Three Roads To Quantum Gravity

THE UNIVERSE IS MADE OF PROCESSES, NOT THINGS
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Is our universe such a causal universe? General relativity
tells us that it is. The description of the universe given by
general relativity is exactly that of a causal universe, because
of the basic lesson of relativity theory: that nothing can travel
faster than light. In particular, no causal effect and no
information can travel faster than light. Keep this in mind,
and consider two events in the history of our universe,
pictured in Figure 9. Let the ®rst be the invention of rock
and roll, which took place perhaps somewhere in Nashville in
the 1950s. Let the second be the fall of the Berlin Wall, in
1989. Did the ®rst causally in¯uence the second? One may
argue about the political and cultural in¯uence of rock and
roll, but what is important is only that the invention of rock
and roll certainly had some effect on the events leading to the
fall of the Berlin Wall. The people who ®rst climbed the wall
in triumph had rock and roll songs in their heads, and so did
the functionaries who made the decisions that led to the
reuni®cation of Germany. So there was certainly a transfer of
information from Nashville in the 1950s to Berlin in 1989.
The fall of the Berlin Wall
The invention of
rock and roll
FIGURE 9
The invention of rock and roll was in the causal past of the fall of the Berlin
Wall because information was able to travel from the ®rst event to the
second.
So in our universe we de®ne the causal future of some event
to consist of all the events that it could send information to,
using light or any other medium. Since nothing can travel
faster than light, the paths of light rays leaving the event
de®ne the outer limits of the causal future of an event. They