# B Entanglement & Wave Function Collapse

#### Lynch101

Follow that line of thought to the end, and you will arrive at the position known as “superdeterminism”. (Google for “superderminism t’Hooft” for more).

On the one hand, superdeterminism is about the only way of reconciling the experimentally confirmed predictions of quantum mechanics with the classical intuition that you so emphatically assert in post #36 of this thread.

On the other hand, superdeterminism is a purely philosophical position that cannot be empirically tested and indeed makes a mockery of the entire notion of empirical science. It’s also not a topic that we can discuss here.
Thank you. I'll check that out

#### DarMM

Gold Member
We might not be able to ascribe a value to it, but that represents a limit to our investigative abilities as opposed to the absence of that property
The Kochen-Specker theorem shows that's not what is happening.

#### Lord Jestocost

Gold Member
2018 Award
I was always taught collapse was just another name for state reduction and thus is a part of the formalism.
State reduction is a fundamental postulate and part of the formalism. As Giancarlo Ghirardi puts it in the entry “Collapse Theories” on the “Stanford Encyclopedia of Philosophy” (https://plato.stanford.edu/entries/qm-collapse/) :

“The fact that when the measurement is completed one can make statements about the outcome is accounted for by the already mentioned WPR postulate (Dirac 1948): a measurement always causes a system to jump in an eigenstate of the observed quantity.” [italics in original, WPR means “wave packet reduction”, LJ]

P. A. M. Dirac writes in “THE PRINCIPLES OF QUANTUM MECHANICS”:

“In this way we see that a measurement always causes the system to jump into an eigenstate of the dynamical variable that is being measured, the eigenvalue this eigenstate belongs to being equal to the result of the measurement.”

Whether one calls this "collapse postulate" or "wave-packet reduction postulate" is merely a matter of taste without any physical consequences.

#### Lynch101

The Kochen-Specker theorem shows that's not what is happening.
I'll try getting my teeth into that but it seems as though there is a limitation on the applicabiliity of mathematics at that level. Could the theorem be influenced by that?

Again, it goes to the idea that there must be something going on in individual experiments as espoused by Smolin and [from just reading Free Will in the Theory of Everything] t'Hooft. I would take it as a given that the universe has properties at all times and therefore a sub-system has properties at all times; and we cannot measure a property that a system doesn't have, so it must therefore have that property before we can measure it.

It could be that the act of measuring gives rise to a property that manifests as a result of the contact between the system and the measuring equipment, which isn't a property of the system in and of itself. To borrow a phrase, it would be a case of "dependent origination" or "dependent arising".

The issue may lie in the definition of the term "properties". If "properties" is defined as those features of a system which have been, or can be measured, then we would require a different term to refer to the makings of the system.

#### DarMM

Gold Member
Whether one calls this "collapse postulate" or "wave-packet reduction postulate" is merely a matter of taste without any physical consequences
True the name has no physical consequences, but the problem is that by collapse some don't mean just state reduction where as some of us do which causes confusion.

#### DarMM

Gold Member
Could the theorem be influenced by that?
No. It's a rigorous theorem.

and we cannot measure a property that a system doesn't have, so it must therefore have that property before we can measure it
The answer here is related to this sentence:
It could be that the act of measuring gives rise to a property that manifests as a result of the contact between the system and the measuring equipment, which isn't a property of the system in and of itself
It doesn't possess the quantities we measure before we measure them, i.e. it doesn't possess Energy, Momentum, Angular Momentum, etc. They seem to arise from contact between equipment and system.

The question then is what are its true properties. Hidden variable theories make different guesses. Copenhagen says the true properties are beyond mathematical description.

#### Lynch101

No. It's a rigorous theorem.
I see. I must be misinterpreting what you meant with the above statement, but I think the below speaks to the point.

The answer here is related to this sentence:

It doesn't possess the quantities we measure before we measure them, i.e. it doesn't possess Energy, Momentum, Angular Momentum, etc. They seem to arise from contact between equipment and system.
This would appear to be analogous to sensory perception.

I guess what I'm trying to get at is the distinction between not having the quantities we measure [before we measure them], while it does have qualities - even if they can't be measured.

Measurement by its very nature seeks to quantify things, so 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 cerrtain qualities.

It might be bundled up in the idea that absolute reality cannot be expressed in relative terms bcos that would be a contradiction in terms.

The question then is what are its true properties. Hidden variable theories make different guesses. Copenhagen says the true properties are beyond mathematical description.
It strikes me that Buddhist (and perhaps Hindu) philosophy is not a million miles from this in the idea that absolute reality is beyond conceptualisation and with the notion of "dependent arising".

#### DarMM

Gold Member
Measurement by its very nature seeks to quantify things, so 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 cerrtain qualities
We've been through this before. It's not a tautology as in classical mechanics the quantities we measure do proceed our measurements.

