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. Varieties of Parallel Universes A striking fact (it''s in part what propelled me to write this book) is that many of the major developments in fundamental theoretical physics-- relativistic physics, quantum physics, cosmological physics, unified physics, computational physics--have led us to consider one or another variety of parallel universe. Indeed, the chapters that follow trace a narrative arc through nine variations on the multiverse theme. Each envisions our universe as part of an unexpectedly larger whole, but the complexion of that whole and the nature of the member universes differ sharply among them. In some, the parallel universes are separated from us by enormous stretches of space or time; in others, they''re hovering millimeters away; in others still, the very notion of their location proves parochial, devoid of meaning. A similar range of possibility is manifest in the laws governing the parallel universes. In some, the laws are the same as in ours; in others, they appear different but have shared a heritage; in others still, the laws are of a form and structure unlike anything we''ve ever encountered.

It''s at once humbling and stirring to imagine just how expansive reality may be. Some of the earliest scientific forays into parallel worlds were initiated in the 1950s by researchers puzzling over aspects of quantum mechanics, a theory developed to explain phenomena taking place in the microscopic realm of atoms and subatomic particles. Quantum mechanics broke the mold of the previous framework, classical mechanics, by establishing that the predictions of science are necessarily probabilistic. We can predict the odds of attaining one outcome, we can predict the odds of another, but we generally can''t predict which will actually happen. This well-known departure from hundreds of years of scientific thought is surprising enough. But there''s a more confounding aspect of quantum theory that receives less attention. After decades of closely studying quantum mechanics, and after having accumulated a wealth of data confirming its probabilistic predictions, no one has been able to explain why only one of the many possible outcomes in any given situation actually happens. When we do experiments, when we examine the world, we all agree that we encounter a single definite reality.

Yet, more than a century after the quantum revolution began, there is no consensus among the world''s physicists as to how this basic fact is compatible with the theory''s mathematical expression. Over the years, this substantial gap in understanding has inspired many creative proposals, but the most startling was among the first. Maybe, that early suggestion went, the familiar notion that any given experiment has one and only one outcome is flawed. The mathematics underlying quantum mechanics--or at least, one perspective on the math-- suggests that all possible outcomes happen, each inhabiting its own separate universe. If a quantum calculation predicts that a particle might be here, or it might be there, then in one universe it is here, and in another it is there. And in each such universe, there''s a copy of you witnessing one or the other outcome, thinking--incorrectly--that your reality is the only reality. When you realize that quantum mechanics underlies all physical processes, from the fusing of atoms in the sun to the neural firing that constitutes the stuff of thought, the far-reaching implications of the proposal become apparent. It says that there''s no such thing as a road untraveled.

Yet each such road-- each reality--is hidden from all others. This tantalizing Many Worlds approach to quantum mechanics has attracted much interest in recent decades. But investigations have shown that it''s a subtle and thorny framework (as we will discuss in Chapter 8); so, even today, after more than half a century of vetting, the proposal remains controversial. Some quantum practitioners argue that it has already been proven correct, while others claim just as assuredly that the mathematical underpinnings don''t hold together. What is beyond doubt is that this early version of parallel universes resonated with themes of separate lands or alternative histories that were being explored in literature, television, and film, creative forays that continue today. (My favorites since childhood include The Wizard of Oz, It''s a Wonderful Life, the Star Trek episode "The City on the Edge of Forever," and, more recently, Sliding Doors and Run Lola Run ). Collectively, these and many other works of popular culture have helped integrate the concept of parallel realities into the zeitgeist and are responsible for fueling much public fascination with the topic. But the mathematics of quantum mechanics is only one of numerous ways that a conception of parallel universes emerges from modern physics.

In fact, it won''t be the first I''ll discuss. Instead, in Chapter 2, I''ll begin with a different route to parallel universes, perhaps the simplest route of all. We''ll see that if space extends infinitely far--a proposition that is consistent with all observations and that is part of the cosmological model favored by many physicists and astronomers--then there must be realms out there (likely way out there) where copies of you and me and everything else are enjoying alternate versions of the reality we experience here. Chapter 3 will journey deeper into cosmology: the inflationary theory, an approach that posits an enormous burst of superfast spatial expansion during the universe''s earliest moments, generates its own version of parallel worlds. If inflation is correct, as the most refined astronomical o.