String Theory: The Skeptic’s Point of View
Even now, with criticism on the rise, it doesn’t appear that the study of string theory has dropped. To understand why physicists continue to study string theory, and why other physicists believe it isn’t delivering as promised, here is a brief recount the general trends in the history of string theory, focusing this time on its shortcomings.
String theory started in 1968 as a theory (called the dual resonance model) to predict the interactions of hadrons (protons and neutrons), but failed at that. Instead of this model, quantum chromodynamics, which said that hadrons were composed of quarks held together by gluons, proved to be the correct model.
Analysis of the early version of string theory showed that it could be viewed as very tiny strings vibrating. In fact, this bosonic string theory had several flaws: fermions couldn’t exist and the theory contained 25 space dimensions, tachyons, and too many massless particles.
These problems were fixed with the addition of supersymmetry, which transformed bosonic string theory into superstring theory. Superstring theory still contained nine space dimensions, though, so most physicists still believed it had no physical reality.
This new version of string theory was shown to contain a massless, spin-2 particle that could be the graviton. Now, instead of a theory of hadron interactions, string theory was a theory of quantum gravity. But most physicists were exploring other theories of quantum gravity, and string theory languished throughout the 1970s.
The first superstring revolution took place in the mid-1980s, when physicists showed ways to construct string theory that made all the anomalies go away. In other words, string theory was shown to be consistent. In addition, physicists found ways to compactify the extra six space dimensions by curling them up into complex shapes that were so tiny they would never be observed.
The rise in work on string theory had great results. In fact, the results were too good, because physicists discovered five distinct variations of string theory, each of which predicted different phenomena in the universe and none of which precisely matched our own.
In 1995, Edward Witten proposed that the five versions of string theory were different low-energy approximations of a single theory, called M-theory. This new theory contained ten space dimensions and strange objects called branes, which had more dimensions than strings.
A major success of string theory was that it was used to construct a description for black holes, which calculated the entropy correctly, according to the Hawking-Bekenstein predictions for black hole thermodynamics. This description applied only to specific types of simplified black holes, although there was some indication that the work might extend to more general black holes.
A problem for string theory arose in 1998, when astrophysicists showed that the universe was expanding. In other words, the cosmological constant of the universe is positive, but all work in string theory had assumed a negative cosmological constant. (The positive cosmological constant is commonly referred to as dark energy.)
In 2003, a method was found to construct string theory in a universe that had dark energy, but there was a major problem with it: A vast number of distinct string theories were possible. Some estimates have been as much as 10500 distinct ways to formulate the theory, which is so absurdly large that it can be treated as if it were basically infinity.
As a response to these findings, physicist Leonard Susskind proposed the application of the anthropic principle as a means of explaining why our universe had the properties it did, given the incredibly large number of possible configurations, which Susskind called the landscape.
This brings us to the current status of string theory, in very broad strokes. You can probably see some chinks in string theory’s armor, where the criticisms seem to resonate particularly strongly.