The theory of special relativity was enunciated by Einstein (1905) and explains in a simple way the ideas or results that already existed in the previous work of various other scientists: Fitzgerald (length contraction), Lorentz (length contraction, time dilation, Lorentz transformations, etc.), Poincaré (variation inertia with speed, group structure of Lorentz transformations, etc.), Michelson and Morley (interferometry experiment showing the invariance of the speed of light). But while some accuse Einstein of base plagiarism, it is undeniable that he had the merit of being the first to propose a clear and precise theory taking into account the disparate results in a uniform manner, and above all basing it on experimental observations and predicting new hitherto unknown effects.

Unlike his predecessors who were looking for complex physical explanations for their various results, Einstein went much further by going back to the basic principles of Newtonian physics. Thus, one of the great pillars of special relativity is the reappraisal of the notions of simultaneity and absolute space, one result of which is to rule out the existence of any special reference frame (thereby making the hypothesis of the existence of the ether obsolete). From the rejection of certain postulates that up to then had been taken for granted, Einstein's theory widened Galileo's principle of relativity by postulating the equivalence of all inertial frames of reference (also called "Galilean" or "Lorenzian" frames of reference), not only for the formulation of the laws of mechanics, but in a general manner for all the laws of physics, including electromagnetism. This enunciation was in agreement with the Michelson Morley experiment since the invariance of the laws of electromagnetism (the Maxwell equations) implies the invariance of the speed of light, which is assumed to have a finite and identical value in all reference frames.

There are numerous consequences of the special theory of relativity, amongst which the most famous and immediate are: the impossibility of instantaneous interactions at a distance, the mass-energy equivalence (which is often expressed by Einstein's celebrated formula, E=mc²), the impossibility for a particle of matter of reaching or exceeding the speed of light, and the existence of antimatter (a result which also depends on quantum physics). There have been many experimental verifications of the special theory of relativity and these have become a daily occurrence, for example in particle accelerators or even in hospitals since positron emission tomography uses antimatter.

It is important to understand that special relativity evolved very rapidly after Einstein first formulated it. The current formulation (which Einstein used to formulate general relativity) depends on the notion of four-dimensional space-time in which time and space cannot be apprehended independently, unlike what happens in Newtonian physics. This formulation is due to one of Einstein's former teachers, Hermann Minkowski, who gave his name to the space-time of special relativity. With this new approach, the notion of invariant space-time distance becomes primordial, relegating the Lorentz transformations and the propagation of light to secondary roles in the theory.