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Glossary / History of physics

Relativity

Einstein's theory that measurements of time and space depend on the observer's motion and on gravity — special relativity (1905) and general relativity (1915–16).

Special relativity (1905) starts from the constancy of the speed of light for every observer and derives effects such as time slowing for fast-moving objects[1]. General relativity (1915–16) describes gravity as the curvature of spacetime and predicts that time runs slower where gravity is stronger[2].

The clocks on GPS satellites run slow because of their speed and fast because of weaker gravity, gaining about 38 microseconds a day net. Uncorrected, positions would drift by roughly 10 km a day, so relativity is indispensable even in everyday devices[3].

Related terms

  • Light quantumThe particle-like packet of energy that light behaves as. Now called the photon.
  • Fine structureThe splitting of what looks like a single line in an atom's spectrum into several closely spaced lines when examined in detail.

On the timeline

Sources

  1. A. Einstein, “Zur Elektrodynamik bewegter Körper,” Ann. Phys. 322, 891–921 (1905).
  2. A. Einstein, “Die Grundlage der allgemeinen Relativitätstheorie,” Ann. Phys. 354, 769–822 (1916).
  3. N. Ashby, “Relativity in the Global Positioning System,” Living Rev. Relativ. 6, 1 (2003).