Transcript: Space-time diagrams are a way of graphically representing space and time. Remember that the universe we live in is made up of three dimensions of space and one of time, also called the space-time continuum. To represent this in a graphical form we have to collapse the three dimensions of space to one dimension; this becomes the x-axis. The other dimension is time, and that’s the y-axis. By convention the slope of a line at forty-five degrees corresponds to the fastest speed that anything can move in the universe, the velocity of light, three hundred thousand kilometers per second. Any other objects other than radiation most move at the speed of light or slower speeds. On the page, therefore, we form a triangle with the point of origin of the events at the apex. The triangle can sweep out a cone, and this is called a light cone in two dimensions. Essentially the light cone defines all the regions within a space-time that are in causal connection or can communicate with each other by electromagnetic radiation or any other means. In a space-time diagram true events that have their origin on the x-axis emit light cones going upward. Unless these light cones overlap, the two regions are not in communication with each other. Space-time diagrams are therefore used to show which regions of a universe or a space-time are in communication with each other and which regions are isolated from each other.