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The Many-Worlds Interpretation is not really an interpretation. It just says that the known laws of Quantum Mechanics are correct, and that any measurement proc

The Many-Worlds Interpretation is not really an interpretation. It just says that the known laws of Quantum Mechanics are correct, and that any measurement process itself has to obey the laws of Quantum Mechanics. In other words, nothing un-Quantum-Mechanical happens during measurement.

Jensson 4y ago
MVI doesn't solve wave function collapses, if it did it would be able to tell exactly when the universe will split into many worlds, but it can't. So it doesn't solve anything, the difficulty with the theory is still there.
- layer8 4y ago
  The perceived wave function collapse is explained by decoherence. There is no extra “split” that happens. “World” is just a term referring to the parts of the universal wave function that are entangled with each other. As to “exactly when the universe will split into many worlds”, it happens zillions of times per second pretty much everywhere. It’s even not clear whether the branching is a discrete and countable occurrence, it could really be a continuous fanning out.
  - Jensson 4y ago
    So when does decoherence happen? Provide a formula for it. That is what people struggle with in quantum mechanics, MWI just deflects the question. Without a formula for when this phenomena happens any "interpretation" is just nonsense, except as a tool to get towards that formula.
    - russdill 4y ago
      There's no yes/no equation as decoherence is statistical in nature.
      - Jensson 4y ago
        Statistics also has formulas... This is the problem with MWI gospel, people come and say that it solves all the math but it leaves gaping holes like this. And you didn't even solve the randomness, since as you say it is "statistical", although that is just a theory we currently have no formula for this. Calling it a wave function collapse is much more honest since it accurately describes the situation, that we have no clue at what it is, why or when it happens. All we know how to do is calculate what possibly happens when we know that a wave function collapse will happen in a certain way.
    - DiogenesKynikos 4y ago
      The mathematical framework for modeling decoherence involves coupling an isolated system to a thermal bath, described by a density matrix. Decoherence is a phenomenon that's been studied mathematically and experimentally, not simply an interpretation or hand-waving.
      - _0w8t 4y ago
        This notion does not explain how the thermal bath appears. I.e. why the system interacts with a particular bath and not a superposition of such baths? At best the decoherence can try to explain how appearance of a sufficiently big classical object may trigger appearance of other objects, but it does not explain how the initial object appears.
        lmm 4y ago
        > This notion does not explain how the thermal bath appears. The point of the bath is it's a generic model of a large system with lots of states and interactions. > I.e. why the system interacts with a particular bath and not a superposition of such baths? The bath can be in a superposition state at the start, that's fine. (Indeed the whole model would be pretty useless otherwise).
        _0w8t 4y ago
        Again, models of decoherence assume existence of an external bath resembling classical state. By interacting with it the system becomes classical itself if it is and the bath is sufficiently big. But it does not explain how that external bath appears. In universe-as-a-wave-function there is no external bath. The system as whole is always coherent and is described by a single wave function. Then the idea of the decoherence is that perhaps during evolution of this big wave function it is possible to find a subsystem that looks like it is approximately decoherent during some time interval. Then one claim that our visible universe is just such subsystem. The trouble is that so far nobody came up with a model where such temporary approximately classical subsystem appears. Perhaps we have not tried hard enough or this is a really difficult problem, but at this point I am rather skeptical that decoherence is the right approach at all.
    - lmm 4y ago
      Decoherence of a system happens when the entropy of entanglement is maximal, so if you want a formula then it's S(Tr_a(p_ab)) -> N/e. In practice a complex system is probably never fully decoherent, but the separability is small enough to not matter; if we're being truly rigorous we work with it as a limit, similar to how we use the "classical limit" in other parts of QM or other contexts. (Analogy: imagine saying "Relativity doesn't explain how we live in a Newtonian world. If relativity is valid, it should be able to say at exactly what speed motion becomes nonrelativistic. Provide a formula for it")
      - Jensson 4y ago
