###### January 23, 2013

# What is Relativity?

Hi, I’m Emerald Robinson, and in this “What Is” video, we’re going to discuss one of science’s most famous theories: Albert Einstein’s Theory of Relativity.

The theory of relativity has two parts – special relativity and general relativity.

Special relativity states that the laws of physics apply no matter how fast you’re moving. For example, the same rules of gravity apply to a brick tossed out of a moving airplane as to one dropped off of a building. Einstein applied this principle to light, stating that the speed of light, represented by “c,” is constant. Light always travels at the same speed for all observers no matter how fast you’re moving, or how fast the source of the light is moving. The light that comes from a car’s headlights always moves at the same speed, whether the car is parked, or moving at sixty five miles an hour.

The theory of special relativity changed the way scientists thought about time. Until Einstein’s theory, it was thought that everyone experienced time the same way. Special relativity determined that the rate at which time passes to you depends on your speed – the faster you’re moving, the slower that time passes.

Special relativity is also the place where one of the world’s most famous mathematical formulas comes from: E equals m c squared. In this equation, e stands for “energy,” m represents mass, and c is the speed of light. In other words, energy and mass are equivalent – one can’t exist without the other.

Based on the theory of special relativity, Einstein became convinced that space and time are not separate. General relativity is a theory that gravity is caused by bending time and space. It’s been used to explain how light bends around objects in space, like the light we see during a solar eclipse.