special theory of relativity

In his special theory of relativity, Einstein showed that time and length are not as absolute as everyday experience would suggest: Moving clocks run slower, and moving objects are shorter. Those are just two of the unusual properties of Einstein's world! Another consequence of special relativity is the most famous formula of all: E=mc², stating that two physical quantities which physicists had defined separately, namely energy and mass, are in fact equivalent

general theory of relativity

In Einstein's general theory of relativity, space and time become even more flexible. "Your mileage may vary," and so may the time intervals you measure, depending on where and when you are. This flexibility has an analogue in the geometry of surfaces like that of a sphere - there is a curvature of space and time. Distorted space and time influence the way that material objects or light move. In fact, there is a direct connection to the cosmic interaction that holds the universe together, makes the earth orbit the sun and keeps our feet on the ground: gravity.