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. That means, the theory proposes to model a certain effect, without proposing to adequately model any of the causes which contribute to the effect.

For example, 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]

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  1. ^ c.f. Stamatescu, Ion-Olimpiu; Seiler, Erhard (2007). Approaches to Fundamental Physics: An Assessment of Current Theoretical Ideas. Lecture Notes in Physics. vol. 721. Springer. p. 47. ISBN 978-3-540-71115-5.