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This isn't quite right, wave function collapse is relative so while within the box an observer's wave function of the box will be in a definite eigenstate, the

This isn't quite right, wave function collapse is relative so while within the box an observer's wave function of the box will be in a definite eigenstate, the wavefunction of the box for an observer isolated from the box will still be in a superposition of its states. In other words, the fact that an observation takes place from within the box and collapses the wave function for an observer in the box does not mean that the wave function for every observer also collapses. A wave function collapse for one observer does not imply a wave function collapse for any other observer.

Jensson 2y ago
> wave function collapse is relative No it isn't, why do you think this? If it was we wouldn't be able to see it in our experiments, as where the particle ends up would depend on which observer is looking, which would get absurd consequences.
- Maxatar 2y ago
  I provided my explanation for why this is the case. A measurement device only collapses the wave function for itself, it does not collapse the wave function for every other observer/measurement device. Of course, a second measurement device that's completely isolated from the first measurement device can eventually interact with the first measurement device and the two will converge and share the same wave function. However, until the second measurement device interacts with the first one or with the original quantum system being observed, then from the point of view of the second measurement device, the first measurement device is in a superposition of all the possible states that it could have observed. This can continue on and on, with a third measurement device which is entirely isolated from the second measurement device having a wavefunction that's a superposition of all the possible observations that the second measurement device will observer...
  - Jensson 2y ago
    > A measurement device only collapses the wave function for itself, it does not collapse the wave function for every other observer/measurement device. Wave function collapse is a real measurable phenomena, what you describe is not measurable so just a belief, don't mix in your beliefs with actual results.
    - Maxatar 2y ago
      Wave function collapse is one of many equivalent (so far as we know) interpretations of quantum mechanics and not all interpretations have wave function collapse whatsoever. With that said even in the Copenhagen interpretation which is the most popular interpretation of quantum mechanics, the moment in time when the wave function collapses is relative to each measurement device. It is not the case that just because some measurement device that has been entirely isolated from me happens to perform a measurement on a quantum system, that my wave function of that quantum system will also collapse. On the contrary my wave function remains exactly as is until the moment I perform a measurement on the quantum system which collapses my wave function of said system.
      - Jensson 2y ago
        Double slit experiment disproves you here, the observed pattern is the same for every observer and shows a collapse happened.
      - danbruc 2y ago
        One common view is that there is THE wave function of the universe. If it collapses, it collapses, there is no way it can collapse for some observers and not others. You might not know that it has collapsed but that does not make it not collapsed. There is a different view that the wave function is just an representation of the knowledge someone has about the state of the universe or some part of it, in which case it can collapse for some and not others. On the other hand there is not really any collapse at all in this case, it is just a fancy term for updating once believe about the state of the universe or some system. It is also important to note that a collapse is not a total collapse into a point. Measuring different things will collapse the wave function along different dimensions, only measuring everything in the universe would collapse the wave function along all dimensions into a point, or as far as the uncertainty principle permits.
        gus_massa 2y ago
