String Theory For Dummies E-Book

Andrew Zimmerman Jones

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 610

Veröffentlichungsjahr: 2009

Beschreibung

A clear, plain-English guide to this complex scientific theory String theory is the hottest topic in physics right now, with books on the subject (pro and con) flying out of the stores. String Theory For Dummies offers an accessible introduction to this highly mathematical "theory of everything," which posits ten or more dimensions in an attempt to explain the basic nature of matter and energy. Written for both students and people interested in science, this guide explains concepts, discusses the string theory's hypotheses and predictions, and presents the math in an approachable manner. It features in-depth examples and an easy-to-understand style so that readers can understand this controversial, cutting-edge theory.

Leseprobe

String Theory For Dummies®

Table of Contents

Introduction

About This Book

Conventions Used in This Book

What You’re Not to Read

Foolish Assumptions

How This Book Is Organized

Part I: Introducing String Theory

Part II: The Physics Upon Which String Theory Is Built

Part III: Building String Theory: A Theory of Everything

Part IV: The Unseen Cosmos: String Theory on the Boundaries of Knowledge

Part V: What the Other Guys Say: Criticism and Alternatives

Part VI: The Part of Tens

normals Used in this Book

Where to Go from Here

Part I: Introducing String Theory

Chapter 1: So What Is String Theory Anyway?

String Theory: Seeing What Vibrating Strings Can Tell Us about the Universe

Using tiny and huge concepts to create a theory of everything

A quick look at where string theory has been

Introducing the Key Elements of String Theory

Strings and branes

Quantum gravity

Unification of forces

Supersymmetry

Extra dimensions

Understanding the Aim of String Theory

Explaining matter and mass

Defining space and time

Quantizing gravity

Unifying forces

Appreciating the Theory’s Amazing (and Controversial) Implications

Landscape of possible theories

Parallel universes

Wormholes

The universe as a hologram

Time travel

The big bang

The end of the universe

Why Is String Theory So Important?

Chapter 2: The Physics Road Dead Ends at Quantum Gravity

Understanding Two Schools of Thought on Gravity

Newton’s law of gravity: Gravity as force

Einstein’s law of gravity: Gravity as geometry

Describing Matter: Physical and Energy-Filled

Viewing matter classically: Chunks of stuff

Viewing matter at a quantum scale: Chunks of energy

Grasping for the Fundamental Forces of Physics

Electromagnetism: Super-speedy energy waves

Nuclear forces: What the strong force joins, the weak force tears apart

Infinities: Why Einstein and the Quanta Don’t Get Along

Singularities: Bending gravity to the breaking point

Quantum jitters: Space-time under a quantum microscope

Unifying the Forces

Einstein’s failed quest to explain everything

A particle of gravity: The graviton

Supersymmetry’s role in quantum gravity

Chapter 3: Accomplishments and Failures of String Theory

Celebrating String Theory’s Successes

Predicting gravity out of strings

Explaining what happens to a black hole (sort of)

Explaining quantum field theory using string theory

Like John Travolta, string theory keeps making a comeback

Being the most popular theory in town

Considering String Theory’s Setbacks

The universe doesn’t have enough particles

Dark energy: The discovery string theory should have predicted

Where did all of these “fundamental” theories come from?

Looking into String Theory’s Future

Theoretical complications: Can we figure out string theory?

Experimental complications: Can we prove string theory?

