"The wavefunction never collapses"

[jbergman]

As far as their claims about "what is really happening", yes. Of course they all make the same predictions for what we can test by experiment, but they don't limit themselves to that.

For example, the MWI says measurements have all possible outcomes, but other interpretations say measurements have single outcomes. Those are incompatible statements; they can't both be true. They both make the same predictions for what we can test by experiment, but their claims go beyond what we can test by experiment, and they do so in incompatible ways.

I would say, "their claims go beyond what we can test by experiment now". For instance with MWI, one could conceivably devise an experiment to measure the gravitational effect of the different worlds which would prove or it. But of course. We don't believe we have a complete theory of gravity compatible with QM so there is your out.

 

[PeterDonis]

with MWI, one could conceivably devise an experiment to measure the gravitational effect of the different worlds

No, you couldn't, because any such effects would just end up being entangled with everything else, and in any given branch, there would be just one gravitational effect, and the branches are decohered so they don't interfere with each other, so there's no experiment that could detect the presence of more than one branch.

 

[Roberto Pavani]

Let me clarify: either
(a) the outcome was already assigned to each branch before measurement; natural, but operationally a hidden variable;
(b) the branches form at measurement and "decide" their outcomes at that moment, correlated with each other (one gets 0 because the other gets 1).
In case (b), this seems like an "entangled" collapse.
I don't see a third option, but I'm open to one.

 

[javisot]

But we are always observers inside one branch, and from that perspective, it is operationally indistinguishable from a hidden variable.

A change to this paragraph:

"we are always observers inside one branch, and from that perspective, it is operationally indistinguishable from Copenhagen."

and, Copenhagen≠hidden variable

 

[PeterDonis]

(a) the outcome was already assigned to each branch before measurement; natural, but operationally a hidden variable;

I don't even know what this means.

(b) the branches form at measurement

In the sense that the measurement interaction is what entangles the particles with the measuring devices and the environment, and that entanglement, spreading among a very large number of untrackable degrees of freedom in the environment is what leads to decoherence, yes.

and "decide" their outcomes at that moment, correlated with each other (one gets 0 because the other gets 1).

No, there is no "decision" involved. The wave function already contains all the outcomes and the correlations between them. There's nothing to decide.

I don't see a third option, but I'm open to one.

I think you need to stop waving your hands and write down some actual math. If you write down the actual math, the things I've been saying should be obvious, and it should also be obvious why things like your a) are just meaningless noise, since as far as I can see your a) doesn't correspond to anything in the actual math.

 

[Matterwave]

If nothing in the theory determines which branch you end up in, yet you end up in one, isn't that precisely the definition of "incomplete" in the EPR sense?

I would recommend you to actually read the original EPR paper! It is eminently readable!

In EPR they define a necessary (not sufficient) condition for completeness as "...every element of the physical reality must have a counterpart in the physical theory."

But they are quite specific on what they mean by "element of reality" as it pertains to that paper. A sufficient (not necessary) requirement is: "If, without in any way disturbing a system, we can predict with certainty (i.e., with probability equal to unity) the value of a physical quantity, then there exists an element of physical reality corresponding to this physical quantity."

They were quite focused on position and momentum as their "element(s) of reality" which QM had no "corresponding object" -- (I.e. standard QM has only a probability distribution for them before any measurement is made and can not tell you what they "really are" in the sense of a hidden/extra variable). It was later Bohm who expanded the EPR to spin to make the example easier to deal with.

Same as PeroK, I see no way where MWI branching fits this definition of aspect of reality and therefore I see no way to directly use EPR argument to say MWI is incomplete.

 

[PeterDonis]

you end up in one

No, you end up in all branches. You do not end up in just one. So there can't be any "hidden variable" that "determines" that you end up in a certain branch--because you don't.

 

[Roberto Pavani]

Thank you for clarifying that it is (b). I accept the correction on EPR.

But I still have a question about the mechanism in (b): you said "the wave function already contains all outcomes and correlations, there is nothing to decide."

If everything was already there before the measurement, what does the measurement actually do? Does it spawn two worlds, one with 0 and one with 1? And from the perspective of World_0: wasn't the outcome already determined? This brings me back to the same question, just without the EPR label.
Sorry for my naive quesitons but i'm trying to be as much rigorous as i can in order to understand the process.

 

[PeterDonis]

If everything was already there before the measurement, what does the measurement actually do?

It entangles the measured system with the measuring device and the environment, as I said in post #125. And this would be obvious to you if you looked at the math.

i'm trying to be as much rigorous as i can

No, you're not, if you're not looking at the actual math. Please go do that before posting any further in this thread.

 

[Roberto Pavani]

Thank you, I'll study the math and will do some simulations with qiskit. I appreciate everyone's patience with my questions.

 

[PeterDonis]

will do some simulations with qiskit

Please note that you should not require any simulations in order to see the basic idea in the math; for a simplified scenario like measuring two entangled qubits, which is what we've been discussing, everything can be written out in closed form and doesn't require any numerical analysis.

 

[bhobba]

for a simplified scenario like measuring two entangled qubits, which is what we've been discussing, everything can be written out in closed form and doesn't require any numerical analysis.

It can be found in Lenny Susskind's book on QM:
https://www.amazon.com.au/Quantum-Mechanics-Theoretical-Art-Friedman/dp/0465062903

Thanks
Bill