A new multiverse theory was proposed last week, and it could be promising. I have to say, the basic idea of the theory is very appealing to me. Basically the idea (as far as I understand it) is that the unpredictable world we live in is actually one of many (but not infinite) parallel worlds that reside in the same physical space and time as ours, right on top of each other, but in different dimensions. These many worlds follow basic, Newtonian laws, and do not, on their own have the principle of Quantum Mechanics.
What does that mean? Well, since these 'worlds', as the theory calls them, aren't separated from each other (like other multi-universe theories claim), they can interact with each other. These interactions are what give rise to what we see as Quantum Mechanical effects.
From Universe Today:
Hall states in the paper that simple Newtonian Physics can explain how all these worlds evolve... Certainly, Einstein's Special and General Theories of Relativity completes the Newtonian equations and are not dismissed by [Many Interacting Worlds Theory]. However, the paper begins with the simpler model using Newtonian physics and even explains that some fundamental behavior of quantum mechanics unfolds from a universe comprised of just two interacting worlds.
This means that, on their own, a given world is just a classical Newtonian environment. Newton's laws apply. Quantum Mechanics arises from the fact that all of these, otherwise clean, Newtonian worlds are stacked together and interact. And I find this idea fascinating. It means that, if this is true, Quantum Mechanics is a consequence of reality, not a builtin principle of it, and that has me very excited.
A pretty straightforward, but by no means perfect metaphor is that the universe can be thought of as a billiards table. We are hitting the cue ball and watching the results as it hits the other balls. Quantum Mechanical effects, in this metaphor, would be the 7 ball randomly veering off in another direction when you hit the 4 ball. Weird right? Well, this theory basically says that the 7 ball isn't moving randomly, instead there's another person playing right next you who's hitting another cue ball at the 7 ball when you hit the 4 ball. Quantum Mechanics is the study of the 4 and 7 balls, whereas MIW theory looks at the 2 players. If we realize that there are two players and not just one, the movement of the balls is relatively easy to explain.
Quantum Mechanics works, we use its principles every day. That's not going to change. What would change, if this theory proves useful, is that Quantum Mechanics would not be thought of as a fundamental principle of the universe, but merely a consequence of other principles.
I'll be keeping my eye on this; hopefully it pans out for further investigation.