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Intermediate Membranes

A little General Relativity

Isaac Newton was the first to recognize that gravity, which causes an apple to fall from a tree, is the same force keeps planets in orbit around the sun. He described it as an attractive force between two objects with mass with the force being stronger when the masses are greater and when the objects are closer together. His description does a great job at describing objects falling to earth and a pretty good job at describing the motion of the planets around the sun.

Einstein came up with a deeper explanation for gravity. He said that massive objects actually curve the fabric of space -- and time-- in their vicinity, similar to the way the surface of mattress is curved when a bowling ball is placed on it. The surface of a mattress is two-dimensional, and our universe has three space and one time (=4 total) dimensions. Clearly they are not exactly the same thing, but we talk about the analogy of a two-dimensional universe because it is much easier to envision something curved in two dimensions than in four.

Einstein proposed that, rather that thinking of some mysterious force reaching out from the center of massive objects and pulling on other objects, it is more correct to think of this curvature as causing the paths of planets to curve around the sun and other gravitational phenomena. Think about what would happen if you drove your car in a straight line. Assuming things like trees and oceans didn't get in your way, if you drove far enough (about 25,000 miles), you would end up where you started. This is because the earth's surface is curved. Einstein used a similar explanation, involving curvature of space (and time) to explain why the planets circle the sun back to their original position.

While Newton's law of gravity usually works pretty well for describing the way things move near small masses like the earth, very close to larger masses like the sun we start to notice a discrepancy between the predictions of Einstein's theory and Newton's theory. And many tests have shown Einstein's predictions to be much more correct.