= String theory
{wiki}
String theory replaces fundamental point-like objects by one-dimensional strings whose motion sweeps out a two-dimensional <worldsheet>. Quantizing the string's vibration modes produces different particle states; the <string tension> fixes the energy cost of stretching the string and the associated excitation scale.
A closed-string spectrum contains a massless spin-two mode identified with a <graviton>, motivating the use of string theory in <quantum gravity>. Consistent models also involve additional fields, dimensions and symmetry constraints. <D-branes> supply dynamical objects on which open-string endpoints can lie, and <string dualities> relate descriptions with different geometrical or coupling interpretations.
These are mathematical and physical structures of the theory, rather than evidence that strings have been directly resolved experimentally. Connecting a specific model to observed low-energy physics requires additional choices of background and <compactification>, with predictions tested against measurements. The <bosonic string theory> gives a useful introductory model but lacks several ingredients needed for a realistic particle spectrum.
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