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.