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This does not help at all, quantum systems - even messy complex ones, boxes containing cats and air - evolve unitarily and do not destroy a superposition. The b

This does not help at all, quantum systems - even messy complex ones, boxes containing cats and air - evolve unitarily and do not destroy a superposition. The box and the air and the cat can interact all they want, you will still have a superposition of cat dead - and box and air in some state - and cat alive - and box and air in some other state. And no point are we addressing how we get rid of one half of the entangled state. And even if I see the box wiggle a bit, we only declare that me watching the box somehow was an measurement and that collapsed the wave function without answering the question why I did not simply get entangled with the box, too.

Jensson 2y ago
> quantum systems - even messy complex ones, boxes containing cats and air - evolve unitarily and do not destroy a superposition This is wrong, they do collapse, you can test this using the double slit experiment. No human has to be there for the wavefunction to collapse and thus break entanglement. If what you said was true then this wouldn't be a part of physics, physics is only about stuff we can measure not nonsense that has no meaning.
- tsimionescu 2y ago
  It's not at all clear how, when, and why wave function collapse happens. The only known quantity there, decoherence, is related to interaction between a coherent quantum system and a noisy decohered environment, which causes the original system to decohere itself. Theoretically, as far as it is known today, it is possible for a quantum system of any size, and at any temperature, to remain in a coherent state, if it could be perfectly isolated from the rest of the world. Any limits to this process, where a perfectly isolated system would undergo wave function collapse without any outside interference, would be a Nobel-prize worthy discovery and would constitute a new theory of physics: quantum mechanics and QFT allow no such process. Edit: and the double slit experiment is unrelated - the screen and the slit are noisy classical systems that are not in any way isolated from the rest of the classical world. In principle, if you performed the experiment using a screen and slits made entirely of atoms entangled with the emitter, and isolated perfectly from the rest of the world and the CMB and everything, you'd get a different result: this is what QM predicts.
  - Jensson 2y ago
    > quantum mechanics and QFT allow no such process Double slit experiments shows collapse happens without any outside interference, nothing outside of the room has to do anything for the resulting measurements to show collapse happened. You can then afterwards enter the room, look at the recorded data and see that collapses did happen. So you must have misunderstood what those theories says about collapses. They don't say exactly when collapses happens or what causes them, that is the unsolved problem, but we can know that they do happen.
    - danbruc 2y ago
      Quantum mechanics, i.e. the Schrödinger equation, and wave function collapse are incompatible. While not measured a quantum system evolves unitarily, when measured the quantum system ends up in the measured Eigenstate which is a non-unitary change. But any measurement device is also a quantum system and therefore the combined system should also evolve unitarily. And now we have a serious problem to which nobody has a generally accepted answer.
      - woopsn 2y ago
        What is deficient about the original explanation that the wave function is not real but rather a scientific theory? The issue has been unresolved for about a century now, it's relatively unsatisfying but hasn't held the science back in any way.
        tsimionescu 2y ago
        The problem with that idea is that we don't have any other model, and little room for it. We already know from Bell's inequality and the experiments that verified it that there can't be underlying variables that we haven't detected yet (or if they exist, we have to rethink special relativity and by extension QFT). It's also quite hard to explain why the wave function is so good at predicting the results of experiments if it's not a part of the underlying reality.
        danbruc 2y ago
EvgeniyZh 2y ago
> evolve unitarily and do not destroy a superposition Unitary evolution can destroy superposition of the small subsystem by introducing negligible amount of superposition into large system.
- danbruc 2y ago
  Not sure if that is true, but assuming it is, does this really count as destruction instead of moving it around and spreading it? After all unitary evolution is reversible and I could therefore reconcentrate the superposition into the small system. It would however seemingly be a good explanation how to get from the decayed non-decayed superposition of a few atoms to an almost alive or almost dead cat. This is however also what makes me doubt your statement, if true, why are we still puzzled about Schrödinger's cat?
  - EvgeniyZh 2y ago
    Well the naming is a different question, but it is consistent with observation of what we call wavefunction collapse. In Schroedinger cat experiment, the evolution is not thermalization but rather entangling the whole cat with atom state. I don't think we're really puzzled about it now, since we know that macroscopic objects can be in superposition. So cat, theoretically, can be in superposition of alive and dead until measured. I don't think there is any contradiction or confusion here physics-wise.

