Wheeler's delayed choice doesn't change the past

[vanhees71]

What's your view of the Wheeler-Feynman absorber theory?

I am not a fan of that or the transactional interpretation, but that scientifically means diddly squat. As usual I agree with you but I don't think the situation is quite as clear cut as you said - or maybe its is - but you can best explain what you mean because I think its a bit context dependent ie we can't send information backward in time - that would be exactly as you say.

Thanks
Bill

The Wheeler-Feynman absorber theory is not acausal, but it's classical. In one of Feynman's semi-autobiographical books (very amusing to read) Feynman tells about it that Wheeler promised the quantum theory, but this was too optimistic. So it's an interesting point of view on classical electrodynamics but a dead end of research towards a reformulation of QED.

 

[Lord Jestocost]

I did however understand what Einstein said - do you think Einstein understood him?

Tagore points out that it is impossible to think “your consciousness” away from reality: “…it is a relative world, depending for its reality upon our consciousness“. Thus, Einstein has finally to admit that his conception of „the world as a reality independent of the human factor“ is nothing but a religion, without any scientific basis.

 

[RUTA]

Keep in mind what we’re trying to explain is the correlation b/w spacelike separated events that violates Bell’s inequality, not the simple fact that the Source emission event resides in their mutual causal past lightcones. The violation of Bell’s inequality says simply that you cannot explain that correlation in strictly forward-time-evolved and timelike fashion. That is generally what people mean by “causal,” although you may have a different definition in which case the discussion is strictly semantics. If you’re willing to drop “timelike fashion” and allow “spacelike fashion,” then you can use superluminal forward-time-evolved causation to explain the correlation. The problem with that is SR tells us the order of spacelike separated events is frame dependent, so you either have to select a preferred frame or allow backward-time-evolved causation. But, if you’re willing to allow backward-time-evolved causation (aka retrocausation), then you can still restrict your time-evolved explanation to the past lightcones of the detection events (“timelike fashion”) anyway. So, you see that forward-time-evolved causation is out unless you want a preferred frame. The only way to maintain time-evolved causation is retrocausation. That’s why some have chosen another route, i.e., an adynamical block universe explanation. Accordingly, the most fundamental explanation for the correlation is a spatiotemporally global rule that, in this case, is just not amenable to forward-time-evolved and timelike storytelling (dynamism). It’s a very powerful explanatory method (as we show in our book) and quite consistent with the Lagrangian formalism of physics, i.e., all current physics is perfectly reasonable in the “God’s-eye view,” as Wilczek calls it. It’s our anthropocentric “ant’s-eye” bias that keeps us from using the more powerful adynamical (“acausal” as defined above) explanation in conjunction with our very successful physics.

 

[bhobba]

Thus, Einstein has finally to admit that his conception of „the world as a reality independent of the human factor“ is nothing but a religion, without any scientific basis.

Its true - I don't think Einstein finally had to admit that - its almost trivially obvious. I get him saying that - that's not the issue. Einstein on many occasions admitted to being quite religious - believing in the god of Spinoza - he was quite open about it. But before jumping to conclusions look up what the god of Spinoza is if you don't already know..

That said things like the below leave me cold:
'When our universe is in harmony with man, the eternal, we know it as truth, we feel it as beauty'

Being nice I would say I don't understand it - but really for me I think the above is mystical nonsense. I think Einstein may have felt the same - but of course we will never know. I get the 'feeling' he was having a bit of 'fun' with this guy - just a feeling - we will never know. Einstein nearly always, unlike some contemporaries like Bohr generally expressed himself clearly - not always correctly - but clearly. This guy did not do that which I think would have irked Einstein. But, as I said, we will never know.

Thanks
Bill

 

[bhobba]

Keep in mind what we’re trying to explain is the correlation b/w spacelike separated events that violates Bell’s inequality, not the simple fact that the Source emission event resides in their mutual causal past lightcones. The violation of Bell’s inequality says simply that you cannot explain that correlation in strictly forward-time-evolved and timelike fashion.

