Mass in special relativity (including the kinetic energy of the gas) only when it is measured in the center of momentum frame. Just as is the case for single particles, the calculated "rest mass" of such a container of gas does not change when it is in motion, although its "relativistic mass" does change. The container may even be subjected to a force which gives it an overall velocity, or else (equivalently) it may be viewed from an inertial frame in which it has an overall velocity (that is, technically, a frame in which its center of mass has a velocity). In this case,
Chirp mass terms of other common mass parameters: where formula_7 is the total mass of the system, formula_8 is the reduced mass of the system, formula_9 is the mass ratio, and formula_10 is the symmetric mass ratio. In general relativity, the phase evolution of a binary orbit can be computed using a post-Newtonian expansion, a perturbative expansion in powers of the orbital velocity formula_11. The first order gravitational wave frequency, formula_12, evolution is described by the differential equation where formula_14 and formula_15 are the speed of light and Newton's gravitational constant, respectively. Integrating equation () with respect to time gives: where C