Part II: The Physics Upon Which String Theory Is Built

Chapter 4: Putting String Theory in Context: Understanding the Method of Science

Exploring the Practice of Science

The myth of the scientific method

The need for experimental falsifiability

The foundation of theory is mathematics

The rule of simplicity

The role of objectivity in science

Understanding How Scientific Change Is Viewed

Old becomes new again: Science as revolution

Combining forces: Science as unification

What happens when you break it? Science as symmetry

Chapter 5: What You Must Know about Classical Physics

This Crazy Little Thing Called Physics

No laughing matter: What we’re made of

Add a little energy: Why stuff happens

Symmetry: Why some laws were made to be broken

All Shook Up: Waves and Vibrations

Catching the wave

Getting some good vibrations

Newton’s Revolution: How Physics Was Born

Force, mass, and acceleration: Putting objects into motion

Gravity: A great discovery

Optics: Shedding light on light’s properties

Calculus and mathematics: Enhancing scientific understanding

The Forces of Light: Electricity and Magnetism

Light as a wave: The ether theory

Invisible lines of force: Electric and magnetic fields

Maxwell’s equations bring it all together: Electromagnetic waves

Two dark clouds and the birth of modern physics

Chapter 6: Revolutionizing Space and Time: Einstein’s Relativity

What Waves Light Waves? Searching for the Ether

No Ether? No Problem: Introducing Special Relativity

Unifying space and time

Unifying mass and energy

Changing Course: Introducing General Relativity

Gravity as acceleration

Gravity as geometry

Testing general relativity

Applying Einstein’s Work to the Mysteries of the Universe

Kaluza-Klein Theory — String Theory’s Predecessor

Chapter 7: Brushing Up on Quantum Theory Basics

Unlocking the First Quanta: The Birth of Quantum Physics

Fun with Photons: Einstein’s Nobel Idea of Light

Waves and Particles Living Together

Light as a wave: The double slit experiment

Particles as a wave: The de Broglie hypothesis

Quantum physics to the rescue: The quantum wavefunction

Why We Can’t Measure It All:The Uncertainty Principle

Dead Cats, Live Cats, and Probability in Quantum Physics

Does Anyone Know What Quantum Theory Means?

Interactions transform quantum systems: The Copenhagen interpretation

If no one’s there to see it, does the universe exist? The participatory anthropic principle

All possibilities take place: The many worlds interpretation

What are the odds? Consistent histories

Searching for more fundamental data: The hidden variables interpretation

Quantum Units of Nature — Planck Units

Chapter 8: The Standard Model of Particle Physics

Atoms, Atoms, Everywhere Atoms: Introducing Atomic Theory

Popping Open the Atomic Hood and Seeing What’s Inside

Discovering the electron

The nucleus is the thing in the middle

Watching the dance inside an atom

The Quantum Picture of the Photon: Quantum Electrodynamics

Dr. Feynman’s doodles explain how particles exchange information

Discovering that other kind of matter: Antimatter

Sometimes a particle is only virtual

Digging into the Nucleus: Quantum Chromodynamics

The pieces that make up the nucleus: Nucleons

The pieces that make up the nucleon’s pieces: Quarks

Looking into the Types of Particles

Particles of force: Bosons

Particles of matter: Fermions

Gauge Bosons: Particles Holding Other Particles Together

Exploring the Theory of Where Mass Comes From

From Big to Small: The Hierarchy Problem in Physics

Chapter 9: Physics in Space: Considering Cosmology and Astrophysics

Creating an Incorrect Model of the Universe

Aristotle assigns realms to the universe

Ptolemy puts Earth at the center of the universe (and the Catholic Church agrees)

The Enlightened Universe: Some Changes Allowed

Copernicus corrects what’s where in the universe

Beholding the movements of heavenly bodies

Introducing the Idea of an Expanding Universe

Discovering that energy and pressure have gravity

Hubble drives it home

Finding a Beginning: The Big Bang Theory

Bucking the big bang: The steady state theory

Going to bat for the big bang: Cosmic microwave background radiation

Understanding where the chemical elements came from

Using Inflation to Solve the Universe’s Problems of Flatness and Horizon

The universe’s issues: Too far and too flat

Rapid expansion early on holds the solutions

Dark Matter: The Source of Extra Gravity

Dark Energy: Pushing the Universe Apart

Stretching the Fabric of Space-Time into a Black Hole

What goes on inside a black hole?

What goes on at the edge of a black hole?

Part III: Building String Theory: A Theory of Everything

Chapter 10: Early Strings and Superstrings: Unearthing the Theory’s Beginnings

Bosonic String Theory: The First String Theory

Explaining the scattering of particles with early dual resonance models

Exploring the first physical model: Particles as strings

Bosonic string theory loses out to the Standard Model

Why Bosonic String Theory Doesn’t Describe Our Universe

Massless particles

Tachyons

No electrons allowed

25 space dimensions, plus 1 of time

Supersymmetry Saves the Day: Superstring Theory

Fermions and bosons coexist . . . sort of

Double your particle fun: Supersymmetry hypothesizes superpartners

Some problems get fixed, but the dimension problem remains

Supersymmetry and Quantum Gravity in the Disco Era

The graviton is found hiding in string theory

The other supersymmetric gravity theory: Supergravity

String theorists don’t get no respect

A Theory of Everything: The First Superstring Revolution

But We’ve Got Five Theories!

