Warped Passages - Lisa Randall [141]
Despite its failure to describe hadrons, we can learn a little about the good features of the string theory of gravity by examining a few of the problems that hadronic string theory faced. Remarkably, the failures of the string theory of hadrons were redeeming features (or at least not obstacles) for the string theory of quantum gravity.
The first problem with the original version of string theory was that it contained a tachyon. People initially thought of tachyons as particles traveling faster than the speed of light (the term comes from the Greek tachos, meaning “speed”). But we now know that a tachyon indicates an instability in a theory that contains it. Regrettably for science fiction fans, tachyons are not real physical particles that appear in nature. If your theory seems to contain a tachyon, you are analyzing it incorrectly. A system that contains a tachyon can (and will) transform itself into a related system with lower energy in which the tachyon is absent. The system with the tachyon doesn’t last long enough for it to have any physical effects; it’s only a feature of the incorrect theoretical description. You need to find a theoretical description of the related stable configuration without a tachyon before you can identify true physical particles and forces. Unless it contains such a configuration, your theory is incomplete.
String theory with a tachyon didn’t seem to make sense. But no one knew how to formulate the theory in a way that eliminated it. This meant that the predictions from string theory, including those for particles other than the tachyon, were not reliable. You might think that this should have been reason enough to abandon hadronic string theory. But physicists held out hope that the tachyon wasn’t real; some thought it might simply be a problem with the mathematical approximations that were made when formulating the theory, but that wasn’t very likely.
However, Ramond, Neveu and Schwarz discovered an alternative supersymmetric version of string: the superstring. Superstring theory’s critically important advantage over the original version of string theory was that it contained spin -½ particles, giving it the potential to describe the Standard Model fermions such as the electron and the different types of quark. But an added bonus of superstring theory was that it did not contain the tachyon that had plagued the original version of string theory. The superstring, which seemed a more promising theory in any case, didn’t have the tachyon instability that would have threatened to hamper its progress.
A second problem for the original string theory of hadrons was that it contained a massless spin-2 particle. Calculations showed that there was no way to eliminate it, but no experimenter had ever discovered this pesky particle. Since experimenters should have been able to observe any massless particle that interacted as strongly as a hadron, hadronic string theory appeared to be in trouble.
Scherk and Schwarz turned string theory on its head when they showed that the “bad” spin-2 particle that confounded hadronic string theory might in fact be the crowning glory of a string theory of gravity; the spin-2 particle could actually be the graviton. They went on to show that the spin-2 particle behaved just as a graviton should. The critical observation that string theory contained a candidate for the graviton made string theory a potential theory of quantum gravity. With a particle description, no one had figured out how to formulate a consistent theory of gravity that worked at all energies. A string theory description, on the other hand, looked like it might do the trick.
There was another indication that although a string theory of hadrons wouldn’t work, Scherk and Schwarz might be on the right track with a string theory of gravity. As we saw in Chapter 7, Friedman, Kendall, and Taylor at the Stanford Linear Accelerator Center (SLAC) showed that electrons dramatically scatter from nuclei, implying the