        Relativity is a real formula, you just say that decoherence seems to correlate with entropy and then gave a formula for entropy. We don't have a formula for decoherence. Any explanation for decoherence that doesn't help us find a formula for it is nonsense. Here is a better theory than MWI: We live in a simulation, the computer is optimized so when there are too complex interactions in an area it simplifies the state into some probable untangled version. This theory predicts that decoherence thus happens at certain computational complexity levels, that is something we could try to find and test experimentally making this theory more scientific than MWI. MWI doesn't lead anywhere, it isn't science, it is just nonsense. I'm not saying my theory here is a good one, but it is better than MWI which isn't a high bar.
        lmm 4y ago
        > Relativity is a real formula Relativity is a real formula and the classical limit is a real phenomenon, but there's no formula for when a relativistic situation "becomes classical". > you just say that decoherence seems to correlate with entropy and then gave a formula for entropy If the observed system and the observer/external universe are fully entangled (i.e. their entropy of entanglement is maximal) then decoherence has occurred and all observations of the observed system by that observer will be indistinguishable from if the system were not in superposition. That is a mathematical fact that falls right out of the equations. So I don't know what else you're asking for.
- russdill 4y ago
  There isn't some physical event in MWI that splits worlds. It's just a consequence of decoherence.
  - goatlover 4y ago
    But we only observe the cat being alive or dead. Where is the other cat? Are the many worlds hiding in the equation?
    - Vecr 4y ago
      You are "in" the wave function still associated with the live cat, but not with the dead cat wave function. Obviously neither you or a cat is a single wave function, but that's basically the explanation.
      - goatlover 4y ago
        I understand that's the interpretation, but I don't observe the dead cat, so I don't feel compelled to accept MWI. I'm not saying it's wrong (who knows), only that it's not scientific (lacking empirical evidence for the dead cat) and one of the other interpretations could be correct (again who knows if any of them are representative of the true state of affairs).
        layer8 4y ago
        The two cats are what the Schroedinger equation implies. The fact that you don't observe the dead cat is perfectly consistent with the Schroedinger equation. To deny the dead cat means that the Schroedinger equation doesn't describe physical reality, and that it has to be modified or added to in some way. There are ways to do so, but it makes the underlying model more complicated and more awkward. Occam's razor would suggest that the Schroedinger equation is the simplest and most parsimonious explanation for what we observe, despite it also predicting the unobservable (for us) other branches of the wave function.
        4y ago
goatlover 4y ago
It’s an interpretation because we don’t have any empirical evidence for the other worlds. Science requires evidence, math alone isn’t enough. MWI is metaphysics, which is fine. We all wax philosophical sometimes as humans. Even the great Feynman couldn’t help himself.
- DiogenesKynikos 4y ago
  There is a massive amount of evidence that Quantum Mechanics is the way the Universe works, and the MWI is simply vanilla Quantum Mechanics, with zero further assumptions. The problem is that the name "Many Worlds Interpretation" sounds grandiose. "Many worlds" is simply an imaginative way of describing the existence of different states of the wavefunction - a concept that is extremely well established and universally accepted. MWI just says that we should assume that Quantum Mechanics holds in all situations, including when we make measurements (i.e., there's nothing special about the measurement process).
  - goatlover 4y ago
    QM isn’t a complete description of the universe because of gravity, dark energy, dark matter, the arrow of time, and it doesn’t explain probability. So it’s a little premature to say that’s how the universe works.
    - DiogenesKynikos 4y ago
      Gravity, dark energy, dark matter or the arrow of time in no way conflict with QM. There isn't yet any successful quantum theory of gravity, but it's universally believed in the physics community that there will eventually be a quantum description of gravity. Dark energy and dark matter are merely components of the universe whose precise character is not yet known, but again, everyone believes that whatever they are, they will be described quantum mechanically. The arrow of time is merely a consequence of entropy being low in the early Universe. The reason why physicists universally believe that the Universe is quantum mechanical is that it's impossible for QM to describe only a part of physics. If one aspect of physics is quantum mechanical, it "infects" every other aspect of physics and forces it to be quantum mechanical. You can't have elections obey quantum mechanical laws, but the gravity they produce be classical. It's really all or nothing.

