Theoretical physics

Theoretical physics is a branch of physics that uses mathematical models and abstractions of physical objects and systems to explain and predict natural phenomena. It is, in the broadest sense, the attempt to say why things happen the way they do, not merely to record that they do. This is in contrast to experimental physics, which tests and refines those explanations through direct measurement and observation. In practice, the two feed each other constantly: a theoretical prediction suggests an experiment, and an unexpected experimental result sends theorists back to the drawing board.

The scope of theoretical physics is enormous. It ranges from the behaviour of quarks and elementary particles at scales far smaller than an atom to the large-scale structure of the universe itself. Where direct experimentation is impossible or simply not yet feasible, theoretical physics advances understanding through mathematical reasoning and thought experiments. This is perhaps the thing that surprises people most about the field: some of its most important results have come from pure reasoning, long before any instrument could test them. General relativity, quantum mechanics, and the Standard Model of particle physics each originated primarily as theoretical constructions, only later confirmed by experiment.