**String theory** is a theory of quantum gravity in which 0d point particles are replaced by 1d objects called strings which propagate through space and interact with each other. Strings can be open like lines (ending in D-branes) or closed like circles. They also have direction and orientation represented by arrows on diagrams.

There are many vibrational states of a string, including:

- one that corresponds to the graviton which carries gravitational force.
- one that corresponds to the photon which carries the electromagnetic force of light.
- one that corresponds to the gluon which carries the strong nuclear force.
- the lowest-energy vibration of an open string is a tachyon.

The length scale of strings is on the order of the Planck length, or 10^{−35} meters, the scale at which the effects of quantum gravity become significant. At such scales, strings are effectively indistinguishable from 0d point particles.

String theories require extra dimensions for their mathematical consistency:

- In bosonic string theory, spacetime is 26-dimensional.
- In superstring theory it is 10-dimensional.
- In M-theory it is 11-dimensional.

Early string theorists used branes to visualize higher dimensions, such as assuming that the observable universe is a 4d subspace of a higher dimensional space. String theorists claim that a variety of higher spatial dimensions are “compactified” or tightly curled up (in structures known as Calabi-Yau spaces) so that the extra dimensions are essentially invisible.

In addition to gravity, string theory is used to study exotic matter, black holes, the Big Bang, and symmetries in mathematics. It eventually leads to the Theory of Everything, which unifies the four fundamental forces of nature.

Quote from Michio Kaku: *"In string theory, all particles are vibrations on a tiny rubber band; physics is the harmonies on the string; chemistry is the melodies we play on vibrating strings; the universe is a symphony of strings, and the "Mind of God" is cosmic music resonating in 11 dimensional hyperspace."*

The founder of M-theory, Edward Witten wrote: *“String theory is twenty-first century physics that fell accidentally into the twentieth century.”*