A blog-lecture version of chapter two section two of Modern Relativity's special relativity by David Waite.
Special relativity (SR) (aka the special theory of relativity (STR)) is the physical theory of measurement in inertial frames of reference proposed in 1905 by Albert Einstein in his article "On the Electrodynamics of Moving Bodies". It generalizes Galileo's principle of relativity - that all uniform motion was relative, and that there is no absolute and well-defined state of rest (no privileged reference frames) - from mechanics to all the laws of physics, including electrodynamics.To stress this point, Einstein not only widened the postulate of relativity, but added the second postulate that all observers will always measure the speed of light to be the same no matter what their state of uniform linear motion.This theory has a variety of surprising consequences that violate common sense, but all have been experimentally verified. Special relativity overthrows Newtonian notions of absolute space and time by stating that time and space are perceived differently in the sense that measurements of length and time intervals depend on the motion of the observer. It yields the equivalence of matter and energy, as expressed in the mass-energy equivalence formula E = mc2, where c is the speed of light in a vacuum. Special relativity agrees with Newtonian mechanics in their common realm of applicability, in experiments in which all velocities are small compared to the speed of light.The theory was called "special" because it applies the principle of relativity only to inertial frames. Einstein developed general relativity to apply the principle generally, that is, to any frame, and that theory includes the effects of gravity. Special relativity does not account for gravity, but it can deal with accelerations.Although special relativity makes some quantities relative, such as time, that we would have imagined to be absolute based on everyday experience, it also makes absolute some others that were thought to be relative. In particular, it states that the speed of light is the same for all observers, even if they are in motion relative to one another. Special relativity reveals that c is not just the velocity of a certain phenomenon - light - but rather a fundamental feature of the way space and time are tied together. In particular, special relativity states that it is impossible for any material object to accelerate to light speed.