Relativity

Special relativity combines equal physical laws in inertial frames with a common vacuum light speed. It selects no privileged state of absolute rest. Observers need not agree on every measured quantity; instead, precise transformation rules relate their measurements of space, time, energy and momentum.

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Elapsed time depends on the events and clock paths being compared. Time dilation has measurable consequences, including the observed survival of fast-moving unstable particles. At ordinary low speeds the corrections are small, which helps explain the continuing usefulness of classical mechanics in many everyday calculations.

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Rest mass corresponds to rest energy E=mc²; a moving body also has kinetic energy. The expression is therefore not a universal formula for kinetic energy alone. Accelerating a massive body toward light speed requires increasing energy without allowing it to attain that speed.

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General relativity connects gravity with dynamical spacetime geometry. Matter and energy affect geometry, which in turn affects motion. Light deflection, black holes and gravitational waves follow within this framework. The theory is a specific account of gravity, not a claim that every statement is merely relative.

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