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There is an often-repeated statement that different quantum mechanical theories are equivalent. This is an over-simplification. Is wave function collapse is a r

• rubidium 4y ago
  Many worlds is just some peoples favorite because they get too weirded out but indeterminism. They prefer a billiard ball universe and so invented the hairbrained many worlds theory.
  • ravi-delia 4y ago
    Or- and hear me out with this one- they prefer an explanation which follows naturally from existing experiments and doesn't require a hacked together non-linear irreversible operation which occurs only under bizarre conditions exactly when needed to patch over experimental results. No one in the whole world cares if the universe is deterministic or not, but collapse is embedded in an entirely deterministic system. MWI may not be right, but collapse is wrong.
    • shadowfox 4y ago
      > MWI may not be right, but collapse is wrong. Such confidence ... I envy that.
    • nil-sec 4y ago
      It’s quite nice to have something irreversible though. It gives you time. Also nobody really thinks QM is the end of it, assuming semi classical physics under the hood is just odd to me. There is something below QM that we don’t understand yet (AdS/CFT looks like a good start to me) and personally I think the whole interpretation of QM debate will look stupid in retrospect. Yeah collapse is odd, but it just shows us this isn’t it. Reality is much weirder than we thought and giving up on realism is just the beginning.
      • ravi-delia 4y ago
        You get time from all sorts of technically reversible things though! Even in a totally classical universe entropy gives us an arrow of time. Under MWI decoherence is reversible, but is functionally irreversible in the same way entropy is. You're right though, I very much doubt QFT as it stands is the bottom. However, that doesn't mean the current debate is stupid. Whatever underlies QM, you'd still expect the measurement effect to also be an emergent property. The debate about whether atoms existed is still meaningful even though we now know that "atom" isn't a perfectly natural category. Indeed there are protons and such underling the physics, but the protons do pretend to be atoms much of the time, and thus pretend to do all the things we use atoms to predict. Similarly, MWI and collapse (as well as, if less so, weirder theories) can be good explanations as to why a quantum phenomenon occurs even if there's also a reason they happen.
    • jobs_throwaway 4y ago
      > No one in the whole world cares if the universe is deterministic or not lol
      • ravi-delia 4y ago
        OK other than the several holy wars and church schisms and most of the physicists of the early 1900s including Einstein. And quantum-woo obsessed people on the internet. The point is that all the quantum stuff still adds up to mostly deterministic classical physics. Maybe only quantum physicists get to have free will, so long as they make decisions based on the exact results of their experiments?
    • tsimionescu 4y ago
      Actually, if you eliminate collapse entirely as a non-linear operation, then you have a new huge problem for QM: there is plenty of non-linearity in the universe (e.g. double pendulum experiments, not to mention GR), and QM predicts that there shouldn't be any non-linearity at all, if you eliminate wave function collapse/the Born postulate.
      • GoblinSlayer 4y ago
        Hmm... is de Broglie wave linear? Trigonometric functions don't look very linear to me.
        • ravi-delia 4y ago
          Ahh, linear in the differential equations sense, not the affine function sense. Trigonometric functions are quite linear- they decompose into a basis! That's why, for instance, Fourier transforms are useful for more than signal processing.
  • macrolocal 4y ago
    Eh, these are models, not reality. The Many-Worlds interpretation makes the fewest assumptions, so it's probably the least prone to over-fitting. Until we find philosophically or experimentally distinguished ways to pare down its ontological cost, working with most general theory is just keeping an open mind.
• lupire 4y ago
  Disproving some form of measurable wave function does not provide evidence for MWI, which is unmeasurable and untestable nearly by definition.
  • fallingfrog 4y ago
    Well, the thing is that the MWI is actually simpler than all the other interpretations, because it simply removes the idea of wave function collapse entirely. Any theory involving wave function collapse is adding something extra to quantum physics that can't be demonstrated experimentally. Because we live in the macro world, it feels like a single unitary reality is simpler, but actually, the MWI makes fewer assumptions. So I would turn that around and put the burden on the other theories to show that collapse is real, the default position should be MWI.
    • TheOtherHobbes 4y ago
      You don't think postulating an unknown number of extra universes with a very poorly defined relationship with each other qualifies as an extra assumption?
      • pdonis 4y ago
        The MWI does not postulate extra universes, although that pop science description is unfortunately common even though it is wrong. The MWI just says that the one universe that exists is actually very, very different from what we perceive. (To be clear, I'm not saying this correct description makes the MWI any more palatable. I am just clarifying exactly what kind of unpalatableness the MWI requires.)
        • ryeights 4y ago
          Can you elaborate on this? The Wikipedia page for MWI says >The many-worlds interpretation implies that there are most likely an uncountably infinite number of universes. https://en.wikipedia.org/wiki/Many-worlds_interpretation https://en.wikipedia.org/wiki/Many-worlds_interpretation
• zhouyisu 4y ago
  A interesting deduction: By making quantum computer, we are making us into Schrödinger’s Cat. Which entangles our destiny to a quantum possibility. Such as a highly sophiscated military quantum computer which decides war and peace on quantum computing. It must be very fun to observe us from a Alien Species's angle.
