QUANTUM PHYSICS

A quantum theory is a theory of physics that uses Planck's constant. In contrast to classical physics, where variables are often continuous, many of the variables in a quantum theory take on discrete values. The quantum was a concept that grew out of the realisation that electromagnetic radiation came in discrete packets, called quanta. The process of converting a classical theory into a quantum theory is called quantisation and is divided into stages: first quantisation, second quantisation, etc depending on the extent to which the theory is quantised.

Quantum physics is the set of quantum theories. There are several of them:

• quantum mechanics -- a first quantised or semi-classical theory in which particle properties are quantised, but not particle numbers, fields and fundamental interactions.
• quantum field theory or QFT -- a second or canonically quantized theory in which all aspects of particles, fields and interactions are quantised, with the exception of gravitation. Quantum electrodynamics, quantum chromodynamics and electroweak theory are examples of relativistic fundamental QFTs which taken together form the Standard Model. Solid state physics is a non-fundamental QFT.
• quantum gravity -- a third quantised theory in which general relativity (i.e. gravity) is also quantised. This theory is incomplete, and is hoped to be finalised within the framework of a theory of everything, such as string theory or M-theory.