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 occurs due to the exchange of virtual photons.

Pairs

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.

Quantum field theory

In quantum field theory the number of particles in an area of space is not a well-defined quantity (because of problems involving Newton-Wigner localization ), 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.

The concept of a virtual particle only makes sense in the interaction picture, which does not exist according to Haag's theorem.

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.

History

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.