  • spywaregorilla 4y ago
    Quantum computing is a means to avoid checking wrong solutions en route to solving certain types of problems. Seems very unlikely for this kind of decision to ever depend on a quantum computer.
• andrewgleave 4y ago
  The Logic of Experimental Tests, Particularly of Everettian Quantum Theory: https://arxiv.org/pdf/1508.02048.pdf https://arxiv.org/pdf/1508.02048.pdf
• criddell 4y ago
  > My feeling is that Everett's "many worlds" interpretation is the clear favorite But what are the worlds? My feeling is that Max Tegmark has it right. Everything is mathematics.
  • spywaregorilla 4y ago
    Mildly different wobbled electrons.
    • criddell 4y ago
      And what are the electrons? AFAIK, they are excitations within a quantum field. And what's a quantum field? Each layer down seems to get closer and closer to pure mathematics. I wish I understood it better.
      • d_tr 4y ago
        > I wish I understood it better. Me too but... It would still be some form of mathematics as soon as you tried to write it down in a neat, precise way for others to understand it. Maybe exotic math, but still math :p
  • im3w1l 4y ago
    We can detect their presence because they cause interference with our own.
    • naasking 4y ago
      Circular. You have to already accept many worlds to take this as evidence of worlds. You can argue for Many-Worlds on grounds of parsimony and clarity, but that's it.
      • im3w1l 4y ago
        No I don't think so. Many worlds is basically the hypothesis that we can put arbitrarily large systems in superposition. Like schroedingers cat that is both dead and alive, but instead it could be the whole lab, the whole planet, the whole galaxy that is simultaneously in two different states. We could test this by trying to actually do it, by putting really large and complex systems in superposition. If they both evolve as we'd predict and cause the interference patterns we predict, then we must conclude that these "other-labs", "other-planets", "other-galaxies", "other-worlds" evolve just like ours, and have a causal effect on ours. Saying that sure, they evolve like ours, and sure they affect ours, but even so they are not real - that is I suppose one stance, but then you are getting pretty close to solipsism.
• herdcall 4y ago
  Check out "superdeterminism," first proposed by Bell (of Bell's inequality fame) himself and currently being vigorously pursued by Sabina Hossenfelder. There is no "collapse" per se according to this model and everything is really classical with hidden variables, and the entanglement comes because everything shared the same origin (the big bang). A very fascinating and in my opinion the best explanation, though it's extremely controversial and taboo because of its implications on free will (e.g., I see no mention of it at all in this article).
  • iainmerrick 4y ago
    I have never fully understood that idea, even after reading Hossenfelder's posts about it. The idea that there's a "block universe" and everything is predetermined is comprehensible, sure; but what's the exact mechanism by which Bell's inequality is violated in experiments? Doesn't that require the universe to have been carefully set up to look as if it's acting in a weird non-local way, on an experiment-by-experiment basis? I assume I am just misunderstanding the idea, though. Any suggested reading material much appreciated!
    • superposeur 4y ago
      Agreed — superdeterminism is not so much a theory as a theory-for-a-theory at this stage (a vague guess at the form such a theory might take). It is a sign of the much greater maturity of the physical collapse models that their parameters can be constrained by experiment. The ability to be constrained is a good sign, not a bad one. I love John Preskill’s standard nod to superdeterminism in his writings: without mentioning it by name he says “I leave it up to the reader to decide how seriously to take this possibility” (get it?)
      • naasking 4y ago
        I think the right way to look at it is that superdeterminism is an umbrella term for a class of theories that have a certain property. Like some interpretations of quantum mechanics are "psi-ontic", meaning the wave function is considered physically real (like many worlds), vs. "psi-epistemic" where the wave function is considered a reflection of our knowledge of reality (Copenhagen).
• jfengel 4y ago
  That's right: the "collapse" interpretation is an approximation. You can derive if from the MWI. We already know that the "worlds" separate and almost completely cease interacting. The interactions decrease to the order of "two to the power of Avogadro's number" almost immediately. The collapse interpretation rounds that to zero (at some arbitrary point). To make that more concrete you have to add a term to the equations, making it theoretically testable, but it's remarkable that anybody could manage to test it in practice. It's clever to use a neutrino detector -- something designed to measure ludicrously small amounts of interactions. I don't think anybody is surprised by the results, just that they could get any. The un-physical arbitrariness of the cutoff caused problems from the get-go. I was going to say that it doesn't make MWI any more palatable, though I suppose it does. They've detected the other branches, albeit in a very indirect way. They're real. But they're just as inaccessible as they ever were. You still need the concept of decoherence to make it work. The unstable equilibrium can't hold for large objects. That's both mathematically and physically sound.
  • pdonis 4y ago
    > You can derive if from the MWI. Not as an actual physical process, no. "Collapse" in the MWI just means that, once the "worlds" have decohered, within each "world" the wave function can be collapsed to the one that describes the measurement result observed in that "world", for purposes of predicting future results. But in the MWI this is just a mathematical convenience and doesn't correspond to anything physically happening. In the theories being tested by the experiments described in this article, though, collapse is an actual physical process: there is only one "world". This contradicts the MWI. > They've detected the other branches, albeit in a very indirect way. What experiments are you referring to here?
