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The principle of relativity requires that the speed vA, B of A with respect to B be the same as the speed vA, B of B with respect to A for all ordered pairs A, B. It also requires that the interval x used to calculate dx/dt = vA, B be defined for a particular experiment by resting endpoints. The rest frame of the endpoints is preferred for the experiment in that only the Lorentz contraction of x applies to dx/dt = vA, B = vB, A. Consequently, relativistic time dilation is asymmetrical, the clocks in the frame of x running faster than the clocks in a moving frame for all observers. This is analogous to the way in which the zero-momentum frame must be used to predict the outcome of dynamic particle experiments. In general, the principle of relativity does not say that for a given experiment E there is no preferred frame R(E). Rather, it states that there is no one preferred frame R for all E; that is, no absolute frame.

By direct substitution from E = mc² and = h(mv)⁻¹, energy becomes E = hc²(v)⁻¹. This reduces to Planck's equation if, and only if, v = c. It follows from Snell's law that a photon undergoing refraction will gain energy E = hc²[(v)⁻¹ − (₀C)⁻¹], where ₀ is its wavelength in vacuo. This very small energy gain arises from work done on the photon's mass by the refracting medium in decelerating the photon from c to v. The medium therefore looses internal energy while the photon is passing through, regaining it when the photon leaves to resume speed c. Photon momentum is a linear, monotone-increasing function of speed, p = hc(₀v)⁻¹. The reason photons do not have rest mass is because they cannot rest in space (v = 0) for the same reason massive particles cannot rest in time (v = c): In either case the energy would be infinite. EPR-type paradoxes can be resolved by replacing the notion of self-interference with recognition of the fact that _x is the extent of the x-axis that is instantaneously occupied by a particle.