it won’t decay.
The possible existence of the graviton in string theory is one of the major motivations for looking toward the theory as a likely solution to the problem of quantum gravity.
Supersymmetry’s role in quantum gravity
Supersymmetry is a principle that says that two types of fundamental particles, bosons and fermions, are connected to each other. The benefit of this type of symmetry is that the mathematical relationships in gauge theory reduce in such a way that unifying all the forces becomes more feasible. (We explain bosons and fermions in greater detail in Chapter 8, while we present a more detailed discussion of supersymmetry in Chapter 10.)
The top graph in Figure 2-2 shows the strengths of the three forces described by the Standard Model modeled at different energy levels. If the three forces met up in the same point, it would indicate that there might be an energy level where these three forces become fully unified into one single force.
Lisa Randall, 2005. Reproduced by permission of HarperCollins Publishers.
FIGURE 2-2: If supersymmetry is added, the strengths of the forces in the Standard Model scale differently with energy, and they may become equal at high enough energy.
However, as the lower graph of Figure 2-2 shows, when supersymmetry is introduced into the equation (literally, not just metaphorically), the three forces meet in a single point. If supersymmetry proves to be true, it’s strong evidence that the three forces of the Standard Model unify at high enough energy.
Many physicists believe that all four forces are a manifestation of the same fundamental laws, which in string theory would be the dynamics of quantum strings. This should be apparent at high energy levels, but as the universe reduced into a lower-energy state, the inherent symmetry between the forces began to break down. This broken symmetry caused the creation of four apparently very different forces of nature.
The goal of a theory of quantum gravity is, in a sense, an attempt to look back in time, to when these four forces were unified into a single structure. If successful, it would profoundly affect our understanding of the first few moments of the universe — the last time the forces joined together in this way.
Chapter 3
Accomplishments and Failures of String Theory
IN THIS CHAPTER
String theory is a work in progress, having captured the hearts and minds of much of the theoretical physics community while being apparently disconnected from any realistic chance of definitive experimental proof. Despite this, it has had some successes — unexpected predictions and achievements that may well indicate string theorists are on the right track.
String theory critics would also point out (and many string theorists would probably agree) that the last couple of decades haven’t been kind to string theory because the momentum toward a unified theory of everything has slowed, and the latest particle colliders have failed to provide any direct evidence for string theory.
In this chapter, you see some of the major successes and failures of string theory, as well as look at the possibilities for where string theory may go from here. The controversy over string theory rests entirely on how much significance physicists give to these different outcomes.
Celebrating String Theory’s Successes
String theory has gone through many transformations since its origins in 1968, when theorists hoped it would be a model of certain types of particle collisions. It initially failed at that goal, but in the 50 years since, string theory has developed into the primary candidate for a theory of quantum gravity. It has driven major developments in mathematics, and theorists have used insights from string theory to tackle other, unexpected problems in physics. In fact, the very presence of gravity within string theory is an unexpected outcome!
Predicting gravity out of strings
The first and foremost success of string theory is the unexpected discovery of objects within the theory that match the properties of the graviton. These objects are a specific type of closed strings that are also massless particles that have a spin of 2, exactly like gravitons. To put it another way, gravitons are a spin-2 massless particle that, under string theory, can be formed by a certain type of vibrating closed string. String theory wasn’t created to have gravitons — they’re a natural and required consequence of the theory.
One of the greatest problems in modern theoretical physics is that gravity seems to be disconnected from all the other forces of physics that are explained by the Standard Model of particle physics. String theory solves this problem because it not only includes gravity but makes gravity a necessary by-product of the theory.
Explaining what happens to a black hole (sort of)
A major motivating factor for the search for a theory of quantum gravity is to explain the behavior of black holes, and string theory appears to be one of the best methods of achieving that goal. String theorists have created mathematical models of black holes that appear similar to predictions made by Stephen Hawking more than 50 years ago and may be at the heart of resolving a long-standing puzzle within theoretical physics: What happens to matter that falls into a black hole?
Scientists’ understanding of black holes has always run into problems, because to study the quantum behavior of a black hole, you need to somehow describe all the quantum states (possible configurations, as defined by quantum physics) of the black hole. Unfortunately, black holes are objects in general relativity, so it’s not clear how to define these quantum states. (See Chapter 2 for an explanation of the conflicts between general relativity and quantum physics.)