The size of the Universe and the Great Filter

by Paul Crowley

In a small Universe, an early Great Filter is unlikely, simply because we’re evidence against it.

Suppose the Filter is early; how severe must it be for us to observe what we observe? By “severe” I mean: what proportion of worlds must it stop? Our existence is evidence towards an upper bound on the severity of an early Great Filter, while the fact that we observe no other life tells us about a lower bound. If the observable Universe is a substantial fraction of the whole Universe, then the two bounds aren’t very far apart, and so to defend an early Filter we have to believe in a great cosmic coincidence in which the severity of the Filter was just right, which is in turn is evidence for a late Filter whose severity we have no upper bound for. This argument has in the past given me real cause to worry that the Filter is late, and very severe.

However, this argument doesn’t hold at all if the observable Universe is a tiny fraction of the whole Universe.  The larger the difference between these two numbers, the bigger the gap between the bounds we have for the severity of the Filter, because intelligent only has to appear once in the whole Universe for us to contemplate this question, while it has to arrive twice in the smaller, observable bubble for us to observe it.

As I understand it, modern cosmology points towards a Universe that is either infinite, or very much larger than the observable Universe, so on those grounds alone we can perhaps worry less. But far more strongly than that, the Many Worlds interpretation of quantum mechanics gives us a many-branched Universe that is just unthinkably larger than the tiny portion we can observe; if intelligent life emerged on as many Everett branches as there are stars in the galaxy, we would still appear alone to our best ability to tell. So I now think that this isn’t a reason to worry that the Filter is late. It is however an excellent reason to expect never to meet an alien. Sorry.