I think I will have to read your book (of which I will get a copy when released) to understand what you are saying.

Just a few points:
1. Right at the very foundations of ordinary QM is the Galilean transformations. So non-locality is built in from the start. You have to go to QFT to discuss locality and it is embodied in the cluster decomposition property:
https://www.physicsforums.com/threads/cluster-decomposition-in-qft.547574/

2. Note the exclusion in that property - correlations. EPR is just a correlation so IMHO is no mystery at all - the key thing is if you want to have CFD you need non-locality. Don't care about CFD - then no issue - we just have some different kind of statistical properties to those correlations than classically. That's hardly surprising because we also know QM as a generalized probability model is different to ordinary probability - ordinary probability is the simplest - QM the next simplest.

3. Regarding delayed choice QM explains it just fine:
http://quantum.phys.cmu.edu/CQT/chaps/cqt20.pdf

Thanks
Bill

 

[vanhees71]

Keep in mind what we’re trying to explain is the correlation b/w spacelike separated events that violates Bell’s inequality, not the simple fact that the Source emission event resides in their mutual causal past lightcones. The violation of Bell’s inequality says simply that you cannot explain that correlation in strictly forward-time-evolved and timelike fashion. That is generally what people mean by “causal,” although you may have a different definition in which case the discussion is strictly semantics. If you’re willing to drop “timelike fashion” and allow “spacelike fashion,” then you can use superluminal forward-time-evolved causation to explain the correlation. The problem with that is SR tells us the order of spacelike separated events is frame dependent, so you either have to select a preferred frame or allow backward-time-evolved causation. But, if you’re willing to allow backward-time-evolved causation (aka retrocausation), then you can still restrict your time-evolved explanation to the past lightcones of the detection events (“timelike fashion”) anyway. So, you see that forward-time-evolved causation is out unless you want a preferred frame. The only way to maintain time-evolved causation is retrocausation. That’s why some have chosen another route, i.e., an adynamical block universe explanation. Accordingly, the most fundamental explanation for the correlation is a spatiotemporally global rule that, in this case, is just not amenable to forward-time-evolved and timelike storytelling (dynamism). It’s a very powerful explanatory method (as we show in our book) and quite consistent with the Lagrangian formalism of physics, i.e., all current physics is perfectly reasonable in the “God’s-eye view,” as Wilczek calls it. It’s our anthropocentric “ant’s-eye” bias that keeps us from using the more powerful adynamical (“acausal” as defined above) explanation in conjunction with our very successful physics.

But in standard QED there are no causally connected spacelike separated events possible, and this is by construction. I don't see any necessity to invent new theories, where the well-established relativistic spacetime and causality structure is violated. The correlations are indeed there due to the preparation of the polarization-entangled biphoton state, and there's no assumption about causal effects between the later measurements necessary. This becomes the more clear if you consider a real postselection setup, which is an extreme way of a delayed-choice experiment. Take Walborn's realization of the quantum eraser. You can do this experiment also by just making a measurement protocol of both detectors, using time marks of registration events. In the eraser setup you can then erase the which-way-information long after all photons are gone by just bringing both measurement protocols together and only select the signal photons for which also the idler has gone through the polarizer (oriented parallel to the one or the other quarter-wave plate in the slits), i.e., you can erase the WWI information even after the measurement is irreversibly fixed in the measurement protocols.

I guess, to understand what you are after with your alternative interpretation, I'd have to read your book.

 

[RUTA]

I think I will have to read your book (of which I will get a copy when released) to understand what you are saying.

Just a few points:
1. Right at the very foundations of ordinary QM is the Galilean transformations. So non-locality is built in from the start. You have to go to QFT to discuss locality and it is embodied in the cluster decomposition property:
https://www.physicsforums.com/threads/cluster-decomposition-in-qft.547574/

2. Note the exclusion in that property - correlations. EPR is just a correlation so IMHO is no mystery at all - the key thing is if you want to have CFD you need non-locality. Don't care about CFD - then no issue - we just have some different kind of statistical properties to those correlations than classically. That's hardly surprising because we also know QM as a generalized probability model is different to ordinary probability - ordinary probability is the simplest - QM the next simplest.

