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To get a better feeling for what is going on, we may graph the situation. We will here have a first look at a space-time diagram. We will reduce the number of time dimensions to one. It is customary to have the time on the vertical axis. It is also common not to have time on the vertical axis, but instead, ct. That would make the units on both axes length units. It also has the advantage of making light move at a 45-degree angle.
In the figure above, the two diagonal lines represent the motion two beams of light would take in space-time if they converged towards the origin and then diverged again. In 4D space-time, this would instead be a 4D cone, but I had a hard time finding commands to draw in 4D in JavaScript, so this will do. Such a cone is called a light cone. The green part is all parts of the space where light, or something moving slower than light, could have reached the origin. The yellow part is all parts where light, or something moving slower than light, could reach. The green part is thus the past, and the yellow part is the future, as seen from the origin. The white parts are parts that could neither be reached from the past, nor in the future, assuming nothing could move faster than light. Those parts are often called elsewhere.
Another possibility is to have time on the vertical axis and x/c on the horizontal axis. In that case, the units on both axes would be time units.
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