An Etymological Dictionary of Astronomy and Astrophysics
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In → plasma physics, → fluid dynamics, and → self-gravitating systems, a particle or ensemble of particles which are affected by the evolution of the rest of the system, but do not affect the rest of the system. The test particle concept is used, e.g., for revealing a flow, describing the path of fluid elements, and so on.

→ test; → particle.

A subatomic particle that, according to the uncertainty principle, comes into being out of energy fluctuations of the "vacuum" and lasts for extremely short periods of time. An electron-positron pair can exist only about 4 x 10^-21 seconds. The lifetime increases as the mass and energy involved decreases. Virtual particles are real and have measurable effects, but cannot be directly observed, according to the uncertainty principle. → vacuum polarization.

→ virtual; → particle

The principle admitted in → quantum mechanics that all particles have a wave-like nature and that waves have a particle aspect. The wave-particle duality is of fundamental importance in obtaining a realistic picture of the → elementary particles.
See also: → particle nature, → wave nature, → de Broglie hypothesis, → Davisson-Germer experiment.

→ wave; → particle; → duality.