Special relativity (SR, also known as the special theory of relativity or STR) is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".

It extends Galileo''s principle of relativity—that all uniform motion is relative, and that there is no absolute and well-defined state of rest (no privileged reference frames)—to account for the constant speed of light.which was previously observed in theMichelson-Morley experiment—and postulates that it holds for all the laws of physics, including both the laws of mechanics and ofelectrodynamics, whatever they may be

This theory has a wide range of consequences which have been experimentally verified, including counter-intuitive ones such aslength contraction, time dilation and relativity of simultaneity. It has replaced the classical notion of invariant time interval for two events with the notion of invariant space-time interval. Combined with other laws of physics, the two postulates of special relativity predict the equivalence of mass and energy, as expressed in the mass–energy equivalence formula E = mc², where c is the speed of light in vacuum.The predictions of special relativity agree well with Newtonian mechanics in their common realm of applicability, specifically in experiments in which all velocities are small compared with the speed of light. Special relativity reveals that c is not just the velocity of a certain phenomenon—namely the propagation of electromagnetic radiation (light)—but rather a fundamental feature of the way space and time are unified as spacetime. One of the consequences of the theory is that it is impossible for any particle that has rest mass to be accelerated to the speed of light.

The theory was originally termed "special" because it applied the principle of relativity only to the special case of inertial reference frames, i.e. frames of reference in uniform relative motion with respect to each other.^[7] Einstein developed general relativity to apply the principle in the more general case, that is, to any frame so as to handle general coordinate transformations, and that theory includes the effects of gravity.

Relativistic quantum mechanics

Special relativity can be combined with quantum theory to form relativistic quantum mechanics. It is an unsolved question how general relativity and quantum mechanics can be unified; quantum gravitation is an active area in theoretical research.

The early Bohr-Sommerfeld atomic model explained the fine structure of alkaline atoms by using both special relativity and the preliminary knowledge on quantum mechanics of the time.^{[citation needed]}

Paul Dirac developed a relativistic wave equation now known as the Dirac equation in his honour, fully compatible both with special relativity and with the final version of quantum theory existing after 1926. This theory explained not only the intrinsic angular momentum of the electrons called spin, a property which can only bestated, but not explained by non-relativistic quantum mechanics, and led to the prediction of the antiparticle of the electron, the positron. Also the fine structure could only be fully explained with special relativity.

On the other hand, the existence of antiparticles leads to the conclusion that a naive unification of quantum mechanics (as originally formulated by Erwin Schrödinger,Werner Heisenberg, and many others) with special relativity is not possible. Instead, a theory of quantized fields is necessary; where particles can be created and destroyed throughout space, as in quantum electrodynamics and quantum chromodynamics. These elements merge in the standard model of particle physics.