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The bestselling author of The Elegant Universe and The Fabric of the Cosmos tackles perhaps the most mind-bending question in modern physics and cosmology: Is our universe the only universe?

There was a time when "universe" meant all there is. Everything. Yet, a number of theories are converging on the possibility that our universe may be but one among many parallel universes populating a vast multiverse. Here, Briane Greene, one of our foremost physicists and science writers, takes us on a breathtaking journey to a multiverse comprising an endless series of big bangs, a multiverse with duplicates of every one of us, a multiverse populated by vast sheets of spacetime, a multiverse in which all we consider real are holographic illusions, and even a multiverse made purely of math--and reveals the reality hidden within each.

Using his trademark wit and precision, Greene presents a thrilling survey of cutting-edge physics and confronts the inevitable question: How can fundamental science progress if great swaths of reality lie beyond our reach? The Hidden Reality is a remarkable adventure through a world more vast and strange than anything we could have imagined.

Chapter Page
Preface vii
1. The Bounds of Reality: On Parallel Worlds 3
2. Endless Doppelgängers: The Quilted Multiverse 11
3. Eternity and Infinity: The Inflationary Multiverse 43
4. Unifying Nature's Laws: On the Road to String Theory 83
5. Hovering Universes in Nearby Dimensions: The Brane and Cyclic Multiverses 119
6. New Thinking About an Old Constant: The Landscape Multiverse 146
7. Science and the Multiverse: On Inference, Explanation, and Prediction 188
8. The Many Worlds of Quantum Measurement: The Quantum Multiverse 217
9. Black Holes and Holograms: The Holographic Multiverse 272
10. Universes, Computers, and Mathematical Reality: The Simulated and Ultimate Multiverses 314
11. The Limits of Inquiry: Multiverses and the Future 353
Notes 371
Suggestions for Further Reading 417
Index 421

Chapter 1

The Bounds of Reality

On Parallel Worlds

If, when I was growing up, my room had been adorned with only a single mirror, my childhood daydreams might have been very different. But it had two. And each morning when I opened the closet to get my clothes, the one built into its door aligned with the one on the wall, creating a seemingly endless series of reflections of anything situated between them. It was mesmerizing. I delighted in seeing image after image populating the parallel glass planes, extending back as far as the eye could discern. All the reflections seemed to move in unison?but that, I knew, was a mere limitation of human perception; at a young age I had learned of light’s finite speed. So in my mind’s eye, I would watch the light’s round-trip journeys. The bob of my head, the sweep of my arm silently echoed between the mirrors, each reflected image nudging the next. Sometimes I would imagine an irreverent me way down the line who refused to fall into place, disrupting the steady progression and creating a new reality that informed the ones that followed. During lulls at school, I would sometimes think about the light I had shed that morning, still endlessly bouncing between the mirrors, and I’d join one of my reflected selves, entering an imaginary parallel world constructed of light and driven by fantasy. It was a safe way to break the rules.

To be sure, reflected images don’t have minds of their own. But these youthful flights of fancy, with their imagined parallel realities, resonate with an increasingly prominent theme in modern science?the possibility of worlds lying beyond the one we know. This book is an exploration of such possibilities, a considered journey through the science of parallel universes.

Universe and Universes

There was a time when “universe” meant “all there is.” Everything. The whole shebang. The notion of more than one universe, more than one everything, would seemingly be a contradiction in terms. Yet a range of theoretical developments has gradually qualified the interpretation of “universe.” To a physicist, the word’s meaning now largely depends on context. Sometimes “universe” still connotes absolutely everything. Sometimes it refers only to those parts of everything that someone such as you or I could, in principle, have access to. Sometimes it’s applied to separate realms, ones that are partly or fully, temporarily or permanently, inaccessible to us; in this sense, the word relegates ours to membership in a large, perhaps infinitely large, collection.

With its hegemony diminished, “universe” has given way to other terms introduced to capture the wider canvas on which the totality of reality may be painted. Parallel worlds or parallel universes or multiple universes or alternate universes or the metaverse, megaverse, or multiverse?they’re all synonymous and they’re all among the words used to embrace not just our universe but a spectrum of others that may be out there.

You’ll notice that the terms are somewhat vague. What exactly constitutes a world or a universe? What criteria distinguish realms that are distinct parts of a single universe from those classified as universes of their own? Perhaps someday our understanding of multiple universes will mature sufficiently for us to have precise answers to these questions. For now, we’ll use the approach famously applied by Justice Potter Stewart in attempting to define pornography. While the U.S. Supreme Court wrestled mightily to delineate a standard, Stewart declared simply and forthrightly, “I know it when I see it.”

In the end, labeling one realm or another a parallel universe is merely a question of language. What matters, what’s at the heart of the subject, is whether there exist realms that challenge convention by suggesting that what we’ve long thought to be the universe is only one component of a far grander, perhaps far stranger, and mostly hidden reality.

During the last half century, science has provided ample ways in which this possibility might be realized.

Parallel Universes and the Deep Laws of the Cosmos

The bestselling author of The Elegant Universe and The Fabric of the Cosmos tackles perhaps the most mind-bending question in modern physics and cosmology: Is our universe the only universe?

There was a time when "universe" meant all there is. Everything. Yet, a number of theories are converging on the possibility that our universe may be but one among many parallel universes populating a vast multiverse. Here, Briane Greene, one of our foremost physicists and science writers, takes us on a breathtaking journey to a multiverse comprising an endless series of big bangs, a multiverse with duplicates of every one of us, a multiverse populated by vast sheets of spacetime, a multiverse in which all we consider real are holographic illusions, and even a multiverse made purely of math--and reveals the reality hidden within each.

Using his trademark wit and precision, Greene presents a thrilling survey of cutting-edge physics and confronts the inevitable question: How can fundamental science progress if great swaths of reality lie beyond our reach? The Hidden Reality is a remarkable adventure through a world more vast and strange than anything we could have imagined.