Superstring

Superstring theory is a framework in theoretical physics in which the fundamental constituents of matter are not point particles but tiny one-dimensional objects called strings. In perturbative superstring theory, strings can be closed or open; closed strings carry gravity, while open strings end on extended objects called D-branes. The theory requires supersymmetry on the two-dimensional worldsheet and yields a spectrum that includes fermions, in addition to bosons.

The theory is formulated in ten spacetime dimensions; consistency conditions impose this dimensionality for perturbative formulations. There are five consistent perturbative superstring theories: Type I, Type IIA, Type IIB, heterotic SO(32), and heterotic E8×E8. These theories differ in their gauge content and chirality but are connected by a network of dualities that reveal them as different limits of a broader non-perturbative framework known as M-theory.

Extra dimensions are typically compactified to yield four-dimensional physics. Common choices include Calabi–Yau manifolds, with the geometry and topology of the compact space determining features of the low-energy theory. The variety of possible compactifications leads to a large number of metastable vacua, a situation referred to as the string theory landscape.

Branes, especially D-branes, play a central role by supporting open strings and providing a mechanism for non-gravitational gauge forces. Phenomenology and challenges include the lack of direct experimental confirmation and the difficulty of deriving unique low-energy predictions, given the vast landscape. String theory also provides tools for quantum gravity, black hole physics, and holography via dualities such as the AdS/CFT correspondence.