Jensson 4y ago

MVI doesn't solve wave function collapses, if it did it would be able to tell exactly when the universe will split into many worlds, but it can't. So it doesn't solve anything, the difficulty with the theory is still there.

layer8 4y ago
The perceived wave function collapse is explained by decoherence. There is no extra “split” that happens. “World” is just a term referring to the parts of the universal wave function that are entangled with each other. As to “exactly when the universe will split into many worlds”, it happens zillions of times per second pretty much everywhere. It’s even not clear whether the branching is a discrete and countable occurrence, it could really be a continuous fanning out.
- Jensson 4y ago
  So when does decoherence happen? Provide a formula for it. That is what people struggle with in quantum mechanics, MWI just deflects the question. Without a formula for when this phenomena happens any "interpretation" is just nonsense, except as a tool to get towards that formula.
  - russdill 4y ago
    There's no yes/no equation as decoherence is statistical in nature.
    - Jensson 4y ago
      Statistics also has formulas... This is the problem with MWI gospel, people come and say that it solves all the math but it leaves gaping holes like this. And you didn't even solve the randomness, since as you say it is "statistical", although that is just a theory we currently have no formula for this. Calling it a wave function collapse is much more honest since it accurately describes the situation, that we have no clue at what it is, why or when it happens. All we know how to do is calculate what possibly happens when we know that a wave function collapse will happen in a certain way.
  - DiogenesKynikos 4y ago
    The mathematical framework for modeling decoherence involves coupling an isolated system to a thermal bath, described by a density matrix. Decoherence is a phenomenon that's been studied mathematically and experimentally, not simply an interpretation or hand-waving.
    - _0w8t 4y ago
      This notion does not explain how the thermal bath appears. I.e. why the system interacts with a particular bath and not a superposition of such baths? At best the decoherence can try to explain how appearance of a sufficiently big classical object may trigger appearance of other objects, but it does not explain how the initial object appears.
      - lmm 4y ago
        > This notion does not explain how the thermal bath appears. The point of the bath is it's a generic model of a large system with lots of states and interactions. > I.e. why the system interacts with a particular bath and not a superposition of such baths? The bath can be in a superposition state at the start, that's fine. (Indeed the whole model would be pretty useless otherwise).
        _0w8t 4y ago
        Again, models of decoherence assume existence of an external bath resembling classical state. By interacting with it the system becomes classical itself if it is and the bath is sufficiently big. But it does not explain how that external bath appears. In universe-as-a-wave-function there is no external bath. The system as whole is always coherent and is described by a single wave function. Then the idea of the decoherence is that perhaps during evolution of this big wave function it is possible to find a subsystem that looks like it is approximately decoherent during some time interval. Then one claim that our visible universe is just such subsystem. The trouble is that so far nobody came up with a model where such temporary approximately classical subsystem appears. Perhaps we have not tried hard enough or this is a really difficult problem, but at this point I am rather skeptical that decoherence is the right approach at all.
  - lmm 4y ago
    Decoherence of a system happens when the entropy of entanglement is maximal, so if you want a formula then it's S(Tr_a(p_ab)) -> N/e. In practice a complex system is probably never fully decoherent, but the separability is small enough to not matter; if we're being truly rigorous we work with it as a limit, similar to how we use the "classical limit" in other parts of QM or other contexts. (Analogy: imagine saying "Relativity doesn't explain how we live in a Newtonian world. If relativity is valid, it should be able to say at exactly what speed motion becomes nonrelativistic. Provide a formula for it")
    - Jensson 4y ago
      Relativity is a real formula, you just say that decoherence seems to correlate with entropy and then gave a formula for entropy. We don't have a formula for decoherence. Any explanation for decoherence that doesn't help us find a formula for it is nonsense. Here is a better theory than MWI: We live in a simulation, the computer is optimized so when there are too complex interactions in an area it simplifies the state into some probable untangled version. This theory predicts that decoherence thus happens at certain computational complexity levels, that is something we could try to find and test experimentally making this theory more scientific than MWI. MWI doesn't lead anywhere, it isn't science, it is just nonsense. I'm not saying my theory here is a good one, but it is better than MWI which isn't a high bar.
      - lmm 4y ago
        > Relativity is a real formula Relativity is a real formula and the classical limit is a real phenomenon, but there's no formula for when a relativistic situation "becomes classical". > you just say that decoherence seems to correlate with entropy and then gave a formula for entropy If the observed system and the observer/external universe are fully entangled (i.e. their entropy of entanglement is maximal) then decoherence has occurred and all observations of the observed system by that observer will be indistinguishable from if the system were not in superposition. That is a mathematical fact that falls right out of the equations. So I don't know what else you're asking for.
russdill 4y ago
There isn't some physical event in MWI that splits worlds. It's just a consequence of decoherence.
- goatlover 4y ago
  But we only observe the cat being alive or dead. Where is the other cat? Are the many worlds hiding in the equation?
  - Vecr 4y ago
    You are "in" the wave function still associated with the live cat, but not with the dead cat wave function. Obviously neither you or a cat is a single wave function, but that's basically the explanation.
    - goatlover 4y ago
      I understand that's the interpretation, but I don't observe the dead cat, so I don't feel compelled to accept MWI. I'm not saying it's wrong (who knows), only that it's not scientific (lacking empirical evidence for the dead cat) and one of the other interpretations could be correct (again who knows if any of them are representative of the true state of affairs).
      - layer8 4y ago
        The two cats are what the Schroedinger equation implies. The fact that you don't observe the dead cat is perfectly consistent with the Schroedinger equation. To deny the dead cat means that the Schroedinger equation doesn't describe physical reality, and that it has to be modified or added to in some way. There are ways to do so, but it makes the underlying model more complicated and more awkward. Occam's razor would suggest that the Schroedinger equation is the simplest and most parsimonious explanation for what we observe, despite it also predicting the unobservable (for us) other branches of the wave function.
        4y ago

