Type I, Type IIA, and Type IIB superstring theories are different formulations of string theory, which is a theoretical framework attempting to describe fundamental particles and forces as tiny vibrating strings.
Type I superstring theory: Type I superstring theory is formulated in ten dimensions and includes both open strings (with endpoints that can interact with other strings) and closed strings (loops with no endpoints). It incorporates supersymmetry, a symmetry that relates particles with different spin properties. Type I superstring theory also includes "D-branes," which are extended objects where open strings can end. It has both gauge bosons (particles associated with fundamental forces) and gravity.
Type IIA superstring theory: Type IIA superstring theory is also formulated in ten dimensions and involves only closed strings. It incorporates supersymmetry and includes "D-branes" as well. It predicts the existence of gauge bosons and gravity. Type IIA theory is non-chiral, meaning it does not distinguish between left- and right-handed particles.
Type IIB superstring theory: Type IIB superstring theory is formulated in ten dimensions and includes only closed strings. It also incorporates supersymmetry and includes "D-branes." Similar to Type IIA theory, it predicts the existence of gauge bosons and gravity. However, Type IIB theory is chiral, meaning it distinguishes between left- and right-handed particles.
M-theory is a more comprehensive framework that encompasses all of these superstring theories. M-theory is an 11-dimensional theory that relates different string theories, including Type IIA, Type IIB, and Type I. In certain limits, M-theory can be approximated by each of these string theories. It provides a unified description of various string theories and incorporates new objects called "M-branes," which generalize the concept of "D-branes" in lower-dimensional theories.
M-theory remains an area of active research, and many details are still being explored and understood. It is considered a promising candidate for a theory of everything, where all fundamental forces and particles are unified. However, the full understanding and formulation of M-theory are still ongoing topics of investigation in theoretical physics.