What does special relativity say?
In 1905 Einstein started from two principles: the laws of physics are the same for all observers moving at constant velocity, and the speed of light is the same for everyone. The consequences are startling: moving clocks run slow (time dilation), moving objects contract, and mass and energy are equivalent—E=mc².
What is general relativity, and how do we know it's right?
General relativity (1915) treats gravity not as a force but as mass curving spacetime. Evidence includes starlight bending around the Sun (1919 eclipse), the precession of Mercury's orbit, and the first direct detection of gravitational waves in 2015. GPS satellites would drift by several kilometers a day if engineers didn't correct for relativistic clock differences of about 38 microseconds per day.
FAQ
- Can anything go faster than light?
- Not anything with mass, according to special relativity.
- So was Newton wrong?
- Newton's laws remain an excellent approximation at everyday speeds and gravity.
Sources reviewed: 2026-10-25
References
- Einstein, A. (1905). Zur Elektrodynamik bewegter Körper. Annalen der Physik, 17, 891–921.
- Einstein, A. (1916). Die Grundlage der allgemeinen Relativitätstheorie. Annalen der Physik, 49, 769–822.
- Abbott, B. P., et al. (2016). Observation of gravitational waves from a binary black hole merger. Physical Review Letters, 116, 061102.