Intro to the cosmos

Preface

Space and time capture the imagination like no other scientific subject. For good reason. They form the arena of reality, the very fabric of the cosmos. Our entire existence-everything we do, think, and experience takes place in some region of space during some interval of time. Yet science is still struggling to understand what space and time actually are. Are they real physical entities or simply useful ideas? If they're real, are they fundamental, or do they emerge from more basic constituents? What does it mean for space to be empty? Does time have a beginning? Does it have an arrow, flowing inexorably from past to future, as common experience would indicate? Can we manipulate space and time? In this book, we follow three hundred years of passionate scientific investigation seeking answers, or at least glimpses of answers, to such basic but deep questions about the nature of the universe. Our journey also brings us repeatedly to another, tightly related question, as encompassing as it is elusive: What is reality ?We humans only have access to the internal experiences of perception and thought, so how can we be sure they truly reflect an external world? Philosophers have long recognized this problem. Filmmakers have popularized it through story lines involving artificial worlds, generated by finely tuned neurological stimulation that exist solely within the minds of their protagonists. And physicists such as myself are acutely aware that the reality we observe-matter evolving on the stage of space and time-may have little to do with the reality, if any, that's out there. Nevertheless, because observations are all we have, we take them seriously. We choose hard data and the framework of mathematics as our guides, not unrestrained imagination or unrelenting skepticism, and seek the simplest yet most wide-reaching theories capable of explaining and predicting the outcome of today's and future experiments. This severely restricts the theories we pursue. (In this book, for example, we won't find a hint that I'm floating in a tank, connected to thousands of brain-stimulating wires, making me merely think that I'm now writing this text.) But during the last hundred years, discoveries in physics have suggested revisions to our everyday sense of reality that are as dramatic, as mind-bending, and as paradigm-shaking as the most imaginative science fiction. These revolutionary upheavals will frame our passage through the pages that follow. Many of the questions we explore are the same ones that, in various guises, furrowed the brows of Aristotle, Galileo, Newton, Einstein, and countless others through the ages. And because this book seeks to convey science in the making, we follow these questions as they've been declared answered by one generation, overturned by their successors, and refined and reinterpreted b!; scientists in the centuries that followed. For example, on the perplexing question of whether completely empty space is, like a blank canvas, a real entity or merely an abstract idea, we follow the pendulum of scientific opinion as it swings between Isaac Newton's seventeenth-century declaration that space is real, Ernst Mach's conclusion in the nineteenth century that it isn't, and Einstein's 20^th century dramatic reformulation of the question itself, in which he merged space and time, and largely refuted Mach. We then encounter subsequent discoveries that transformed the question once again by redefining the meaning of "empty," envisioning that space is unavoidably suffused with what are called quantum fields and possibly a diffuse uniform energy called a cosmological constant-modern echoes of the old and discredited notion of a space-filling anther. What's more, we then describe how upcoming space-based experiments may confirm particular features of Mach's conclusions that happen to agree with Einstein's general relativity, illustrating well the fascinating and tangled web of scientific development. In our own era we encounter inflationary cosmology's gratifying insights into time's error, string theory's rich assortment of extra spatial dimensions, his-theory's radical suggestion that the space we inhabit may be but a sliver floating in a grander cosmos, and the current wild speculation that the universe we see may be nothing more than a cosmic hologram. We don't yet know if the more recent of these theoretical proposals are right. But outrageous as they sound, we take them seriously because they are where our dogged search for the deepest laws of the universe leads. Not only can a strange and unfamiliar reality arise from the fertile imagination of science fiction, but one may also emerge from the cutting-edge findings of modern physics. e Cosmos is intended primarily for the general reader who has little or no formal training in the sciences but whose desire to understand the workings of the universe provides incentive to grapple with a number of complex and challenging concepts. As in my first book, The Elegant Universe, I've stayed close to the core scientific ideas throughout, while stripping away the mathematical details in favor of metaphors, analogies, stories, and illustrations. When we reach the book's most difficult sections, I forewarn the reader and provide brief summaries for those who decide to skip or skim these more involved discussions. In this way, the reader should be able to walk the path of discovery and gain not just knowledge of physics' current worldview, but an understanding of how and why that worldview has gained prominence. Students, avid readers of general-level science, teachers, and professionals should also find much of interest in the book. Although the initial chapters cover the necessary but standard background material in relativity and quantum mechanics, the focus on the corporeality of space and time is somewhat unconventional in its approach. Subsequent chapters cover a wide range of topics-Bell's theorem, delayed choice experiments, quantum measurement, accelerated expansion, the possibly of producing black holes in the next generation of particle accelerators, fanciful wormhole time machines, to name a few-and so will bring such readers up to date on a number of the most tantalizing and debated advances. Some of the material I cover is controversial. For those issues that remain up in the air, I've discussed the leading viewpoints in the main text. For the points of contention that I feel have achieved more of a consensus, I've relegated differing viewpoints to the notes. Some scientists, especially those holding minority views, may take exception to some of my judgments, but through the main text and the notes, I've striven for a balanced treatment. In the notes, the particularly diligent reader will also find more complete explanations, clarifications, and caveats relevant to points I've simplified, as well as (for those so inclined) brief mathematical counterparts to the equation-free approach taken in the main text. A short glossary provides a reference for some of the more specialized scientific terms. Even a book of this length can't exhaust the vast subject of space and time. I've focused on those features I find both exciting and essential to forming a full picture of the reality painted by modern science. No doubt, many of these choices reflect personal taste, and so I apologize to those who feel their own work or favorite area of study is not given adequate attention.