String Theory For Dummies. Andrew Zimmerman Jones

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.

Part 1

      Introducing String Theory

IN THIS PART …

       Understand the basics of string theory.

       Grasp the fundamentals of quantum gravity.

       Explore the accomplishments and failures of string theory.

Chapter 1

      So What Is String Theory Anyway?

      IN THIS CHAPTER

      

Knowing that string theory is based on vibrating strings of energy

      

Understanding the key elements of string theory

      

Hoping to explain the entire universe with string theory

      

Studying string theory could be the driving scientific goal of the 21st century

      String theory is a work in progress, so trying to pin down exactly what string theory is, or what its fundamental elements are, can be kind of tricky. Regardless, that’s exactly what we try to do in this chapter.

      In this chapter, you gain a basic understanding of string theory. We outline the key elements of string theory, which provide the foundation for most of this book. We also discuss the possibility that string theory is the starting point for a “theory of everything,” which would define all of our universe’s physical laws in one simple (or not so simple) mathematical formula. Finally, we look at the reasons why you should care about string theory.

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

      String theory is a physics theory that models the fundamental particles and interactions in the universe by representing everything in terms of vibrating filaments of energy, called strings. Like all modern physical theories, this image is actually expressed in a precise mathematical language that eventually results in quantitative as well as qualitative predictions.

      In this theory, strings of energy represent the most fundamental aspect of nature. String theory also predicts other fundamental objects, called branes, which emerge as a natural generalization of the strings. All the matter in our universe consists of the vibrations of these strings (and branes). One important result of string theory is that gravity is a natural consequence of the theory, which is why scientists believe that string theory may hold the answer to possibly uniting gravity with the other forces that affect matter.

      

We want to reiterate something important: String theory is a mathematical theory. It’s based on mathematical equations that can be interpreted in certain ways. If you’ve never studied physics before, this may seem odd, but all physical theories are expressed in the language of mathematics. In this book, we avoid the mathematics and try to get to the heart of what the theory is telling us about the physical universe.

      

At present, no one knows exactly what the “final” version of string theory, which will precisely reproduce the universe as we know it, should look like. Scientists have some vague notions about the general elements that will exist within the theory, but no one has come up with the final list of equations that represents all of string theory in our universe, and experiments haven’t yet been able to confirm it (though they haven’t successfully refuted it, either). Physicists have created simplified versions of a stringy universe, but none quite describes our universe … yet.

      Using tiny and huge concepts to create a theory of everything

String theory is a type of high-energy theoretical physics, practiced largely by particle physicists. It’s an evolution of quantum field theory (see the sidebar “What is quantum field theory?”), which is the current framework that describes the particles and forces in our universe (except gravity). String theory famously predicts that the universe should have more spatial dimensions than the three we observe. It also shows that, in principle, the extra dimensions within the theory can be wrapped up into a very small size (a process called compactification) in a way that reproduces fundamental particles like the photon or the electron. This is the power of string theory — using the fundamental strings, and the way extra dimensions are compactified, to provide a unified description of all the particles and forces known to modern physics.

      Among the forces that need to be described is, of course, gravity. Superficially, gravity has been the simplest force for humans to grasp since the time of Galileo. There is, however, more than meets the eye, as Einstein discovered: Gravity is a theory of the geometry of space and time. For this reason, it’s notoriously hard to marry the ideas of quantum physics with gravity. String theory does incorporate both gravity and quantum physics in a natural way. You can even say that string