3. Regarding delayed choice QM explains it just fine:
http://quantum.phys.cmu.edu/CQT/chaps/cqt20.pdf

Thanks
Bill

Regarding point 1, here is an excerpt from our book:

Before continuing, we should point out that using a block universe interpretation of QM is supported by more than the Feynman path integral. (kaiser, BohrUlfbeck, anandan) all showed independently that the non-commutivity of the position and momentum operators in QM follows from the non-commutivity of the Lorentz boosts and spatial translations in SR, i.e., the relativity of simultaneity. Per Kaiser:
begin{quote}
For had we begun with Newtonian spacetime, we would have the Galilean group instead of [the restricted Poincar{\'e} group]. Since Galilean boosts commute with spatial translations (time being absolute), the brackets between the corresponding generators vanish, hence no canonical commutation relations (CCR)! In the [c $\rightarrow\infty$ limit of the Poincar{\'e} algebra], \textit{the CCR are a remnant of relativistic invariance where, due to the nonabsolute nature of simultaneity, spatial translations do not commute with pure Lorentz transformations} \cite[p. 706]{kaiser}. [Italics in original].
end{quote}

title ={Phase-space approach to relativistic quantum mechanics III: Quantization, relativity, localization and gauge freedom},
author ={Kaiser, G.},
journal ={Journal of Mathematical Physics},
volume ={22},
page ={705--714},
year ={1981}

title ={Laws, Symmetries, and Reality},
author ={Anandan, J.},
journal ={International Journal of Theoretical Physics},
volume ={42},
pages ={1943--1955},
year ={2003},
url ={https://arxiv.org/abs/quant-ph/0304109}

title ={Primary manifestation of symmetry. Origin of quantal indeterminacy},
author ={Bohr, A., and Ulfbeck, O.},
journal ={Reviews of Modern Physics},
volume ={67},
number ={},
pages ={1--35},
year ={1995},

I’m in a hurry to make a meeting, so I don’t have time to clean up the LaTeX more than that, but I know you’re interested in math, so I wanted to get that to you.

 

[vanhees71]

Regarding point 1, here is an excerpt from our book:

Before continuing, we should point out that using a block universe interpretation of QM is supported by more than the Feynman path integral. (kaiser, BohrUlfbeck, anandan) all showed independently that the non-commutivity of the position and momentum operators in QM follows from the non-commutivity of the Lorentz boosts and spatial translations in SR, i.e., the relativity of simultaneity. Per Kaiser:
begin{quote}
For had we begun with Newtonian spacetime, we would have the Galilean group instead of [the restricted Poincar{\'e} group]. Since Galilean boosts commute with spatial translations (time being absolute), the brackets between the corresponding generators vanish, hence no canonical commutation relations (CCR)! In the [c $\rightarrow\infty$ limit of the Poincar{\'e} algebra], \textit{the CCR are a remnant of relativistic invariance where, due to the nonabsolute nature of simultaneity, spatial translations do not commute with pure Lorentz transformations} \cite[p. 706]{kaiser}. [Italics in original].
end{quote}

Well, you can in full generality get position observables only for all kinds of massive particles or massless particles with spin $\leq 1/2$. Of course, you are right concerning the commutation relations of position and momentum, if position observables exist, because they are simply the Lie algebra of spatial translations, which are both part of the continuous parts of the Poincare (proper orthochronous Poincare) group and the Galilei group.

