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because it’s impossible to construct any string theory without gravity.

      Still, not every aspect of gravity is understood from string theory. Importantly, the established theory of gravity, general relativity, has a fluid, dynamic space-time, and one aspect of string theory that’s still being worked on is getting that type of space-time to emerge out of the theory.

The major achievements of string theory are concepts you can’t see, unless you know how to interpret the physics equations. String theory deals with rather extreme amounts of energy; that’s why it’s hard to test its predictions directly with experiments. Yet it has revealed profound mathematical relationships within the equations, which leads physicists to believe that they must be true. We discuss these properties and relationships — known by jargon that describes various symmetries and dualities, the cancellation of anomalies, and the explanation of black hole entropy — in Chapters 10 and 11.

      In recent years, there has been much public debate over string theory, waged within newsrooms and across the internet. We address these issues in Part 5, but they come down to fundamental questions about how science should be pursued. String theorists believe that their methods are sound, while the critics believe they’re questionable because they stray too far from contact with experimentation — the true core of physics. Time, and experimental evidence, will tell which side has made the better argument.

      WHAT IS QUANTUM FIELD THEORY?

      Physicists use fields to describe the things that don’t just have a particular position but exist at every point in space. For example, you can think about the temperature in a room as a field — it may be different near an open window than near a hot stove, and you could imagine measuring the temperature at every single point in the room. A field theory, then, is a set of rules that tell you how some field will behave, such as how the temperature in the room changes over time.

      In Chapters 7 and 8, you find out about one of the most important achievements of the 20th century: the development of quantum theory. This refers to principles that lead to seemingly bizarre physical phenomena that nonetheless appear to occur in the subatomic world.

      When you combine these two concepts, you get quantum field theory: a field theory that obeys the principles of quantum theory. All modern particle physics is described by quantum field theories.

      A quick look at where string theory has been

      String theory was originally developed in 1968 as an attempt to explain the behavior of hadrons (such as protons and neutrons, the particles that make up an atomic nucleus) inside particle accelerators. Physicists later realized this theory could also be used to explain some aspects of gravity. For more than a decade, string theory was abandoned by most physicists, mainly because it required a large number of extra, unseen dimensions. It rose to prominence again in the mid-1980s, when physicists were able to prove it was a mathematically consistent theory.

      In the mid-1990s, string theory was updated to become a more complex theory, called M-theory, which contains more objects than just strings. These new objects were called branes, and they could have anywhere from zero to nine dimensions. The earlier string theories (which now also include branes) were seen as approximations of the more complete M-theory.

      

Technically, the modern M-theory is more than the traditional string theory, but the name “string theory” is still often used for M-theory and its various offspring theories. (Even the original superstring theories have been shown to include branes.) Our convention in this book is to refer to theories that contain branes, which are variants of M-theory and the original string theories, using the term “string theory.”

Introducing the Key Elements of String Theory

      Five key ideas are at the heart of string theory and come up again and again. It’s best for you to become familiar with these key concepts right off the bat.

       String theory predicts that all objects in our universe are composed of vibrating filaments (and membranes) of energy.

       String theory attempts to reconcile general relativity (gravity) with quantum physics.

       String theory provides a way of unifying all the fundamental forces of the universe.

       String theory predicts a new connection (called supersymmetry) between two fundamentally different types of particles, bosons and fermions.

       String theory predicts a number of extra (usually unobservable) dimensions to the universe.

      We introduce you to the very basics of these ideas in the following sections.

      Strings and branes

      When the theory was originally developed in the 1970s, the filaments of energy in string theory were considered to be one-dimensional objects: strings. (One-dimensional indicates that a string has only one dimension, length, as opposed to, say, a square, which has both length and height dimensions.)

These strings came in two forms: closed strings and open strings. An open string has ends that don’t touch each other, while a closed string is a loop with no open end. It was eventually found that these early strings, called Type I strings, could go through five basic types of interactions, as Figure 1-1 shows.

      FIGURE 1-1: Type I strings can go through five fundamental interactions, based on different ways of joining and splitting.

      

The interactions are based on a string’s ability to have its ends join and split apart. Because the ends of open strings can join together to form closed strings, you can’t construct a string theory without closed strings. This is a manifestation of the dualities of string theory, which you will encounter in Chapter 11 and that resulted in the proposal of M-theory.

      

This proved to be important because closed strings have properties that make physicists believe they might describe gravity! In other words, physicists began to realize that instead of just being a theory of matter particles, string theory may be able to explain gravity and the behavior of particles.

Over the years, it was discovered that the theory required objects other than strings. These objects can be seen as sheets, or branes. Strings can attach at one or both ends to these branes. Figure 1-2 shows a 2-dimensional brane (called a 2-brane). (See Chapter 11 for more about branes.)

      FIGURE 1-2: In string theory, strings attach themselves to branes.

      Quantum

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