The Relationship of Space and Time
To make an ultimate theory of physics one needs to understand the true nature not only of space but also of time. And I believe that here again the idea of thinking in terms of programs provides some crucial insights.
In our everyday experience space and time seem very different. For example, we can move from one point in space to another in more or less any way we choose. But we seem to be forced to progress through time in a very specific way. Yet despite such obvious apparent differences, almost all models in present-day fundamental physics have been built on the idea that space and time somehow work fundamentally the same.
But for most of the systems based on programs that I have discussed in this book this is certainly not true. And thus for example in a cellular automaton moving from one point in space to another just corresponds to shifting from one cell to another. But moving from one point in time to another involves actually applying the cellular automaton rule.
When we make a picture of the behavior of a cellular automaton, however, we do nevertheless tend to represent space and time in the same visual kind of way—with space going across the page and time going down. And in fact the basic notion of extending the idea of position in space to an idea of position in time has been common in scientific thought for more than five centuries.
But in the past century what has happened is that space and time have come to be thought of as being much more fundamentally similar. As we will discuss later in this chapter, the main origin of this is that in relativity theory certain aspects of space and time seem to become interchangeable. And from this there emerged the idea of thinking in terms of a spacetime continuum in which time appears merely as a fourth dimension just like the three ordinary dimensions of space.
So while in a system like a cellular automaton one typically imagines that a new and separate state of the system is somehow produced at each step in time, present-day physics more tends to think of the complete history of the universe throughout time as being just a single structure laid out in the four dimensions of spacetime.
So what then might determine the form of this structure?