    • whimsicalism 4y ago
      > doesn't correspond to anything physically happening It corresponds to the inner product of the two branches of the wavefunction going to 0 due to physical phenomenon.
      • 4y ago
• TheOtherHobbes 4y ago
  I don't see how relabelling "collapse" to "separation" and adding more or less infinitely proliferating universes solves the problem. MWI also contradicts itself. Supposedly the universes are independent, but if their influence doesn't define the wave equation they do nothing to help explain it. It's a very strong and exceptional claim with absolutely no evidence to support it. "Feeling" isn't enough.
  • krohling 4y ago
    I don't believe "feeling" has anything to do with the MWI interpretation. At least I've never heard it described that way. Given the equations that describe quantum mechanics (ie Schrodinger equation) MWI is essentially the "null hypothesis". No equations that describe collapse have ever passed the rigor of experiment and all collapse theories require modifications to the mathematics of QM. The burden of proof here is on theorists that support collapse theories not proponents of MWI.
    • kgwgk 4y ago
      > MWI is essentially the "null hypothesis" Null hypothesis in the sense of not producing any predicition? /joking I agree with the grand-parent that substituting a "collapse" that we cannot understand with a "separation" that we cannot understand doesn't seem a big step forward.
  • Lichtso 4y ago
    > I don't see how relabelling "collapse" to "separation" and adding more or less infinitely proliferating universes solves the problem. MWI isn't just renaming the collapse. Copenhagen is fundamentally different in that exactly one outcome is somehow chosen / selected and all others cease to exist. In other words Copenhagen has to add / invent some information: Which world to pick and when to do so. MWI simply avoids having to pick by continuing every branch simultaneously and recursively. Meaning that MWI is actually a simpler theory and shows that the selection is unnecessary and all problems that come with it can be avoided. In that sense, the burden of proof lies with Copenhagen IMO and it just gives handwavy answers that the selection process somehow involves a conscious observer, whatever that is ... > MWI also contradicts itself. Supposedly the universes are independent, but if their influence doesn't define the wave equation they do nothing to help explain it. In MWI the wave function IS the integral of all possible outcomes / worlds / branches. In that sense they don't just influence it, they define it. Not sure how that is contradictory. Btw, the same goes for Copenhagen in the state of super position as well. So, they are identical up to the point where Copenhagen selects one (collapses) and MWI simply carries on.
• bnralt 4y ago
  > There is an often-repeated statement that different quantum mechanical theories are equivalent. Worse than that, I'd say that "physical collapse" is usually presented to the general public as established scientific fact, even by prominent scientists that know better.
• pred_ 4y ago
  > Is wave function collapse is a real thing or just an approximation of what happens when a large system is entangled? I've listened to enough Carroll to convince myself that I think MWI makes sense, but what I never got is that it doesn't seem to offer much in terms of explanation of how the entangled state ends up being what it is: We see a particular result of an experiment with many possible a priori outcomes, but is there a mechanism that determines which particular entangled state we end up in? Something like the phases of the wave functions on individual Hilbert space sectors conspiring to produce a certain outcome, so that an omnipotent observer with full access to both parts would be able to tell what happens a priori, no dice being rolled? Or is that just hidden variable theory?
  • krohling 4y ago
    > it doesn't seem to offer much in terms of explanation of how the entangled state ends up being what it is: According to MWI, all possible results of an experiment are manifest and real. You're asking "which particular entangled state we end up in?". You end up in all of them. For a 2-state superposition system, there are 2 versions of "you" that exist after the experiment, both of which are equally real. Subsequent measurements of the quantum system will appear to be "collapsed" for both versions of you but each will see different and opposing values.
    • lamontcg 4y ago
      > You end up in all of them. Except that I've only ever experienced ending up in one of them, so exactly how did the "I" that is typing this right now wind up here, and not in some other branch? There's an unexplained bifurcation of consciousness implied by MWI which it cannot explain (although I think it suggests the appeal of MWI since it gives enough wiggle room that people think that their free will could control which universe they wind up in, which appeals to everyone's inner Malcolm Gladwell).
      • GoblinSlayer 4y ago
        You end up in all branches and your consciousness ends up in all branches, where your mind works. MWI works best with physicalism, yep :) Maybe your copy in another branch even posted the same comment.
• pmoriarty 4y ago
  "Some smart people have tried to make theories where wave function collapse is a real thing." Aa a function is a mathematical abstraction, I don't see how it could be real... unless you believe the world is made of math..
• ummonk 4y ago
  That oft repeated statement is really a lie except inasmuch as much as all interpretations / theories are intended to be compatible with reality as we observe it (and thus yield equivalent predictions to classical quantum mechanics for everyday quantum experiments).
• isitmadeofglass 4y ago
  The MWI isn’t “no wave function collapse” it’s an infinite number of universes all collapsing with different outcomes.