3 ms·

> maybe superposition describes the real physical situation It may or may not. What really needs explaining IMO is the mechanism of collapse of psi, something

> maybe superposition describes the real physical situation It may or may not. What really needs explaining IMO is the mechanism of collapse of psi, something standard QM is pretty much silent upon.

Joker_vD 2y ago
Maybe it doesn't collapse, and the world is quantum through and through?
- coffeeaddict1 2y ago
  Then why do we observe a collapse?
  - evanb 2y ago
    Decoherence may be enough. The idea is that the starting wavefunction of us and some experiment (say, an electron) is a tensor product | us > ⊗ | electron in superposition > and unitary time evolution governed by the Schrödinger equation evolves that state to | us seeing spin up in a detector > ⊗ | spin up > + | us seeing spin down > ⊗ | spin down > (I elided the amplitudes since we're just interested in the idea and not some particular values). In each 'branch' of the wavefunction (meaning: in each addend) we're left wondering "wow, how did the wavefunction collapse to the value that I am seeing", because we're inside the wf. But that's just a question we have that we've been tricked into by our limited sight; if we could see the whole wf of the universe, with our own superposition included, we'd see there is no collapse but just unitary Schrödinger evolution forever.
    - GoblinSlayer 2y ago
      Quantum physics doesn't use normalization much and these formulas work better without normalization. Normalization can be provided by the Bayes formula, so postulating it is redundant. Also I heard QFT has some problem with global normalization, because everything must be local there, so normalization is replaced by Stokes' formula: the flux of state norm through closed spacetime surface is zero, i.e. the norm only conserves locally. And the evolution formula U(t)|0,0> = |1,t> + |2,t> can be seen as local conservation of the norm.
    - tsimionescu 2y ago
      This runs into two separate but maybe related problems. One is called the preferred basis problem: while you're right that the world could be described by |us seeing spin up in a detector> ⊗ |spin up> + |us seeing spin down> ⊗ |spin down> it can equally validly be described by (|us seeing spin up in a detector> + |us seeing spin down>) ⊗ (|spin up> + |spin down>) + (|us seeing spin up in a detector> - |us seeing spin down>) ⊗ (|spin up> - |spin down>) (again leaving off all of the coefficients - each of the states should have a 1/sqrt(2) coefficient to be a valid superpositon). But even though in the math these are perfectly equivalent and valid solutions, we only ever observe solution 1 and never solution 2. We don't even have words to describe what decomposition 2 would mean. The second problem is one of arriving from these values to actual probabilities. In this interpretation, all values of the wave function are phsycially realized, so all possible outcomes have probability 1. Now, in a simple binary with equal probabilities on both sides (if the electron is equally like to be spin up or spin down), this can sort of work. But if the Spin Up state has a higher amplitude than the spin down state (which is easy to do in experiments), then we have a problem. If the electron is both spin up and spin down every time we run this experiment, how come we observe the spin up version more often than the spin down version? In more technical terms, how can we justify the exact shape of the Born rule without postulating any extra processes? In some variants of many worlds, they do add an extra postulate: that the amplitude of the wavefunction of a particular state corresponds to the number of "worlds" in which that state occurs. An observer can then deduce what are the odds that they live in one of the worlds where the outcome is spin up versus the worlds where it is spin down. But more modern versions of MWI like to say that no separate "worlds" actually exist (since that brings about a whole host of other problems), but then justifying these probabilities, without even more complicated postulates, comes back.
      - GoblinSlayer 2y ago
        >If the electron is both spin up and spin down every time we run this experiment, how come we observe the spin up version more often than the spin down version? Because after a series of measurements your state asymptotically approaches |we observe the spin up version more often than the spin down version>, which is in fact a mere tautology: if it didn't, you wouldn't conclude it. You don't even need any interpretation for this, you can just calculate it, and asymptotic certainty is trivial to interpret. In this sense MWI is a shut up and calculate interpretation, the only difficulty is to formulate your question in a computable form. If your question has no computable form, it probably doesn't match reality.

