Three Roads To Quantum Gravity

THE UNIVERSE IS MADE OF PROCESSES, NOT THINGS
59
see from Figure 10 also that there are many other events
which lie outside both the past and future light cones of our
particular event. These are events that took place so far from
our event that light could not have reached it. For example,
the birth of the worst poet in the universe, on a planet in a
galaxy thirty billion light years from us is, fortunately, outside
both our future and past light cones. So in our universe,
specifying the paths of all the light rays or, equivalently,
drawing the light cones around every event, is a way to
describe the structure of all possible causal relations. Together,
these relations comprise what we call the causal
structure of a universe.
Many popular accounts of general relativity contain a lot of
talk about `the geometry of spacetime'. But actually most of
that has to do with the causal structure. Almost all of the
information needed to construct the geometry of spacetime
consists of the story of the causal structure. So not only do we
live in a causal universe, but most of the story of our universe
is the story of the causal relations among its events. The
metaphor in which space and time together have a geometry,
called the spacetime geometry, is not actually very helpful in
understanding the physical meaning of general relativity.
That metaphor is based on a mathematical coincidence that is
helpful only to those who know enough mathematics to make
use of it. The fundamental idea in general relativity is that the
causal structure of events can itself by in¯uenced by those
events. The causal structure is not ®xed for all time. It is
dynamical: it evolves, subject to laws. The laws that
determine how the causal structure of the universe grows
in time are called the Einstein equations. They are very
complicated, but when there are big, slow moving klutzes of
matter around, like stars and planets, they become much
simpler. Basically, what happens then is that the light cones
tilt towards the matter, as shown in Figure 11. (This is what is
often described as the curvature, or distortion of the geometry
of space and time.) As a result, matter tends to fall towards
massive objects. This is, of course, another way of talking
about the gravitational force. If matter moves around, then
waves travel through the causal structure and the light cones