String theory and quantum field theory are both theoretical frameworks in physics, but they have different approaches and describe different aspects of the universe.
Quantum field theory (QFT) is a framework that combines quantum mechanics with special relativity. It describes the behavior of elementary particles as fields that permeate all of space and time. In QFT, particles are represented as excitations or quanta of these fields. The theory allows for the creation and annihilation of particles and describes their interactions through various forces and interactions.
Quantum field theory has been highly successful in describing the electromagnetic force (quantum electrodynamics) and the weak and strong nuclear forces (quantum chromodynamics). It is the basis for the standard model of particle physics, which provides a comprehensive description of the known elementary particles and their interactions.
On the other hand, string theory is a theoretical framework that aims to provide a unified description of all fundamental forces and particles, including gravity, within a single theoretical framework. It postulates that the fundamental entities of the universe are not point-like particles but tiny, vibrating strings. Different vibrations or modes of these strings correspond to different particles and their properties.
String theory incorporates quantum mechanics and general relativity, and it requires the existence of additional dimensions beyond the familiar four (three spatial dimensions and one time dimension) to be consistent. These extra dimensions are usually compactified or curled up at microscopic scales, making them undetectable by current experiments.
One of the major motivations behind string theory is its potential to resolve the theoretical inconsistencies between quantum mechanics and general relativity, which arise in certain extreme conditions such as the singularities inside black holes or the early moments of the universe (the Big Bang).
While quantum field theory is a well-developed framework with experimental support, string theory is still a subject of active research and has not yet made definitive experimental predictions that can be tested. It has, however, provided valuable insights and connections between different branches of physics and mathematics.
In summary, quantum field theory describes the behavior of particles and forces based on quantum mechanics and special relativity, while string theory is a more ambitious attempt to unify all fundamental forces and particles, including gravity, within a framework that treats particles as vibrating strings in higher-dimensional space.