Quantum field theory is a theoretical framework that **combines quantum mechanics and special relativity** to describe the behavior of particles and their interactions.

It is a fundamental theory in physics that has had a profound impact on our understanding of the fundamental nature of the universe.

**Classical vs Quantum Physics**

In classical physics, particles are described as point-like objects with well-defined properties, such as mass and charge.

However, in quantum mechanics, particles are described by wave functions, which can be thought of as a mathematical representation of the probability of finding a particle in a certain location.

In addition, quantum mechanics allows for the phenomenon of superposition, where a particle can exist in multiple states at the same time.

**Quantum Field Theory (QFT)**

Quantum field theory takes these ideas a step further by **treating particles as excitations of a field, rather than as point-like objects.**

A field can be thought of as a mathematical entity that exists throughout all space and time and is associated with a particular type of particle. For example, the electromagnetic field is associated with photons, and the Higgs field is associated with Higgs bosons.

In quantum field theory, particles are created and destroyed continuously, and their interactions are described by the exchange of virtual particles. These virtual particles are short-lived and cannot be directly observed, but their presence can be inferred through the effects they have on other particles.

**Renormalization**

One of the key concepts in quantum field theory is the idea of renormalization, which is a mathematical technique used to deal with infinities that can arise in calculations. Renormalization allows physicists to make predictions about the behavior of particles and their interactions that are in good agreement with experimental observations.

**Applications**

Quantum field theory has had a number of important applications, including the **prediction of the Higgs boson** and the **development of the Standard Model**, which is a theoretical framework that describes the fundamental particles and forces of nature.

It has also been used to study a wide range of phenomena, including the behavior of materials at very high temperatures and densities, and the behavior of matter in extreme conditions, such as those found in the early universe.