AG, You're making the classic mistake of conflating what's mathematically rigorous with what's immediately observable. The universal wavefunction isn't a mere hypothesis; it's a direct consequence of applying Schrödinger's equation universally. If you take the equation seriously and reject ad hoc collapses, you get the Many-Worlds Interpretation (MWI).
> "If it's not a world, why do advocates of the MWI refer to worlds with > copies of experimenters doing quantum measurements, or maybe flying > insects changing directions?" Because "worlds" are emergent patterns within the wavefunction, not fundamental ontological entities. Just like temperature emerges from molecular motion but isn't a single particle property, "worlds" emerge from decoherence but aren't fundamental themselves. The term "world" is a linguistic convenience, but the reality is just unitary evolution. > "Why should anyone experience superpositions? What would it feel like?" You're already in a superposition; you just don’t experience it as such because measurement entangles you with an outcome, and decoherence suppresses interference. It's like asking why you don’t "experience" being in two places at once when your wavefunction technically spreads over a region. The question misunderstands how observation works in quantum mechanics. > "In a macro experiment like throwing dice, there's no interference, > so what role has decoherence have in this situation or generally?" Decoherence ensures that macroscopic randomness behaves classically. The dice don’t interfere with themselves because they are entangled with countless environmental degrees of freedom. That's decoherence in action— destroying coherence between outcomes so that they evolve independently. > "Maybe 'collapse' isn't a good way to describe what happens when a > measurement occurs. ISTM, that the wf evolves into a delta function > centered at the measurement value. Physically this is what happens, > even though we don't know why." That's just Copenhagen in disguise. The wavefunction doesn't evolve into a delta function unless you force a collapse postulate, which is an artificial addition. Without collapse, all components continue to evolve, just without interference. That’s precisely what the MWI states—nothing more, nothing less. > "I am content to acknowledge that it's an unsolved problem, rather > than postulating other worlds and a universal wf, both of which are > totally undetectable. This can't be compared to a BH which has > something detectable in the region of its horizon." You say "totally undetectable" as if collapse is any more detectable. The difference is that MWI follows directly from the Schrödinger equation without arbitrary assumptions. You claim to be content with an "unsolved problem," but what you call "unsolved" is just an unwillingness to accept the straightforward implications of quantum mechanics. And the black hole analogy stands—if you refuse to acknowledge something just because it's not directly observable, you might as well deny the existence of the singularity inside a black hole. The math tells us it's there. Just like the math tells us all components of the wavefunction persist. You're clinging to a philosophical preference, not physics. Quentin Le ven. 14 févr. 2025, 23:48, Alan Grayson <[email protected]> a écrit : > > > On Friday, February 14, 2025 at 1:04:49 PM UTC-7 Quentin Anciaux wrote: > > AG, > > A branch is just part of the universal wavefunction evolving under > Schrödinger’s equation. It’s not a "world" with clear boundaries but a > region of the wavefunction that has decohered, making interference > impossible. > > > If it's not a world, why do advocates of the MWI refer to worlds with > copies of experimenters doing quantum measurements, or maybe flying insects > changing directions? AG > > You don’t experience other branches for the same reason you don’t > experience superpositions, > > > Why should anyone experience superpositions? What would it feel like? AG > > > decoherence isolates them. > > > In a macro experiment like throwing dice, there's no interference, so what > role has decoherence have in this situation or generally? AG > > > Saying they "don’t exist" because they’re inaccessible is like denying the > other side of a black hole. MWI isn’t excess, it’s just following the math > without adding collapse. > > > Maybe "collapse" isn't a good way to describe what happens when a > measurement occurs. ISTM, that the wf evolves into a delta function > centered at the measurement value. Physically this is what happens, even > though we don't know why. I am content to acknowledge that it's an unsolved > problem, rather than postulating other worlds and a universal wf, both of > which are totally undetectable. This can't be compared to a BH which has > something detectable in the region of its horizon. AG > > > Quentin > > Le ven. 14 févr. 2025, 20:58, Alan Grayson <[email protected]> a écrit : > > > > On Friday, February 14, 2025 at 8:36:52 AM UTC-7 Quentin Anciaux wrote: > > > > Le ven. 14 févr. 2025, 16:23, Alan Grayson <[email protected]> a écrit : > > > > On Friday, February 14, 2025 at 2:41:44 AM UTC-7 Quentin Anciaux wrote: > > > > Le ven. 14 févr. 2025, 10:29, Alan Grayson <[email protected]> a écrit : > > > > On Friday, February 14, 2025 at 2:17:37 AM UTC-7 Quentin Anciaux wrote: > > > > Le ven. 14 févr. 2025, 10:08, Alan Grayson <[email protected]> a écrit : > > > On Friday, February 14, 2025 at 1:37:20 AM UTC-7 Quentin Anciaux wrote: > > Le ven. 14 févr. 2025, 06:13, Brent Meeker <[email protected]> a écrit : > > > > On 2/13/2025 4:57 AM, John Clark wrote: > > On Wed, Feb 12, 2025 at 5:41 PM Brent Meeker <[email protected]> wrote: > > *>> Schrodinger's Equation is 100% deterministic, so why is it necessary > to resort to probability at all?