> maybe superposition describes the real physical situation It may or may not. What really needs explaining IMO is the mechanism of collapse of psi, something standard QM is pretty much silent upon.

Joker_vD 2y ago

Maybe it doesn't collapse, and the world is quantum through and through?

coffeeaddict1 2y ago
Then why do we observe a collapse?
- evanb 2y ago
  Decoherence may be enough. The idea is that the starting wavefunction of us and some experiment (say, an electron) is a tensor product | us > ⊗ | electron in superposition > and unitary time evolution governed by the Schrödinger equation evolves that state to | us seeing spin up in a detector > ⊗ | spin up > + | us seeing spin down > ⊗ | spin down > (I elided the amplitudes since we're just interested in the idea and not some particular values). In each 'branch' of the wavefunction (meaning: in each addend) we're left wondering "wow, how did the wavefunction collapse to the value that I am seeing", because we're inside the wf. But that's just a question we have that we've been tricked into by our limited sight; if we could see the whole wf of the universe, with our own superposition included, we'd see there is no collapse but just unitary Schrödinger evolution forever.
  - GoblinSlayer 2y ago
    Quantum physics doesn't use normalization much and these formulas work better without normalization. Normalization can be provided by the Bayes formula, so postulating it is redundant. Also I heard QFT has some problem with global normalization, because everything must be local there, so normalization is replaced by Stokes' formula: the flux of state norm through closed spacetime surface is zero, i.e. the norm only conserves locally. And the evolution formula U(t)|0,0> = |1,t> + |2,t> can be seen as local conservation of the norm.
  - tsimionescu 2y ago
    This runs into two separate but maybe related problems. One is called the preferred basis problem: while you're right that the world could be described by |us seeing spin up in a detector> ⊗ |spin up> + |us seeing spin down> ⊗ |spin down> it can equally validly be described by (|us seeing spin up in a detector> + |us seeing spin down>) ⊗ (|spin up> + |spin down>) + (|us seeing spin up in a detector> - |us seeing spin down>) ⊗ (|spin up> - |spin down>) (again leaving off all of the coefficients - each of the states should have a 1/sqrt(2) coefficient to be a valid superpositon). But even though in the math these are perfectly equivalent and valid solutions, we only ever observe solution 1 and never solution 2. We don't even have words to describe what decomposition 2 would mean. The second problem is one of arriving from these values to actual probabilities. In this interpretation, all values of the wave function are phsycially realized, so all possible outcomes have probability 1. Now, in a simple binary with equal probabilities on both sides (if the electron is equally like to be spin up or spin down), this can sort of work. But if the Spin Up state has a higher amplitude than the spin down state (which is easy to do in experiments), then we have a problem. If the electron is both spin up and spin down every time we run this experiment, how come we observe the spin up version more often than the spin down version? In more technical terms, how can we justify the exact shape of the Born rule without postulating any extra processes? In some variants of many worlds, they do add an extra postulate: that the amplitude of the wavefunction of a particular state corresponds to the number of "worlds" in which that state occurs. An observer can then deduce what are the odds that they live in one of the worlds where the outcome is spin up versus the worlds where it is spin down. But more modern versions of MWI like to say that no separate "worlds" actually exist (since that brings about a whole host of other problems), but then justifying these probabilities, without even more complicated postulates, comes back.
    - GoblinSlayer 2y ago
      >If the electron is both spin up and spin down every time we run this experiment, how come we observe the spin up version more often than the spin down version? Because after a series of measurements your state asymptotically approaches |we observe the spin up version more often than the spin down version>, which is in fact a mere tautology: if it didn't, you wouldn't conclude it. You don't even need any interpretation for this, you can just calculate it, and asymptotic certainty is trivial to interpret. In this sense MWI is a shut up and calculate interpretation, the only difficulty is to formulate your question in a computable form. If your question has no computable form, it probably doesn't match reality.