String Theory
From Srafopedia
String theory is a scientific model of fundamental physics which states that the basic building blocks of nature are one-dimensional objects called super-strings. According to this theory all the fundamental particles are different vibrations of these peculiar objects. The attraction of the string theory is that it can result in the quantum theory of gravity which is one of the biggest problems in modern science. String theory has some peculiar characteristics, for example it postulates that there are in fact ten or eleven dimensions, although we only experience four (three spatial and time) of them because the rest are so called "compact dimensions", meaning that they are so small that they cannot be observed. There are six kinds of string theory five of which are unified in the so called M-theory. String theory hasn't however been confirmed because it so far hasn't been able to make predictions that would differ from ones given by other theories. The energies required to really test it cannot be achieved with modern technology.
In His Dark Materials
In the world of His Dark Materials string theory is the most likely explanation of the nature of the Clouded Mountain. In The Amber Spyglass, when Mrs. Coulter approaches the Clouded Mountain it is said that it reminded her of a certain theory, proposed by one of the experimental theologians, that there are seven or eight extra spatial dimensions on the very small scale and that they are impossible to examine directly. This is almost exactly what the string theory says. The experimental theologian in question is also said to have constructed a model of the extra dimensions which was described as "folds within folds, corners and edges both containing and being contained: its inside was everywhere and its outside was everywhere else". This is probably a reference to Calabi-Yau manifolds that are indeed used in string theory as a model of extra dimensions.
Including string theory in His Dark Materials is consistent with Philip Pullman's tendency to involve relevant scientific ideas of modern physics in explaining how his world works.
See also: Dark Matter, Quantum Physics
