Mass in special relativity what causes these particles to be termed "massless." However, even massless particles have a relativistic mass, which varies with their observed energy in various frames of reference, The invariant mass is the ratio of four-momentum (the four-dimensional generalization of classical momentum) to four-velocity: and is also the ratio of four-acceleration to four-force when the rest mass is constant. The four-dimensional form of Newton's second law is: The relativistic expressions for "E" and "p" obey the relativistic energy–momentum relation: where the m is the rest mass, or the invariant mass for systems, and "E" is the total energy. The equation is
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