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.)
This is the closest that string theory has come to an experimental prediction. Unfortunately, there’s nothing experimental about it because scientists can’t directly observe black holes to this level of detail. It’s a theoretical prediction that unexpectedly matches another (well-accepted) theoretical prediction about black holes. And, beyond that, the prediction only holds for certain types of black holes and hasn’t yet been successfully extended to all black holes.
For a more detailed look at black holes and string theory, check out Chapters 9, 13, and 16.
Explaining quantum field theory using string theory
One of the major successes of string theory is something called the Maldacena conjecture, or the AdS/CFT correspondence. (We get into what this means in Chapter 13.) Developed in 1997 and later expanded, this correspondence appears to give insights into gauge theories, like those at the heart of quantum field theory, and their relation to gravity. (See Chapter 2 for an explanation of gauge theories.)
The original AdS/CFT correspondence, written by Juan Maldacena, argues that strings (that is, quantum gravity) in certain D-dimensional universes are equivalent to certain quantum field theories (without gravity) in a (D-1)-dimensional universe. This sounds confusing (it is), but in a nutshell, it means that quantum gravity is a bit like the Standard model (but for a universe in one dimension less), and the Standard model is a bit like quantum gravity (but for a universe in one dimension more). This is a very surprising way to think about quantum gravity (first anticipated by Nobel laureate Gerard ’t Hooft), which finds its most precise realization in Maldacena’s AdS/CFT correspondence.
More precisely, Maldacena proposed that a certain 3-dimensional (three space dimensions, like our universe) gauge theory, with the most supersymmetry allowed, describes the same physics as a string theory in a 4-dimensional (four space dimensions) world. This means that questions about string theory can be asked in the language of gauge theory, which is a quantum theory that physicists know how to work with!
String theory keeps making a comeback
String theory has suffered more setbacks than probably any other scientific
