|
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.
Pairs
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 holes evaporate.
Quantum field theory
In quantum field theory 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. 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.
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.
|