line of a particle, and has a length equal to one unit of time in the frame of the particle. An accelerated particle does not have an inertial frame in which it is always at rest. However, an inertial frame can always be found which is momentarily comoving with the particle. This frame, the momentarily comoving reference frame (MCRF), enables application of special relativity to the analysis of accelerated particles. Since photons move on null lines, for a photon, and a cannot be defined. There is no frame in which a photon is at rest, and no MCRF can be established along a photon's path. Energy–momentum 4-vector: As indicated before, there are varying treatments for the energy-momentum so that one may also see it expressed as or The first component is the total energy (including mass) of the particle (or system of particles) in a given frame, while the remaining components are its spatial momentum. The energy-momentum is a conserved quantity. Acceleration 4-vector: This results from taking the derivative of the velocity with respect to Force 4-vector: This is the derivative of the momentum with respect to As expected, the final components of the above are all standard corresponding to spatial , etc. 4-vectors and physical law The first postulate of special relativity declares the equivalency of all inertial frames. A physical law holding in one frame must apply in all frames, since otherwise it would be possible to differentiate between frames. Newtonian momenta fail to behave properly under Lorentzian transformation, and Einstein preferred to change the definition of momentum to one involving rather than give up on conservation of momentum. Physical laws must be based on constructs that are frame independent. This means that physical laws may take the form of equations connecting scalars, which are always frame independent. However, equations involving require the use of tensors with appropriate rank, which themselves can be thought of as being built up from . Acceleration It is a common misconception that special relativity is applicable only to inertial frames, and that it is unable to handle accelerating objects or accelerating reference frames. Actually, accelerating objects can generally be analyzed without needing to deal with accelerating frames at all. It is only when gravitation is significant that general relativity is required. Properly handling accelerating frames does require some care, however. The difference between special and general relativity is that (1) In special