String theory is a theoretical framework in physics that attempts to reconcile quantum mechanics and general relativity, two fundamental but seemingly incompatible theories that describe how the universe works at very small and very large scales. The core idea of string theory is that the fundamental building blocks of the universe are not point-like particles, as traditionally thought, but rather tiny, vibrating strings of energy.
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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.
