3 ms·

> 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

> 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.
  - tsimionescu 2y ago
    There is nothing in the double slit experiment that makes collapse necessarily an objective reality. There is in fact no way to check if collapse of the wave function has happened or not. I imagine you're thinking of the version of the double slit experiment with a detector installed next to one of the slits, which prevents the interference pattern from forming. However, this only applies for a "measurement device" installed at the slits. That is, a classical system interacting with the photon/electron/etc before it hits the classical screen. But this is not mysterious at all: the classical device causes decoherence of the wave function and thus prevents self-interference of the beams traveling through the two slots. That is true even in interpretations that consider that wave function collapse is not a physical phenomenon, such as Many Worlds. But this doesn't prove much. We already know that a photon hitting an atom doesn't cause wave function collapse, while a photon hitting a detector does. We can say that an atom is not a measurement device and an observer is, but this immediately raises the question of why. Or, we can choose the MWI route and say that the wave function never collapses, and the only difference between an atom and a detector is that we are not entangled with that atom, but we are entangled with the detector. And if so, it immediately follows that if we could get a detector that we are not entangled with, we would not see the same results as we do in the traditional double slit + detector on a slit experiment.
tsimionescu 2y ago
After a measurement, the wave function of a particle, and of any particles entangled with it, collapses. But this isn't itself a measurable difference. For example, when performing measurements on two entangled particles at very far away places, the wave function collapses instantly, across any distance, even light years away. After that experiment, the result on the other side is 100% determined (if performed in the same basis). But there is no experiment whatsoever that could be done to tell if the collapse has happened or not from the other side. This is the resolution of the famous EPR paradox: wave function collapse is non-local (instantaneous), but it is probably impossible to send classical information with it faster than the speed of light limit.

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.
- tsimionescu 2y ago
  There is nothing in the double slit experiment that makes collapse necessarily an objective reality. There is in fact no way to check if collapse of the wave function has happened or not. I imagine you're thinking of the version of the double slit experiment with a detector installed next to one of the slits, which prevents the interference pattern from forming. However, this only applies for a "measurement device" installed at the slits. That is, a classical system interacting with the photon/electron/etc before it hits the classical screen. But this is not mysterious at all: the classical device causes decoherence of the wave function and thus prevents self-interference of the beams traveling through the two slots. That is true even in interpretations that consider that wave function collapse is not a physical phenomenon, such as Many Worlds. But this doesn't prove much. We already know that a photon hitting an atom doesn't cause wave function collapse, while a photon hitting a detector does. We can say that an atom is not a measurement device and an observer is, but this immediately raises the question of why. Or, we can choose the MWI route and say that the wave function never collapses, and the only difference between an atom and a detector is that we are not entangled with that atom, but we are entangled with the detector. And if so, it immediately follows that if we could get a detector that we are not entangled with, we would not see the same results as we do in the traditional double slit + detector on a slit experiment.

tsimionescu 2y ago

After a measurement, the wave function of a particle, and of any particles entangled with it, collapses. But this isn't itself a measurable difference. For example, when performing measurements on two entangled particles at very far away places, the wave function collapses instantly, across any distance, even light years away. After that experiment, the result on the other side is 100% determined (if performed in the same basis). But there is no experiment whatsoever that could be done to tell if the collapse has happened or not from the other side. This is the resolution of the famous EPR paradox: wave function collapse is non-local (instantaneous), but it is probably impossible to send classical information with it faster than the speed of light limit.