Following is a brief review of these topics. This diagram plots the position of a particle as it moves through space-time. In reality, a true space-time diagram would have four dimensions including three space dimensions and one time dimension. This type of diagram is very difficult to illustrate and visualize. Wherever possible we use a simpler convention to illustrate concepts in space-time.
Lets look at the example of a particle starting at rest on the origin of the y and t axes.
So for a particle at rest in some observers frame of reference, its space-time diagram for that observer is a vertical world line. This is illustrated in the space-time diagram to the right showing a particle at rest. The world line represents the path the particle follows in space-time as a whole. If a particle experiences acceleration, its world line will curve away from the vertical time axis. If the same particle experiences decceleration, its world line will curve back towards the vertical time axis.