Glossary

Speed of light

c ≈ 299,792 km/s — the same for every observer and the universe's ultimate speed limit.

Image: LucasVB. Public domain, via Wikimedia Commons (Source)

The speed of light in a vacuum, denoted c, is exactly 299,792,458 metres per second. Its constancy for all observers — regardless of how they or the source move — is the bedrock postulate of special relativity.

Because reaching c would require infinite energy for anything with mass, it acts as an unbreakable cosmic speed limit.

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 Six patent files Made-up inventions on Einstein’s desk: find the impossible part and stamp “Patent rejected”. 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 The light-clock Send a clock made of light on a journey and watch it tick more slowly. 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
Source: Wikipedia: Speed of light — Read the full article on Wikipedia.

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