I've had parallel universes on the brain this week, thanks to a bit of convergence between Job Number 1 and Job Number 2. First, courtesy of Job Number 1, there's the many worlds interpretation of quantum mechanics, which suggests that what we call wavefunction collapse is, in reality, the branching off of a new universe.
Then, over at Job Number 2, there's the "multiverse" advanced by cosmologists like Tufts' Alexander Vilenkin, in which our Big Bang is just one of many which occur at other points in space and time; with inflation continuing to pop the cork on new universes all the time--for eternity--there must be infinitely many universes in existence. As Vilenkin says in one interview:
...[I]f you have a finite number of histories that are unfolding in an infinite number of regions, it follows that every history that can possibly happen will happen and it will happen an infinite number of times. So there are an infinite number of regions where we have exactly the same planet, like our Earth, with exactly the same people and we are having exactly the same conversation. This is not something that would affect your daily life, but it's something to ponder.
All of which makes me think of those Choose Your Own Adventure books that used to populate the elementary school library; at the end of each chapter, you had to make a multiple-choice selection (e.g. "If you decide to slay the tiger, turn to page 23; If you decide to run away from the tiger, turn to page 41; If you decide to gently pet the tiger, turn to page 45") that would create a unique story.
Of course, there was a finite number of stories in those little books, but if you agree that there is a fininte number of particles in the universe, then one could conceivably create a "choose your own adventure" book that encompassed every possible configuration of the universe. It would be very long and very boring ("If this particle is spin up, turn to page 1,000,002; if this particle is spin down, turn to page 132,332,837,113"), but it would be finite.
In the many worlds interpretation, each "adventure" in this book plays out in a private universe which can't touch any of the others. But in Vilenkin's view, there is the potential for two independent storylines to intersect, as "bubble universe" collide.
In fact, Vilenkin and his collaborators Jaume Garriga and Alan Guth (who, incidentally, first proposed the theory of cosmic inflation), have calculated how often bubble universes should bump in to each other--and how often a bubble's inhabitants should notice the collision. The answer is complicated, but boils down to: it depends.
"It depends" on things like where you live inside the bubble, which is actually pretty surprising, since one of the basic tenets of modern cosmology is that no location in the universe is more "special" than any other. (Kind of like what you learned in 7th grade health class.)
Can it explain why a bubble universe hasn't yet crashed in to our own? Why we haven't had a momentary glimpse of some alternate universe in which, say, dinosaurs are driving around in hovercars, chatting on rotary cell phones--before that universe slammed into us and knocked us out of existence? Ask me a week from Wednesday!