This is just a riff – obviously prompted by the Deutsch debate – to lay out my original point – “I don’t buy Many Worlds”.
Many worlds thought of as the Everettian interpretation of quantum theory has (had) an easy to visualise metaphor, even if it might also be easy to discount it as a world-view of what really exists. The idea that every quantum event – each wave-function collapse – creates a bifurcation in the universe(s) into one where what actually happened happens and (an)other(s) where what was possible didn’t happen here but (maybe) did happen there. Without any deep understanding of the maths and mechanisms of such theory(ies) it is a simple way to visualise that somehow the statistical distribution of possibilities existed in this world up to that point and then didn’t. Where did they go?
It’s nevertheless almost impossible to hold in your minds-eye world-view that all these gazillions of alternate worlds really exist in any meaningful way? To even entertain that thought is also mentally related to what we hold in mind as the world, the cosmos, the / a universe in the first place. And whilst Many Worlds is completely unrelated physically to The Multiverse(s) – the idea of a whole universe evolving out of a (near-)singularity suffers from the same credulity problems.
Now what’s actually going on here is very close to the most fundamental question of philosophy, let alone physics. Why / how-come something out of nothing? So any physical answer is going to be dubious, even if we manage to establish a mental picture of what we might believe. If that something – seed – existed before this universe, where did it exist and where is that (time &) place now? What does “now” even mean? (also part of Deutsch’s video essay).
One of my “priors” is that “The Universe” is unitary – it is the entire cosmos of possibility and reality. That’s what we mean by “the”. As soon as we entertain the idea of multiple universes we need different names for different things. The entire super-(multi-uni)-verse idea must contain any subsidiary universes, if they really exist. If we open up thought of time itself, not just now, as having no beginning or end – and that “The” (super) universe has always existed, it is not difficult to imagine “This” (current) universe – and other subsidiary universe(s) – evolving from there.
No less credible than the big-bang idea itself, anyway.
The crucial idea that makes this credible is that of an horizon. Whether a tiny compressed “black hole” of a cosmic seed, or the furthest reaches of a universe in time and space, they have a horizon beyond which no information can pass. The contents of a black hole or a a complete universe can never be known to another “region” of space-time the other side of such a horizon. A seed forming a new universe may inherit some information from the existing region in which it formed, but after that – incommunicado. That’s it.
So we may “imagine” the reality of multiple universes and/or many worlds in our thought experiments – different thought experiments, in different unrelated parts of physics, remember – but never have to treat them as part of reality in the universe we inhabit. They can never have any meaning in this universe.
Not quite right, but close enough for now. “True enough” to use Nick Cave / Sean O’Hagan’s humane “Irish” expression.
Need to relate this to my point about Deutsch and his Everettian “World View”.