Type I string theory

Type IIA string theory

Type IIB string theory

Two strings in one: Heterotic strings

How to Fold Space: Introducing Calabi-Yau Manifolds

String Theory Loses Steam

Chapter 11: M-Theory and Beyond: Bringing String Theory Together

Introducing the Unifying Theory:M-Theory

Translating one string theory into another: Duality

Using two dualities to unite five superstring theories

The second superstring revolution begins: Connecting to the 11-dimensional theory

Branes: Stretching Out a String

The discovery of D-branes: Giving open strings something to hold on to

Creating particles from p-branes

Deducing that branes are required by M-theory

Uniting D-branes and p-branes into one type of brane

Using branes to explain black holes

Getting stuck on a brane: Brane worlds

Matrix Theory as a Potential M-Theory

Gaining Insight from the Holographic Principle

Capturing multidimensional information on a flat surface

Connecting the holographic principle to our reality

Considering AdS/CFT correspondence

String Theory Gets Surprised by Dark Energy

Considering Proposals for Why Dimensions Sometimes Uncurl

Measurable dimensions

Infinite dimensions: Randall-Sundrum models

Understanding the Current Landscape: A Multitude of Theories

The anthropic principle requires observers

Disagreeing about the principle’s value

Chapter 12: Putting String Theory to the Test

Understanding the Obstacles

Testing an incomplete theory with indistinct predictions

Test versus proof

Testing Supersymmetry

Finding the missing sparticles

Testing implications of supersymmetry

Testing Gravity from Extra Dimensions

Testing the inverse square law

Searching for gravity waves in the CMBR

Disproving String Theory Sounds Easier Than It Is

Violating relativity

Mathematical inconsistencies

Could Proton Decay Spell Disaster?

Looking for Evidence in the Cosmic Laboratory: Exploring the Universe

Using outer space rays to amplify small events

Analyzing dark matter and dark energy

Detecting cosmic superstrings

Looking for Evidence Closer to Home: Using Particle Accelerators

Relativistic Heavy Ion Collider (RHIC)

Large Hadron Collider (LHC)

Colliders of the future

Part IV: The Unseen Cosmos: String Theory On the Boundaries of Knowledge

Chapter 13: Making Space for Extra Dimensions

What Are Dimensions?

2-Dimensional Space: Exploring the Geometry of Flatland

Euclidean geometry: Think back to high school geometry

Cartesian geometry: Merging algebra and Euclidean geometry

Three Dimensions of Space

A straight line in space: Vectors

Twisting 2-dimensional space in three dimensions: The Mobius strip

More twists in three dimensions:Non-Euclidean geometry

Four Dimensions of Space-Time

Adding More Dimensions to Make a Theory Work

Sending Space and Time on a Bender

Are Extra Dimensions Really Necessary?

Offering an alternative to multiple dimensions

Weighing fewer dimensions against simpler equations

Chapter 14: Our Universe — String Theory, Cosmology, and Astrophysics

The Start of the Universe with String Theory

What was before the bang?

What banged?

Explaining Black Holes with String Theory

String theory and the thermodynamics of a black hole

String theory and the black hole information paradox

The Evolution of the Universe

The swelling continues: Eternal inflation

The hidden matter and energy

The Undiscovered Country: The Future of the Cosmos

A universe of ice: The big freeze

From point to point: The big crunch

A new beginning: The big bounce

Exploring a Finely Tuned Universe

Chapter 15: Parallel Universes: Maybe You Can Be Two Places at Once

Exploring the Multiverse: A Theory of Parallel Universes

Level 1: If you go far enough, you’ll get back home

Level 2: If you go far enough, you’ll fall into wonderland

Level 3: If you stay where you are, you’ll run into yourself

Level 4: Somewhere over the rainbow, there’s a magical land

Accessing Other Universes

A history of hyperspace

How quantum mechanics can get us from here to there

Chapter 16: Have Time, Will Travel

Temporal Mechanics 101: How Time Flies

The arrow of time: A one-way ticket

Relativity, worldlines, and worldsheets: Moving through space-time

Hawking’s chronology protection conjecture: You’re not going anywhere

Slowing Time to a Standstill with Relativity

Time dilation: Sometimes even the best watches run slow

Black hole event horizons: An extra-slow version of slow motion

General Relativity and Wormholes: Doorways in Space and Time

Taking a shortcut through space and time with a wormhole

Overcoming a wormhole’s instability with negative energy

Crossing Cosmic Strings to Allow Time Travel

A Two-Timing Science: String Theory Makes More Time Dimensions Possible

Adding a new time dimension

Reflecting two-time onto a one-time universe

Does two-time physics have any real applications?

