Wednesday, September 26, 2007

Multiverses -- Physical and Metaphysical

I feel I’ve been inconsistent. On the one hand, wearing a philosophy hat, I’ve endorsed a form of modal realism, where our actual world is a subset of a full expanse of metaphysical possibilities. On the other hand, when I wrote about multiverse models offered by theoretical physicists, I downplayed the "actual" reality of the distant regions described in such theories.

I’m thus returning to the old question of what, if any, is the relationship between the philosopher’s space of possible worlds, and the multiverse described by some physical theories. What should one’s stance be toward each of these? I think I have a clearer thought on this; but first let me digress briefly to say what led me to it.

For awhile I’ve wanted to identify the “actual” world with the region we have causal contact with. The reason for this is motivated by my preferred model of causation and ontology, which says that the familiar concrete world consists of events which are actualized possibilities. In an earlier post I stated this and thus proposed that physical models which contained models of the universe or multiverse beyond this region were describing things which were not actual, but only possible. The clear implication to the reader is that the regions so described were “less real” than our neighborhood. Clark, his comments, questioned whether I could justify discounting all the various multiverse theories. Alejandro described my stance as anti-Copernican: the idea being that our particular neighborhood shouldn’t be viewed as special in this way, given a sound theory which describes our region as well as points beyond. I think this was good criticism.

Now, however, in the context of modal realism, I have this idea that it is consistent with my ideas that “actual” be considered an indexical term (see prior post). In this case I can still denote our causal region as actual, to fit my idea of causation as the process of actualizing possibilities or propensities. However, there is no intent to say our region is special. The terms actual and possible are relative to a local point of view. There is nothing special about our region – from the point of view of an observer elsewhere, the concrete events familiar to us are unactualized. And I think this stance can be appropriate in the context of physical multiverse models as well as in the metaphysical context where it arose. The actual world has a different status then the regions beyond our contact, but this is a relational distinction, not an absolute one, and thus not in itself a rejection of these attempts to model distant reality.

So, given that conclusion, what can be said about the relationship between the metaphysical and physical multiverses? The philosophically motivated idea is that of a space where every metaphysical possibility exists. This is equivalent to saying every logical possibility exists if we endorse modal rationalism. Our actual world is one island in this expanse. I see multiverse theories offered by physicists as attempts to model particular subsets of this space by allowing for the extension or variation of our physical events and laws (as best we know them so far). Such theories widen the realm of nomological possibility – and as you widen this scope more and more you begin to converge toward the ultimate space of metaphysical/logical possibility.

Monday, September 17, 2007

Actual as Indexical, After All?

David Lewis’ theory of modal realism features a collection of concrete worlds, which are spatio-temporally connected societies in themselves but wholly isolated from one other. We live in one of these, which we call the actual world; the others we call possible worlds. But there is nothing special about being “actual”; it just refers to where we reside: actual is an indexical term.

In sketching my own ideas, I’ve rejected Lewis’ theory, but I’m now considering that his sense of actual may be the right one for my model as well. What follows is some “work-in-progress” thinking on this. (My modal realism posts are here).

Lewis’ worlds are completely separate, and they are static entities; his treatment of causation is Humean in spirit. On the other hand, in my preferred view, causation is a process of actualizing possibilities. The actual (concrete) world consists of the events we are already in causal contact with. At any given moment we are surrounded by adjacent (abstract) possible events which are available for our actualizing. More distant possibilities (and possible “worlds”) are rational constructions based on our experience with possibility (they are also accurate in their representations, if we embrace modal rationalism).

To those modal realists who disagreed with his account, and preferred to think of possible worlds as abstract objects, Lewis demanded an account of what magic accounted for the instantiation of one of the worlds as actual. But in my idea, it is not an entire world that is instantiated as actual all at once, rather a series of events are actualized through an active causal process. So it wasn’t clear that Lewis’ objections would apply in the same way.

We are on a journey through a space of possible events, actualizing certain ones as we go. At first, I thought this meant we are something magic or special in that we are blazing a causal path of actualization against a backdrop of a space of unactualized possibilities, and so I was feeling the bite of Lewis’s critique of his opponents: how can I account for our actualized “world” being thus special vs. the rest of the possibilia?

Then I realized that the mistake was to picture the space of possibilities as static. This was a hangover from Lewis’ model. In my model, a “possible” event is one we have not been in causal contact with from our particular perspective. But, from a different point of view this event could be actualized. Possibilities and actualities exist relative to a particular perspective. We just label the events unfolding in our local causal nexus “actual”, and the rest “possible”. All the events of modal space can be thought of as in an active process of becoming actual. The multiverse is alive, not static. And we are not special, after all.