Entanglement & Wave Function Collapse

[vanhees71]

I don't know, what I commented as "highly problematic", but Laloe's statement is of course not problematic in any way. It's just standard QT and in full accordance with the minimal statistical interpretation.

 

[Nugatory]

My intuitive response to this - which I have learned to expect to be wrong - is that just because we lose the capability of measuring it, surely doesn't mean that it doesn't have that property?

That is the essential difference between classical and quantum mechanics. Classical mechanics says that your intuition is right, quantum mechanics says that your intuition is wrong. Experiments have fairly convincingly shown that the quantum mechanics is right.

 

[DrChinese]

But what if either Bob or Alice simply don't make a measurement? Let's say Bob just doesn't bother going to the lab that day, or whenever. Does the wave function of his particle still collapse?

Presumably Bob's particle interacts with something sometime. When that happens, there is - in effect - a measurement of some kind. Of course, recording it is not necessary.

... it would be tautological to say that it doesn't have those quantities before we attempt to ascribe those quantities to it. Whereas it would have certain qualities.

What is tautological is assuming that which you seek to prove. Bell and other no-go theorems clearly rule your hypothesis out, unless there are currently unknown mechanisms for action at a distance.

 

[vanhees71]

We don't lose the capability of measuring anything. Why should we?

The difference between classical and quantum theory is that in the classical theory all observables have determined values before they are measured, no matter in which state the system is prepared in.

According to QT there's no state you can prepare the system in, in which all observables have determined values. Almost always the preparation of a state in which one observable takes a determined value, another observable that is not compatible to the first, cannot take a determined value.

This has nothing to do with our capability to measure this indetermined observable. We can always measure any well-defined observable of any system. That's what defines what an observable is, namely something that can be observed, and observation in the exact sciences means it can be measured.

 

[Lord Jestocost]

I don't know, what I commented as "highly problematic", but Laloe's statement is of course not problematic in any way. It's just standard QT and in full accordance with the minimal statistical interpretation.

In my comment #53, I have merely cited remarks by Ghirardi and Dirac which are related to the wave-packet reduction postulate. Referring to comment #53, you start your comment #59 with “But that's highly problematic”. I do not have any clue what’s now highly problematic.

The wave-packet reduction postulate has to put in “by hand” to relate the mathematical formalism of quantum theory to our perceived reality – actual outcomes of single measurements events. It addresses measurements on single systems and on entangled systems.

 

[vanhees71]

The collapse postulate is highly problematic for the known reasons. It's formally contradicting the very construction of microcausal relativistic QFTs in simply assuming "spooky action at a distance". It's an unnecessary assumption. I don't need to repeat all the arguments a made already above.

 

[atyy]

The collapse postulate is highly problematic for the known reasons. It's formally contradicting the very construction of microcausal relativistic QFTs in simply assuming "spooky action at a distance". It's an unnecessary assumption. I don't need to repeat all the arguments a made already above.

This is not correct. The collapse postulate is consistent with microcausality (no superluminal transmission of information).

 

[Lord Jestocost]

The collapse postulate is highly problematic for the known reasons. It's formally contradicting the very construction of microcausal relativistic QFTs in simply assuming "spooky action at a distance". It's an unnecessary assumption. I don't need to repeat all the arguments a made already above.

For me, ideas of "superluminal signals" or "spooky actions at a distance" were always nonsensical ideas and helpless attempts to “explain” something which cannot be explained. But, what have "superluminal signals" or "spooky actions at a distance" to do with the collapse postulate. You are mashing up a lot of things.

Now, regarding the micro-causality condition of relativistic quantum field theory:

As Peter Mittelstaedt remarks in “Quantum Holism, Superluminality, and Einstein Causality”:

“Finally, we analyze these arguments and show that the micro-causality condition of relativistic quantum field theory excludes entanglement induced superluminal signals but that this condition is justified by the exclusion of superluminal signals. Hence, we are confronted here with a vicious circle, and the question whether there are superluminal EPR-signals cannot be answered in this way.”

