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

Recent developments in cosmology and particle physics, such as the string landscape picture, have led to the remarkable realization that our universe - rather than being unique - could be just one of many universes. The multiverse proposal helps to explain the origin of the universe and some of its observational features. Since the physical constants can be different in other universes, the fine-tunings which appear necessary for the emergence of life may also be explained. Nevertheless, many physicists remain uncomfortable with the multiverse proposal, since it is highly speculative and perhaps untestable. In this volume, a number of active and eminent researchers in the field - mainly cosmologists and particle physicists but also some philosophers - address these issues and describe recent developments. The articles represent the full spectrum of views, providing for the first time an overview of the subject. They are written at different academic levels, engaging lay-readers and researchers alike.

NATIONAL BESTSELLER • From one of the world’s leading physicists and author of the Pulitzer Prize finalist The Elegant Universe, comes “an astonishing ride” through the universe (The New York Times) that makes us look at reality in a completely different way. Space and time form the very fabric of the cosmos. Yet they remain among the most mysterious of concepts. Is space an entity? Why does time have a direction? Could the universe exist without space and time? Can we travel to the past? Greene has set himself a daunting task: to explain non-intuitive, mathematical concepts like String Theory, the Heisenberg Uncertainty Principle, and Inflationary Cosmology with analogies drawn from common experience. From Newton’s unchanging realm in which space and time are absolute, to Einstein’s fluid conception of spacetime, to quantum mechanics’ entangled arena where vastly distant objects can instantaneously coordinate their behavior, Greene takes us all, regardless of our scientific backgrounds, on an irresistible and revelatory journey to the new layers of reality that modern physics has discovered lying just beneath the surface of our everyday world.

“Multiverse” cosmologies imagine our universe as just one of a vast number of others. While this idea has captivated philosophy, religion, and literature for millennia, it is now being considered as a scientific hypothesis—with different models emerging from cosmology, quantum mechanics, and string theory. Beginning with ancient Atomist and Stoic philosophies, Mary-Jane Rubenstein links contemporary models of the multiverse to their forerunners and explores their current emergence. One reason is the so-called fine-tuning of the universe: nature’s constants are so delicately calibrated, it seems they have been set just right to allow life to emerge. For some theologians, these “fine-tunings” are proof of God; for others, “God” is an insufficient explanation. One compelling solution: if all possible worlds exist somewhere, then it is no surprise one of them happens to be suitable for life. Yet this hypothesis replaces God with an equally baffling article of faith: the existence of universes beyond, before, or after our own, eternally generated yet forever inaccessible. In sidestepping metaphysics, multiverse scenarios collide with it, producing their own counter-theological narratives. Rubenstein argues, however, that this interdisciplinary collision provides the condition of its scientific viability, reconfiguring the boundaries among physics, philosophy, and religion.

One of the most controversial, cutting-edge ideas in cosmology—the possibility that there exist multiple parallel universes—in fact has a long history. Tom Siegfried reminds us that the size and number of the heavens have been contested since ancient times. His story offers deep lessons about the nature of science and the quest for understanding.

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.

Max Tegmark leads us on an astonishing journey through past, present and future, and through the physics, astronomy and mathematics that are the foundation of his work, most particularly his hypothesis that our physical reality is a mathematical structure and his theory of the ultimate multiverse. In a dazzling combination of both popular and groundbreaking science, he not only helps us grasp his often mind-boggling theories, but he also shares with us some of the often surprising triumphs and disappointments that have shaped his life as a scientist. Fascinating from first to last—this is a book that has already prompted the attention and admiration of some of the most prominent scientists and mathematicians.