Jensson 2y ago

> quantum systems - even messy complex ones, boxes containing cats and air - evolve unitarily and do not destroy a superposition This is wrong, they do collapse, you can test this using the double slit experiment. No human has to be there for the wavefunction to collapse and thus break entanglement. If what you said was true then this wouldn't be a part of physics, physics is only about stuff we can measure not nonsense that has no meaning.

tsimionescu 2y ago
It's not at all clear how, when, and why wave function collapse happens. The only known quantity there, decoherence, is related to interaction between a coherent quantum system and a noisy decohered environment, which causes the original system to decohere itself. Theoretically, as far as it is known today, it is possible for a quantum system of any size, and at any temperature, to remain in a coherent state, if it could be perfectly isolated from the rest of the world. Any limits to this process, where a perfectly isolated system would undergo wave function collapse without any outside interference, would be a Nobel-prize worthy discovery and would constitute a new theory of physics: quantum mechanics and QFT allow no such process. Edit: and the double slit experiment is unrelated - the screen and the slit are noisy classical systems that are not in any way isolated from the rest of the classical world. In principle, if you performed the experiment using a screen and slits made entirely of atoms entangled with the emitter, and isolated perfectly from the rest of the world and the CMB and everything, you'd get a different result: this is what QM predicts.
- Jensson 2y ago
  > quantum mechanics and QFT allow no such process Double slit experiments shows collapse happens without any outside interference, nothing outside of the room has to do anything for the resulting measurements to show collapse happened. You can then afterwards enter the room, look at the recorded data and see that collapses did happen. So you must have misunderstood what those theories says about collapses. They don't say exactly when collapses happens or what causes them, that is the unsolved problem, but we can know that they do happen.
  - danbruc 2y ago
    Quantum mechanics, i.e. the Schrödinger equation, and wave function collapse are incompatible. While not measured a quantum system evolves unitarily, when measured the quantum system ends up in the measured Eigenstate which is a non-unitary change. But any measurement device is also a quantum system and therefore the combined system should also evolve unitarily. And now we have a serious problem to which nobody has a generally accepted answer.
    - woopsn 2y ago
      What is deficient about the original explanation that the wave function is not real but rather a scientific theory? The issue has been unresolved for about a century now, it's relatively unsatisfying but hasn't held the science back in any way.
      - tsimionescu 2y ago
        The problem with that idea is that we don't have any other model, and little room for it. We already know from Bell's inequality and the experiments that verified it that there can't be underlying variables that we haven't detected yet (or if they exist, we have to rethink special relativity and by extension QFT). It's also quite hard to explain why the wave function is so good at predicting the results of experiments if it's not a part of the underlying reality.
      - danbruc 2y ago

EvgeniyZh 2y ago

> evolve unitarily and do not destroy a superposition Unitary evolution can destroy superposition of the small subsystem by introducing negligible amount of superposition into large system.

danbruc 2y ago
Not sure if that is true, but assuming it is, does this really count as destruction instead of moving it around and spreading it? After all unitary evolution is reversible and I could therefore reconcentrate the superposition into the small system. It would however seemingly be a good explanation how to get from the decayed non-decayed superposition of a few atoms to an almost alive or almost dead cat. This is however also what makes me doubt your statement, if true, why are we still puzzled about Schrödinger's cat?
- EvgeniyZh 2y ago
  Well the naming is a different question, but it is consistent with observation of what we call wavefunction collapse. In Schroedinger cat experiment, the evolution is not thermalization but rather entangling the whole cat with atom state. I don't think we're really puzzled about it now, since we know that macroscopic objects can be in superposition. So cat, theoretically, can be in superposition of alive and dead until measured. I don't think there is any contradiction or confusion here physics-wise.