*can*add apples to oranges if you can find some way of converting between them. If a grocer is selling apples for 30 cents and oranges for 10 cents, then I can buy three oranges for the same price as 1 apple, or six oranges for the same price as 2 apples, or 9 oranges for the same price as 3 apples etc. The ratio of the value of apples to oranges is 3. If I were to construct a graph relating the number of apples I could exchange for oranges it would be a straight line, and the slope of this line would be 3. So that the slope of the apples-oranges curve is the exchange rate or conversion factor between the two.

^{-1}relative to my position, then you see me moving at 10 m s

^{-1}relative to your position. At the moment we don't know what the value of this exchange rate velocity is so we will just call it '

*c*' (note: at this stage we are making no assumptions that

*c*is the speed of light). So now to convert distance, x, to time t we use this exchange rate c.

*events*in spacetime. Now spacetime consists of four dimensions: three space dimensions and one time dimension but I can't draw 4 dimensions on a two dimensional screen (in fact if I am honest I cant really imagine what four dimensions would look like) but there is no problem describing it with maths. To represent it in two dimensions and to keep the maths simple we will roll up the space dimensions into one axis labelled

*x*. This shouldn't worry you too much if you remember that our aim is to come up with a way of measuring the 'distance' between 'events' in spacetime, and if we know the location of any two points in space we can measure or calculate the distance between them to come up with a single value,

*x*, for the space component of their distance. For example, if I constructed a three dimensional map of my office, the distance between my chair and the bin can be represented by a single value of 2.1 m.

*x*, and another representing time,

*ct*(remember that c is just the exchange rate we need to use to convert time into the same units as space). On this diagram we can plot some 'events' that occur at a particular locations in space and time. Our first spacetime diagram has three points, R, G and B. R lies directly to the left G so they both have the same value for their time component but a different value for the space component. In other words, events R and G occur at the same time at different places. Similarly, G lies directly below B so we can say that these two events occur at different times in the same place.