black hole.
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 theory in human history, but those hiccups don’t seem to last very long. Every time it seems that some flaw in the theory comes along, the mathematical resiliency of string theory seems to not only save it, but bring it back stronger than ever.
When extra dimensions came into the theory in the 1970s, string theory was abandoned by many, but it had a comeback in the first superstring revolution. It then turned out that there were five distinct versions of string theory, but a second superstring revolution was sparked by unifying them. When string theorists realized a vast number of solutions to string theories (each solution to string theory is called a vacuum, while many solutions are called vacua) were possible, they turned this into a virtue instead of a drawback.
Still, even after so many years, some scientists believe that string theory is failing at its goals. (See “Considering String Theory’s Setbacks” later in this chapter.)
Being the most popular theory in town
Many young physicists feel that string theory, as the primary theory of quantum gravity, is the best (or only) avenue for making a significant contribution to our understanding of this topic. Over the last three decades, high-energy theoretical physics (especially in the United States) has become dominated by string theorists. In the high-stakes world of “publish-or-perish” academia, this is a major success.
Why do so many physicists turn toward this field when it offers no experimental evidence? Some of the brightest theoretical physicists of either the 20th or the 21st centuries — Edward Witten, John Henry Schwarz, Leonard Susskind, and others you meet throughout this book — continually return to the same common reasons in support of their interest:
If string theory were wrong, it wouldn’t provide the rich structure that it does, such as with the development of the heterotic string (see Chapter 10), which allows for an approximation of the Standard Model of particle physics within string theory.
If string theory were wrong, it wouldn’t lead to better understanding of quantum field theory, quantum chromodynamics (see Chapter 8), or the quantum states of black holes, as presented by the work of Leonard Susskind, Andrew Strominger, Cumrun Vafa, and Juan Maldacena (see Chapters 11, 13, and 16).
If string theory were wrong, it would have collapsed in on itself well before now, instead of passing many mathematical consistency checks (such as those discussed in Chapter 10) and providing more and more elaborate ways to be interpreted, such as the dualities and symmetries that allowed for the presentation of M-theory (discussed in Chapter 11).
This is how theoretical physicists think, and it’s why so many of them continue to believe that string theory is the place to be. The mathematical beauty of the theory, the fact that it’s so adaptable, is seen as one of its virtues. The theory continues to be refined, and it hasn’t been shown to be incompatible with our universe. There has been no brick wall where the theory failed to provide something new and — in some eyes, at least — meaningful, so those studying string theory have had no reason to give up and look somewhere else. (The history of string theory in Chapters 10 and 11 offers a better appreciation of these achievements.)
Whether this resilience of string theory will translate someday into proof that the theory is fundamentally correct remains to be seen, but for the majority of those working on the problems, confidence is high.
As you can read in Chapter 18, this popularity is also seen by some critics as a flaw. Physics thrives on the rigorous debate of conflicting ideas, and some physicists are concerned that the driving support of string theory, to the exclusion of all other ideas, isn’t healthy for the field. For some of these critics, the mathematics of string theory has indeed already shown that the theory isn’t performing as expected (or, in their view, as needed to be a fundamental theory) and string theorists are in denial.
Considering String Theory’s Setbacks
Because string theory has made so few specific predictions, it’s hard to disprove it, but the theory has fallen short of some of the hype about how it will be a fundamental theory to explain all the physics in our universe, a “theory of everything.” This failure to meet that lofty goal seems to be the basis of many (if not most) of the attacks against it.
In Chapter 18, you find more detailed criticisms of string theory. Some of them cut to the very heart of whether string theory is even scientific or whether it’s being pursued in the correct way. For now, we leave those more abstract questions