For non-relativistic particles, the massless case doesn't exist and thus you have always a position operator. The main difference between Poincare and Galilei group is the notion of mass: In the Poincare group it's defined as a Casimir operator via $p \cdot p=m^2$ (in natural units, $\hbar=c=1$) and for the Galilei group it's a non-trivial central charge of the corresponding Lie algebra. The case $m=0$ doesn't lead to physically sensible dynamics, as shown for the first time in the famous paper by Inönü and Wigner [1]. That's why the naive commutation relations between boosts and momentum operators are not applying, but you have the mass as a central charge, i.e., you have to lift the 10D classical Galilei group to the 11D quantum Galilei group, and then the commutators read
$$[\hat{K}_i,\hat{P}_j]=\mathrm{i} \delta_{ij} \mathbb{1}.$$
For a detailed analysis, see my QM manuscript (which however is in German)

http://theory.gsi.de/~vanhees/faq/quant/node74.html

or the textbook by Ballentine, which gives the same derivation in a bit different form.

[1] E. Inönü and E. P. Wigner, Representations of the Galilei group, Il Nuovo Cimento, 9 (1952), p. 705–718.
http://dx.doi.org/10.1007/BF02782239

 

[bhobba]

Regarding point 1

Its over my head - I will wait for the book.

But Ballentine showed, chapter 3, using Galilean transformations (see page 66) the dynamics of standard QM follow eg the form of the Schrodinger equation. This is hardly surprising because from the the path integral approach the PLA immediately follows and Landau - in Mechanics - showed exactly the same thing. Take the Hamiltonian used in hydrogen atom. Move the object and the force changes instantaneously. IMHO its part of the reason standard QM fails with things like spontaneous emission - its basis is wrong - or at least just approximate. Of course for many purposes an excellent approximation like classical mechanics itself - but still fundamentally wrong.

Thanks
Bill

 

[vanhees71]

Of course. @RUTA 's statement in #77 doesn't apply, because in QT a symmetry isn't realized by proper unitary transformations of the symmetry group but by ray representations, and the Galilei group has non-trivial ray representations, which are precisely the physically relevant ones, and the proper unitary representations do not lead to a physically interpretable particle dynamics (see my previous posting). You can of course always lift any ray representation of the symmetry group to unitary representation of a central extension of the original group, and that's precisely the 11 dimensional quantum Galilei group. The latter interpretation of Galilei invariance leads to the mass superselection rule of non-relativistic QT. If I remember right, Ballentine's book has all this in great detail.

 

[bhobba]

Of course. @RUTA If I remember right, Ballentine's book has all this in great detail.

You are right - it does. And I even got the book, found the key page and double checked. Commutation relation etc - the lot - follows from the Galilean transformations. One must go to QFT for locality to be a worry.

Of course it in no way falsifies, changes, or in anyway alters Bell - just adds a slight twist in interpretation.

Thanks
Bill

 

[vanhees71]

Sure, all the Bell tests today are completely understandable using standard QFT (mostly QED since the experiments are usually using photons). I don't see, what one should seek for an acausal theory as long as we can explain everything with the standard theory!

 

[Stephen Tashi]

Now let's stay on topic with this thread.

That would be a miracle. As far as topics go, the thread seems to be in a superposition.

In the delayed choice experiment on the surface it looks like something really weird is going on - but when analysed carefully that isn't he case:
http://quantum.phys.cmu.edu/CQT/chaps/cqt20.pdf

That's one chapter of an entire book http://quantum.phys.cmu.edu/CQT, which, in previous chapters develops an approach to QM not found in all textbooks. I found the review of the book by Faris http://math.arizona.edu/~faris/consis.pdf helpful.

So (on the topic of that book) there are at least two questions. 1) Is the general method advocated in the book good, correct, nice etc ? and 2) Does the general method advocated in the book successfully resolve the paradoxes it considers in chapter 20 ?

 

[bhobba]

Is the general method advocated in the book good, correct, nice etc

Yes - it resolves all known supposed paradoxes.