* > > > > > *Because one thing of many possible happens.* > > > > *Why is that "one" thing special? I can answer that; because it's not > special, many things happen, everything that is not forbidden happens. You > have no answer to that question other than "because it is". * > > The only thing special about is that it's the one that happened. If > everything not forbidden happens then you're going to need to explain what > probabilty means. > > > * > I can write an equation for the toss of die that shows that the > probability of each face is 1/6. That equation is deterministic. It > determines probabilities. And probabilities tell you that some things > happen and some don't. Not that every face of the die comes up on every > throw.* > > > > *Schrodinger's equation produces a complex-valued wave that evolves in > time, the square of the absolute value of the amplitude of that wave > determines probabilities. You just take the Born Rule as a given because > experimenters tell you that it works. Many Worlds can tell you why it works > and why you need it. * > > So you say. But all attempts to derive it, assuming MWI, have failed. I > look forward to your paper. > > > *And unlike Schrodinger's Equation your dice equation directly determines > a probability* > > Not as directly as Schrodinger's equation determines QM proability > amplitudes. > > *; classical physics doesn't have or need a counterpart to the Born Rule > (although the square of the absolute value of an electromagnetic wave is > proportional to its energy). Classical physics can provide us with an > excellent approximation of how the orientation of the die will change in > time, so why do we need to use probability? The reason for that is > practical not fundamental, sometimes in classical physics tiny changes in > initial conditions lead to exponentially diverging trajectories over time, > and you're never going to know the initial conditions exactly, and even if > you did you don't have the computing capacity to use them.* > > *> And you have no answer to what probability means, until you resort to > "uncertainty of self-location",* > > > *Resort to? If I'm not allowed to give the correct answer then my answer > is going to be wrong. Many Worlds says everything always obeys > Schrodinger's equation including the observer, therefore there will always > be self-location uncertainty, it can't be avoided. * > > And how does that result in uncertainty, when you are located in every > branch. It's just the problem of what does probability mean when > everything happens. You're just pushing the problem around. > > Brent, > > The problem isn’t that "everything happens"—it’s *how often* different > things happen from the perspective of an observer. Probability in MWI > doesn’t mean "some branches exist and others don’t" but rather that an > observer finds themselves in certain branches *proportionally* to their > measure. > > Saying "you’re just pushing the problem around" ignores that probability > in any framework is about *expectations for future experience* based on > structure. In a single-world view, you justify probabilities by appeal to > hypothetical ensembles or repeated trials that never actually happen. In > MWI, the structure of the wavefunction provides the ensemble *within* > reality, and measure determines where most instances of an observer exist. > > Also, I’m not specifically advocating for MWI. I lean more towards *a > computational theory of reality*, where measure and probability emerge > from an underlying informational structure. But I do favor frameworks where > *everything > happens* rather than a single unique history set in stone forever. > > *Then you'll like this: * > *In a horse race, according to the MWI, multiple worlds come into > existence for all possible winners in a particular race. But for one given > race, are there are not multiple worlds, possibly countably infinite, which > come into existence for every possible way in which the winner wins, while > retaining the finishing order of the losing horses? I think so, and is the > reason I find the MWI and its devotees, lacking in discrimination. But for > a discerning eye, it's in the eye of the beholder, of Schrodinger's > equation. AG * > > > AG, > > That’s exactly the point, MWI doesn’t just split for the winner, it splits > for every possible microscopic detail of the race, including variations in > how each horse crosses the finish line, fluctuations in the crowd, air > molecules, and so on. The number of branches isn’t just countably infinite; > it follows the continuous evolution of the wavefunction. > > But calling that "lacking in discrimination" misses the core idea. It’s > not about choosing which worlds are "important", it’s about unitary > evolution preserving all possible outcomes. The structure is dictated by > Schrödinger’s equation, not by human intuition about what "should" count as > a distinct event. > > If you think that level of detail makes MWI