String Theory, in physics, a theory that describes the most elementary objects in nature as extremely small, one-dimensional structures called strings. Conventional theories in physics, in contrast, describe the most elementary objects in nature as point-like particles. The goal of string theory is to combine quantum mechanics (which describes the behavior of and forces acting between atoms and between subatomic particles) with the general theory of relativity (which describes the force of gravity). Such an all-encompassing theory of nature is sometimes called a theory of everything. A major advance made in the 1990's was the discovery of how various string theories relate to each other.
In supersymmetric string theories, or superstring theories, strings and their interactions form the basis of all matter and all physical forces in the universe. Elementary strings are theorized to vibrate in various patterns, or excitation modes. The specific vibration pattern of an elementary string determines the unit of matter or force it generates in roughly the same way the vibration of a violin string determines the musical tone it makes. Mathematical equations used in string theory require that the universe be formed by at least six dimensions in addition to the four commonly experienced dimensions of length, width, height, and time. Physicists do not agree on the nature of these extra dimensions.