Page 17 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 2

Figure 2:4

Given an event P, one can divide the other events in the universe into three classes. Those events that can be
reached from the event P by a particle or wave traveling at or below the speed of light are said to be in the
future of P. They will lie within or on the expanding sphere of light emitted from the event P. Thus they will lie
within or on the future light cone of P in the space-time diagram. Only events in the future of P can be affected
by what happens at P because nothing can travel faster than light.
Similarly, the past of P can be defined as the set of all events from which it is possible to reach the event P
traveling at or below the speed of light. It is thus the set of events that can affect what happens at P. The
events that do not lie in the future or past of P are said to lie in the elsewhere of P Figure 2:5.

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