Glossary

Special relativity

Einstein's 1905 theory: the speed of light is the same for everyone, so space and time are relative.

Image: Unknown photographer (ETH-Bibliothek Zürich). Public domain, via Wikimedia Commons (Source)

Special relativity, published by Einstein in 1905, rests on two postulates: the laws of physics are the same in every inertial (non-accelerating) frame, and the speed of light in a vacuum is constant for all observers. From these follow startling consequences — moving clocks run slow (time dilation), moving objects contract along their motion (length contraction), and simultaneity is relative.

The theory unified space and time into a single four-dimensional spacetime and set the cosmic speed limit: nothing carrying information or mass can travel faster than light.

Try it yourself

Train and lightning Two lightning strikes, simultaneous – or not? Find out why “now” depends on where you stand. On the page: Special Relativity tried Synchronising the Bern clocks Set three station clocks by signal – and see why a moving train has a different “now”. On the page: patent-office tried Chasing a beam of light Chase a beam of light at 90 % of the speed of light – how fast does it fly past you? On the page: Special Relativity tried Cycling through slow light Ride through a town where light travels at 30 km/h – and watch houses shrink and clocks drift apart. On the page: slow-light tried Knowledge quiz: life & physics Questions on Einstein’s life and physics – with an explanation and a source for every answer. On the page: Learn
Source: Wikipedia: Special relativity — Read the full article on Wikipedia.

← All glossary terms