A test particle is a concept used in physics, particularly in fields such as classical mechanics, general relativity, and astrophysics. It refers to an idealized particle that is used to probe the effects of a force field or gravitational field without being affected by its own gravitational influence.
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A test particle is an idealized body whose own fields and effect on its environment are negligible. Its motion probes an externally prescribed field. In general relativity, a free massive particle in this limit follows a timelike geodesic, while a free massless particle follows a null geodesic. Nongravitational forces can change either path; appreciable self-gravity requires a more complete treatment.