But if that isn't to your taste ie its within a specific interpretation, there are tons of papers explaining what's going on. This one concentrates on the so called separation fallacy:
http://philsci-archive.pitt.edu/10216/1/SeparationFallacy-rev.pdf
We have seen the same fallacy of interpretation in two-slit experiments, which-way interferometer experiments, polarization analyzers, and Stern-Gerlach experiments. The common element in all the cases is that there is some separation apparatus that puts a particle into a certain superposition of spatially-entangled eigenstates in such a manner that when an appropriately spatially-positioned detector induces a collapse to an eigenstate, then the detector will only register one of the eigenstates. The separation fallacy is that this is misinterpreted as showing that the particle was already in that eigenstate in that position as a result of the previous "separation." In fact the superposition evolves until some distinction is made that constitutes a measurement, and only then is the state reduced to an eigenstate. In general, when a measurement shows a specific eigenstate, it should not be assumed that the quantum system was already in that eigenstate (e.g., "goes through upper slit/arm or through lower slit/arm") prior to the measurement. The quantum erasers are more elaborate versions of these simpler experiments, and a similar separation fallacy arises in that context.

Thanks
Bill

 

[RUTA]

But in standard QED there are no causally connected spacelike separated events possible, and this is by construction. I don't see any necessity to invent new theories, where the well-established relativistic spacetime and causality structure is violated. The correlations are indeed there due to the preparation of the polarization-entangled biphoton state, and there's no assumption about causal effects between the later measurements necessary. This becomes the more clear if you consider a real postselection setup, which is an extreme way of a delayed-choice experiment. Take Walborn's realization of the quantum eraser. You can do this experiment also by just making a measurement protocol of both detectors, using time marks of registration events. In the eraser setup you can then erase the which-way-information long after all photons are gone by just bringing both measurement protocols together and only select the signal photons for which also the idler has gone through the polarizer (oriented parallel to the one or the other quarter-wave plate in the slits), i.e., you can erase the WWI information even after the measurement is irreversibly fixed in the measurement protocols.

I guess, to understand what you are after with your alternative interpretation, I'd have to read your book.

The analysis of a standard Bell-inequality-violating experiment requires only simple Hilbert space analysis (see Dehlinger, D., and Mitchell, M.W.: Entangled photons, nonlocality, and Bell inequalities in the undergraduate laboratory. American Journal of Physics 70(9), 903--910 (2002), for example), which must follow from any QFT in the proper limit (by definition). And, there is nothing wrong with the physics that fits the data so well in these experiments; it's not an issue with the physics. So, there is nothing in the formalism of QFT that resolves the ontological mysteries of quantum nonlocality -- you have to develop an ontology to resolve ontological mysteries.

 

[RUTA]

Of course. @RUTA 's statement in #77 doesn't apply, because in QT a symmetry isn't realized by proper unitary transformations of the symmetry group but by ray representations, and the Galilei group has non-trivial ray representations, which are precisely the physically relevant ones, and the proper unitary representations do not lead to a physically interpretable particle dynamics (see my previous posting). You can of course always lift any ray representation of the symmetry group to unitary representation of a central extension of the original group, and that's precisely the 11 dimensional quantum Galilei group. The latter interpretation of Galilei invariance leads to the mass superselection rule of non-relativistic QT. If I remember right, Ballentine's book has all this in great detail.

The statement follows from the results of the cited papers (all of which I have checked). Nothing you've said here refutes that work.

 

[Stephen Tashi]

there are tons of papers explaining what's going on. This one concentrates on the so called separation fallacy:
http://philsci-archive.pitt.edu/10216/1/SeparationFallacy-rev.pdf

If paper P1 explains paradox W one way and paper P2 explains paradox W a different way then has paradox W really been explained?

The paper linked above offers a conceptually simple explanation -given the we accept the idea of a Quantum state as simple. By contrast, what is Griffiths' consistent histories approach trying to explain?

The above paper deals with the fallacy of assuming that a device which creates a superposition of states also separates particles into being in one state or the other. It doesn't deal with a paradox claiming that the evolution of superposed quantum states after the particle exits the device is different or evolves differently in time depending on the event of a detector being used at a later time. Is that what Griffths is explaining?