 

[vanhees71]

The collapse postulate says that, when I have a polarization-entangled photon pair and measure at point A the polarization of one of the photons, then the state changes immediately by some magic outside of the well-defined quantum(-field-)theoretical dynamics such that also the 2nd photon, to be registered lightyears away gets a definite polarization. This doesn't make sense indeed and it contradicts the very theory it pretends to interpret.

It's completely irrelevant however for anything concerned with physics: there's no magic dynamics outside of the well-established rules of QFT only because a piece of matter is used as measurement device by some physicist but there's only the well-established rules of QFT which explain quite well, why the photo detector clicks, and this click is a localized event at the place of the detector. Nothing happens instantaneously to some other far-distantly registered photon that's entangled with the just measured photon.

The non-classical correlations described by entanglement are correlations, which are imposed on the system by the initial preparation. It's not caused by the measurements done on parts of this system.

 

[DrChinese]

It's completely irrelevant however for anything concerned with physics: there's no magic dynamics outside of the well-established rules of QFT only because a piece of matter is used as measurement device by some physicist but there's only the well-established rules of QFT which explain quite well, why the photo detector clicks, and this click is a localized event at the place of the detector. Nothing happens instantaneously to some other far-distantly registered photon that's entangled with the just measured photon.

The non-classical correlations described by entanglement are correlations, which are imposed on the system by the initial preparation. It's not caused by the measurements done on parts of this system.

I have tried to remain quiet through this, hoping that basic answers would be limited to things we all agree about. But this is a very advanced level discussion topic which vanhees71's position has previously been demonstrated to be at odds with most top experimentalists and theoreticians in this area of entanglement (Weinberg, Zeilinger, etc). Bell shows there is no local realistic explanation for entanglement. Period. No one questions this (past a few fringe authors). QFT may be constructed to be local, but clearly other physicists do not accept the premise that "nothing happens elsewhere" but "correlations imposed on the system by initial preparation". This is local realism, plain and simple. Vanhees71 should not be pushing QFT as a local realistic theory, which is in essence what he is saying.

Weinberg: "There is a troubling weirdness about quantum mechanics. Perhaps its weirdest feature is entanglement, the need to describe even systems that extend over macroscopic distances in ways that are inconsistent with classical ideas." And: ...according to present ideas a measurement in one subsystem does change the state vector for a distant isolated subsystem... "

My position being: As a consequence of Bell and subsequent experiments, Quantum non-locality is accepted as standard physics at this time. The primary point of contention is the mechanism, which is unknown at this time. Interpretations exist to address this, which clearly would not be needed if QFT already had those answers.

It doesn't make sense for our discussions to repeatedly devolve from the OP into one in which vanhees71 posts his minority non-standard position, regardless of his leaning on QFT; his reading does not match a single other source I have encountered. I have asked repeatedly for him to post quotes from other reputable sources, and to date there has not been a single suitable quote other than "all papers ever written on QFT". So I again ask: where does Zeilinger say that quantum non-locality is absurd in the face of QFT? Where does Weinberg say that? Are all Bohmians wrong because QFT has all the answers? How can you reconcile an assertion that QFT is local realistic with Bell? I don't think that calling correlations "non-classical" is going to do the trick.

Finally, I point out that the formula for polarization correlation between remote measurements depends on a single variable: the difference in the measurement angles. That renders absurd the assertion that nothing happens as a result of a measurement. Every interpretation essentially says the measurement is a critical element. And in fact there is no variable in the equation that in any way relates to vanhees71's "initial preparation" other than it is entangled.

I will not comment further on vanhees71's statements in this thread, so as to prevent it from being further hijacked.

 

[PeterDonis]

Thread closed for moderation.

 

[fresh_42]

It seems the discussion came to a kind of end. As we already have many threads with this topic or similar, this one will remain closed.