Greene has a way about him that is at once knowledgeable and endearing; he starts his description of parallel universes with a story from his childhood. It isn't forced or unnatural. He wants you to understand.
Though Greene has substantially more space to take tangents (141 more pages than Hawking, though I can't attest to the font sizes or line spacing being the same...), he stays on topic. No random digressions into Newton's church attendance or arguments against philosophy or religion (he does tell some funny side-stories, including one about Gamow's notorious sense of humor, but they are all couched in the point at hand: he introduces a person before introducing that person's key insight into the scientific theory being described), no vehement arguments over free will or aversion to individual scientists due to their beliefs (in fact, Greene even describes some groundbreaking work by Weinberg, among others, who used as his motivation the so-called "anthropic principle"). Greene uses amusing anecdotes, relevant references to popular culture (Cartman makes what may be his scientific debut), and his own insight from years of personal experience to describe what would otherwise be completely inaccessible to anyone without a graduate degree in mean field theory. And despite being more in-depth than the recent Hawking publication, he doesn't mince words or add fluff. There's nothing wasted in that extra volume.
Nine versions of possible multiverses (multiple/parallel universes) are presented, each with appropriate historical and scientific background, as well as - gasp! - possible scientific experiments which could test for each of them (not much can be done with the current level of technology, but the future is bright and the possibilities copious). In Greene's terminology, they are: the Quilted Multiverse, Inflationary Multiverse, Brane Multiverse, Cyclic Multiverse, Landscape Multiverse, Quantum Multiverse, Holographic Multiverse, Simulated Multiverse and Ultimate Multiverse (parts a and b). Briefly - and really, you should read the book - they can be explained as follows:
- Quilted Multiverse: if the universe is infinite (in space), conditions will necessarily repeat across space, which yields an infinite number of parallel worlds. We wouldn't "see" these parallel universes-within-the-universe, because they'd fall outside of our cosmic horizon (the distance limit we're able to observe due to the finite speed of light). This multiverse proposal arises mainly from general relativity, which doesn't require that the universe be finite.
- Inflationary (or Bubble) Multiverse: with the advent of inflationary theory, with its "inflaton" field (it can essentially be thought of as an antigravity field), to describe the beginnings of our own universe, an interesting tangent arose. The inflaton field could yield different areas of inflation (due to quantum fluctuations; this is how Cartman got involved, believe it or not), each forming a "bubble" within which a universe would form.
- Brane Multiverse: our first true string theory contender, "branes" are higher-dimensional objects within the string theory framework. A universe like ours could exist on a three-dimensional brane (for reasons he explains, the universe is essentially "trapped" on the brane), but more branes could exist elsewhere in the multi-dimensional space which carried their own universes.
- Cyclic Multiverse: the "braneworlds" from the Brane Multiverse, since they can move, have the potential to collide within the larger-dimensional space. If and when they do, the universes on each are completely annihilated and begin anew. Since branes are gravitationally attracted like any massive object, they could be pulled in and collide over and over, creating many new universes.
- Landscape Multiverse: delving deeper into string theory and adding in inflationary theory, we find that the multiple dimensions allowed by string theory subsequently allow an entire N-dimensional "landscape" upon which an infinite number of inflationary bubble universes could form.
- Quantum Multiverse: this may be the most familiar of the list. It refers to the Many Worlds interpretation of quantum mechanics, which flew in the face of the historical Copenhagen interpretation. In essence, each quantum possibility (is the electron here or there?) is actually manifested in a parallel universe (in our universe, it's here, but simultaneously in a parallel universe, it's there).
- Holographic Multiverse: strange, cutting edge science from string theory, quantum field theory and black holes suggests that our universe may just be a mirrored or projected "image" from a distant, differently-dimensioned (2D instead of 3D?) but physically equivalent parallel universe.
- Simulated Multiverse: the most banal but perhaps most immediately relevant, technological growth may make it possible for entire, self-consistent, self-aware and self-propagating simulated universes to be created.
- Ultimate Multiverse: Part (a), if you will, is actually philosophical - the "principle of fecundity" asserts that every possible universe is a real universe (even the ones made entirely of gorgonzola or the ones made of nothing), thereby nullifying the question of why our particular universe is the one which exists. Part (b) trims this slightly by introducing a level of self-consistency, proposing that every possible mathematically consistent universe is a universe, or, more correctly, that "these universes instantiate all possible mathematical equations."
"The breadth of multiverse proposals... might suggest a panorama of hidden realities [essentially, what Hawking/Mlodinow were suggesting]. But I've titled this book in the singular [The Hidden Reality] to reflect the unique and uniquely powerful theme that underlies them all: the capacity of mathematics to reveal secreted truths about the workings of the world. Centuries of discovery have made this abundantly evident; monumental upheavals in physics have emerged time and again from vigorously following mathematics' lead.... [The] multiverse proposals similarly rely on a belief that mathematics is tightly stitched into the fabric of reality."However, he continues:
"Our universe is not the only one possible [if these theories are true]. Its properties could have been different.... In turn, seeking a fundamental explanation for why certain things are the way they are would be pointless....Lastly, on a bit of a schadenfreude note, I wished to make a point about philosophy. Unlike Hawking and Mlodinow, who seem to believe that philosophy is dead, Greene takes a far more reasonable standpoint. During a philosophy course as a freshman at Harvard, he was challenged by his professor: "Let's say you find the unified theory," he said. "Would that really provide the answers you're looking for?" The answer is, of course, no. The philosophy professor was right, and Greene knew it. It was this encounter which prompted Greene to consider the concept of the Ultimate Multiverse - proving, it would seem, that instead of philosophy being dead, it is alive and well and helping to push forward the frontiers of science.
I don't know if this is how things will turn out. No one does. But it's only through fearless engagement that we can learn our own limits. It's only through the rational pursuit of theories, even those that whisk us into strange and unfamiliar domains, that we stand a chance of revealing the expanse of reality."
And one last poke, at those Dawkins-ites:
"I understand well the impulse to tether scientific investigations to those propositions that can be tested now, or in the near future; this is, after all, how we built the scientific edifice. But I find it parochial to bound our thinking by the arbitrary limits imposed by where we are, when we are, and who we are. Reality transcends these limits, so it's to be expected that sooner or later the search for deep truths will too.... Sometimes [science] challenges us to reexamine our views of science itself."