Editing Virtual particle (section)

== Pair production ==
{{Main article|Pair production}}
Virtual particles are often popularly described as coming in pairs, a [[Elementary particle|particle]] and  [[antiparticle]] which can be of any kind. These pairs exist for an extremely short time, and then mutually annihilate, or in some cases, the pair may be boosted apart using external energy so that they avoid annihilation and become actual particles, as described below.

This may occur in one of two ways. In an accelerating [[frame of reference]], the virtual particles may appear to be actual to the accelerating observer; this is known as the [[Unruh effect]].  In short, the vacuum of a stationary frame appears, to the accelerated observer, to be a warm [[gas]] of actual particles in [[thermodynamic equilibrium]].

Another example is pair production in very strong electric fields, sometimes called [[vacuum decay]]. If, for example, a pair of [[atomic nucleus|atomic nuclei]] are merged to very briefly form a nucleus with a charge greater than about 140, (that is, larger than about the inverse of the [[fine-structure constant]], which is a [[dimensionless quantity]]), the strength of the electric field will be such that it will be energetically favorable{{explain|reason=In a closed system, energy is conserved, so what is meant by {{''}}energetically favorable{{''}} and why does this lead to positron–electron pair creation?|date=November 2021}} to create positron–electron pairs out of the vacuum or [[Dirac sea]], with the electron attracted to the nucleus to annihilate the positive charge. This pair-creation amplitude was first calculated by [[Julian Schwinger]] in 1951.