Virtual particle

In quantum field theory, the uncertainty principle implies the number of particles in an area of space is not a well-defined quantity, but like other quantum observables is represented by a probability distribution. Since these particles do not have a permanent existence, they are called virtual particles or vacuum fluctuations.

Even though we can't see them, we know that these virtual particles are "really there" in empty space because they leave a detectable trace of their activities. One effect of virtual photons, for example, is to produce a tiny shift in the energy levels of atoms. They also cause an equally tiny change in the magnetic moment of electrons. These minute but significant alterations have been very accurately measured using spectroscopic techniques. The Casimir effect is an attraction between two uncharged parallel metal plates because fewer virtual particles can be created between the plates than in the surrounding space.

Virtual particles are always created as a pair of particle-antiparticle. In some cases though one particle type gets preferetially absobed (see: black hole evaporation, black hole).

Examining normal physical processes with knowledge of this particle - anti-particle phenomena can lead to interesting insight suchh as quantum electro-dynamics; where a changing electromagnetic field is seen as a consequence of changes in the energy density of space-time and thus the rate at which virtual particles are being created (and self anihilating).