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 is never the end result of such a process. Using the language of Feynman diagrams, a virtual particle is associated with the internal lines of the diagrams, the so-called propagators. These virtual particles are responsible for the interactions of the external particles (external legs of the diagram). For example, the electromagnetic interaction (referred to as photons) occurs due to the exchange of virtual photons.
The fact that the particle number operator does not commute with the Hamiltonian 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 of vacuum energy.
Virtual particles are often popularly described as coming in pairs, a particle and antiparticle, which can be of any kind. These pairs exist for an extremely short time, 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 one way of describing the process by which black holes evaporate.
The restriction to particle-antiparticle pairs is actually only necessary if the particles in question carry a conserved quantity, such as electric charge, which is not present in the initial or final state. Otherwise, other situations can arise. For instance, the beta decay of a neutron can happen through the emission of a single virtual, negatively charged W particle that almost immediately decays into a real electron and antineutrino; the neutron turns into a proton when it emits the W particle. The evaporation of a black hole is a process dominated by photons, which are their own antiparticles and are uncharged.
Black holes and Big Bangs
Noted scientists, such as Stephen Hawking, have postulated that radiation emitting from black holes are the likely result of particle pairs where one has fallen into the event horizon of the black hole. The other, without a pair to annihilate it, travels away in the form of emitting radiation.
A similar theory suggests that the universe, in the method of the Big Bang, could have formed from vacuum fluctuations, or virtual particles, appearing in the original nothing to interact in such a way with other appearing particles so that the result is not a complete annihilation of matching pairs.
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. Template:Physics-footer