Test particle

In physical theories, a test particle is an idealized model of an object whose physical properties (usually mass, charge, or size) are assumed to be negligible except for the property being studied, which is considered to be insufficient to alter the behavior of the rest of the system. The concept of a test particle often simplifies problems, and can provide a good approximation for physical phenomena. In addition to its uses in the simplification of the dynamics of a system in particular limits, it is also used as a diagnostic in computer simulations of physical processes.

Classical gravity

The easiest case for the application of a test particle arises in Newtonian gravity. The general expression for the gravitational force between two masses and is:

where and represent the position of each particle in space. In the general solution for this equation, both masses rotate around their center of mass, in this specific case:

In the case where one of the masses is much larger than the other (), one can assume that the smaller mass moves as a test particle in a gravitational field generated by the larger mass, which does not accelerate. By defining the gravitational field as