ABSTRACT
One of the most surprising effects of relativity is that times and lengths depend on who measures them. Imagine that one person stays on Earth while her twin goes on a long spaceflight almost as fast as the speed of light. The person on Earth would see the spacecraft become shorter as it accelerates toward the speed of light. This is called Lorentz contraction. Also, time runs more slowly as speed increases. When the astronaut twin returns to Earth, she would be younger than her earthbound twin. This is known as the “twins paradox.”
