### Theory of Relativity Concepts

- Einstein's theory of special relativity - localized behavior of objects in inertial frames of reference, generally only relevant at speeds very near the speed of light
- Lorentz transformations - the transformation equations used to calculate the coordinate changes under special relativity
- Einstein's theory of general relativity - the more comprehensive theory, which treats gravity as a geometric phenomenon of a curved spacetime coordinate system, which also includes noninertial (i.e. accelerating) frames of reference
- Fundamental principles of 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