        Imagine you have a lamp that sends a single photon, a double slit, and a screen with silver or something to catch the photon. All of that is inside a black box. You press the button to send the photon, open the box and see the screen. There are 5 possible scenarios: 1) The wave function collapsed when the photon colides with the screen. You can calculate the time using distance/c. All the other steps are classic. You just open the box and see where the spot was. 2) When the photon colides with the screen there is no colapse. You get a superposition of two screens. A few milliseconds later when the gas inside the box hits the screen there is a collapse and ow you have a classic spot in a classic screen. Later you just open the box. 3) When the photon colides with the screen there is no colapse. You get a superposition of two screens. A few milliseconds later when the gas inside the box hit the screen, the gas gets entangled and later the walls of the box get entangled. There is no collapse until you open the box. Anyway, when you open the box everything get's classic and you see the usual result. 4) When the photon colides with the screen there is no colapse. You get a superposition of two screens. A few milliseconds later when the gas inside the box hit the screen, the gas gets entangled and later the walls of the box get entangled. There is no collapse. When you open the box your eyes and brain get entangled. There is no collapse! Anyway, each of the versions of you see the expected result because each one is entangled with a screen that has a spot as expected. 5) Shut up and calculate! I'm a strong proponent of 5 because people pay me to calculate the distribution of the probability of the spot and not to tell weird stories. The easy way to get the result of 5 is to use the scenario in 1, and that is what is teach, and that is essentially the Copenhagen interpretation and anyone that is not insane will use that scenario to make the calculations because it's "obvious" that all the other add negligidle corrections and are infinitely more difficult. Here the problem is to define what is a measurement. It's "obvious" that the screen is good enough, but there is no good technical definition of what "obvious" means. The problem is that the only non magical scenario is 4, where there is no collapse and you get entangled too. There are still many technical details to solve, but if it's real you would see the same experimental results than in 1, 2, 3 and 4, but the advantage is that there is no magic rule of collapse. In this scenario, measurement does not collapse wave functions, it just an illusion when some of the parts is big enough to get decoherence and make all the versions so different that you get (almost) no interference effect. Anyway we are still not sure, so you can believe any of them until someone solves the problem, probably in 50 or 100 years.
- tsimionescu 2y ago
  This is actually an open problem in QM. We don't understand what constitutes a measurement, and so we don't know if two perfectly isolated observers could disagree on the "state" of a system in this way (as in, collapsed or not collapsed). And given special relativity, it is definitely possible that two different observers could disagree on the order that two independent measurements of the same system happened. Do note that the result of the experiments is not going to differ, it is guaranteed to be consistent by the fact that both experiments are measuring the same quantum system. It's just that one observer is unsure of what result they will get for a subsequent experiment, while the other observer can know it exactly. And in the many worlds interpretation, it is indeed believed that apparent wave function collapse is a relative phenomenon, caused by entanglement with the classical environment, which spreads out at the speed of light.
  - Jensson 2y ago
    > We don't understand what constitutes a measurement, and so we don't know if two perfectly isolated observers could disagree on the "state" of a system in this way (as in, collapsed or not collapsed) We do know that they can't disagree as collapsed particles behave differently than uncollapsed particles. > It's just that one observer is unsure of what result they will get for a subsequent experiment, while the other observer can know it exactly. Quantum systems aren't about knowledge or statistics, the wave function is an actual physical thing that changes how the particle behaves. If the wave function has collapsed it no longer behaves the same as before, so what you said here is wrong.
    - Maxatar 2y ago
      You're now arguing against claims no one has made, which is apparent from the fact that you seem to only quite very tiny snippets of what's said while ignoring the main argument. You seem to just want to argue for the sake of arguing.
      - Jensson 2y ago
        Wasn't the argument that two observers can disagree whether a collapse has happened? I offered evidence that they can't, as otherwise they would see different results in the double slit experiment.

