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Again, models of decoherence assume existence of an external bath resembling classical state. By interacting with it the system becomes classical itself if it i

by _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.

layer8 4y ago
I don’t think it’s black & white, either coherent or decoherent, for subsystems. It’s rather a scale. In that sense there may not be discrete “worlds”, but rather a continuum of them, matching the degree to which particles are entangled or not entangled, and in superposition or not in superposition.
lmm 4y ago
> models of decoherence assume existence of an external bath resembling classical state They don't assume it's classical. They assume it's a system with lots of states and interactions, because if you want to have a model of "the external universe" then you have to have some kind of concept of what that looks like. > 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. Sure; the point is that if you split the universe into "these two entangled particles" and "the rest of the universe", because you want to understand how we observe that entanglement "decaying" as the particles interact with the rest of the universe, then you can model that as "these two entangled particles" interacting with "a thermal bath". > The trouble is that so far nobody came up with a model where such temporary approximately classical subsystem appears. This is nonsense? Wavefunctions generally have a lot of structure and decomposing them into smaller subsystems and functions happens all the time.
- _0w8t 4y ago
  What is necessary is to find in the big wave function a subsystem consisting of, say, a particle with a spin and something approximating a classical measurement device. Such 2-part subsystem should explain then how the classically-looking part gets the value it measures and the nature of probabilities. Despite attempts nobody was able to find such subsystem.
  - lmm 4y ago
    For a sufficiently simplified model of a measuring device you can do that. I mean there's no getting away from having to use the Born rule, and I'm not as sanguine as Aaronson about the idea that that's adequately justified, but it's not like there's a better alternative.
    - _0w8t 4y ago
      So far nobody was able to get such model unless I missed something. And the whole point of the decoherence exercise is to avoid Born rule as it implies a truly classical subsystem and we are back to where we started.
      - lmm 4y ago
        > So far nobody was able to get such model unless I missed something. If you model it as e.g. a quantum system with a state space that describes what you're measuring, coupled to a thermal bath, then it works. What kind of model are you looking for? Real measuring instruments are probably too big to ever model the full quantum state space of all the particles they're made up of, for example. > And the whole point of the decoherence exercise is to avoid Born rule as it implies a truly classical subsystem What do you mean by "truly classical"? The Born rule implies that our experience is the same as if we found ourselves within one of an ensemble of possible classical-like states with a certain probability, but, well, experimentally that's what we do; any interpretation of quantum mechanics needs to be able to recover that reality.

by _0w8t 4y ago

layer8 4y ago

I don’t think it’s black & white, either coherent or decoherent, for subsystems. It’s rather a scale. In that sense there may not be discrete “worlds”, but rather a continuum of them, matching the degree to which particles are entangled or not entangled, and in superposition or not in superposition.

lmm 4y ago

> models of decoherence assume existence of an external bath resembling classical state They don't assume it's classical. They assume it's a system with lots of states and interactions, because if you want to have a model of "the external universe" then you have to have some kind of concept of what that looks like. > 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. Sure; the point is that if you split the universe into "these two entangled particles" and "the rest of the universe", because you want to understand how we observe that entanglement "decaying" as the particles interact with the rest of the universe, then you can model that as "these two entangled particles" interacting with "a thermal bath". > The trouble is that so far nobody came up with a model where such temporary approximately classical subsystem appears. This is nonsense? Wavefunctions generally have a lot of structure and decomposing them into smaller subsystems and functions happens all the time.

_0w8t 4y ago
What is necessary is to find in the big wave function a subsystem consisting of, say, a particle with a spin and something approximating a classical measurement device. Such 2-part subsystem should explain then how the classically-looking part gets the value it measures and the nature of probabilities. Despite attempts nobody was able to find such subsystem.
- lmm 4y ago
  For a sufficiently simplified model of a measuring device you can do that. I mean there's no getting away from having to use the Born rule, and I'm not as sanguine as Aaronson about the idea that that's adequately justified, but it's not like there's a better alternative.
  - _0w8t 4y ago
    So far nobody was able to get such model unless I missed something. And the whole point of the decoherence exercise is to avoid Born rule as it implies a truly classical subsystem and we are back to where we started.
    - lmm 4y ago
      > So far nobody was able to get such model unless I missed something. If you model it as e.g. a quantum system with a state space that describes what you're measuring, coupled to a thermal bath, then it works. What kind of model are you looking for? Real measuring instruments are probably too big to ever model the full quantum state space of all the particles they're made up of, for example. > And the whole point of the decoherence exercise is to avoid Born rule as it implies a truly classical subsystem What do you mean by "truly classical"? The Born rule implies that our experience is the same as if we found ourselves within one of an ensemble of possible classical-like states with a certain probability, but, well, experimentally that's what we do; any interpretation of quantum mechanics needs to be able to recover that reality.