Because of this, it is often employed in particle physics for systems which consist of widely separated high-energy particles. As such, it may also be applied to systems of unbound particles in high-speed relative motion. The concept of invariant mass does not require bound systems of particles, however. Under such circumstances the invariant mass is equal to the relativistic mass (discussed below), which is the total energy of the system divided by c 2 (the speed of light squared). Thus, invariant mass is a natural unit of mass used for systems which are being viewed from their center of momentum frame (COM frame), as when any closed system (for example a bottle of hot gas) is weighed, which requires that the measurement be taken in the center of momentum frame where the system has no net momentum. The more general invariant mass (calculated with a more complicated formula) loosely corresponds to the 'rest mass' of a 'system'. The invariant mass is another name for the rest mass of single particles. The term mass in special relativity usually refers to the rest mass of the object, which is the Newtonian mass as measured by an observer moving along with the object. The concept is generalized in mass in general relativity. For example, photons have zero rest mass but contribute to the inertia (and weight in a gravitational field) of any system containing them. The measurable inertia and the warping of spacetime by a body in a given frame of reference is determined by its relativistic mass, not merely its invariant mass. In contrast, 'invariant mass' is usually preferred over rest energy. The term 'relativistic mass' tends not to be used in particle and nuclear physics and is often avoided by writers on special relativity, in favor of referring to the body's relativistic energy. According to the concept of mass–energy equivalence, invariant mass is equivalent to rest energy, while relativistic mass is equivalent to relativistic energy (also called total energy). The word ' mass' has two meanings in special relativity: invariant mass (also called rest mass) is an invariant quantity which is the same for all observers in all reference frames, while the relativistic mass is dependent on the velocity of the observer.

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