Testtömege

Testtömege, or test mass, is an idealized small mass used to probe a field in physics, with the crucial assumption that its own properties do not appreciably alter the field. It serves as a simplified probe to study how a field influences motion or to characterize the field itself.

In gravitation and general relativity, a testtömege is assumed to be negligible compared with the sources that generate the gravitational field. The motion of a test mass in a given field is described, in general relativity, by a geodesic of the spacetime metric, so its trajectory reveals the geometry of spacetime rather than the field being affected by the mass. The equivalence principle suggests that all sufficiently small test masses follow the same paths in a given gravitational field, independent of their mass or composition, provided non-gravitational forces are absent.

In electromagnetism, a testtömege is embodied by a test charge—an infinitesimally small charge used to measure an electric field without meaningfully perturbing it. The field is inferred from the force experienced by the test charge under the assumption that the charge’s presence does not alter the field.

Limitations: A real object cannot be perfectly massless, so the test mass is an approximation. The assumption fails if the field sources are comparable in strength to the test mass, or if non-gravitational forces are significant.

Applications: The concept is widely used in theoretical analyses, experimental design, and numerical simulations to simplify problems and interpret measurements, including studies of orbital dynamics, field mapping, and the operation of gravitational-wave detectors.