Free Three Roads to Quantum Gravity by Lee Smolin Page A

A had not occurred, B could not have. In this case we can say that A was a
contributing cause of the event B. An event may have more than one contributing cause, and an event may also contribute to causing more than one future event.
    Given any two events, A and B, there are only three possibilities: either A is a cause of B, or B is a cause of A, or neither is the cause of the other. We say that in the first case A is in the causal past of B, in the second, B is in the causal past of A, and in the third case neither is in the causal past of the other. This is illustrated in Figure 6 , in which each event is indicated by a point and each arrow represents a causal relation. Such a picture is a picture of the universe as a process. Figure 7 shows a more complicated universe, consisting of many events, with a complicated set of causal relationships. These pictures are stories told visually - diagrams of the history of a universe.
    Such a universe has time built into it from the beginning. Time and change are not optional, for the universe is a story and it is composed of processes. In such a world, time and causality are synonymous. There is no meaning to the past of an event except the set of events that caused it. And there is no meaning to the future of an event except the set of events it will influence. When we are dealing with a causal universe, we can therefore shorten ‘causal past’ and ‘causal future’ to simply ‘past’ and ‘future’. Figure 8 shows the causal past and future of a particular event in Figure 7 . A causal universe is not a series of stills following on, one after the other. There is time, but there is not really any notion of a moment of time. There are only process that follow one another by causal necessity. It makes no sense to say what such a universe is. If one wants to talk about it, one has no alternative but to tell its story.

    FIGURE 6
    The three possible causal relations between two events, A and B: (a) A is to the future of B; (b) B is to the future of A; (c) A and B are neither to the future nor to the past of each other (though they may have other causal relations, for example both being in the past of event C, as shown).

    FIGURE 7
    One volley in a tennis game, represented by the causal relations of a few of its events.
    One way to think of such a causal universe is in terms of the transfer of information. We can think of the content of each arrow in Figures 6 to 8 as a few bits of information. Each event is then something like a transistor that takes in information from events in its past, makes a simple computation and sends the result to the events in its future. A computation is
then a kind of story in which information comes in, is sent from transistor to transistor, and is occasionally sent to the output. If we were to remove the inputs and outputs from modern computers, most of them would continue to run indefinitely. The flow of information around the circuits of a computer constitutes a story in which events are computations and causal processes are just the flow of bits of information from one computation to the next. This leads to a very useful metaphor - the universe as a kind of computer. But it is a computer in which the circuitry is not fixed, but can evolve in time as a consequence of the information flowing through it.

    FIGURE 8
    The future and past of Olga’s second return. Note that Sam being confused is in neither set of events.

    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 first be the invention of rock and roll, which took place perhaps somewhere in