Editing Mass in special relativity (section)

== Rest mass ==
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 ''[[invariant mass]]'' is another name for the ''rest mass'' of single particles. The more general invariant mass (calculated with a more complicated formula) loosely corresponds to the "rest mass" of a "system". 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. 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''<sup>2</sup> (the [[speed of light]] squared).

The concept of invariant mass does not require bound systems of particles, however. As such, it may also be applied to systems of unbound particles in high-speed relative motion. Because of this, it is often employed in particle physics for systems which consist of widely separated high-energy particles. If such systems were derived from a single particle, then the calculation of the invariant mass of such systems, which is a never-changing quantity, will provide the rest mass of the parent particle (because it is conserved over time).

It is often convenient in calculation that the invariant mass of a system is the total energy of the system (divided by {{math|''c''<sup>2</sup>}}) in the COM frame (where, by definition, the momentum of the system is zero). However, since the invariant mass of any system is also the same quantity in all inertial frames, it is a quantity often calculated from the total energy in the COM frame, then used to calculate system energies and momenta in other frames where the momenta are not zero, and the system total energy will necessarily be a different quantity than in the COM frame. As with energy and momentum, the invariant mass of a system cannot be destroyed or changed, and it is thus conserved, so long as the system is closed to all influences. (The technical term is [[isolated system]] meaning that an idealized boundary is drawn around the system, and no mass/energy is allowed across it.)