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What is special relativity?
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Quick answer
Special relativity is Einstein's 1905 theory describing how space and time behave for objects moving at constant high speeds. Its core idea — that the speed of light is the same for everyone — forces time to slow and lengths to contract for fast-moving objects.
Key things to understand
- Its two postulates: the laws of physics are the same for all non-accelerating observers, and light's speed in a vacuum is constant for everyone.
- Time dilation: a fast-moving clock ticks slower as seen by a stationary observer.
- Length contraction: fast-moving objects appear shortened in their direction of motion.
- Mass and energy are equivalent, captured by E = mc² — a little mass holds enormous energy.
- Nothing with mass can reach the speed of light; it would take infinite energy.
People also ask
What does E = mc² actually mean?
Mass and energy are two forms of the same thing: mass can convert to energy and back, and because c² is huge, a tiny mass yields enormous energy — the principle behind nuclear power.
Is time dilation real?
Yes — it's measured routinely. Precise clocks on fast aircraft and satellites tick slightly differently from ground clocks, and GPS must correct for it to stay accurate.
Why can't anything go faster than light?
As an object with mass speeds up, the energy needed to accelerate it further grows without limit, becoming infinite at light speed — so it can never quite get there.