Did you know that there were four dimensions? We are living in a four dimensional world…all the time. And time is the fourth dimension.

It is impossible to imagine a four dimensional space. However we know it exists.
In old theories, space and time were independent from each other. In relativity however time and space cannot be separated. This theory was established by Einstein and cosmology, astronomy, and physics are all based on it today.

Space-time is based on events. An event is something that happens at a unique spot in space-time.
Look at the image. Here space is combined in two dimensions.
Time goes up the vertical axis, and space is the horizontal axis. The event is the place on the diagram where the two cones join. Imagine a flash of light being emitted at that event. Lets call that event Y. That light being emitted will go up in time. But light has a particular speed. The more time passes, the more it has evolved in space. The cone symbolizes the spreading of the light with time. Any other object than light affected by that event will travel at a lower speed in space-time (This isn’t true with the Quantum World, Read the Quantum Theory Database for more information.) Its axis, instead of being at 45° for instance like light will be strictly below 45° because nothing can go faster than light. The top cone is the future light cone. Above the x axis we have the future light cone, and below it is the past light cone.

The past light cone is the set of events from
which a pulse of light is able to reach Y. One can predict Y if he knows all the events in the past light cone of that event. The past light cone is the past, Y is the present and the future light cone is the future.
Imagine that at Y the sun dies. The earth is located at another point of space at the moment where Y occurred. As light from the sun takes 8 minutes to reach us, the sun’s explosion won’t affect us immediately. As we go up in time we will enter the future light cone of the event. Only then will we be affected by the explosion of the sun. Y can only be influenced by another event in its past light cone. If an event is not in Y’s past light cone, then Y cannot be affected by it.

You might have to read the article several times to understand it :/. I’ve tried to make this as simple as possible, but this is some quite complicated business!

Read the article from wikipedia if you’re interested: Light Cones at Wikipedia

Cheers :D!
Clement