Definition · Plain-language
Theory of relativity
The theory of relativity is Einstein’s account of how space, time and gravity work, comprising special relativity and general relativity.
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Special relativity
Special relativity, published by Albert Einstein in 1905, rests on two ideas: the laws of physics are the same for all observers moving at constant velocity, and the speed of light in a vacuum is the same for everyone, no matter how they move. These simple postulates have startling consequences. Time runs slower for a fast-moving clock (time dilation), moving objects contract along their direction of motion (length contraction), and mass and energy are equivalent, captured in the famous equation E = mc². It is called "special" because it applies to the special case of motion without acceleration or gravity.
General relativity
In 1915 Einstein extended his ideas to include gravity and acceleration, producing general relativity. Its central insight is that gravity is not a pull transmitted across space but the result of mass and energy bending the fabric of spacetime. Objects follow the straightest possible paths through this curved geometry, which we experience as gravitational attraction. A common picture is a heavy ball resting on a stretched sheet: it makes a dip, and smaller balls roll toward it. The theory predicts that light bends near massive bodies, time runs slower in strong gravity, and ripples called gravitational waves can spread through spacetime.
How we know it is right
Relativity is not just elegant; it has passed every experimental test. The bending of starlight by the Sun was measured during a 1919 eclipse. Atomic clocks flown on aircraft and satellites run measurably slower or faster as relativity predicts, an effect that satellite navigation systems must correct for to stay accurate. Gravitational waves from colliding black holes were detected directly in 2015, a century after Einstein predicted them. These confirmations make relativity one of the best-tested theories in all of science.
Key facts
At a glance
- Definition: Einstein’s theory of space, time and gravity
- Two parts: special relativity (1905) and general relativity (1915)
- Special: light speed is constant; time and space are relative to motion
- General: gravity is the curvature of spacetime by mass and energy
- Famous result: mass–energy equivalence, E = mc²
- Confirmed by: starlight bending, atomic clocks, gravitational waves
Common misconceptions
What people often get wrong
Often heard: Relativity means everything is just a matter of opinion.
Actually: It is the opposite. Relativity says the laws of physics and the speed of light are absolute — the same for everyone. What is relative is the measurement of time and space, in a precise, predictable way.
Often heard: General relativity replaced Newton’s gravity and proved it wrong.
Actually: Newton’s law of gravity remains an excellent approximation for everyday situations. General relativity refines it, mattering most for strong gravity and high precision, and reduces to Newton’s description in ordinary conditions.
Often heard: E = mc² is only about nuclear bombs.
Actually: The equation states a general equivalence of mass and energy that applies everywhere, including stars, particle physics and even chemical reactions, where the mass change is just far too small to notice.
Going deeper
