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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 predicted

Astronomers found evidence in 1998 that the expansion of the universe was actually accelerating. This accelerated expansion is caused by the dark energy that you hear about so often in the news. Not only did string theory not predict the existence of dark energy, but its attempts to use science’s best theories to calculate the amount of dark energy come up with a number that’s vastly larger than the one observed by astronomers. The theory just absolutely failed to initially make sense of dark energy.

      Claiming this as a flaw of string theory is a bit more controversial than the other two problems, but there’s some (albeit questionable) logic behind it. The goal of string theory is nothing less than the complete rewriting of gravitational law, so it’s not unreasonable to think that string theory should’ve anticipated dark energy in some way.

      When Einstein constructed his theory of general relativity, the mathematics indicated that space could be expanding (later proved to be true). When Paul Dirac formulated a quantum theory of the electron, the mathematics indicated an antiparticle existed (later proved to actually exist). A profound theory like string theory can be expected to illuminate new facts about our universe, not be blindsided by unanticipated discoveries.

      Of course, no other theory anticipated an accelerating expansion of the universe either. Prior to the observational evidence, cosmologists (and string theorists) had no reason to assume that the expansion rate of space was increasing. Years after dark energy was discovered, it was shown that string theory could be modified to include it, which string theorists count as a success (although the critics continue to be unsatisfied).

      Where did all these “fundamental” theories come from?

      Unfortunately, as string theorists performed more research, they had a growing problem (pun intended). Instead of narrowing in on a single vacuum (solution) that could be used to explain the universe, it began to look like there were an absurdly large number of vacua. Some physicists’ hopes that a unique, fundamental version of string theory would fall out of the mathematics effectively dissolved.

      In truth, such hype was rarely justified in the first place. In general relativity, for example, an infinite number of ways to solve the equations exist, and the goal is to find solutions that match our universe. The overly ambitious string theorists (the ones who expected a single vacuum to fall out of the sky) soon realized that they, too, would end up with a rich string theory landscape, as Leonard Susskind calls the range of possible vacua (see Chapter 12 for more on Susskind’s landscape idea). The goal of string theory has since become to figure out which set of vacua applies to our universe.

STRING THEORY IS … INEVITABLE?

      A modern take on the study of string theory turns the tables and asks, what else could be a good theory of quantum gravity? In this approach, physicists don’t try to construct new theories. Instead, they try to list all the properties that any reasonable theory of quantum gravity should have.

      For instance, a reasonable quantum gravity should contain the graviton, and it should look like ordinary (classical) gravity when the masses and distances involved have the right size. There are additional requirements that come from quantum physics and are a little more technical. They ensure that the various symmetries of the theory are realized and that all probabilities always add up to 100 percent. (This may sound silly, but it’s one of the most important concepts in quantum theory, called unitarity.)

      The hope is that by considering all these constraints, we can pin down what quantum gravity should look like. This approach is sometimes called bootstrapping (as in “pulling yourself up by your own bootstraps”). It’s not clear if the various requirements that physicists have imposed can nail down a single theory of quantum gravity. However, these constraints actually rule out large families of theories, leaving relatively little space beyond string theory.

      If this bootstrap approach can be completed, it might eventually show that any reasonable theory of quantum gravity is inevitably string theory. You’ll find more about this approach in Chapter 14, in the section “Bootstrapping Our Way into String Theory”.

Looking into String Theory’s Future

      At present, string theory faces two hurdles. The first is the theoretical hurdle, which is whether a model that describes our own universe can be formulated. The second hurdle is the experimental one, because even if string theorists are successful in modeling our universe, they’ll then have to figure out how to make a distinct prediction from the theory that’s testable in some way.

      Right now, string theory falls short on both counts, and it’s unclear whether it can ever be formulated in a way that will be uniquely testable. The critics claim that growing disillusionment with string theory is rising among theoretical physicists, while the supporters continue to talk about how string theory is being used to resolve the major questions of the universe.

Only time will tell whether string theory is right or wrong, but regardless of the answer, string theory has driven scientists for years to ask fundamental questions about our universe and explore the answers to those questions in new ways. Even an alternative theory would partly owe its success to the hard work performed by string theorists.

      Theoretical complications: Can we figure out string theory?

      The current version of string theory is called M-theory. Introduced in 1995, M-theory is a comprehensive theory that includes the five supersymmetric string theories and exists in 11 dimensions. There’s

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