Science
What is Special relativity?
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.
See it, don’t just read it.
Watch a 2-minute lesson with voice + animation that explains special relativity.
Key things to understand
- 1Its 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.
- 2Time dilation: a fast-moving clock ticks slower as seen by a stationary observer.
- 3Length contraction: fast-moving objects appear shortened in their direction of motion.
- 4Mass and energy are equivalent, captured by E = mc² — a little mass holds enormous energy.
- 5Nothing with mass can reach the speed of light; it would take infinite energy.
Frequently asked questions
- 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.