Einstein Relativity: Special and General

Two interrelated theories: special and general relativity

Einstein’s theory of relativity consists of two interrelated theories: special relativity (proposed in 1905) and general relativity (published in 1915). Special relativity applies to all physical phenomena in the absence of gravity and is based on two postulates that replace classical mechanics’ assumptions with Lorentz transformations. It leads to effects such as relativity of simultaneity, time dilation, length contraction, a finite maximum speed (the speed of light), and mass–energy equivalence (E = mc2). General relativity extends these ideas to gravitation and explains the law of gravitation as a feature of spacetime itself. It begins with the equivalence principle, treating accelerated motion and free-fall in a gravitational field as physically identical, which implies that free fall is inertial motion. Einstein then formulated gravity using curved spacetime and the Einstein field equations, linking spacetime curvature to mass, energy, and momentum. The theory predicts phenomena including gravitational time dilation, orbital precession, light deflection, frame-dragging, and cosmic expansion, and it has been repeatedly confirmed by experiments and observations. Both theories are “principle-theories” grounded in experimentally verified regularities, making testable predictions. Special relativity was validated through key experiments such as Michelson–Morley, Kennedy–Thorndike, and Ives–Stilwell, while general relativity has been confirmed by classic tests like Mercury’s perihelion precession, deflection of light by the Sun, and gravitational redshift. Relativity also became essential for practical technologies (e.g., GPS) and for explaining major astrophysical discoveries such as neutron stars, black holes, and gravitational waves.

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Special relativity: no gravity regime

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