theoretical physics Theoretical physics is a branch of physics that employs mathematical models and abstractions of physical objects and systems to rationalize, explain and predict natural phenomena. This is in contrast to experimental physics, which uses experim ...

, ultraviolet completion, or UV completion, of a

quantum field theory In theoretical physics, quantum field theory (QFT) is a theoretical framework that combines classical field theory, special relativity, and quantum mechanics. QFT is used in particle physics to construct physical models of subatomic particles and ...

is the passing from a lower energy

to a more general quantum field theory above a threshold value known as the cutoff. In particular, the more general high energy theory must be well-defined at arbitrarily high energies. The word "ultraviolet" in this so-called "ultraviolet regime" is only figurative, and refers to energies much higher than

ultraviolet light Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...

''per se''. Rather, by analogy to the relationship between ultraviolet and visible light, it refers to energies higher than (and wavelengths shorter than) those "visible" to laboratory experiment. The ultraviolet theory must be

renormalizable Renormalization is a collection of techniques in quantum field theory, the statistical mechanics of fields, and the theory of self-similarity, self-similar geometric structures, that are used to treat infinity, infinities arising in calculated ...

; it can have no

Landau pole In physics, the Landau pole (or the Moscow zero, or the Landau ghost) is the momentum (or energy) scale at which the coupling constant (interaction strength) of a quantum field theory becomes infinite. Such a possibility was pointed out by the ph ...

s; and most typically, it enjoys

asymptotic freedom In quantum field theory, asymptotic freedom is a property of some gauge theories that causes interactions between particles to become asymptotically weaker as the energy scale increases and the corresponding length scale decreases. Asymptotic fr ...

in the case that it is a quantum field theory (or at least has a nontrivial fixed point). However, it may also be a background of

string theory In physics, string theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings. String theory describes how these strings propagate through space and interac ...

whose ultraviolet behavior is at least as good as that of renormalizable quantum field theories. Besides these two known examples (QFT and string theory), it could be a completely different theory than string theory that behaves well at very high energies. There is an analogous phrase "infrared completion", which applies to length scales longer than those "visible" to normal experiment, particularly

cosmology Cosmology () is a branch of physics and metaphysics dealing with the nature of the universe. The term ''cosmology'' was first used in English in 1656 in Thomas Blount (lexicographer), Thomas Blount's ''Glossographia'', and in 1731 taken up in ...

distances.

See also