With the advent of relativity and quantum mechanics in the early 20th century, this all changed. Special Relativity dealt with velocities far beyond that which any human could ever experience. General Relativity dealt with interactions on a cosmic scale. And quantum mechanics dealt with scales far smaller than that which could be experienced or observed- even by physicists. What these new disciplines shared was that they they could only be truly understood by someone conversant with the mathematics involved. Although mass-induced curvature of space (for example) is commonly explained by analogy to a weight on a rubber sheet; that's at best, a weak metaphor. A ball bearing rolling on a rubber sheet is still being pulled down by gravity; it is not tracing a path in curved space that minimizes action.

As modern theories physics have become more complex, more purely mathematical, and further removed from the experience of the perceivable world, the books that attempt to explain things like inflationary cosmologies, string theory and supersymmetry have become increasingly less satisfying. And that brings us to the central failure of almost every popular book on modern physics I've ever read- the inability to actually explain the why and how. After reading countless books by popular authors like Tim Ferris, I realized that although many were, indeed, excellent writers, none of them actually understood the physics they were purporting to explain. At beast, they were simply repeating the metaphors they'd been given. They didn't understand the physics well enough to explain it.

There were a few exceptions- popular books written by actual physicists who also had a particular gift for teaching and explanation. To date, I've only found three who both have a deep understanding of modern physics, and who can convey more than a metaphorical understanding of this to a reasonably intelligent, but non-specialist, reader: Richard Feynman, Alan Guth, and Brian Greene. True, there are other physicists who write popular books, but most aim pretty low. They're satisfied to give a general sort of metaphorical explanation- curved space is like a curved rubber sheet, expansion is like inflating a beach ball, and strings are like... little strings. But Feynman, Guth and Greene each tried to really convey the real science.

The late Richard Feynman is still the master. His lectures- especially "The Character of Physical Law"- did a magnificent job of making clear even such difficult concepts as the quantum explanation of diffraction. Guth's "The Inflationary Universe" does a superb job of explaining topics like tension and negative energy in telling his story of the origins of cosmic inflation theory. And Brian Greene, author of the current volume under discussion, has now produced his third book attempting to explain some very difficult ideas to the lay reader. In "The Hidden Reality", Green tackles string theory, the multiverse, symmetry, group theory, and dozens of other topics, and he does so without resort to any "it's just like..." metaphors. He uses graphic representations when possible, to illustrate mathematical relationships without math when possible (although much of the real math can be found in the appendix.) He explains where and how contemporary cosmological theories originated, and gives the reader a good sense of exactly how we arrived at a position in which physics is largely dominated by untestable theories that make few predictions about the measurable universe- and why this is not necessarily a problem.

Greene is one of the principle authors of modern string theory, and he does a superlative job of conveying, for the lay reader, both the state of string theory, and its genesis. While to fully understand such notions as (say) the role of Calabi-Yu shapes in defining the topology of the multidimensional universe would no doubt require a real familiarity of topology, I think Greene comes as close as possible (or at least as close as I've seen) in conveying to the reader why it is that these shapes play a role in defining space, and how it is that physicists came to propose their existence. His explanation of quantum uncertainly and of Schrodinger's probability wave is probably the best non-mathematical one I've read.

This is not an easy book to read. I went as far as a few calculus courses and a semester of physics back in my undergraduate days, and I found this book fairly hard going. It's not terribly mathematical (except in the appendices) but the concepts are not easy, and there's little if any fluff to be found. This is not the sort of breezy reading found in the typical popular physics book (here's the atom, here's a quark, wasn't that cool?) The reader who attempts to simply skim through without trying to follow Greene's narration and really understand what he's trying to explain will quickly find themselves lost, reading words without a clue of what they mean. I've been reading it for two weeks, attacking a chapter (or part of a chapter) each day, and often backtracking to make sure I understand what Greene is trying to convey. The reader who is prepared to take this approach, and spend a lot of time reading, pausing, think about what they've read, and rereading each section to make sure they really understand what's going on, will find this a very rewarding book.