The framework of Quantum Field Theory (QFT) encompasses the mathematical structure of a wide variety of physical systems, including systems of particle physics, general relativity, statistical physics, and the early universe. QFT unifies Einstein's theory of relativity with quantummechanics, permitting us to describe the physics of the very small and the very fast in one unifiedframework. In QFT the fundamental object is a field that is valued at every point in spacetime, and

particles are excitations of these fundamental fields. One reason we must switch to thinking aboutfields rather than particles is that in relativistic processes, particles can be created or destroyed.When particle number is not conserved, we need a theory that can describe more than singleparticle dynamics. Another fundamental reason for a quantum theory of fields is that we need toconstruct laws of nature that are local, which requires taking into account the fact that informationtravels at a finite speed. QFT provides a natural framework for doing this