At the intersection of physics and philosophy of science, this book outlines the philosophical challenge to theoretical physics in a measured, well-grounded manner. Cosmologists, high energy physicists, and philosophers including graduate students and researchers will find a systematic exploration of such questions in this important book.

The structure of the multiverse, the nature of each universe within it and the relationships among the various constituent universes, depend on the specific multiverse hypothesis considered. Multiple universes have been hypothesized in cosmology, physics, astronomy, religion, philosophy, transpersonal psychology, and fiction, particularly in science fiction and fantasy. In these contexts, parallel universes are also called "alternate universes," "quantum universes," "interpenetrating dimensions," "parallel dimensions," "parallel worlds," "alternate realities," "alternate timelines," and "dimensional planes," among other names. The physics community continues to fiercely debate the multiverse hypothesis. Prominent physicists disagree about whether the multiverse may exist, and whether it is even a legitimate topic of scientific inquiry. Serious concerns have been raised about whether attempts to exempt the multiverse from experimental verification may erode public confidence in science and ultimately damage the nature of fundamental physics. Some have argued that the multiverse question is philosophical rather than scientific because it lacks falsifiability; the ability to disprove a theory by means of scientific experiment has always been part of the accepted scientific method. Paul Steinhardt has famously argued that no experiment can rule out a theory if it provides for all possible outcomes. This book discusses the numerous concepts and theories concerning multiple universes.

If the laws of nature are fine-tuned for life, can we infer other universes with different laws? How could we even test such a theory without empirical access to those distant places? Can we believe in the multiverse of the Everett interpretation of quantum theory or in the reality of other possible worlds, as advocated by philosopher David Lewis? At the intersection of physics and philosophy of science, this book outlines the philosophical challenge to theoretical physics in a measured, well-grounded manner. The origin of multiverse theories are explored within the context of the fine-tuning problem and a systematic comparison between the various different multiverse models are included. Cosmologists, high energy physicists, and philosophers including graduate students and researchers will find a systematic exploration of such questions in this important book.

The Emergent Multiverse presents a striking new account of the 'many worlds' approach to quantum theory. The point of science, it is generally accepted, is to tell us how the world works and what it is like. But quantum theory seems to fail to do this: taken literally as a theory of the world, it seems to make crazy claims: particles are in two places at once; cats are alive and dead at the same time. So physicists and philosophers have often been led either to give up on the idea that quantum theory describes reality, or to modify or augment the theory. The Everett interpretation of quantum mechanics takes the apparent craziness seriously, and asks, 'what would it be like if particles really were in two places at once, if cats really were alive and dead at the same time'? The answer, it turns out, is that if the world were like that—if it were as quantum theory claims—it would be a world that, at the macroscopic level, was constantly branching into copies—hence the more sensationalist name for the Everett interpretation, the 'many worlds theory'. But really, the interpretation is not sensationalist at all: it simply takes quantum theory seriously, literally, as a description of the world. Once dismissed as absurd, it is now accepted by many physicists as the best way to make coherent sense of quantum theory. David Wallace offers a clear and up-to-date survey of work on the Everett interpretation in physics and in philosophy of science, and at the same time provides a self-contained and thoroughly modern account of it—an account which is accessible to readers who have previously studied quantum theory at undergraduate level, and which will shape the future direction of research by leading experts in the field.

Physicists argue from different perspectives for and against the idea of the existence of multiple universes.

INSTANT NEW YORK TIMES BESTSELLER “Most appealing... technical accuracy and lightness of tone... Impeccable.”—Wall Street Journal “A porthole into another world.”—Scientific American “Brings science dissemination to a new level.”—Science The most trusted explainer of the most mind-boggling concepts pulls back the veil of mystery that has too long cloaked the most valuable building blocks of modern science. Sean Carroll, with his genius for making complex notions entertaining, presents in his uniquely lucid voice the fundamental ideas informing the modern physics of reality. Physics offers deep insights into the workings of the universe but those insights come in the form of equations that often look like gobbledygook. Sean Carroll shows that they are really like meaningful poems that can help us fly over sierras to discover a miraculous multidimensional landscape alive with radiant giants, warped space-time, and bewilderingly powerful forces. High school calculus is itself a centuries-old marvel as worthy of our gaze as the Mona Lisa. And it may come as a surprise the extent to which all our most cutting-edge ideas about black holes are built on the math calculus enables. No one else could so smoothly guide readers toward grasping the very equation Einstein used to describe his theory of general relativity. In the tradition of the legendary Richard Feynman lectures presented sixty years ago, this book is an inspiring, dazzling introduction to a way of seeing that will resonate across cultural and generational boundaries for many years to come.

