In the description of the interaction between elementary particles in quantum field theory, a virtual particle is a temporary elementary particle, used to describe an intermediate stage in the interaction. A virtual particle never is the end result of such a process.
Virtual particles always come in pairs, a particle and antiparticle, which can be of any kind. They exist for an extremely short time. Virtual particles are always created as a particle-antiparticle pair, and mutually annihilate in short order. In some cases, however, it is possible to boost the pair apart using external energy so that they avoid annihilation and become real particles. This is the process by which black holesevaporate.
In quantum field theory the number of particless 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 of vacuum energy. Mathematically, this is expressed by the fact that the particle number operator does not commute with the Hamiltonian.
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. Examining normal physical processes with knowledge of this particle-antiparticle phenomenon can lead to interesting insights such as quantum electrodynamics. 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.
Paul Dirac was the first to propose that empty space (the vacuum) can be visualized as consisting of a sea of virtual electron-positron pairs that can only be released or separated when sufficient energy is made available.
