Three Roads To Quantum Gravity

60 THREE ROADS TO QUANTUM GRAVITY
space
time
History of a star
FIGURE 11
A massive object such as a star causes the light cones in its vicinity to tip
towards it. This has the effect of causing freely falling particles to appear to
accelerate towards the object.
oscillate back and forth, as shown in Figure 12. These are the
gravitational waves.
So, Einstein's theory of gravity is a theory of causal
structure. It tells us that the essence of spacetime is causal
structure and that the motion of matter is a consequence of
alterations in the network of causal relations. What is left out
from the notion of causal structure is any measure of quantity
or scale. How many events are contained in the passage of a
signal from you to me, when we talk on the telephone? How
many events have there been in the whole history of the
universe in the past of this particular moment, as you ®nish
reading this sentence? If we knew the answers to these
questions, and we also knew the structure of causal relations
among the events in the history of the universe, then we