I have to admit that I had to struggle to get through this book. The theme of the book was exciting, but the writing was dull and wandered off into corners of such extreme "science" that it didn't look like science to me so my attention drifted and I had to work hard to keep up the level of attention necessary to plow through the book.
The book looks at 7 theories of "parallel" universes and 1 theory of off-the-wall "we all live in a simulated universe". That last one didn't belong in the book and was the capstone sealing the fate of this book in my mind: crap. The other seven theories were mildly interesting but struck me as a waste of brilliant minds:
- The Quilted Multiverse - the idea that we can only see part of the universe and if it is infinite then there can be infinite patches like ours that "quilt" the infinite fabric of the universe. And that in each of these the physical constants can be set slightly differently so you get an infinite variety. In fact, you can get an infinite number of patches are are exact mimics of the patch we inhabit.
- The Inflationary Universe - uses the idea of inflation to talk about how new universes can pop into existence as the inflationary universe continues its infinite expansion.
- Brane Universe - the idea that string theory requires a 10 dimension (or 11 dimension, depends on how you count them) universe in which we live stuck on a 3D "brane". There can exist many branes so there are multiple parallel universes.
- Cyclic Multiverse - uses the string theory multi-dimensional brane world as a way to allow branes to bang up against another branes. Each "bang" is in fact a "big bang" that generates a universe. This allows an eternal cycle of big bang then big collapse then big bang then big collapse ad infinitum without breaking the 2nd law of thermodynamics that requires entropy to increase. The entropy gets spread over the other dimensions.
- Landscape Multiverse - uses the fact that string theory allows 10^500 power different possible universes allowed by the underlying Calabi-Yau curled up dimensions of string pace, and you let the hypothesized "inflaton" field of the inflationary universe create all the "bubble universes" which give you every possible 10^500 different string-based universes replicated an infinite number of times.
- Quantum Multiverse - this takes the crazy "many worlds" interpretation of Hugh Everett III and populates an infinity of infinite universes that are hived off each time a quantum probability wave "decoheres" and collapses into the actual measured result in a world. Since the wave can decohere is many, many ways, then at each instant all over our current universe we are hiving off practically infinite numbers of new univereses where the decoherence gave a different result. Crazy stuff because it takes puzzle in our world and propagates an unimaginable number of multiple universes as "the solution" to this problem. Talk about rabbits out of a hat!
- Holographic Multiverse - takes some physics developed by Leonard Susskind that finds a duality between 3D objects within a universe to 2D objects on the event horizon of a something like a black hole. Mix this with string theory and you can speculate about bubble universes where their 3D innards are "determined" by 2D entities on the skin of the bubble universe.
Most of these theories in Greene's book are about stuff that -- by assumption -- are things we can never see. That strikes me as nonsense. It isn't physics. The only place where he can get my interest up is when he claims that the proposed universe beyond our own might have experimental and measurable impacts on our universe. That I can accept as potential science, but even here he ends up talking about "possibilities" and nothing tangible.
So I'm left wondering... why waste all that brilliance chasing fairy dust? Is it really the case that there are no interesting physical phenomena in the world we actually inhabit? I've been waiting for 30 years for a competent theory of superconductors. Nothing. Instead a bevy of high-powered physicists have wasted thousands of man-years on string theory. Understanding dark matter and dark energy which exist in our universe strikes me as important, but all this effort is diverted into chasing ideas that probably will never have any impact on real science. This book, and all this work on string theory, strikes me as the same tragedy as the last 30 years of Einstein's life: he chased dreams rather than do solid physics. It was a terrible waste of an excellent mind.
I don't recommend this book. Too much wild-eyed speculation. Far, far too little real physics. I've read Brian Greene's two previous books. They are fine. They are readable and they include real science. But neither is a "stand out" book that excites the reader. I do admire scientists who take the time to write for the public, but only a few have a real feel for the public and are able to write a book that sets a non-scientist's mind ablaze with possibilities and a glimpse into the real science. Partly this is because science is so highly technical these days. But also because it takes a peculiar writing talent to capture a reader's imagination. Brian Green has created "workmanlike" books in the past but nothing that blazed with brilliance. And, sadly, this latest book is quite a dud.
Update 2011mar04: Here is an audio of an interview of Brian Greene by NPR's Ira Flatow.