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 and focus on three issues that even most string theorists aren’t particularly happy about:
Because of supersymmetry, string theory requires a large number of particles beyond what scientists have ever observed.
This new theory of gravity was unable to predict the accelerated expansion of the universe that was detected by astronomers.
A vastly large number of mathematically feasible string theory vacua (solutions) currently exist, so it seems virtually impossible to figure out which could describe our universe.
The following sections cover these dilemmas in more detail.
The universe doesn’t have enough particles
For the mathematics of string theory to work, physicists have to assume a symmetry in nature called supersymmetry, which creates a correspondence between different types of particles. One problem with this is that instead of the 25 fundamental particles in the Standard Model, supersymmetry requires at least 36 fundamental particles (which means that nature allows 25 more particles that scientists have never seen!). In some ways, string theory does make things simpler — the fundamental objects are strings and branes or, as predicted by matrix theory, 0-dimensional branes called partons. These strings, branes, or possibly partons make up the particles that physicists have observed (or the ones they hope to observe). But that’s on a very fundamental level; from a practical standpoint, string theory doubles the number of particles allowed by nature from 25 to 50.
One of the biggest possible successes for string theory would be to experimentally detect these missing supersymmetric partner particles. Many theoretical physicists hoped that when the Large Hadron Collider particle accelerator at the European Organization for Nuclear Research in Switzerland went online, it would detect supersymmetric particles. This hasn’t happened — yet.
Even if it’s found, proof of supersymmetry doesn’t inherently prove string theory, so the debate would continue to rage on, but at least one major objection would be removed. Supersymmetry might well end up being true, whether or not string theory as a whole is shown to accurately describe nature.
Dark energy: The discovery string theory should have
