Physics
Relativity
Quick fact
The speed of light in a vacuum is the universe's ultimate speed limit—approximately 300,000 km per second—and it never changes, regardless of the observer's motion.
Why this is interesting
You've probably noticed that when you're in a moving car, the trees outside seem to fly past. But what if I told you that time itself can slow down and space can shrink, all depending on how fast you're moving? Relativity reveals this hidden world.
Read the full explanation
Understanding Relativity
Imagine you and a friend are on two different trains passing each other at high speed. You both see the other's watch ticking slower. This isn't an illusion—it's real time dilation. Similarly, a spaceship zooming past Earth would appear squashed along its direction of motion (length contraction). These effects become noticeable only at speeds close to light speed. What's happening? The laws of physics are the same for all observers, and the speed of light is constant. To reconcile different viewpoints, space and time must become flexible—forming a unified four-dimensional fabric called spacetime.
A deeper explanation
Einstein's special relativity (1905) is built on two postulates: (1) The laws of physics are identical in all inertial reference frames; (2) The speed of light in vacuum is the same for all observers. From these, the Lorentz transformations show that time and space are relative—moving clocks tick slower, and moving objects contract. The famous equation E=mc² emerges as a consequence. General relativity (1915) extends this to accelerated frames and gravity. The equivalence principle states that gravity and acceleration are indistinguishable, leading to the idea that mass warps spacetime. This curvature dictates how objects move—'falling' along geodesics. Relativity explains why GPS satellites must adjust for both special (time dilation) and general (gravitational time dilation) effects to provide accurate positioning. It also predicts black holes, gravitational waves, and the expansion of the universe.