[Veedrac]

Maybe I am just out of my depth, but I don't understand what problem quantum Darwinism is solving. The Schrödinger equation already explains why observers seem to agree: the ones that don't are separated from each other.

This article is making some pilot-wave-like claim on top of quantum Darwinism that while the Schrödinger equation is real, all the 'real realness' exists in some pointer to a specific location inside it. Why does it do this? Where does this claim come from? At least collapse theories allow that the thing the Schrödinger equation is modelling is actually real up until the part God gets out his frustum culler.

[anon291]

I think the claim is this: the wave function never collapses. However, the effect of the wave function on the environment quickly converges to only one of the two states. We could not know the difference because we cannot directly observe the wave function. We only can see the result as it is magnified onto a macro scale by our observation equipment (or, lacking that, our eyes, which themselves turn a tiny microscopic phenomenon into macro signals). Once that particular outcome has been 'selected' for, the probability of the other outcome quickly becomes vanishingly small very fast. Thus, all future outcomes are that outcome, even though the underlying reality is still that fully entangled state.

Photons (and other objects that seem to behave 'quantumly') do not seem subject to this (and thus we can use them to understand quantum behavior) because they have particular properties wherein their behavior is not as affected by these macroscopic drop-offs quite as badly.

[Veedrac]

My confusion is that this is just Many Worlds / the Schrödinger equation, and Quantum Darwinism doesn't seem to add anything that wasn't already obvious by inspection. But after reading more, I think that's kind of the point? It's ultimately just an argument for why the Schrödinger equation produces these locally classical regions, plus a bunch of overly flowery prose and dressing up in invented jargon that can mostly be ignored. I think the article failed to ignore that second part and ended up confused.

[anon291]

Many worlds is not the Schrodinger equation. No I don't think this is many worlds. The decision is made uniquely and then is amplified.

[Veedrac]

Many worlds is just the claim that the Schrödinger equation holds in actuality.

I don't think QD makes decisions 'uniquely'. Take this quote,

> The step from the epistemic (“I have evidence of |π17〉”.) to ontic (“The system is in the state |π17〉”.) is then an extrapolation justified by the nature of ρS⁢ℰ: Observers who detected evidence consistent with |π17〉 will continue to detect data consistent with |π17〉 when they intercept additional fragments of ℰ. So, while the other branches may be in principle present, observers will perceive only data consistent with the branch to which they got attached by the very first measurement. Other observers that have independently “looked at” S will agree.

https://pmc.ncbi.nlm.nih.gov/articles/PMC9689795/

Emphasis on "the other branches may be in principle present" — the claim at least in this paper can't be that all branches agree, just that they agree locally.

[anon291]

Without defining what 'actuality' is, then there's no meaning to 'the Schrodinger equation holds in actuality'. In their own way, all interpretations of quantum mechanics claim the Schrodinger equation holds in 'actuality'. Some view probability and potential as a claim on 'actuality'. Others dismiss this and instead view probability skeptically and claim it must thus be true. This is an ontological argument, not a scientific one.

[Veedrac]

If you don't like the word 'actuality', I can rephrase. Many worlds is just the claim that physical reality materially evolves in correspondence with the Schrödinger equation.

If you want to quibble over what it means for something to be material, go ahead, but unless you can tie it to some specific claim being made about QD I don't really know what the exercise gets you.