Sending Messages through Time

Time Travel Paradoxes

The twin paradox

The grandfather paradox

Where are the time travelers?

Part V: What the Other Guys Say: Criticisms and Alternatives

Chapter 17: Taking a Closer Look at the String Theory Controversy

The String Wars: Outlining the Arguments

Thirty years and counting: Framing the debate from the skeptic’s point of view

A rise of criticisms

Is String Theory Scientific?

Argument No. 1: String theory explains nothing

Argument No. 2: String theory explains too much

Turning a Critical Eye to String Theorists

Hundreds of physicists just can’t be wrong

Holding the keys to the academic kingdom

Does String Theory Describe Our Universe?

Making sense of extra dimensions

Space-time should be fluid

How finite is string theory?

A String Theory Rebuttal

Chapter 18: Loop Quantum Gravity: String Theory’s Biggest Competitor

Taking the Loop: Introducing Another Road to Quantum Gravity

The great background debate

What is looping anyway?

Making Predictions with Loop Quantum Gravity

Gravity exists (Duh!)

Black holes contain only so much space

Gamma ray burst radiation travels at different speeds

Finding Favor and Flaw with Loop Quantum Gravity

The benefit of a finite theorem

Spending some time focusing on the flaws

So Are These Two Theories the Same with Different Names?

Chapter 19: Considering Other Ways to Explain the Universe

Taking Other Roads to Quantum Gravity

Causal dynamical triangulations (CDT): If you’ve got the time, I’ve got the space

Quantum Einstein gravity: Too small to tug

Quantum graphity: Disconnecting nodes

Internal relativity: Spinning the universe into existence

Newton and Einstein Don’t Make All the Rules: Modifying the Law of Gravity

Doubly special relativity (DSR): Twice as many limits as ordinary relativity

Modified Newtonian dynamics (MOND): Disregarding dark matter

Variable speed of light (VSL): Light used to travel even faster

Modified gravity (MOG): The bigger the distance, the greater the gravity

Rewriting the Math Books and Physics Books at the Same Time

Compute this: Quantum information theory

Looking at relationships: Twistor theory

Uniting mathematical systems: Noncommutative geometry

Part VI: The Part of Tens

Chapter 20: Ten Questions a Theory of Everything Should (Ideally) Answer

The Big Bang: What Banged (and Inflated)?

Baryon Asymmetry: Why Does Matter Exist?

Hierarchy Issues: Why Are There Gaps in Forces, Particles, and Energy Levels?

Fine-Tuning: Why Do Fundamental Constants Have the Values They Do?

Black Hole Information Paradox: What Happens to Missing Black Hole Matter?

Quantum Interpretation: What Does Quantum Mechanics Mean?

Dark Mystery No. 1: What Is Dark Matter (and Why Is There So Much)?

Dark Mystery No. 2: What Is Dark Energy (and Why Is It So Weak)?

Time Symmetry: Why Does Time Seem to Move Forward?

The End of the Universe: What Comes Next?

Chapter 21: Ten Notable String Theorists

Edward Witten

John Henry Schwarz

Yoichiro Nambu

Leonard Susskind

David Gross

Joe Polchinski

Juan Maldacena

Lisa Randall

Michio Kaku

Brian Greene

by Andrew Zimmerman Jones

with Daniel Robbins, PhD in Physics

String Theory For Dummies®

Published byWiley Publishing, Inc.111 River St.Hoboken, NJ 07030-5774www.wiley.com

Copyright © 2010 by Wiley Publishing, Inc., Indianapolis, Indiana

Published by Wiley Publishing, Inc., Indianapolis, Indiana

Published simultaneously in Canada

No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, except as permitted under Sections 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923, [phone], fax [phone]. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, [phone], fax [phone], or online at http://www.wiley.com/go/permissions.

