Superstrings

Superstrings, or superstring theory, is a theoretical framework in which the fundamental constituents of matter are not point particles but one-dimensional objects called strings. The different vibrational modes of these strings correspond to the spectrum of particles, with gravity emerging from a massless spin-2 mode. The theory is built on supersymmetry, a proposed symmetry between bosons and fermions, which leads to a richer particle spectrum and cancellations of certain quantum inconsistencies.

Strings can be closed loops or open strands with endpoints. The requirement of mathematical consistency in superstring theory sets the spacetime dimensionality to ten. To connect with the four-dimensional world, the extra six dimensions are typically assumed to be compactified on small, complex geometries such as Calabi–Yau manifolds.

Five consistent perturbative superstring theories exist: Type I, Type IIA, Type IIB, and the two heterotic theories with gauge groups SO(32) and E8×E8. These are related by dualities, and nonperturbative objects called D-branes enable open strings to end on higher-dimensional surfaces.

Remarkably, an overarching framework called M-theory was proposed to unify the five theories within an eleven-dimensional setting. M-theory provides a broader picture in which strings can appear as limiting cases of membranes and other extended objects.

As of now, there is no experimental evidence for superstrings. The energy scales required to probe string effects are far beyond current particle accelerators, and the theory faces challenges such as the landscape of vacua and issues of testability. Nevertheless, superstring theory has influenced broad areas of theoretical physics and mathematics, including quantum gravity, black hole thermodynamics, and gauge–gravity dualities.