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# Einstein's Theory of Relativity

## What is Relativity?

Einstein's theory of relativity is a famous theory, but it's little understood. Basically, the theory of relativity refers to two different elements of the same theory: general relativity and special relativity. The theory of special relativity was introduced first, and was later considered to be a special case of the more comprehensive theory of general relativity.

### What is Relativity?

Classical relativity (defined initially by Galileo Galilei and refined by Sir Isaac Newton) involves a simple transformation between a moving object and an observer in another inertial frame of reference. If you are walking in a moving train, and someone stationary on the ground is watching, your speed relative to the observer will be the sum of your speed relative to the train and the train's speed relative to the observer. You're in one inertial frame of reference, the train itself (and anyone sitting still on it) are in another, and the observer is in still another.

The problem with this is that light was believed, in the majority of the 1800s, to propagate as a wave through a universal substance known as the ether, which would have counted as a separate frame of reference (similar to the train in the above example). The famed Michelson-Morley experiment, however, had failed to detect Earth's motion relative to the ether and no one could explain why. Something was wrong with the classical interpretation of relativity as it applied to light ... and so the field was ripe for a new interpretation when Einstein came along.

### Einstein's Theory of Relativity - Index

##### Einstein's Other Work

Andrew Zimmerman Jones