Trademarks: Wiley, the Wiley Publishing logo, For Dummies, the Dummies Man logo, A Reference for the Rest of Us!, The Dummies Way, Dummies Daily, The Fun and Easy Way, Dummies.com, Making Everything Easier, and related trade dress are trademarks or registered trademarks of John Wiley & Sons, Inc. and/or its affiliates in the United States and other countries, and may not be used without written permission. All other trademarks are the property of their respective owners. Wiley Publishing, Inc., is not associated with any product or vendor mentioned in this book.

Limit of Liability/Disclaimer of Warranty: The publisher and the author make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation warranties of fitness for a particular purpose. No warranty may be created or extended by sales or promotional materials. The advice and strategies contained herein may not be suitable for every situation. This work is sold with the understanding that the publisher is not engaged in rendering legal, accounting, or other professional services. If professional assistance is required, the services of a competent professional person should be sought. Neither the publisher nor the author shall be liable for damages arising herefrom. The fact that an organization or Website is referred to in this work as a citation and/or a potential source of further information does not mean that the author or the publisher endorses the information the organization or Website may provide or recommendations it may make. Further, readers should be aware that Internet Websites listed in this work may have changed or disappeared between when this work was written and when it is read.

For general information on our other products and services, please contact our Customer Care Department within the U.S. at 877-762-2974, outside the U.S. at 317-572-3993, or fax [phone].

For technical support, please visit www.wiley.com/techsupport.

Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books.

Library of Congress Control Number: 2009937832

ISBN: 978-0-470-59584-8

Manufactured in the United States of America

10 9 8 7 6 5 4 3 2 1

About the Author

Andrew Zimmerman Jones is the Physics Guide at About.com, a New York Times Company, where he writes and edits news and articles on all areas of physics. He spends his days working as an editor for an educational assessment company. He holds a bachelor’s degree in physics from Wabash College, where he also studied mathematics and philosophy, and a master’s degree in mathematical education from Purdue University.

In addition to work for About.com, Andrew has written a number of nonfiction essays and reviews, which have appeared in The Internet Review of Science Fiction, EpicSFF.com, Pink Floyd and Philosophy, Black Gate, and Heroes and Philosophy. His fiction credits include short stories in Abyss and Apex, KidVisions, The Four Bubbas of the Apocalypse, and International House of Bubbas.

He has been a member of Mensa since the eighth grade and has been intensely interested in both science and science fiction since even earlier. Along the way, he’s also become an Eagle Scout, a Master Mason in the Freemasons, and won the Harold Q. Fuller Prize in Physics at Wabash College. His plan for world domination nears completion with the publication of this book.

Andrew lives in central Indiana with his beautiful wife, Amber, and son, Elijah. When he’s not writing or editing, he is most often found reading, playing games, watching television, investigating bizarre scientific phenomena, or updating his personal Web page, which can be found at www.azjones.info. Andrew also regularly reports on any new string theory implications on his Web site at physics.about.com.

Dedication

This book is dedicated to my loving and lovely wife, Amber Eckert-Jones. While physicists still search for a law to unify all of the forces in the physical universe, I don’t need to, because all the forces in my universe come together in you.

Author’s Acknowledgments

I must first profoundly thank my agent, Barb Doyen, for approaching me with this project. My deepest thanks and appreciations go out to the wonderful editorial staff at Wiley: Alissa Schwipps for her valuable input at every step in the process, Vicki Adang for her ability to turn my scientific babble into coherent explanations, and Stacy Kennedy for gathering together such a great team in the first place. I also very much appreciated the constructive and at times critical input of Dr. Rolf Schimmrigk of Indiana University, South Bend, who provided initial technical editing on the book. In addition, I’m profoundly thankful for the extremely detailed technical expertise, review, and frequent discussions offered by Dr. Daniel Robbins of the Weinberg Theory Group at the University of Texas at Austin.

Without the wonderful staff at About.com, notably the Education Channel editor Madeleine Burry, I would never have had the opportunity to grow as a writer in this field. Also to author Robert J. Sawyer, for his mentorship and friendship over the years. Thanks to you all!

