Supersymmetry Saves the Day: Superstring Theory

Despite bosonic string theory’s apparent failures, some brave physicists stayed committed to their work. Why? Well, physicists can be a passionate bunch (nearly obsessive, some might say). Another reason was that by the time these problems were fully realized, many string theorists had already moved on from bosonic string theory anyway.
With the development of supersymmetry in 1971, which allows for bosons and fermions to coexist, string theorists were able to develop supersymmetric string theory, or, for short, superstring theory, which took care of the major problems that destroyed bosonic string theory. This work opened up whole new possibilities for string theory.
Almost every time you hear or read the phrase “string theory,” the person probably really means “superstring theory.” Since the discovery of supersymmetry, it has been applied to virtually all forms of string theory. The only string theory that really has nothing to do with supersymmetry is bosonic string theory, which was created before supersymmetry. For all practical discussion purposes (with anyone who isn’t a theoretical physicist), “string theory” and “superstring theory” are the same term.

Fermions and bosons coexist… sort of

Symmetries exist throughout physics. A symmetry in physics is basically any situation where two properties can be swapped throughout the system and the results are precisely the same.
The notion of symmetry was picked up by Pierre Ramond in 1970, followed by the work of John Schwarz and Andre Neveu in 1971, to give hope to string theorists. Using two different techniques, they showed that bosonic string theory could be generalized in another way to obtain non-integer spins. Not only were the spins non-integer, but they were precisely half-integer spins, which characterize the fermion. No spin >4 particles showed up in the theory, which is good because they don’t exist in nature.
Including fermions into the model meant introducing a powerful new symmetry between fermions and bosons, called supersymmetry. Supersymmetry can be summarized as

Every boson is related to a corresponding fermion. Every fermion is related to a corresponding boson.
In topic 11, I discuss the reasons to believe that supersymmetry is true, as well as ways that it can be proved. For now, it’s enough to know that it’s needed to make string theory work.

Who discovered supersymmetry?

The origins of supersymmetry are a bit confusing, because it was discovered around the same time by four separate groups.
In 1971, Russians Evgeny Likhtman and Yuri Golfand created a consistent theory containing supersymmetry. A year later, they were followed by two more Russians, Vladimir Akulov and Dmitri Volkov. These theories were in only two dimensions, however.
Due to the Cold War, communication between Russia and the non-communist world wasn’t very good, so many physicists didn’t hear about the Russian work. European physicists Julius Wess and Bruno Zumino were able to create a 4-dimensional supersymmetric quantum theory
in 1973, probably aware of the Russian work. Theirs was noticed by the Western physics community at large.
Then, of course, we have Pierre Ramond, John Schwarz, and Andre Neveu, who developed supersymmetry in 1970 and 1971, in the context of their superstring theories. It was only on later analysis that physicists realized their work and the later work hypothesized the same relationships.
Many physicists consider this repeated discovery as a good indication that there’s probably something to the idea of supersymmetry in nature, even if string theory itself doesn’t prove to be correct.
Of course, as you’ll anticipate if you’re looking for trends in the story of string theory, things didn’t quite fall out right. Fermions and bosons have very different properties, so getting them to change places without affecting the possible outcomes of an experiment isn’t easy.
Physicists know about a number of bosons and fermions, but when they began looking at the properties of the theory, they found that the correspondence didn’t exist between known particles. A photon (which is a boson) doesn’t appear to be linked by supersymmetry with any of the known fermions.
Fortunately for theoretical physicists, this messy experimental fact was seen as only a minor obstacle. They turned to a method that has worked for theorists since the dawn of time. If you can’t find evidence of your theory, hypothesize it!

Double your particle fun: Supersymmetry hypothesizes superpartners

Under supersymmetry, the corresponding bosons and fermions are called superpartners. The superpartner of a standard particle is called a sparticle.
Because none of the existing particles are superpartners, this means that if supersymmetry is true, there are twice as many particles as we currently know about. For every standard particle, a sparticle that has never been detected experimentally must exist. The detection of sparticles will be one of the key pieces of evidence the Large Hadron Collider will look for.
If I mention a strangely named particle that you’ve never run into, it’s probably a sparticle. Because supersymmetry introduces so many new particles, it’s important to keep them straight. Physicists have introduced a Dr. Seuss-like naming convention to identify the hypothetical new particles:
The superpartner of a fermion begins with an “s” before the standard particle name; so the superpartner of an “electron” is the “selectron,” and the superpartner of the “quark” is the “squark.”
The superpartner of a boson ends in an “-ino,” so the superpartner of a “photon” is the “photino” and of the “graviton” is the “gravitino.”
Table 10-1 shows the names of standard particles and their corresponding superpartner.

Even though there is an elementary superpartner called a “sneutrino,” there exists no elementary particle called a “sneutron.”

Some problems get fixed, but the dimension problem remains

The introduction of supersymmetry into string theory helped with some of the major problems of bosonic string theory. Fermions now existed within the theory, which had been the biggest problem. Tachyons vanished from superstring theory. Massless particles were still present in the theory, but weren’t seen as a major issue. Even the dimensional problem improved, dropping from 26 space-time dimensions down to a mere ten.
The supersymmetry solution was elegant. Bosons — the photon, graviton, Z, and W bosons — are units of force. Fermions — the electron, quarks, and neutrinos — are units of matter. Supersymmetry created a new symmetry, one between matter and forces.
In 1972, Andre Neveu and Joel Scherk resolved the massless particle issue by showing that string vibrational states could correspond to the gauge bosons, such as the massless photon.
The dimensional problem remained, although it was better than it had been. Instead of 25 spatial dimensions, superstring theory became consistent with a “mere” nine spatial dimensions (plus one time dimension, for a total of ten dimensions). Many string theorists of the day believed this was still too many dimensions to work with, so they abandoned the theory for other lines of research.
One physicist who turned his back on string theory was Michio Kaku, one of today’s most vocal advocates of string theory. Kaku’s PhD thesis involved completing all the terms in the Veneziano model’s infinite series. He’d created a field theory of strings, so he was working in the thick of string theory. Still, he abandoned work on superstring theory, believing that there was no way it could be a valid theory. That’s how serious the dimensional problem was.
For the handful of people who remained dedicated to string theory after 1974, they faced serious issues about how to proceed. With the exception of the dimensional problem, they had resolved nearly all the issues with bosonic string theory by transforming it into superstring theory.
The only question was what to do with it.