1-1: Type I strings can go through five fundamental interactions, based on different ways of joining and splitting.
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 gravity
Modern physics has two basic scientific laws: quantum physics and general relativity. These two scientific laws represent radically different fields of study. Quantum physics studies the very smallest objects in nature, while relativity tends to study nature on the scale of planets, galaxies, and the universe as a whole. (Obviously, gravity affects small particles too, and relativity accounts for this as well, but the effect is usually tiny.) Theories that attempt to unify quantum physics and relativity are theories of quantum gravity, and the most promising of all such theories today is string theory.
The closed strings of string theory (see the preceding section) correspond to the behavior expected for gravity. Specifically, they have properties that match the long-sought-after graviton, a particle that would carry the force of gravity between objects.
Quantum gravity is the subject of Chapter 2, where we cover this idea in much greater depth.
Unification of forces
Hand in hand with the question of quantum gravity, string theory attempts to unify the four forces in the universe — electromagnetic force, the strong nuclear force, the weak nuclear force, and gravity — together into one unified theory. In our universe, these fundamental forces appear as four different phenomena, but string theorists believe that in the early universe (when there were incredibly high energy levels), these forces are all described by different types of strings interacting with each other.
That such a unification may be possible isn’t entirely surprising to physicists because they discovered 50 years ago that two of the forces are actually one and the same: The electromagnetic force and the weak force can be combined in the “electroweak” force. (If you’ve never heard of some of these forces, don’t worry! We discuss them individually in greater detail in Chapter 2 and throughout Part 2.)
Supersymmetry
All particles in the universe can be divided into two types: bosons and fermions. (These types of particles are explained in more detail in Chapter 8.) String theory predicts that a type of connection, called supersymmetry, exists between these two particle types. Under supersymmetry, a fermion must exist for every boson and a boson for every fermion. Unfortunately, experiments have not yet detected these extra particles. (The latest particle that physicists have found is the Higgs boson, which is not one of the supersymmetric partners.)
Supersymmetry is a specific mathematical relationship between certain elements of physics equations. It was discovered outside string theory, although its incorporation into string theory transformed the theory into supersymmetric string theory (or superstring theory) in the mid-1970s. (See Chapter 10 for more specifics about supersymmetry.)
One benefit of supersymmetry is that it balances out string theory’s equations by allowing certain terms to cancel out. Without supersymmetry, the equations result in physical inconsistencies, such as infinite values and imaginary energy levels.
Because scientists haven’t observed the particles predicted by supersymmetry, this is still a theoretical assumption. Many physicists believe that the reason no one has observed the particles is because it takes a lot of energy to generate them. (Energy is related to mass by Einstein’s famous E = mc2 equation, so it takes energy to create a particle.) They may have existed in the early universe, but as the universe cooled off and energy spread out after the big bang, these particles would have collapsed into the lower-energy states that we observe today. (We may not think of our current universe as particularly low energy, but compared to the intense heat of the first few moments after the big bang, it certainly is.)
Scientists hope that astronomical observations or experiments with particle accelerators will uncover some of these higher-energy supersymmetric particles, providing support for this prediction of string theory.
Extra dimensions
Another mathematical result of string theory is that the theory makes sense only in a world with more than three space dimensions! (Our universe has three dimensions of space: left/right, up/down, and front/back.) Two possible explanations currently exist for the location of the extra dimensions.
The extra space dimensions (generally six of them) are curled up (compactified, in string theory terminology) to incredibly small sizes, so we never perceive them.
We are stuck on a 3-dimensional brane, and the extra dimensions extend off of it and are inaccessible to us.
A major area of research among string theorists is on mathematical models of how these extra dimensions could be related to our own. Some of the recent results have predicted that scientists may soon be able to detect the extra dimensions (if they exist) in upcoming experiments because they may be larger than previously expected. (See Chapter 15 for more about extra dimensions.)
Understanding the Aim of String Theory
To many physicists, the goal of string theory is to be a “theory
