# Effective theory

In science, an **effective theory** is a scientific theory which proposes to describe a certain set of observations, but explicitly without the claim or implication that the mechanism employed in the theory has a direct counterpart in the actual causes of the observed phenomena to which the theory is fitted.
I.e. the theory proposes to model a certain *effect*, without proposing to adequately model any of the *causes* which contribute to the effect.

Thus, an effective field theory is a theory which describes phenomena in solid-state physics, notably the BCS theory of superconduction, which treats vibrations of the solid-state lattice as a "field" (i.e. without claiming that there is "really" a field), with its own field quanta, called phonons. Such "effective particles" derived from effective fields are also known as quasiparticles.

In a certain sense, quantum field theory, and any other currently known physical theory, could be described as "effective", as in being the "low energy limit" of an as-yet unknown "Theory of Everything".[1]

## References

- c.f. e.g. Ion-Olimpiu Stamatescu, Erhard Seiler,
*Approaches to fundamental physics: an assessment of current theoretical ideas*, Lecture Notes in Physics, vol. 721, Springer, 2007, ISBN 978-3-540-71115-5, p. 47