We know that it is possible to travel into the future by moving at great speeds, the next problem is discovering how to travel in time a respectable amount without having to sit in a fast moving spaceship for years. This problem is solved
 by the theoretical existence of what are known as closed timelike curves.

Einstein's special and general theories of relativity combine three-dimensional space with time to form four dimensional space-time. Space-time consists of points or events that represent a particular place at a particular time. Below is a space-time diagram. It shows the position of a particle as it moves through space and time. The path the particle takes is called the "world line" of the particle. The world line shown below is one of a particle at rest.

As the particle begins to move, its world line plot will resemble the diagram below. Notice that the graph is not a straight line. In actual space-time a particle accelerates and moves back and forth through space. But, the particle cannot move back and forth through time.

By giving the time axis the units of seconds and the space axis the units of light-seconds, we can create boundaries on the graph which allow for more meaningful analysis of word line curves.

A light second is the distance that light travels in one second, or 300,000 km. Accepting that photons travel a constant speed of light, and agreeing that no particle with mass can travel faster than the speed of light, we can set linear boundaries at x=y and x=-y. The area within these boundaries is where we should expect to find all the world lines of every particle in the universe.

A true space-time diagram would be in four dimensions, but such a graph is difficult to visualize or draw. We can, however, add a third dimension to our present model. This transforms the two intersecting lines x=y and x=-y lines into a pair of cones that wrap around the time axis.

The point at the origin (x=0, y=0, t=0) is called the "Here-Now." Also known as the present. All points in the top cone are in the future of Here-Now and all points in the lower cone are in the past of Here-Now. A particle at the origin can move to any point in the future of Here-Now at some speed less than the speed of light. Similarly, a particle at any point in the past of Here-Now can move to the origin at some speed less than the speed of light.

World lines found within the space cone are known as "time-like." Time-like intervals have a speed of less than the speed of light. A second type of world line is called "light-like." These are intervals that occur at an angle of 45 degrees to the time axis. Particles with these world lines travel at exactly the speed of light. A third type of world line occurs at an angle greater than 45 degrees from the time axis. These are called "space-like" intervals.