String Theory and Einstein’s Law of Gravity
Since string theory is related to the theory of quantum gravity, a good place to begin is by giving an overview of the scientific understanding of gravity, which is defined by Einstein’s theory of general relativity.
Albert Einstein would revolutionize the way physicists saw gravity. Instead of gravity as a force acting between objects, Einstein instead envisioned a universe in which each object’s mass caused a slight bending of space (actually space-time) around it. The movement of an object along the shortest distance in this space-time was gravity. Instead of being a force, gravity was actually an effect of the geometry of space-time itself.
Einstein proposed that motion in the universe could be explained in terms of a coordinate system with three space dimensions — up/down, left/right, and backward/forward, for example — and one time dimension. This 4-dimensional coordinate system, developed by Einstein’s old professor Hermann Minkowski, was called space-time, and came out of Einstein’s 1905 theory of special relativity.
As Einstein generalized this theory, creating the theory of general relativity in 1916, he was able to include gravity in his explanations of motion. In fact, the concept of space-time was crucial to it. The space-time coordinate system bent when matter was placed in it. As objects moved within space and time, they naturally tried to take the shortest path through the bent space-time.
We follow our orbit around the sun because it’s the shortest path (called a geodesic in mathematics) through the curved space-time around the sun.