4 ms·

Again, just take a look at the postulates: Schrödinger equation describes the unitary time evolution of the system under a given Hamiltonian, wavefunction colla

Again, just take a look at the postulates: Schrödinger equation describes the unitary time evolution of the system under a given Hamiltonian, wavefunction collapse describes the outcome of the non-unitary measurement process. The outcome distribution of a measurement is described by the Born rule, the collapse postulate on the other hand tells us that the wave function will change to the actually measured state. Related but not the same. For the state |0> + |1> (unnormalized) the Born rule tells us that a measurement will yield |0> or |1> with 50 % probability each. The collapse postulate tells us that the state will change from |0> + |1> to either |0> or |1> depending on the measurement outcome. This change is non-unitary and therefore incompatible with the Schrödinger equation which tells us that quantum systems evolve unitarily. If you want, consider the combined quantum system of the system under investigation and the measurement device, why should that system not evolve unitarily?

bowsamic 3y ago
> The outcome distribution of a measurement is described by the Born rule, the collapse postulate on the other hand tells us that the wave function will change to the actually measured state. Related but not the same. Born rule is also a postulate. > This change is non-unitary and therefore incompatible with the Schrödinger equation Yes that's why unitary evolution has a separate postulate > If you want, consider the combined quantum system of the system under investigation and the measurement device, why should that system not evolve unitarily? Well, because that's the postulate
- danbruc 3y ago
  The postulates are roughly quantum system evolve unitarily except when they are measured, then a non-unitary wave function collapse happens. That is inconsistent. For the system under investigation this works, it evolves unitarily until measured, then the wave function collapses non-unitarily. The combined system of system under investigation and measurement device however is never measured, therefore must evolve unitarily. This is a conflict, the combined system can not evolve unitarily and also have some subsystem, the system under investigation, evolve non-unitarily, i.e. undergo a wave function collapse. This problem has been recognized for the better part of a century, that is nothing I am pulling out of thin air.
  - bowsamic 3y ago
    I mean, it's objectively not inconsistent. If you take the formal postulates you will not produce an inconsistent result.
    - danbruc 3y ago
      It is objectively inconsistent, the postulates contradict each other. But for almost a century we did not make any substantial progress, neither resolving the issue theoretically or philosophically, nor experimentally demonstrating any problems. Or at least nothing reached broad consensus.
      - bowsamic 3y ago
        It's objectively consistent since the postulates do not contradict each other
        danbruc 3y ago
        This seems debatable, but let us assume they are not obviously inconsistent. Those postulates only really mean something if you apply them in some situation, and if you apply them as laid out a few comments before, they make inconsistent claims. If applied to the subsystems, they postulate non-unitary evolution of the combined system, if applied to the combined system, they postulate unitary evolution. If inconsistent is to strong of a claim, I guess this could be changed to incomplete, maybe there are missing postulates resolving the issue.

bowsamic 3y ago

> The outcome distribution of a measurement is described by the Born rule, the collapse postulate on the other hand tells us that the wave function will change to the actually measured state. Related but not the same. Born rule is also a postulate. > This change is non-unitary and therefore incompatible with the Schrödinger equation Yes that's why unitary evolution has a separate postulate > If you want, consider the combined quantum system of the system under investigation and the measurement device, why should that system not evolve unitarily? Well, because that's the postulate

danbruc 3y ago
The postulates are roughly quantum system evolve unitarily except when they are measured, then a non-unitary wave function collapse happens. That is inconsistent. For the system under investigation this works, it evolves unitarily until measured, then the wave function collapses non-unitarily. The combined system of system under investigation and measurement device however is never measured, therefore must evolve unitarily. This is a conflict, the combined system can not evolve unitarily and also have some subsystem, the system under investigation, evolve non-unitarily, i.e. undergo a wave function collapse. This problem has been recognized for the better part of a century, that is nothing I am pulling out of thin air.
- bowsamic 3y ago
  I mean, it's objectively not inconsistent. If you take the formal postulates you will not produce an inconsistent result.
  - danbruc 3y ago
    It is objectively inconsistent, the postulates contradict each other. But for almost a century we did not make any substantial progress, neither resolving the issue theoretically or philosophically, nor experimentally demonstrating any problems. Or at least nothing reached broad consensus.
    - bowsamic 3y ago
      It's objectively consistent since the postulates do not contradict each other
      - danbruc 3y ago
        This seems debatable, but let us assume they are not obviously inconsistent. Those postulates only really mean something if you apply them in some situation, and if you apply them as laid out a few comments before, they make inconsistent claims. If applied to the subsystems, they postulate non-unitary evolution of the combined system, if applied to the combined system, they postulate unitary evolution. If inconsistent is to strong of a claim, I guess this could be changed to incomplete, maybe there are missing postulates resolving the issue.