[mgraczyk]

What percentage of practicing physicists actually think there is something "missing" in the quantum formalism as it relates to "wavefunction collapse"?

I'm not a physicist, but from the few years of QM I took in college my take is that there is nothing special about "measurement", it's just a label we apply to certain states becoming entnagled. As long as you don't believe there is anything magical about humans or other "conscious" observers, then there doesn't seem to be anything to figure out about collapse.

[Xcelerate]

> then there doesn't seem to be anything to figure out about collapse.

Then why is quantum state evolution seemingly continuous and unitary some of the time, and sharply discontinuous at other times?

[im3w1l]

It seems continuous and unitary when the experimenter takes a lot of care to create a simple and very isolated system. When it's brought into contact with the mess that is the rest of the world it changes character very quickly, and interactions with the rest of the world completely swamp the internal dynamics.

[Orlanthai]

Measurement is not just the same as entanglement. If you try that you get paradoxes and you don't match actual observations.

If you treat a measurement as entanglement then by the Kochen-Specker theorem you can't condition on the measurement outcome. However we seem to be able to do this in actual experiments.

Thus measurement is not entanglement alone, but also the elimination of other bases.

[mgraczyk]

I interpret KS backward from what you're saying. From my understanding, KS says that there is no sense in which the experiment involves an objective revelation of hidden state.

After the experiment, the experimenter did not "learn" or "reveal" some objective fact about reality, ie that the true state was UP rather than DOWN. Instead, after the experiment the experimenter becomes entangled with the UP/DOWN system in such a way that the experimenter measured both UP and DOWN, but all observables relating to the experimenter are either wholly consistent with UP or wholly consistent with DOWN.

[Orlanthai]

The lack of noncontextual hidden variables is the main implication of the Kochen-Specker theorem from which the inability to condition follows. It's not "backward" from the conclusion of no noncontextual hidden state, it's just another consequence there of.

So the Kochen-Specker theorem says there was no pre-existent noncontextual state for the particle. However that doesn't in any way imply the particle was both UP and DOWN or that the device measured both UP and DOWN. Especially for the device as the Kochen-Specker theorem is proved in a context where observable outcomes are assumed to be single-valued.

However in real practice we can condition on the states of our devices following measurement, hence they don't seem to be susceptible to a Kochen-Specker result when viewed as the system for some "super"-observational device. Which they would be if they simply entered an entangled state. Thus the assumption of measurement as simple entanglement does not match actual observed reality. This is a point made in many texts such as those of Schlosshauer, Omnès, Peres and at a very rigorous level in the theory of C* algebras and Category theory by Fröhlich and Landsman respectively.

[fsh]

In the lab we do measure things and we do observe wavefunction collapse. There is no satisfying explanation why "us becoming entangled with the quantum system" or whatever explanation one chooses looks like collapse to us.

[mgraczyk]

I've always been content to know that after measurement, the quantum system that includes both me and the device has a certain quantum state which is a superposition of all the measurement eigenstates, but by construction that state couldn't be measured in such a way that I would have ever experienced an inconsistency.