Relativistic speed
Relativistic speed refers to speed at which relativistic effects become significant to the desired accuracy of measurement of the phenomenon being observed. Relativistic effects are those discrepancies between values calculated by models considering and not considering relativity. Related words are velocity, rapidity, and celerity which is proper velocity. Speed is a scalar, being the magnitude of the velocity vector which in relativity is the four-velocity and in three-dimension Euclidean space a three-velocity. Speed is empirically measured as average speed, although current devices in common use can estimate speed over very small intervals and closely approximate instantaneous speed. Non-relativistic discrepancies include cosine error which occurs in speed detection devices when only one scalar component of the three-velocity is measured and the Doppler effect which may affect observations of wavelength and frequency.Relativistic effects are highly non-linear and for everyday purposes are insignificant because the Newtonian model closely approximates the relativity model. In special relativity the Lorentz factor is a measure of time dilation, length contraction and relativistic mass increase of a moving object.
Speed of light
The speed at which light waves propagate in vacuum is independent both of the motion of the wave source and of the inertial frame of reference of the observer. This invariance of the speed of light was postulated by Einstein in 1905, after being motivated by Maxwell's theory of electromagnetism and the lack of evidence for the luminiferous aether; it has since been consistently confirmed by many experiments. It is only possible to verify experimentally that the two-way speed of light (for example, from a source to a mirror and back again) is frame-independent, because it is impossible to measure the one-way speed of light (for example, from a source to a distant detector) without some convention as to how clocks at the source and at the detector should be synchronized. However, by adopting Einstein synchronization for the clocks, the one-way speed of light becomes equal to the two-way speed of light by definition. The special theory of relativity explores the consequences of this invariance of "c" with the assumption that the laws of physics are the same in all inertial frames of reference. One consequence is that "c" is the speed at which all massless particles and waves, including light, must travel in vacuum.
Special relativity has many counterintuitive and experimentally verified implications. These include the equivalence of mass and energy , length contraction (moving objects shorten), and time dilation (moving clocks run more slowly). The factor "γ" by which lengths contract and times dilate is known as the Lorentz factor and is given by , where "v" is the speed of the object. The difference of "γ" from 1 is negligible for speeds much slower than "c", such as most everyday speeds—in which case special relativity is closely approximated by Galilean relativity—but it increases at relativistic speeds and diverges to infinity as "v" approaches "c". For example, a time dilation factor of "γ" = 2 occurs at a relative velocity of 86.6% of the speed of light ("v" = .866"c"). Similarly, a time dilation factor of "γ" = 10 occurs at "v" = 99.5% "c".