relativity

Time slows down, space bends, and mass becomes energy. Einstein's relativity is strange but beautifully logical.

If the speed of light were dramatically lower — say, 100 km/h — relativistic effects would dominate everyday life. Cars would gain mass as they accelerated, time would dilate for joggers, and the universe would look profoundly different.

Nothing with mass can reach the speed of light. But shadows, spots of light, and the expansion of the universe can move faster than light without violating physics. Here's the strange truth about superluminal motion.

Time travel forward already happens — astronauts do it, in tiny amounts. Time travel backward is much harder, but Einstein's equations don't strictly forbid it. Here's what physics actually says.

Astronauts return from space slightly younger than they would have been on Earth. The explanation is real physics — Einstein's theory of relativity — and it has measurable consequences for everything from GPS to interstellar travel.