Three Roads To Quantum Gravity

58 THREE ROADS TO QUANTUM GRAVITY
form what we call the future light cone of an event (Figure 10).
We call it a cone because, if we draw the picture so that space
has only two dimensions, as in Figure 10, it looks like a cone.
The causal past of an event consists of all the events that
could have in¯uenced it. The in¯uence must travel from some
event in the past at the speed of light or less. So the light rays
arriving at an event form the outer boundary of the past of an
event, and make up what we call the past light cone of an
event. One is pictured in Figure 10. We can see that the
structure of the causal relations around any event can be
pictured in terms of both the past and future light cones. We
An event in the future of A
Future light cone of A
time
space
length
A
E, An event not
causally related to A
width
Past light cone of A
An event in the past of A
FIGURE 10
The past and future light cones of an event, A. The future light cone is made
up of the paths of all light signals from A to any event in A's future. Any
event inside the cone is in the future of A, causally, because an in¯uence
could travel from A to that event at less than the speed of light. We also see
the past light cone of A, which contains all the events that may have
in¯uenced A. We also see another event, E, which is in neither the past nor
the future of A. The diagram is drawn as if space had two dimensions.