Although gravity is often described as a force that pulls matter together across space, the question of how this force is transmitted is unanswered. Most scientists believe that part of the solution lies in Einstein's Special Theory of Relativity, published in 1916, which explained the significance of the acceleration produced by the force of gravity. Although not accepted by all scientists immediately after its publication, the theory has come into its own in recent decades, with the discovery of the way space curves, time dilation and black holes.
Important in the General Theory is the notion of space-time. Conventionally, space is viewed only in the familiar three dimensions: height, width and depth. But space and time are intimately bound, so space-time includes time as an extra dimension, and objects are thus located in time as well as in space. Tracing an object's space-time path therefore involves plotting its journey in both time and space.
Einstein's theory also shows that gravity works by bending space-time; matter simply follows the bend. If space-time is seen as a stretched rubber sheet, massive objects on it tug it down locally – the more massive the object, the bigger the distortion – and matter in the vicinity simply rolls into one of these dips. It is wrong, however, to think of gravity as a mysterious force pulling space-time into a curve; gravity is the curvature of space-time.
It is hard to imagine curved space. But what it means in practice is that gravity makes moving objects (everything in space is moving) follow a curved path. However, it is important to remember that gravity is a curve in space-time, not just in space. The Earth's path through space is drawn into a curve by the gravity of the Sun. The curvature is not just Earth's elliptical three-dimensional orbit, but its spiral orbit through space-time. This is because the Earth returns to the same place again and again, but each year it moves farther on in time.