Understanding Gravity

When most people think of gravity, they think of a force that keeps keeps things together: it keeps people on the surface of the Earth, it keeps the Earth in orbit around the Sun, and it even keeps entire galaxies together. This way of thinking about gravity — as a long range force of attraction — was firmly established in the 17th century by Isaac Newton. Newton's law of gravity is a spectacular example of how some simple mathematical rules can accurately explain what we observe in nature, but it isn't the end of the story. By the end of the 19th century, people had found several situations in which the classical physical laws, such as Newton's law of gravity, didn't quite work. Newton's theory isn't totally wrong, but it is incomplete. Few people realized just how profoundly a more complete law of gravity would change our view of the Universe, but that is exactly what happened after Albert Einstein weighed in.

In 1916, Einstein published his general theory of relativity, a completely new way of thinking about gravity. In general relativity, or GR, we think of gravity as a distortion, or curvature, of the fabric of space and time itself (called space-time). In this context, space means the distance between two objects, or the shortest path you could take to get between point A and point B. This is not the same thing as "outer space" — every thing in the Universe exists in space-time, including the Earth and everything on (and in) it. In the concept of space-time, time refers to that which is measured by clocks.