There was a time when 'universe' meant all there is. Everything. Yet, as Brian Greene's extraordinary book shows, ours may be just one universe among many, like endless reflections in a mirror. He takes us on a captivating exploration of parallel worlds - from a multiverse where an infinite number of your doppelg ngers are reading this sentence, to vast oceans of bubble universes and even multiverses made of mathematics - showing just how much of reality's true nature may be hidden within them.

A novel interpretation of quantum mechanics, first proposed in brief form by Hugh Everett in 1957, forms the nucleus around which this book has developed. In his interpretation, Dr. Everett denies the existence of a separate classical realm and asserts the propriety of considering a state vector for the whole universe. Because this state vector never collapses, reality as a whole is rigorously deterministic. This reality, which is described jointly by the dynamical variables and the state vector, is not the reality customarily perceived; rather, it is a reality composed of many worlds. By virtue of the temporal development of the dynamical variables, the state vector decomposes naturally into orthogonal vectors, reflecting a continual splitting of the universe into a multitude of mutually unobservable but equally real worlds, in each of which every good measurement has yielded a definite result, and in most of which the familiar statistical quantum laws hold. The volume contains Dr. Everett's short paper from 1957, "'Relative State' Formulation of Quantum Mechanics," and a far longer exposition of his interpretation, entitled "The Theory of the Universal Wave Function," never before published. In addition, other papers by Wheeler, DeWitt, Graham, and Cooper and Van Vechten provide further discussion of the same theme. Together, they constitute virtually the entire world output of scholarly commentary on the Everett interpretation. Originally published in 1973. The Princeton Legacy Library uses the latest print-on-demand technology to again make available previously out-of-print books from the distinguished backlist of Princeton University Press. These editions preserve the original texts of these important books while presenting them in durable paperback and hardcover editions. The goal of the Princeton Legacy Library is to vastly increase access to the rich scholarly heritage found in the thousands of books published by Princeton University Press since its founding in 1905.

The award-winning former editor of Science News shows that one of the most fascinating and controversial ideas in contemporary cosmology—the existence of multiple parallel universes—has a long and divisive history that continues to this day. We often consider the universe to encompass everything that exists, but some scientists have come to believe that the vast, expanding universe we inhabit may be just one of many. The totality of those parallel universes, still for some the stuff of science fiction, has come to be known as the multiverse. The concept of the multiverse, exotic as it may be, isn’t actually new. In The Number of the Heavens, veteran science journalist Tom Siegfried traces the history of this controversial idea from antiquity to the present. Ancient Greek philosophers first raised the possibility of multiple universes, but Aristotle insisted on one and only one cosmos. Then in 1277 the bishop of Paris declared it heresy to teach that God could not create as many universes as he pleased, unleashing fervent philosophical debate about whether there might exist a “plurality of worlds.” As the Middle Ages gave way to the Renaissance, the philosophical debates became more scientific. René Descartes declared “the number of the heavens” to be indefinitely large, and as notions of the known universe expanded from our solar system to our galaxy, the debate about its multiplicity was repeatedly recast. In the 1980s, new theories about the big bang reignited interest in the multiverse. Today the controversy continues, as cosmologists and physicists explore the possibility of many big bangs, extra dimensions of space, and a set of branching, parallel universes. This engrossing story offers deep lessons about the nature of science and the quest to understand the universe.

Multiverse theory posits the existence of multiple, possibly infinite, universes that exist parallel to our own, collectively forming what is known as the "multiverse." The origins of this theory are rooted in quantum mechanics and cosmology. In 1957, physicist Hugh Everett III proposed the "Many-Worlds Interpretation" of quantum mechanics, suggesting that all possible outcomes of quantum measurements are realized in some "world" or universe, leading to a vast number of parallel universes. In cosmology, theoretical models involving inflation (a rapid expansion of the universe after the Big Bang) suggest that different regions of space could stop inflating at different times, creating separate "bubble universes" with potentially different physical properties. There are various types of multiverses proposed by scientists. A Level I multiverse includes regions beyond our observable universe, where regions sufficiently far away might have different physical conditions and histories. A Level II multiverse consists of universes with different physical constants and laws of physics, arising from different inflationary conditions. The Many-Worlds Interpretation of quantum mechanics is considered a Level III multiverse, where every quantum event branches into new universes. A Level IV multiverse encompasses universes with fundamentally different mathematical structures, including all possible mathematical realities.