Lesson #12 of 18 in the Brain Makeover collaboration with Professor James Trefil/GMU, the 76ers Cheerleaders and the Science Cheerleader. See Brain Makeover Series.

Every observer sees the same laws of nature.

This is the main tenet of the theory of relativity, developed by Albert Einstein in the early 20th century. The theory comes in two parts: special relativity, which deals with observers moving at constant velocities, and general relativity, which deals with observers that are accelerating.

The most familiar results of special relativity are that (1) moving clocks slow down, (2) moving objects become more massive, (3) moving objects shorten in their direction of motion, and (4) mass and energy are equivalent, as stated in the famous equation E=mc2 .

General relativity is our best current theory of gravitation. It pictures space-time as a kind of membrane that can be distorted by the presence of massive objects.
(Image courtesy of NASA)