Jensson 2y ago

> wave function collapse is relative No it isn't, why do you think this? If it was we wouldn't be able to see it in our experiments, as where the particle ends up would depend on which observer is looking, which would get absurd consequences.

Maxatar 2y ago
I provided my explanation for why this is the case. A measurement device only collapses the wave function for itself, it does not collapse the wave function for every other observer/measurement device. Of course, a second measurement device that's completely isolated from the first measurement device can eventually interact with the first measurement device and the two will converge and share the same wave function. However, until the second measurement device interacts with the first one or with the original quantum system being observed, then from the point of view of the second measurement device, the first measurement device is in a superposition of all the possible states that it could have observed. This can continue on and on, with a third measurement device which is entirely isolated from the second measurement device having a wavefunction that's a superposition of all the possible observations that the second measurement device will observer...
- Jensson 2y ago
  > A measurement device only collapses the wave function for itself, it does not collapse the wave function for every other observer/measurement device. Wave function collapse is a real measurable phenomena, what you describe is not measurable so just a belief, don't mix in your beliefs with actual results.
  - Maxatar 2y ago
    Wave function collapse is one of many equivalent (so far as we know) interpretations of quantum mechanics and not all interpretations have wave function collapse whatsoever. With that said even in the Copenhagen interpretation which is the most popular interpretation of quantum mechanics, the moment in time when the wave function collapses is relative to each measurement device. It is not the case that just because some measurement device that has been entirely isolated from me happens to perform a measurement on a quantum system, that my wave function of that quantum system will also collapse. On the contrary my wave function remains exactly as is until the moment I perform a measurement on the quantum system which collapses my wave function of said system.
    - Jensson 2y ago
      Double slit experiment disproves you here, the observed pattern is the same for every observer and shows a collapse happened.
    - danbruc 2y ago
      One common view is that there is THE wave function of the universe. If it collapses, it collapses, there is no way it can collapse for some observers and not others. You might not know that it has collapsed but that does not make it not collapsed. There is a different view that the wave function is just an representation of the knowledge someone has about the state of the universe or some part of it, in which case it can collapse for some and not others. On the other hand there is not really any collapse at all in this case, it is just a fancy term for updating once believe about the state of the universe or some system. It is also important to note that a collapse is not a total collapse into a point. Measuring different things will collapse the wave function along different dimensions, only measuring everything in the universe would collapse the wave function along all dimensions into a point, or as far as the uncertainty principle permits.
      - gus_massa 2y ago
        Imagine you have a lamp that sends a single photon, a double slit, and a screen with silver or something to catch the photon. All of that is inside a black box. You press the button to send the photon, open the box and see the screen. There are 5 possible scenarios: 1) The wave function collapsed when the photon colides with the screen. You can calculate the time using distance/c. All the other steps are classic. You just open the box and see where the spot was. 2) When the photon colides with the screen there is no colapse. You get a superposition of two screens. A few milliseconds later when the gas inside the box hits the screen there is a collapse and ow you have a classic spot in a classic screen. Later you just open the box. 3) When the photon colides with the screen there is no colapse. You get a superposition of two screens. A few milliseconds later when the gas inside the box hit the screen, the gas gets entangled and later the walls of the box get entangled. There is no collapse until you open the box. Anyway, when you open the box everything get's classic and you see the usual result. 4) When the photon colides with the screen there is no colapse. You get a superposition of two screens. A few milliseconds later when the gas inside the box hit the screen, the gas gets entangled and later the walls of the box get entangled. There is no collapse. When you open the box your eyes and brain get entangled. There is no collapse! Anyway, each of the versions of you see the expected result because each one is entangled with a screen that has a spot as expected. 5) Shut up and calculate! I'm a strong proponent of 5 because people pay me to calculate the distribution of the probability of the spot and not to tell weird stories. The easy way to get the result of 5 is to use the scenario in 1, and that is what is teach, and that is essentially the Copenhagen interpretation and anyone that is not insane will use that scenario to make the calculations because it's "obvious" that all the other add negligidle corrections and are infinitely more difficult. Here the problem is to define what is a measurement. It's "obvious" that the screen is good enough, but there is no good technical definition of what "obvious" means. The problem is that the only non magical scenario is 4, where there is no collapse and you get entangled too. There are still many technical details to solve, but if it's real you would see the same experimental results than in 1, 2, 3 and 4, but the advantage is that there is no magic rule of collapse. In this scenario, measurement does not collapse wave functions, it just an illusion when some of the parts is big enough to get decoherence and make all the versions so different that you get (almost) no interference effect. Anyway we are still not sure, so you can believe any of them until someone solves the problem, probably in 50 or 100 years.
tsimionescu 2y ago
This is actually an open problem in QM. We don't understand what constitutes a measurement, and so we don't know if two perfectly isolated observers could disagree on the "state" of a system in this way (as in, collapsed or not collapsed). And given special relativity, it is definitely possible that two different observers could disagree on the order that two independent measurements of the same system happened. Do note that the result of the experiments is not going to differ, it is guaranteed to be consistent by the fact that both experiments are measuring the same quantum system. It's just that one observer is unsure of what result they will get for a subsequent experiment, while the other observer can know it exactly. And in the many worlds interpretation, it is indeed believed that apparent wave function collapse is a relative phenomenon, caused by entanglement with the classical environment, which spreads out at the speed of light.
- Jensson 2y ago
  > We don't understand what constitutes a measurement, and so we don't know if two perfectly isolated observers could disagree on the "state" of a system in this way (as in, collapsed or not collapsed) We do know that they can't disagree as collapsed particles behave differently than uncollapsed particles. > It's just that one observer is unsure of what result they will get for a subsequent experiment, while the other observer can know it exactly. Quantum systems aren't about knowledge or statistics, the wave function is an actual physical thing that changes how the particle behaves. If the wave function has collapsed it no longer behaves the same as before, so what you said here is wrong.
  - Maxatar 2y ago
    You're now arguing against claims no one has made, which is apparent from the fact that you seem to only quite very tiny snippets of what's said while ignoring the main argument. You seem to just want to argue for the sake of arguing.
    - Jensson 2y ago
      Wasn't the argument that two observers can disagree whether a collapse has happened? I offered evidence that they can't, as otherwise they would see different results in the double slit experiment.