Einstein's Theory of Special Relativity

What is relativity? Succinctly put, it is the notion that the laws of physics are the same everywhere. We here on Earth obey the same laws of light and gravity as someone in a far off corner of the universe.

The universality of physics means that history is provincial. Different viewers will see the timing and spacing of events differently. What for us is a million years may just be a blink of an eye for someone flying in a high speed rocket or falling into a black hole.

It's all relative.

Definition

Special relativity came first and is based on the speed of light being constant for everyone. Einstein came to this conclusion in 1905 after experimental evidence showed that the speed of light didn't change as the Earth swung around the Sun. This result was surprising to physicists because the speed of most other things does depend on what direction the observer is moving.

Einstein said that all observers will measure the speed of light to be 186,000 miles per second, no matter how fast and what direction they are moving."If you are in a spaceship that is traveling at the speed of light, and you turn on the headlights, does anything happen?"

The answer is the headlights turn on normally, but only from the perspective of someone inside the spaceship. For someone standing outside watching the ship fly by, the headlights do not appear to turn on: light comes out but it takes an eternity for the beams to get ahead of the spaceship.

These contradictory versions arise because rulers and clocks —If the speed of light is to be held constant as Einstein said, then time and space cannot be absolute; they must be subjective. For instance, a 100-foot-long spaceship traveling at 99.99 percent the speed of light will appear one foot long to a stationary observer, but it will remain its normal length for those onboard.

Perhaps even weirder, time passes slower the faster one goes. If a twin rides in the speeding spaceship to some distant star and then comes back, she will be younger than her sister who stayed on Earth.

Mass, too, depends on speed. The faster an object moves, the more massive it becomes. In fact, no spaceship can ever reach 100 percent of the speed of light because its mass would grow to infinity. This relationship between mass and speed is often expressed as a relationship between mass and energy: E=mc^2, where E is energy, m is mass and c is the speed of light.