Warped Passages - Lisa Randall [1]
This book doesn’t skimp on the most advanced and intriguing theoretical ideas, but I’ve tried my best to make it self-contained. I’ve included both key conceptual advances and the physical phenomena to which they apply. The chapters are organized so that readers can tailor the book to their own backgrounds and interests. To help this process, I’ve bulleted the points that I’ll refer to later on when I present more recent ideas about extra dimensions. I’ve also used bullets at the end of the extra-dimensional chapters to clarify what distinguishes each of the possible options for extra-dimensional universes.
Because the idea of extra dimensions is probably new to many readers, in the first few chapters I’ve explained what I mean when I use these words and why extra dimensions can exist but be invisible and intangible. After that, I’ve outlined the theoretical methods with which particle physicists approach their work to clarify the kind of thinking that enters into this admittedly very speculative research.
The recent work on extra dimensions relies on both more traditional and more modern theoretical physics concepts to motivate the questions it answers and its methods. In order to explain what is driving such research, I’ve included an extensive review of twentieth-century physics. Feel free to skim through this review if you like. But if you do, you’ll miss a lot of good stuff!
The review begins with general relativity and quantum mechanics before turning to particle physics and the most important concepts that particle physicists employ today. I’ve presented some rather abstract ideas that are often neglected—in part because they are so abstract—but these concepts are now confirmed by experiment and enter into all research that we do today. Although not all of this material is essential for understanding the ideas you’ll see later on about extra dimensions, I believe many readers will be glad to get a more complete picture.
After this, I’ve described some newer, more speculative notions that have been studied for the last thirty years—namely supersymmetry and string theory. Traditionally, physics has involved an interplay between theory and experiment. Supersymmetry is an extension of known particle physics concepts and has a good chance of being tested in forthcoming experiments. String theory is different. It is based solely on theoretical questions and ideas and isn’t even completely mathematically formulated yet, so we can’t yet be certain of its predictions. As for me, I’m an agnostic on this subject—I don’t know what string theory will ultimately be or whether it will solve the questions of quantum mechanics and gravity it sets out to address. But string theory has been a rich resource for new ideas, some of which I’ve exploited in my own research on extra dimensions of space. These ideas exist independently of string theory, but string theory gives us a good reason to think some of their underlying assumptions could be right.
Having established the context, I’ll finally return to the many exciting new developments about extra dimensions. They tell us remarkable things, such as that extra dimensions can be infinite in size yet remain unseen, or that we can be living in a three-spatial-dimensional sinkhole in a higher-dimensional universe. We now also know reasons why there can be unseen parallel worlds with very different properties from our own.
Throughout the text, I’ve explained physics concepts without equations. But for those who are interested in more mathematical detail, I’ve included a mathematical appendix. In the text itself, I’ve tried to expand the range