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I was reading about the MWI and how there would be vastly different versions of "you" in the different branches.

I'm just wondering, though, which quantum events lead to such different outcomes? Whether an electron spins left in one universe and right in another doesn't seem like it would lead to significantly different outcomes for me. Particularly as the quantum butterfly effect doesn't exist.

So, I'm wondering which events in our macro world are significantly impacted by macro events such that one world would be vastly different to another.

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    $\begingroup$ How are you drawing the line between a quantum effect and a macro effect? A quantum game of russian roulette, with a gun tied to the decay of a single particle, would have macroscopic effects that quickly balloon in the classical world. $\endgroup$
    – Cort Ammon
    Jul 12, 2021 at 18:24
  • $\begingroup$ @CortAmmon Yeah, Id assume that counts. Things that arent covered by the determinism of newtonian physics and the ripple effects of their randomness. I find it difficult to think how Im having curry in one universe, and Pizza in another (Sean Carroll example) just because a certain atom decayed slightly earlier rather than later when theres a trillion other atoms and the sum of which become quite predictable as to when the entire material will decay $\endgroup$
    – Raheemsterling
    Jul 12, 2021 at 19:30
  • $\begingroup$ The MWI is highly speculative in that it conjectures parts of reality that are not observable (the "other worlds"). Finding it difficult to think about unreal things is not an uncommon psychological phenomenon among scientists. $\endgroup$
    – oliver
    Jul 12, 2021 at 19:34
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    $\begingroup$ My personal favorite chaotic system is the motion of the planets in the sky. On human time scales, the motion of the planets is enormously predictable. However, on the scale of millions of years, their motion is chaotic (as is the nature of the N-body problem). Thus, the decay of some particle several hundred million years ago changed the motion of the planets. We don't know which way it changed things -- it looks random to us. But the decay of some particle substantially changed where the planets were in the sky at your birth. How much your... $\endgroup$
    – Cort Ammon
    Jul 12, 2021 at 19:56
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    $\begingroup$ ... life is affected by this would be based on how much you believe in astrology, but I find it fascinating to think that the planets could quite literally be in a different part of their orbit (strange attractor, more precisely) due to the tiniest change at the subatomic scale, millions of years in the past. Then I shrug it off, and call it random chance until I get all red-lights going to work, and assert a cosmic conspiracy against me! $\endgroup$
    – Cort Ammon
    Jul 12, 2021 at 19:57

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That phys.org press release seems to refer to this paper (arXiv).

Quantum mechanics is linear at the level of Schrödinger evolution, and linear systems can't be chaotic. Classical mechanics is also linear at the analogous "level" of probability distributions. But quantum mechanics is nonlinear and chaotic at the level of ordinary particle dynamics, just like classical mechanics. It makes little sense to apply the term "quantum butterfly effect" to an effect that doesn't exist, when there is a real butterfly effect in a quantum world.

It seems very likely that every deviation from perfect homogeneity and isotropy in the universe is due to amplified quantum fluctuations. (That's true in inflationary cosmology, and I'd expect it to be true in practically any alternative to it.) For example, the shape of Earth's land masses was probably determined by quantum fluctuations, and has had an enormous influence on human history.

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  • $\begingroup$ Thank you so much, that was really helpful! It just seemed to me like macro physics was largely deterministic because the quantum effects canceled out due to the large number of atoms i.e. the flip a billion coins and the randomness is largely canceled out example. $\endgroup$
    – Raheemsterling
    Jul 12, 2021 at 19:20
  • $\begingroup$ @Raheemsterling The randomness is largely canceled out, except in the case of unstable systems which magnify the effects of any perturbations, no matter how small. $\endgroup$
    – Cort Ammon
    Jul 12, 2021 at 19:52

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