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Introduction

Why are scientists so excited about string theory? Because string theory is the most likely candidate for a successful theory of quantum gravity — a theory that scientists hope will unite two major physical laws of the universe into one. Right now, these laws (quantum physics and general relativity) describe two totally different types of behavior in totally different ways, and in the realm where neither theory works completely, we really don’t know what’s going on!

Understanding the implications of string theory means understanding profound aspects of our reality at the most fundamental levels. Is there only one law of nature or infinitely many? Why does our universe follow the laws it does? Is time travel possible? How many dimensions does our universe possess? Physicists are passionately seeking answers to these questions.

Indeed, string theory is a fascinating topic, a scientific revolution that promises to transform our understanding of the universe. As you’ll see, these types of revolutions have happened before, and this book helps you understand how physics has developed in the past, as well as how it may develop in the future.

This book contains some ideas that will probably, in the coming years, turn out to be completely false. (We can guarantee this was true about the first edition, and we have no reason to expect it won’t also be true of this second edition.) It contains other ideas that may ultimately prove to be fundamental laws of our universe, perhaps forming the foundation for entirely new fields of science and technology. No one knows what the future holds for string theory.

About This Book

In this book, we aim to give a clear understanding of the ever-evolving scientific subfield known as string theory. The media is abuzz with talk about this “theory of everything,” and when you’re done with this book, you should know what they’re talking about (probably better than they do, most of the time).

In writing this book, we’ve attempted to serve several masters. First and foremost among them has been scientific accuracy, followed closely by entertainment value. Along the way, we’ve also done our best to use language that you can understand no matter your scientific background, and we’ve certainly tried to keep any mathematics to a minimum.

We set out to achieve the following goals with this book:

 Provide the information needed to understand string theory (including established physics concepts that predate string theory).

 Establish the successes of string theory so far.

 Lay out the avenues of study that are attempting to gain more evidence for string theory.

 Explore the bizarre (and speculative) implications of string theory.

 Present the critical viewpoints in opposition to string theory, as well as some alternatives that may bear fruit if string theory proves to be false.

 Have some fun along the way.

 Avoid mathematics at all costs. (You’re welcome!)

We hope you, good reader, find that we’ve been successful at meeting these goals.

And while time may flow in only one direction (Or does it? We explore this in Chapter 17), your reading of this book may not. String theory is a complex scientific topic that includes a lot of interconnected concepts, so jumping between concepts isn’t quite as easy as it may be in some other For Dummies reference books. We’ve tried to help you out by including quick reminders and providing cross-references to other chapters where necessary. So feel free to wander the pages to your heart’s content, knowing that if you get lost, you can work your way back to the information you need.

Foolish Assumptions

About the only assumption we’ve made in writing this book is that you’re reading it because you want to know something about string theory. We’ve even tried not to assume that you enjoy reading physics books. (We do, but we try not to project our own strangeness onto others.)

We have assumed that you have a passing acquaintance with basic physics concepts — maybe you took a physics class in high school or have watched some of the scientific programs about gravity, light waves, black holes, or other physics-related topics on cable channels or your local PBS station. You don’t need a degree in physics to follow the explanations in this book, although without a degree in physics you may be amazed that anyone can make sense of any theory so disconnected from our everyday experience. (Even with a physics degree, it can boggle the mind.)

As is customary in string theory books for the general public, the mathematics has been avoided. You need a graduate degree in mathematics or physics to follow the mathematical equations at the heart of string theory, and we’ve assumed that you don’t have either one. Don’t worry — while a complete understanding of string theory is rooted firmly in the advanced mathematical concepts of geometry and quantum field theory, we’ve used a combination of text and figures to explain the fascinating ideas behind string theory.

Icons Used in This Book

Throughout the book, you’ll find icons in the margins that are designed to help you navigate the text. Here’s what these icons mean:

Although everything in this book is important, some information is more important than other information. This icon points out information that will definitely be useful later in the book.

In science, theories are often explained with analogies, thought experiments, or other helpful examples that present complex mathematical concepts in a way that is more intuitively understandable. This icon indicates that one of these examples or hints is being offered.

Sometimes we go into detail that you don’t need to know to follow the basic discussion and that’s a bit more technical (or mathematical) than you may be interested in. This icon points out that information, which you can skip without losing the thread of the discussion.

Beyond the Book

In addition to what you're reading right now, this book also comes with a free access-anywhere Cheat Sheet. To get it, simply go to www.dummies.com and look for String Theory for Dummies Cheat Sheet in the Search box.

If you want to learn more about some of the ideas that laid the basis or string theory, you can also check out Einstein for Dummies by Carlos I. Calle.

Where to Go from Here

The For Dummies books are organized in such a way that you can surf through any of the chapters and find useful information without having to start at Chapter 1. We (naturally) encourage you to read the whole book, but this structure makes it very easy to start with the topics that interest you the most.

If you have no idea what string theory is, then we recommend looking at Chapter 1 as a starting point, then moving through Chapters 2-3 for a basic overview of what we’re talking about. Chapter 4 focuses on laying some foundational ideas about how theoretical science advances. If your physics is rusty, pay close attention to Chapters 5–9, which cover the history and current status of the major physics concepts that pop up over and over again.

If you’re familiar with string theory but want some more details, jump straight to Chapters 10 and 11, where we explain how string theory came about and reached its current status. Chapters 12 and 13 go a bit deeper into the specifics, including the recent insights from the holographic principle. Chapter 14 offers some ways of testing the theory, while Chapters 15–17 take concepts from string theory and apply them to some fascinating topics in theoretical physics.

Some of you, however, may want to figure out what all the recent fuss is with people arguing across the blogosphere about string theory. For that, we recommend jumping straight to Chapter 18, which addresses some of the major criticisms of string theory. Chapters 19 and 20 focus heavily on other theories that may either help expand or replace string theory, so they’re a good place to go from there.