unreasonable, you should also > reject classical probability, where every possible dice roll, coin flip, or > weather pattern is part of a notional ensemble. The only difference is that > MWI doesn’t assume unrealized outcomes "disappear" without explanation. > > Quentin > > > *So every wiggle of your finger or toe results in perhaps uncountable > worlds coming into existence, as well as every random turn of a flying > insect? It just doesn't pass the smell test. AG * > > > AG, > > Yes, every quantum interaction leads to branching—whether it’s a photon > reflecting off your skin or an insect flapping its wings. But the mistake > is thinking of this as “new worlds popping into existence.” MWI doesn’t add > anything extra, it simply follows unitary evolution, where all possible > outcomes exist in superposition. > > What doesn’t pass the smell test is the idea that only one history is > mysteriously “chosen” while the rest, dictated by the same Schrödinger > equation, vanish without explanation. > > > What's the difference between an outcome not realized, and a branch which > is disjoint from this world (and BTW, what is a branch)? AG > > > AG, > > An unrealized outcome in a single-world interpretation means it never > happened, it was just a hypothetical, a calculation tool that never > manifested in reality. In MWI, an alternative outcome is not "unrealized" > but real in a disjoint branch, it exists, but in a different part of the > wavefunction that no longer interferes with ours. > > A branch is simply a part of the universal wavefunction that has decohered > from others due to interactions with the environment. Once decoherence > happens, branches stop interfering, effectively behaving as separate > classical-like worlds. > > So the key difference is: in single-world views, unrealized outcomes are > fiction, never happened, never will, never experienced, never interacted; > in MWI, they are real but inaccessible due to decoherence. > > Quentin > > > Does a branch have any physical reality? Is it anything like a "world"? If > so, where does this world begin and end? If it can't be detected, it's > operationally equivalent to non-existing. JC persists in his claim that > it's what S's equation says. I don't experience it that way. I don't > believe that if a horse wins a race, all possible winners win in some > defacto imaginary world or branch. The MWI is like Nietszche's critique of > Christianity; Rococo; Excessively Flamboyant. AG > > > > > If you accept quantum mechanics, you already accept that reality operates > in a way that defies classical intuition. Rejecting MWI because it feels > excessive is just favoring one form of weirdness over another. > > Quentin > > > > Saying 'some things happen and others don’t, just because' is not an > explanation—it’s an arbitrary assertion, no better than saying 'God did it > that way.' A real theory should provide a mechanism for why certain things > are observed rather than simply declaring them to be the case. > Quentin > > -- > > You received this message because you are subscribed to the Google Groups > "Everything List" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > > To view this discussion visit > https://groups.google.com/d/msgid/everything-list/481e369b-3123-4a86-814d-e1c4edf869a9n%40googlegroups.com > <https://groups.google.com/d/msgid/everything-list/481e369b-3123-4a86-814d-e1c4edf869a9n%40googlegroups.com?utm_medium=email&utm_source=footer> > . > > -- > You received this message because you are subscribed to the Google Groups > "Everything List" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > > To view this discussion visit > https://groups.google.com/d/msgid/everything-list/d7b79b51-84da-4144-a04f-a0abbc05b250n%40googlegroups.com > <https://groups.google.com/d/msgid/everything-list/d7b79b51-84da-4144-a04f-a0abbc05b250n%40googlegroups.com?utm_medium=email&utm_source=footer> > . > > -- > You received this message because you are subscribed to the Google Groups > "Everything List" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > > To view this discussion visit > https://groups.google.com/d/msgid/everything-list/a2708f1c-c023-43ca-876a-e0eb3e4218b1n%40googlegroups.com > <https://groups.google.com/d/msgid/everything-list/a2708f1c-c023-43ca-876a-e0eb3e4218b1n%40googlegroups.com?utm_medium=email&utm_source=footer> > . > > -- > You received this message because you are subscribed to the Google Groups > "Everything List" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > > To view this discussion visit > https://groups.google.com/d/msgid/everything-list/b7621fa4-975f-4a0f-b734-d03952ae456fn%40googlegroups.com > <https://groups.google.com/d/msgid/everything-list/b7621fa4-975f-4a0f-b734-d03952ae456fn%40googlegroups.com?utm_medium=email&utm_source=footer> > . > > -- > You received this message because you are subscribed to the Google Groups > "Everything List" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To view this discussion visit > https://groups.google.com/d/msgid/everything-list/a4f26c82-41d6-4378-884a-e868ff5e299cn%40googlegroups.com > <https://groups.google.com/d/msgid/everything-list/a4f26c82-41d6-4378-884a-e868ff5e299cn%40googlegroups.com?utm_medium=email&utm_source=footer> > . > -- You received this message because you are subscribed to the Google Groups "Everything List" group. 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