Relativity views space-time as a continuum; that is, time becomes another dimension of space, making space four-dimensional. This is really common sense-we know that it takes time to move through the physical dimensions of space. (length, width, and heigth) Time is a property of space that is distinct from yet closely related to the other three. (Althought we know we can move through space in any direction, we can only move forward in time, at least until wormholes are discovered.)

Curvature of space-time can be more difficult to understand. Although the rubber sheet model gives a picture of how it happens, one cannot easily picture the so-called nothingness of space being curved or a non-physical property like time being warped. But relativity (a theory which has yet to be disproved) predicts this curvature and uses gravity as proof of it. In fact, relativity predicts singularities, points in space-time where the curvature reaches infinity. Singularities are the centers of black holes, points of gravity so strong that nothing, not even light, can escape.

According to relativity, the curvature of space-time is gravity, and this raises an intersting paradox: the amount of curvature is governed by the distribution of matter and energy in the univers, but this distribution is determined by the curvature of space-time. Thinking back to the rubber sheet model, this becomes an obvious truth. A large object, such as a shot put, would cause a large indentation that would cause a marble to roll in. Or, if an area of rubber sheet space contained only small objects, several marbles might roll together to form an area of 'strong gravity', which would in turn attract more objects.

Besides creating the all-important force of gravity, space-time curvature also makes a major dent (no pun intended) in one of our most common beliefs from geometry and family vacations: in curved space-time, a straight line is NOT always the shortest distance between two points. But that's a different topic.

In order to move through the physical dimensions of space, one must also move through time. In this graph, time surrounds the axes, and although the lines could be extended so that we could move in any physical direction, we do not extend them because we do not know how to move backwards in time.