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.