It would not be fair to describe the experience of reading the renowned physicist Brian Greene as a battle of wits. It’s no battle. Most of the wits are on one side, no matter how nicely Mr. Greene tries to soft-pedal his brilliance. After all, he is the scientist who has written so enticingly about superstring theory, Calabi-Yau manifolds and the goings-on at the Large Hadron Collider. You are the one who gets agitated when Mr. Greene makes reference to “conformally invariant supersymmetric quantum gauge field theory” and such.
But there is very good reason to go mano a mano with Mr. Greene when he delivers a new book. He has already written “The Elegant Universe” and “The Fabric of the Cosmos,” two heady but surprisingly accessible explanations of thrillingly arcane research. Reading them is far more edifying than baffling, even if they have patches of authorial quicksand here and there. Over all, Mr. Greene has a gift for elucidating big ideas and knowing that a bombardment of too many small ones might make the armchair physicist implode.

“The art of theoretical physics lies in simplifying the horrendously complex so as to preserve essential physical features while making the theoretical analysis tractable,” he writes encouragingly in his latest mind-bender, “The Hidden Reality.” To put that in even more user-friendly fashion, Mr. Greene values “the art of knowing what to ignore.”

He says this at the very the start of the new book. And it is a necessary reassurance, especially when Mr. Greene offers a preliminary idea of what subjects he will be taking on. This book explores the idea of parallel universes, the array of different forms they might take, the wigginess of their implications (“this would blow Newton’s mind”), the wild extremes that can be extrapolated from such conjectures and the challenge of backing up theory with scientific proof. Yet his book’s first page promises that it will take “no expertise in physics or mathematics on the part of the reader” to keep up.

He lays more groundwork for the readability of “The Hidden Reality” when he puts certain groundbreaking, now basic ideas in their proper perspective. In 1919, when astronomical observations validated Einstein’s 1915 predictions about planetary motion, The New York Times ran an article with the headline “Lights All Askew in the Heavens, Men of Science More or Less Agog.” Now, Mr. Greene points out, nobody’s agog — and you’re apt to be walking around with a hand-held device that has a GPS, the accuracy of which can be traced to Einstein. Perhaps future generations will similarly take in stride the thought of parallel universes — and not just the kinds that are a mainstay of comic books and science fiction.

“The Hidden Reality” starts small (sort of) by raising the question of whether space is infinite or finite. Then it segues to the cosmological principle (“the assumed homogeneity of the cosmos”) and that principle’s implications for how a multiverse (a plural for “universe”) might be configured. A little further on in this same early chapter, Mr. Greene, who relies on earthly reference points like “South Park” and “The Honeymooners” to simplify difficult concepts, imagines a woman who has many shoes. Calling her Imelda, he uses her wardrobe permutations to make a more abstract point: “an infinite number of appearances with a finite number of outfits ensures infinite repetition.”

This chapter is called “Endless Doppelgängers.” It builds upon the idea that infinite variations of ourselves, our lives and our solar system are within the theorist’s realm of possibility. Got that? Mr. Greene then moves to “Eternity and Infinity,” and soon he is introducing both inflationary cosmology, which is one thing, and the inflaton (sic) field, which is quite another, in a book that desperately needs a glossary. (It doesn’t have one. And its illustrations aren’t as helpful as they might be.)

Then he sets forth colossal numbers that are “so extreme that they defy analogy.” But he offers an analogy anyway, and it’s the kind that make his book so startling: “They imply that a region of space the size of a pea would be stretched larger than the observable universe in a time interval so short that the blink of an eye would overestimate it by a factor larger than a million billion billion billion.”

By now, the 11-dimension string theory models of his earlier books (to which Mr. Greene helpfully steers the reader for background material) are looking downright commonsensical. “The Hidden Reality” moves on to increasingly speculative and exotic discussions of a bubble multiverse (“Think of the universe as a gigantic block of Swiss cheese. ...”) a holographic one, a brane-world scenario (courtesy of string theory), computer-driven simulations, questions of how probability relates to infinity, and the Many Worlds view of quantum mechanics. “A frequent criticism of the Many Worlds approach is that it’s just too baroque to be true,” Mr. Greene writes. Readers aren’t apt to disagree.

“My taste is for the expansive,” Mr. Greene writes late in the book. “But I draw the line at ideas that have no possibility of being confronted meaningfully by experiment or observation, not because of human frailty or technological hurdles but because of the proposals’ inherent nature.” Thus, the ninth and last multiverse under discussion (“the Ultimate Multiverse”) is seen as so far beyond the bounds of scientific proof that “The Hidden Reality” has reached its limit. That’s where it leaves Mr. Greene. Where will this book leave you?

Bottom line: It’s exciting and rewarding to read him even when the process is a struggle. This book is significantly more difficult than his earlier ones, but it still captures and engages the imagination. It can veer from eye-glazing passages to simple, crystal-clear thoughts (bubble universes appear finite from the inside but infinite from the outside; got it!), only to get lost again when stringy geometry and sticky branes enter the fray. It can make you wish Tom Lehrer were setting this stuff to music. And it can make you marvel at the thought of a parallel universe in which you read “The Hidden Reality” and every word makes perfect sense to you. That won’t happen in this one.

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