Virtual particles are subatomic particles that form out of "nothing" (vacuum fields conceptually analogous to lines of force between magnetic poles) for extremely short periods of time and then disappear again. Such particles permeate space, mediate particle decay, and mediate the exchange of the fundamental forces (electromagnetic, weak, strong, and—in accord with quantum theory—gravititational forces). Virtual particles are real and have measurable effects, but the same uncertainty principle that allows them to come into existence dictates that they cannot be directly observed.
Heisenberg's uncertainty principle, which explains the virtual particle phenomenon, is most commonly stated as follows: It is impossible to exactly and simultaneously measure both the momentum and position of a particle. There is always an uncertainty in momentum and an uncertainty in position. More importantly, these two uncertainties cannot be reduced to zero together.
One consequence of Heisenberg's uncertainty principle is that the energy and duration of a particle are also characterized by complementary uncertainties. There is always, at every point in space and time, even in a perfect vacuum, an uncertainty in energy and an uncertainty in duration, and these two complementary uncertainties cannot be reduced to zero simultaneously.
The meaning of Heisenberg's uncertainty principle is that "something" can arise from "nothing" if the "something" returns to the "nothing" after a very short time—an interval too short in which to be observed. These micro-violations of energy conservation are not only allowed to happen, they do, and so "empty" space is seething with particle-antiparticle pairs that come into being and then annihilate each other again after a very short interval. Although these particles cannot be observed individually, their existence can be demonstrated.
Normally, a metal plate experiences a storm of fleeting impacts from virtual particles on both of its surfaces; this "vacuum pressure" is equal on both sides of the plate, and so cancels out. If, however, two parallel metal plates are too closely spaced to allow the formation of relatively large virtual particles between them, the vacuum pressure between the plates is less than that on their outer surfaces, and they experience a net force pushing them together. This force is termed the Casimir effect after Dutch physicist Hendrik Casimir (1909–2000), who predicted its existence in 1948, and was experimentally measured in 1997.
The Casimir effect is only one manifestation of the reality of virtual particles. Virtual particles also mediate the exchange of all forces between particles. For example, when an electron experiences electrical repulsion from another electron (electrons are negatively charged, and like charges repel), it is actually exchanging virtual photons with that other electron. Higher-energy virtual photons are only allowed by the uncertainty principle to exist for shorter periods of time, as shown by the uncertainty equation, and thus cannot travel as far as lower-energy virtual photons; this explains why the electric force is stronger at short distances. (In fact, all the basic forces—electric, strong, weak, and gravitational—diminish with distance for this reason. Gravity, however, has not been satisfactorily integrated with the equations that describe the other three forces.)
A third role for virtual particles is in decay mediation. When an unstable subatomic particle decays (i.e., breaks down into two or more other subatomic particles), it does so by first taking the form of a virtual particle. The virtual particle then completes the decay process. In some cases, the intermediate virtual particle has more mass than the initial particle or the final set of decay products; this does not violate the conservation of mass because the intermediate particle is virtual, that is, exists for such a short period of time that it falls within the uncertainty bounds prescribed for the system's energy by the Heisenberg uncertainty principle.
This list of phenomena does not describe all the properties of virtual particles, but does indicate their prevalence.
See also Quantum mechanics.
Barnett, R. Michael, Henry Mühry, and Helen R. Quinn. The Charm of Strange Quarks. New York: Springer-Verlag, 2000.
Ne'eman, Yuval, and Yoram Kirsh. The Particle Hunters. Cambridge, UK: Cambridge University Press, 1996.
Lambrecht, Astrid. "The Casimir Effect: A Force From Nothing." PhysicsWeb. September 2002 [cited February 14, 2003]. <http://physicsweb.org/article/world/15/9/6>.