General relativity is, fundamentally, the study of the metric tensor: it is the object that connects the geometric representation of space and time with mass an

General relativity is, fundamentally, the study of the metric tensor: it is the object that connects the geometric representation of space and time with mass and energy. Different metric tensors can produce wildly different results, from flat space to black holes to bubbles in space time that provide inertial reference frames that can travel "faster" than than the speed of light. You could spend your entire life studying small variations in metric tensors. Some people do. More simply: 1. Light travels from point A to point B via the shortest path. 2. In the absence of mass (aka where the Minkowski metric applies), the shortest path is a "straight line". 3. In the presence of mass (where a non-trivial metric exists), space itself curves. Light, which follows the shortest path between two points, must follow this curved space. As a lower-dimensional analogy, think of what a "straight" line means on the surface of a sphere[1]. The shortest path between two points is, by definition, a "straight" line, but if the space itself that the path is embedded in is curved, then when viewed from a higher dimension (i.e. on the surface of the sphere it's straight for you, but if you were in space) it would appear curved. The analogy fails when you attempt to think about it in higher dimensions, since humans have a very difficult time perceiving anything greater than the standard 3+1 spatial & time dimensions. But the math still holds. Light will always travel in a straight line, it just so happens that gravity redefines what "straight" actually means. 1. http://mathworld.wolfram.com/GreatCircle.html http://mathworld.wolfram.com/GreatCircle.html