Many thanks to physicists Lee Smolin and John W. Moffat of the Perimeter Institute for Theoretical Physics, Leonard Susskind of Stanford University, and Sylvester James Gates, director of the University of Maryland’s Center for String and Particle Theory, for e-mail exchanges that have helped to clarify various points throughout the writing of this book.

Finally, my thanks go out to my wife, Amber, and son, Elijah, for putting up with me, even when I was driven frantic by deadlines. Thanks also to my mother, Nancy Zimmerman, and mother-in-law, Tina Lewis, for their help in keeping the family entertained while I worked feverishly in the basement.

Publisher’s Acknowledgments

We’re proud of this book; please send us your comments through our Dummies online registration form located at http://dummies.custhelp.com. For other comments, please contact our Customer Care Department within the U.S. at 877-762-2974, outside the U.S. at 317-572-3993, or fax [phone].

Some of the people who helped bring this book to market include the following:

Acquisitions, Editorial, and Media Development

Senior Project Editor: Alissa Schwipps

Acquisitions Editor: Stacy Kennedy

Senior Copy Editor: Victoria M. Adang

Assistant Editor: Erin Calligan Mooney

Editorial Program Coordinator: Joe Niesen

Technical Editor: Rolf Schimmrigk

Senior Editorial Manager: Jennifer Ehrlich

Editorial Assistants: Jennette ElNaggar, David Lutton

Art Coordinator: Alicia B. South

Cover Photo: © Laguna Design /Photo Researchers, Inc.

Cartoons: Rich Tennant (www.the5thwave.com)

Composition Services

Project Coordinator: Lynsey Stanford

Layout and Graphics: Ana Carrillo, Melissa K. Jester, Christin Swinford, Christine Williams

Special Art: Precision Graphics

Proofreaders: Melissa Cossell, Sossity R. Smith

Indexer: Infodex Indexing Services, Inc.

Special Help:Matthew Headrick

Publishing and Editorial for Consumer Dummies

Diane Graves Steele, Vice President and Publisher, Consumer Dummies

Kristin Ferguson-Wagstaffe, Product Development Director, Consumer Dummies

Ensley Eikenburg, Associate Publisher, Travel

Kelly Regan, Editorial Director, Travel

Publishing for Technology Dummies

Andy Cummings, Vice President and Publisher, Dummies Technology/General User

Composition Services

Debbie Stailey, Director of Composition Services

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. Are there parallel universes? 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 sorts 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. It contains other ideas that may ultimately prove to be fundamental laws of our universe, perhaps forming the foundation for whole new forms of science and technology. No one knows what the future holds for string theory.

About This Book

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

In writing this book, I set out to achieve the following goals:

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 it proves to be false.

Have some fun along the way.

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

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

And while time may flow in only one direction (Or does it? I explore this in Chapter 16), your reading of this book may not. String theory is a complex scientific topic that has a lot of interconnected concepts, so jumping between concepts is not quite as easy as it may be in some other For Dummies reference books. I’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.

Conventions Used in This Book

The following conventions are used throughout the text to make things consistent and easy to understand:

I use monofont for Web sites. Note: When this book was printed, some Web addresses may have needed to break across two lines of text. If that happened, rest assured that I haven’t put in any extra characters (such as hyphens) to indicate the break. So, when using one of these Web addresses, just type in exactly what you see in this book, as though the line break doesn’t exist.

I’ve done my best not to fill the book with technical jargon, which is hard to do in a book on one of the most complex and mathematically driven scientific topics of all time. When I use a technical term, it’s in italics and closely followed by an easy-to-understand definition.

Bold is used to highlight key words and phrases in bulleted lists.

Finally, one major convention used in this book is in the title: I use the term “string theory.” In Chapter 10, you discover that string theory is actually called superstring theory. As you see in Chapter 11, in 1995 physicists realized that the various “string theories” (five existed at the time) included objects other than strings, called branes. So, strictly speaking, calling it by the name “string theory” is a bit of a misnomer, but people (including string theorists themselves) do it all the time, so I’m treading on safe ground. Many physicists also use the name M-theory to describe string theory after 1995 (although they rarely agree on what the “M” stands for), but, again, I will mostly refer to it just as “string theory” unless the distinction between different types matters.

