Special Relativity
Einstein's 1905 theory of space and time, built on the constancy of the speed of light and the equivalence of all inertial frames.
Special relativity is the theory of space and time published by Albert Einstein in 1905. It resolves a conflict that had emerged at the end of the 19th century: Maxwell's equations imply that light travels at a fixed speed, but classical mechanics assumed that speeds add between moving observers. The theory rests on two postulates: the laws of physics are the same in all inertial (non-accelerating) frames, and the speed of light in vacuum is the same in every inertial frame, independent of the motion of the source or observer.
The consequences follow from those postulates. There is no absolute simultaneity: two events that are simultaneous for one observer occur at different times for another observer moving relative to the first. Time itself dilates — a moving clock runs slow by the Lorentz factor γ = 1/√(1 − v2/c2) — and lengths contract along the direction of motion. The effects are negligible at everyday speeds (γ ≈ 1.000000000005 for a jet aircraft) and become extreme as speed approaches the speed of light.
The most famous consequence is mass–energy equivalence, E = mc^2, which states that the energy content of a body equals its mass times the square of the speed of light. Nuclear reactions, where a small fraction of mass becomes energy, are direct confirmations; the Sun's power and nuclear power plants both draw on this relation.
The "twin paradox" is the classic puzzle of the theory: a twin who travels at high speed and returns is younger than the one who stayed. The resolution is that the traveling twin accelerates — changing frames — while the stay-at-home twin remains in a single inertial frame, so the situation is not symmetric. Special relativity is confirmed by experiments every day: muons created in the upper atmosphere reach the ground only because time dilation extends their lifetime, GPS satellites must correct their clocks for relativistic effects, and particle accelerators could not operate without relativistic kinematics.
In 1908 Hermann Minkowski reformulated the theory geometrically: space and time are not separate, but coordinates of a single four-dimensional spacetime, in which the interval between events — not their separation in space or time alone — is invariant. This geometric view became the foundation for general relativity, Einstein's 1915 theory of gravity.