This is the most elegant theory of a multiverse that I’ve heard of and the first that I could accept. Why? Because these universes are not over-determined in the sense that there is necessarily a universe for every event in our space-time and its corresponding non-event. It’s like ‘universes that beg to differ’, as much a popular trope of sci-fi as a more recent and utterly pointless interpretation of quantum mechanics. Ethan’s universes have just branched out at a given moment and go their own way from then on. There is even no necessity that the natural constants vary between these universes, even though you write “…with a different starting point and different specific initial conditions for each region.” Why the latter? Yes, it could be, at least in some cases. But I don’t see any physical necessity for this deviation from the set of natural constants that we know. This said, these universes could be surprisingly similar to ours. Intuitively, this is a solution that I could imagine of being a physical reality.

Yet, the part that I don’t buy (yet) is the equation of a particle in a coherent quantum state with inflation altogether, the epitome of decoherence, all the more you later admit: “… even though we have no proof that inflation itself behaves as a quantum field.” This as the mechanism that selects if and how multiple universes come into being is not convincing, too subtle and too fragile.

You might say, decoherence doesn’t prevent stars like the sun to run a nuclear fusion engine exclusively under the laws of quantum mechanics. Otherwise there would be no fusion at all. That’s true. But in this case, interestingly, quantum mechanics explains how and why it is possible that within the core of the sun, a sphere of the size that alone almost fills the orbit of the Moon around the Earth, in some very rare cases a fusion can take place. Correspondingly, it would suit your model better if with the quantum properties of inflation it could be explained how and why ONLY SO FEW universes came (and still come?) into being. I just think that your model needs a stronger and better defined mechanism or theoretical term of selection.

Post Scriptum: Funnily enough, just two days after writing the above contentions to Ethan’s proposal, I find this article in Scientific American about a new move in string theory that dramatically enhances its selectivity regarding the number of possible universes: https://www.scientificamerican.com/article/string-theory-may-create-far-fewer-universes-than-thought/

Reginald Gruenenberg

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I'm a German novelist (www.east-pole.com), philosopher and screenwriter. My homepage is www.reggies.world