Introduction

Soon after Einstein developed special relativity, his teacher Herman Minkovski found a simple way to illustrate the strange effects predicted by the theory. If you are comfortable using diagrams and graphs, then learning how to interpret spacetime diagrams will deepen your understanding of relativity.

Relativity theory says that our world is, in some sense, fourdimensional. Time is a dimension and somehow like the three spatial dimensions of height, width and depth. We are used to the idea that three-dimensional objects like cars and houses can be pictured on a flat, two-dimensional piece of paper. But how can we picture a fourdimensional object? Some say that it is impossible, and that our minds are incapable of conceiving four-dimensional objects. Spacetime diagrams use a simple trick: they just omit some of the spatial dimensions. Instead of displaying a three-dimensional object together with its time dimension, they display only one space dimension and one time dimension.

Perhaps you have used graphs where the horizontal axis was labelled “x” and the vertical axis was labelled “y”. In a spacetime diagram, we keep the x-axis but label the vertical axis “t” for “time”. The diagram thus has one space dimension and one time dimension. Each point on the diagram has two coordinates, (x, t), which can be various numbers like (x = 4, t = 3). This point thus corresponds to the place marked four (centimetres) and the moment of time when the clock says three (hours).