#### vanhees71

Gold Member
State reduction is a fundamental postulate and part of the formalism. As Giancarlo Ghirardi puts it in the entry “Collapse Theories” on the “Stanford Encyclopedia of Philosophy” (https://plato.stanford.edu/entries/qm-collapse/) :

“The fact that when the measurement is completed one can make statements about the outcome is accounted for by the already mentioned WPR postulate (Dirac 1948): a measurement always causes a system to jump in an eigenstate of the observed quantity.” [italics in original, WPR means “wave packet reduction”, LJ]

P. A. M. Dirac writes in “THE PRINCIPLES OF QUANTUM MECHANICS”:

“In this way we see that a measurement always causes the system to jump into an eigenstate of the dynamical variable that is being measured, the eigenvalue this eigenstate belongs to being equal to the result of the measurement.”

Whether one calls this "collapse postulate" or "wave-packet reduction postulate" is merely a matter of taste without any physical consequences.
But that's highly problematic. I'd not use collapse or state reduction (it doesn't change when you simply rename it). The important point in connection with entanglement between observables is that the "state collapse/reduction" is NOT caused by an action at a distance. It's just he adjustment of the probability description after the measurement.

Take two polarization entangled photons, i.e., the state
$$|\Psi \rangle=\frac{1}{2} [\hat{a}_H^{\dagger}(\vec{p}_1) \hat{a}_V^{\dagger}(\vec{p}_2)-\hat{a}_{H}^{\dagger}(\vec{p}_2) \hat{a}_{V}^{\dagger}(\vec{p}_1)]|\Omega \rangle.$$
Then you can put two detectors measuring the polarization (using a polarizing beam splitter) at the appropriately far distant places A and B (determined by the direction given by the photon momenta $\vec{p}_1$ and $\vec{p}_2$). These places A and B may be 1 light-minute away from each other.

Now A and B will simply measure a stream of completely unpolarized photons.

If now A finds one of her photons in the polarization state $H$, she immediately knows that B's photon must be in the polarization state $V$. For sure, however nothing has happened to Bob's photon (at least not before any signal could have travelled from A to B, which takes at least 1 minute). According to QED, which is microcausal, A's photon detection is guarganteed to be a local measurement NOT affecting B's photon by some spooky action at a distance. Thus the update,
$$|\Psi \rangle \rightarrow |\Psi_{\text{A's photon is H-polarized}}' \rangle=\frac{1}{\sqrt{2}} \hat{a}_H^{\dagger}(\vec{p}_1) \hat{a}_V^{\dagger}(\vec{p}_2) |\Omega \rangle$$
is just the description of the partial ensemble, for which $A$ finds her photon to be $H$ polarized, no more no less. For that subensemble of course B's photon is determined to be V-polarized, but that's not due to A's measurement but simply because the two-photon state was prepared to be in the (maximally entangled) Bell state. There's no spooky action at a distance whatsoever and thus no collapse. The above "state reduction" is possible due to local measurements on A's photon and is completely epistemic. The only thing it says is that B will measure with certainty his photon to be V polarized, when one only looks at photons for which A has measured her photon to be H polarized.

It doesn't even matter whether A measures here photon's polarization before or after B measures that of his photon. The two measurements can also take place as space-like separated "click events" of A's and B's photodetectors. Thus indeed A's and B's measurements of their photons' polarization cannot be the cause for the other's measurement outcome.

Also A cannot send a faster-than light signal to B, because she can in no way control, which polarization her photon will have before the measurement is actuatlly done. To the contrary due to the state preparation of the entangled pair A's as well as B's photons are completely unpolarized (it's likely to be the most accurate way to produce a source of completely unpolarized single-photon beams ever). All A and B measure is a random squence of polarizations with probabilities 50:50 for either outcome.

Finaly one can think of this most straightforward Bell test as done in the way that A's and B's detectors simply store their polarization-measurement result with accurate time stamps independently on some storage, and then A and B share their information (which they never can do with faster-than-light signals whatsoever) and then one can evaluate the correlations between the photons coming always from one of the entangled pairs, and then they can postselect all photons for which A has measured H-polarization and check that for this subsenemble (which is half as large as the full ensemble of course) B has with certainty found his photon to be V-polarized.

Note that such delayed-choice experiments are for quite some time no longer gedanken experiments but realized with utmost precision in the quantum-optics labs around the world, always with the result that QED is accurate in predicting the probabilities as well as the strong correlations.

One can with the same setup, but measuring certain combinations of polarizations in different directions at A and B, also demonstrate that Bell's inequality is violated precisely as predicted by QT, excluding the validity of any local deterministic hidden-variable theory.

#### Lord Jestocost

Gold Member
2018 Award
But that's highly problematic.
That’s not problematic in case one accepts quantum non-separability. Franck Laloë, in “Do We Really Understand Quantum Mechanics?”:

The idea is that different quantum systems, when they have interacted in the past, no longer have in general their own physical properties; they are both part of a larger system, which is the only one possessing physical properties. One should then not try to separate (conceptually) the whole system into two smaller physical systems and attribute them properties; the whole system is non-separable.

#### vanhees71

Gold Member
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

Mentor
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

Gold Member
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

Gold Member
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

Gold Member
2018 Award
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

Gold Member
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

Gold Member
2018 Award
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

Gold Member
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

Gold Member
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.

Mentor

#### fresh_42

Mentor
2018 Award
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.

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