physical theory that, at the most fundamental level, describes all of physical reality. If successful, string theory could explain many of the fundamental questions about our universe.
To others, the goal is more modest: String theory is a working theory of quantum gravity, and arguably the only one we truly understand. Studying string theory can produce important insights into the nature of quantum gravity, one of the key open questions in physics.
Quantizing gravity
The major accomplishment of string theory is providing a quantum theory of gravity. The current theory of gravity, general relativity, doesn’t allow for the results of quantum physics. Because quantum physics places limitations on the behavior of small objects, it creates major inconsistencies when we’re trying to examine the universe at extremely small scales. (See Chapter 7 for more on quantum physics.)
Therefore, the fact that string theory manages to marry general relativity and quantum physics is by itself remarkable. Not only that, but it has also led to spectacular advances in our understanding of quantum gravity, including the holographic principle, which you find in Chapter 13.
Unifying forces
Currently, four fundamental forces (more precisely called “interactions” by physicists) are known to physics: gravity, electromagnetic force, weak nuclear force, and strong nuclear force. String theory creates a framework in which all four of these interactions were once a part of the same unified force of the universe.
Under this theory, as the early universe cooled off after the big bang, this unified force began to break apart into the different forces we experience today. Experiments at high energies may someday allow us to detect the unification of these forces, although such experiments are well outside our current realm of technology.
Explaining matter and mass
One of the major goals of current string theory research is to construct a solution of string theory that contains the particles that actually exist in our universe.
String theory started out as a theory to explain particles, such as hadrons, as the different higher vibrational modes of a string. In most current formulations of string theory, the matter observed in our universe comes from the lowest-energy vibrations of strings and branes. (The higher-energy vibrations represent more energetic particles that don’t currently exist in our universe if not for a very short time.)
The mass of these fundamental particles comes from the ways that these strings and branes are wrapped in the extra dimensions that are compactified within the theory, in ways that are rather messy and detailed.
For example, consider a simplified case where the extra dimensions are curled up in the shape of a donut (called a torus by mathematicians and physicists), as in Figure 1-3.
FIGURE 1-3: Strings wrap around extra dimensions to create particles with different masses.
A string has two ways to wrap once around this shape.
A short loop around the tube, through the middle of the donut
A long loop wrapping around the entire length of the donut (like a string wraps around a yo-yo)
It’s even possible (though harder to visualize) for a string to wrap in both directions simultaneously — which would, again, give us yet another particle with yet another mass. Branes can also wrap around extra dimensions, creating even more possibilities.
Defining space and time
In many versions of string theory, the extra dimensions of space are compactified into a very tiny size, so they’re unobservable to our current technology. Trying to look at space smaller than this compactified size would provide results that don’t match our understanding of space-time. (As you see in Chapter 2, the behavior of space-time at these small scales is one of the reasons for a search for quantum gravity.) One of string theory’s major obstacles is attempting to figure out how space-time can emerge from the theory.
As a rule, though, string theory is built upon Einstein’s notion of space-time (see Chapter 6). Einstein’s theory has three space dimensions and one time dimension. String theory predicts a few more space dimensions but doesn’t change the fundamental rules of the game all that much, at least at low energies.
Some proposals for how to address this have been developed, mainly focusing on space-time as an emergent phenomenon — that is, the space-time comes out of the sum total of all the string interactions in a way that hasn’t yet been completely worked out within the theory.
However, these approaches don’t meet some physicists’ bar for compelling scientific evidence, leading to criticism of the theory. String theory’s biggest competitor, loop quantum gravity, uses the quantization of space and time as the starting point of its own theory, as Chapter 19 explains. Some believe that this will ultimately be another approach to the same basic theory.
Appreciating the Theory’s Amazing (and Controversial) Implications
Although string theory is fascinating in its own right, what may prove to be even more intriguing are the possibilities that result from it. These topics are explored in greater depth throughout the book and are the focus of Parts 3 and 4.
Landscape of possible theories
One of the most unexpected and disturbing
