Curvature of Space

A hallmark of gravity is that is causes the same acceleration no matter what the mass of the object. For example, a baseball and a cannon ball have very different masses, but if you drop them side-by-side, they accelerate downward at exactly the same rate. To explain this, Einstein envisioned gravity as being caused by a curvature of space. In fact, his general theory of relativity describes gravity entirely in terms of the geometry of both space and time. Far from a source of gravity, like a planet or a star, space is "flat" and clocks tick at their normal rate. Closer to a source of gravity, however, clocks slow down and space is curved. A useful analogy is to imagine that the space near a massive object such as the Sun becomes curved like a surface in figure.

Imagine a ball rolling along this surface. Far from the "well" that represents the Sun, the ball would move in straight line since the surface is fairly flat. If it passes the well, however, it would curve in toward the well. If it is moving at an appropriate speed with respect to the well, it might move in an orbit aroundthe sides of the well.In this analogy,it is the curvatureof the surface that makes the ball follow a curved path near the well. The curvature has the same effect on a ball of any size, which explains why gravity produces the same acceleration on objects of different mass.