String theory is actually a name for a class of theories which may eventually give us a theory of everything. The basic principle of string theory is that all of the elementary particles of the standard model are just different modes of vibration of a more fundamental object: a tiny, one-dimensional “string.” These are not literally strings, of course, but a sort of quantization of spacetime which comes in two forms: linear objects with two free ends (strings) and closed loops.
We know in classical mechanics that ordinary strings will vibrate at certain frequencies, determined by boundary conditions (such as the length of the string, and whether either end is free or attached to something). Each normal mode of vibration corresponds to a standing wave on the string, and each mode has its own frequency. Since frequency is related to energy, and energy is related to mass, it is not too farfetched to postulate that a string with the right properties could have normal modes that correspond to the masses and quantum numbers of all the elementary particles.
In order to account for all the properties of all the known particles, however, these strings have to be extremely tiny (like 10-35 meters long) and be able to wiggle in 10 or 11 dimensions. Mathematicians have no problem working in any number of dimensions, but physically, we have a hard time dealing with any more than the familiar three spatial dimensions plus time. String theory proponents either say the extra dimensions are “compactified” (i.e., made too small to detect) or they say that the dimensions we perceive are simply a subset of the true, larger number of dimensions, and that our perception is limited for some reason.
String theory has been called the ultimate unification, since all of the elementary particles, including the photon and other exchange bosons, are just different vibrational modes of the same object, the one-dimensional string. String theory, combined with supersymmetry, very naturally includes gravity as a quantum field theory.
There are some major problems with string theory, however. One is that there are at least five different versions of the theory that are all able to fit themselves to the standard model. We only want or need one theory of everything, and don’t quite know what to do with five. The other problem is that beyond mimicking the standard model, string theory provides no other predictions that can be tested in the foreseeable future. We can’t imagine any way of directly detecting something that is 20 factors of 10 smaller than a proton.
String theory is definitely a work in progress. Some theorists have become discouraged by a lack of new results in recent years, but many others are still actively exploring the possibilities, trying to find ways that some part of the theory can be tested experimentally.
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