What You’re Not to Read

All the chapters provide you with important information, but some sections offer greater detail or tidbits of information that you can skip for now and come back to later without feeling guilty:

Sidebars: Sidebars are shaded boxes that give detailed examples or explore a tangent in more detail. Ignoring these won’t compromise your understanding of the rest of the material.

Anything with a Technical Stuff normal: This normal indicates information that’s interesting but that you can live without. Read these tidbits later if you’re pressed for time.

Foolish Assumptions

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

I 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 might be amazed that anyone can make sense of any theory this disconnected from our everyday experience. (Even with 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 while I have a graduate degree in mathematics, I’ve assumed that you don’t. Don’t worry — while a complete understanding of string theory is rooted firmly in the advanced mathematical concepts of quantum field theory, I’ve used a combination of text and figures to explain the fascinating ideas behind string theory.

How This Book Is Organized

String Theory For Dummies is written so you can easily get to the information you need, read it, and understand it. It’s designed to follow the historical development of the theory as much as possible, though many of the concepts in string theory are interconnected. Although I’ve attempted to make each chapter understandable on its own, I’ve included cross-references where concepts repeat to get you back to a more thorough discussion of them.

Part I: Introducing String Theory

This first part of the book introduces the key concepts of string theory in a very general way. You read about why scientists are so excited about finding a theory of quantum gravity. Also, you get your first glimpse into the successes and failures of string theory.

Part II: The Physics Upon Which String Theory Is Built

String theory is built upon the major scientific developments of the first 70 years or so of the 20th century. In this part, you find out how physicists (and scientists in general) learn things and what they’ve learned so far. Part II includes chapters on how science develops, classical physics (before Einstein), Einstein’s theory of relativity, quantum physics, and the more recent findings in particle physics and cosmology. The questions raised in these chapters are those that string theory attempts to answer.

Part III: Building String Theory: A Theory of Everything

You get to the heart of the matter in this part. I discuss the creation and development of string theory, from 1968 to early 2009. The amazing transformations of this theory are laid out here. Chapter 12 focuses on ways that the concepts of string theory can be tested.

Part IV: The Unseen Cosmos: String Theory on the Boundaries of Knowledge

Here I take string theory out for a spin in the universe, exploring some of the major concepts in greater detail. Chapter 13 focuses on the concept of extra dimensions, which are at the core of much of string theory study. Chapter 14 explores the implications for cosmology and how string theory could explain certain properties of our universe. Even more amazing, in Chapters 15 and 16, you discover what string theory has to say about possible parallel universes and the potential for time travel.

Part V: What the Other Guys Say: Criticism and Alternatives

The discussion gets heated in this part as you read about the criticisms of string theory. String theory is far from proven, and many scientists feel that it’s heading in the wrong direction. Here you find out why and see what alternatives they’re posing, such as loop quantum gravity (string theory’s biggest competitor). If string theory is wrong, scientists will continue to look for answers to the questions that it seeks to resolve.

Part VI: The Part of Tens

In the For Dummies tradition, the final chapters of this book present lists of ten topics. Chapter 20 sums up ten outstanding physics questions that scientists hope any “theory of everything” (including string theory) will answer. Chapter 21 focuses on ten string theorists who have done a lot to advance the field, either through their own research or by introducing string theory concepts to the world through popular books.

normals Used in this Book

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

Although everything in this book is important, some information is more important than other information. This normal 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 normal indicates that one of these examples or hints is being offered.

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

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. I (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 I recommend looking at Chapter 1 as a starting point. 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 I explain how string theory came about and reached its current status. Chapter 12 offers some ways of testing the theory, while Chapters 13–16 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, I recommend jumping straight to Chapter 17, which addresses some of the major criticisms of string theory. Chapters 18 and 19 focus heavily on other theories that may either help expand or replace string theory, so they’re a good place to go from there.

Part I

Introducing String Theory

In this part . . .

Meet string theory, a bold scientific theory that attempts to reconcile all the physical properties of our universe into a single unified and coherent mathematical framework.

String theory’s goal is to make quantum physics and Einstein’s theory of gravity (called general relativity) play nice. In this part, I explain why scientists want to find a theory of quantum gravity, and then I review the successes and failures at applying string theory to this search.

This part is something of an overview for the entire book, so stick with me. The foundation laid here may help explain the entire universe.