goatlover 4y ago

It’s an interpretation because we don’t have any empirical evidence for the other worlds. Science requires evidence, math alone isn’t enough. MWI is metaphysics, which is fine. We all wax philosophical sometimes as humans. Even the great Feynman couldn’t help himself.

DiogenesKynikos 4y ago
There is a massive amount of evidence that Quantum Mechanics is the way the Universe works, and the MWI is simply vanilla Quantum Mechanics, with zero further assumptions. The problem is that the name "Many Worlds Interpretation" sounds grandiose. "Many worlds" is simply an imaginative way of describing the existence of different states of the wavefunction - a concept that is extremely well established and universally accepted. MWI just says that we should assume that Quantum Mechanics holds in all situations, including when we make measurements (i.e., there's nothing special about the measurement process).
- goatlover 4y ago
  QM isn’t a complete description of the universe because of gravity, dark energy, dark matter, the arrow of time, and it doesn’t explain probability. So it’s a little premature to say that’s how the universe works.
  - DiogenesKynikos 4y ago
    Gravity, dark energy, dark matter or the arrow of time in no way conflict with QM. There isn't yet any successful quantum theory of gravity, but it's universally believed in the physics community that there will eventually be a quantum description of gravity. Dark energy and dark matter are merely components of the universe whose precise character is not yet known, but again, everyone believes that whatever they are, they will be described quantum mechanically. The arrow of time is merely a consequence of entropy being low in the early Universe. The reason why physicists universally believe that the Universe is quantum mechanical is that it's impossible for QM to describe only a part of physics. If one aspect of physics is quantum mechanical, it "infects" every other aspect of physics and forces it to be quantum mechanical. You can't have elections obey quantum mechanical laws, but the gravity they produce be classical. It's really all or nothing.