Decoupling

In physics, decoupling is the general phenomenon in which the interactions between some physical objects (such as elementary particles) disappear.

The term "decoupling" is used in many different contexts.

In gauge theories, there are unobserved polarizations of elementary particles, such as the longitudinal photon. The experiments as well as theoretical consistency dictates that these polarizations cannot be produced by collisions of other particles. Consequently, their interactions with other (physical) particles must be equal to zero, and quantum electrodynamics confirms this expectation: decoupling is a consequence of gauge symmetry.

In cosmology, the term "decoupling" is often used for the moment of recombination when electrons became bound to atomic nuclei, producing Cosmic Microwave Background as we know it.