Einstein Relativity: Special and General

Two interrelated theories of relativity

Albert Einstein’s theory of relativity consists of two interrelated theories: special relativity (published in 1905) and general relativity (published in 1915). Special relativity applies to all physical phenomena in the absence of gravity and is built on two postulates that replace the classical (Galilean) transformation rules with Lorentz transformations. It leads to core 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 the framework to include gravity. It begins with the equivalence principle, which treats accelerated motion and free-fall in a gravitational field as physically identical, implying that free fall is inertial motion. Einstein then reformulated gravity not as a conventional force but as the curvature of spacetime, expressed through the Einstein field equations that relate spacetime curvature to mass, energy, and momentum. This theory explains and predicts phenomena such as gravitational time dilation, orbital precession, light deflection, frame-dragging, and the expansion of the universe, and it has been repeatedly confirmed by experiments and observations.

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Special relativity: no gravity regime

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