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Quantum systems evolve unitarily, measurement devices collapse the wave function which is not unitary. And there is no accepted consensus how those two things f

Quantum systems evolve unitarily, measurement devices collapse the wave function which is not unitary. And there is no accepted consensus how those two things fit together, just several ideas like Everett's many worlds which solves the problem by declaring that the wave function never actually collapses but instead splits the cosmos into many worlds.

Joker_vD 2y ago
Everett's idea was merely that the measurement device gets entangled with the thing being measured, so we now the wave-function of the (combined) device-thing system is the superposition of states <thing is in pure state Ai| ⊗ <device is in "measured Ai" state|; but for the measurement device it of course "looks" like it measured a pure state. That's it. That's basically what the decoherence suggests anyway. The "cosmos splitting" was proposed by Bryce DeWitt which IMHO turned a perfectly straight-forward and reasonable interpretation into a needlessly philosophical statement.
- layer8 2y ago
  Entanglement means that the states become macroscopic in noisy (and not too high-temperature, apparently) environments. Whether you call those states “worlds” or not doesn’t change the philosophical implications.
- kgwgk 2y ago
  > Everett's idea was merely that the measurement device gets entangled with the thing being measured That idea is almost as old as quantum mechanics itself. (von Neumann, 1932 - if not earlier.)
- danbruc 2y ago
  The measurement device gets entangled with the spin - which is in a superposition - and then I get entangled with the measurement device and for got measure the entire environment. How does this avoid the many worlds? If the measurement device only sees half of the entangled spin state and I only see half of the entangled measurement device - say I see the spin measured up because the measurement device saw the spin up - how does this avoid the necessity of a second world where the spin was down? What breaks the symmetry? What makes the spin down half of the entangled state go away? There is also another weird asymmetry. For the spin we say that its superposition exists in my [slice of the] world but for the measurement device we say only half of the entangled state, the outcome I become aware of, is part of [my slice of the] world, what justifies this?
  - Joker_vD 2y ago
    > What breaks the symmetry? Nothing. It keeps existing, and (1/sqrt(2) * <I think spin is down|⊗<device shows spin is down|⊗<spin was down| + 1/sqrt(2) * <I think spin is up|⊗<device shows spin is up|⊗<spin was up|) keeps evolving, and since QM is linear, the summands evolve independently. Yes, to the outside observer (the one who could look at the whole of the universe from the side/above without interacting with it) this does look very similar to MWI. But we are not outside of the universe, we exist inside it as a part of it, so for us it looks like spontaneous collapse of the wave function.
    - danbruc 2y ago
      Maybe I am wrong, but that is exactly what I always assumed the many world interpretation to be. Especially I think many worlds also requires an outside of the cosmos observer to look at all the worlds, I do not think I ever heard someone claim that many worlds allows you and me to see or even move between different worlds.
dmboyd 2y ago
Isn’t this just ontological then. It doesn’t seem like a novel observation that “measurement devices” collapse a wave function (Schrödinger) if you’ve defined a measurement device as such.
- Maxatar 2y ago
  Yes, as far as anyone knows the different interpretations of quantum mechanics are just ontological.
  - woopsn 2y ago
    If I recall there was nothing ontological about the actual Copenhagen group's interpretation. They aimed to develop a predictive theory and nothing more. One of their key insights was that ontology simply doesn't play a role anymore. This led quickly to the discovery of QM, while ~100 years of fussing over the "reality" of the wave function has led to nothing. The "Copenhagen interpretation" as we understand it, in which some thing "collapses" when measured, trades under the same name but was invented later. As a result the original interpretations of Bohr/Heisenberg and their philosophy of science have somewhat exited the discussion, even though they had the only defensible epistemology -- the rest as you say is just ontology.

Joker_vD 2y ago

Everett's idea was merely that the measurement device gets entangled with the thing being measured, so we now the wave-function of the (combined) device-thing system is the superposition of states <thing is in pure state Ai| ⊗ <device is in "measured Ai" state|; but for the measurement device it of course "looks" like it measured a pure state. That's it. That's basically what the decoherence suggests anyway. The "cosmos splitting" was proposed by Bryce DeWitt which IMHO turned a perfectly straight-forward and reasonable interpretation into a needlessly philosophical statement.

layer8 2y ago
Entanglement means that the states become macroscopic in noisy (and not too high-temperature, apparently) environments. Whether you call those states “worlds” or not doesn’t change the philosophical implications.
kgwgk 2y ago
> Everett's idea was merely that the measurement device gets entangled with the thing being measured That idea is almost as old as quantum mechanics itself. (von Neumann, 1932 - if not earlier.)
danbruc 2y ago
The measurement device gets entangled with the spin - which is in a superposition - and then I get entangled with the measurement device and for got measure the entire environment. How does this avoid the many worlds? If the measurement device only sees half of the entangled spin state and I only see half of the entangled measurement device - say I see the spin measured up because the measurement device saw the spin up - how does this avoid the necessity of a second world where the spin was down? What breaks the symmetry? What makes the spin down half of the entangled state go away? There is also another weird asymmetry. For the spin we say that its superposition exists in my [slice of the] world but for the measurement device we say only half of the entangled state, the outcome I become aware of, is part of [my slice of the] world, what justifies this?
- Joker_vD 2y ago
  > What breaks the symmetry? Nothing. It keeps existing, and (1/sqrt(2) * <I think spin is down|⊗<device shows spin is down|⊗<spin was down| + 1/sqrt(2) * <I think spin is up|⊗<device shows spin is up|⊗<spin was up|) keeps evolving, and since QM is linear, the summands evolve independently. Yes, to the outside observer (the one who could look at the whole of the universe from the side/above without interacting with it) this does look very similar to MWI. But we are not outside of the universe, we exist inside it as a part of it, so for us it looks like spontaneous collapse of the wave function.
  - danbruc 2y ago
    Maybe I am wrong, but that is exactly what I always assumed the many world interpretation to be. Especially I think many worlds also requires an outside of the cosmos observer to look at all the worlds, I do not think I ever heard someone claim that many worlds allows you and me to see or even move between different worlds.

dmboyd 2y ago

Isn’t this just ontological then. It doesn’t seem like a novel observation that “measurement devices” collapse a wave function (Schrödinger) if you’ve defined a measurement device as such.

Maxatar 2y ago
Yes, as far as anyone knows the different interpretations of quantum mechanics are just ontological.
- woopsn 2y ago
  If I recall there was nothing ontological about the actual Copenhagen group's interpretation. They aimed to develop a predictive theory and nothing more. One of their key insights was that ontology simply doesn't play a role anymore. This led quickly to the discovery of QM, while ~100 years of fussing over the "reality" of the wave function has led to nothing. The "Copenhagen interpretation" as we understand it, in which some thing "collapses" when measured, trades under the same name but was invented later. As a result the original interpretations of Bohr/Heisenberg and their philosophy of science have somewhat exited the discussion, even though they had the only defensible epistemology -- the rest as you say is just ontology.