1 What is time?

Most people probably don’t think too much about what time ‘is’. It just flows. Ray Cummings, the American science fiction writer, commented that time ‘is what keeps everything from happening at once’, which may be as good a definition as any.

Whatever it is, time certainly seems quite different from space. Time has a direction – it moves forward, and there’s no choice but to move forward with it – whereas in space you can choose to move in any direction. Time is measured with a clock; space is measured with a ruler. An event can be assigned and described by an instant in time and a location in space, but the two are quite distinct. This was the scientific view too, until about a century ago. This week, you’ll learn how time and space are no longer regarded as separate, but as inextricably intertwined.