 

[RUTA]

Sure, all the Bell tests today are completely understandable using standard QFT (mostly QED since the experiments are usually using photons). I don't see, what one should seek for an acausal theory as long as we can explain everything with the standard theory!

Again, there is nothing in the quantum physics that needs to be changed (as we explain in our book, QM and QFT don't need to be replaced, they work very well). The entangled states of the Hilbert space formalism that can be used so effectively to fit the data in Bell-inequality-violating experiments follow from QFT in the proper limit. There is no mystery in that, the physics works. On the flip side, the physics cannot resolve the ontological mysteries associated with quantum nonlocality. I use Zeilinger's delayed choice experiment to show students the mystery of quantum entanglement precisely because it requires no formalism to understand (Anton Zeilinger, ``Why the quantum? `It' from `bit'? A participatory universe? Three far-reaching challenges from John Archibald Wheeler and their relation to experiment,'' in Science and Ultimate Reality: Quantum Theory, Cosmology and Complexity, John D. Barrow, Paul C.W. Davies and Charles L. Harper, Jr. (eds.), (Cambridge Univ Press, Cambridge, 2004), pp 201-220). In that case, it is obvious that the measurement setting the experimentalist controls is associated with the later measurement outcome. So, could the experimentalist's (delayed) choice have been otherwise? That's the mystery. No formalism dispels that mystery (on the contrary, the formalism predicts the outcomes that generate it!). The mystery has to do with ontology and ontology is always underdetermined by the formalism. Thus, more sophisticated experiments have not reduced the number of QM interpretations, indeed they have proliferated. Everyone picks their personal favorite and continues doing physics accordingly, i.e., everyone is using the same formalism and successfully fitting the same data, but offering different explanations afterwards. That's fine for the status quo, the book argues that to advance physics from the status quo (e.g., obtain quantum gravity and explain dark energy and dark matter data) we should consider moving to the "God's-eye view" per Wilczek. It's that simple.

 

[bhobba]

If paper P1 explains paradox W one way and paper P2 explains paradox W a different way then has paradox W really been explained?

Why would it not. We can use Newtons laws or the PLA to solve mechanics problems. Is that an issue as well?

Griffiths concentrates on Histories which is what Consistent Histories is all about. The other doesn't - but I think on separability. Many ways to skin a cat. Griffiths is the more complex answer because of that:
Let us summarize the results obtained by using a quantum coin and studying various consistent families related to the counterfactual statement of the delayed choice paradox. We have looked at three different frameworks, (20.22), (20.23), and (20.24), and found that they give somewhat different answers to the question of what would have happened if the beam splitter had been left in place, when what actually happened was that the photon was detected in E with the beam splitter out. (Such a multiplicity of answers is typical of quantum and—to a lesser degree—classical stochastic counterfactual questions; see Sec. 19.4.) In the end, none of the frameworks supports the original paradox, but each framework evades it for a somewhat different reason. Thus (20.22) does not have photon states localized in the arms of the interferometer, (20.23) has such states, but they cannot be used as a pivot for the counterfactual argument, and remedying this last problem by using (20.24) results in the counterfactual question being answered in terms of MQS states, which were certainly not in view in the original statement of the paradox.

Sometimes Newtons laws make mechanics problems easier to solve - sometime the PLA. Here Consistent Histories is the more complex actually giving slightly different solutions depending on the framework chosen.

Just goes to show it's probably a good idea to understand a number of different interpretations.

Thanks
Bill

 

[zonde]

The problem with that is SR tells us the order of spacelike separated events is frame dependent

You attach physical meaning to convention. You can not blame reality for not respecting particular set of conventions.

 

[zonde]

But in standard QED there are no causally connected spacelike separated events possible, and this is by construction.

How can QED say this is impossible when QED does not speak about individual events? Or I am missing something? In QT's minimal interpretation as given by Ballentine statistical nature of predictions is clearly stated. Is it somehow different in QED? Can QED go beyond that statistical nature and say something about individual realizations of experiment (except in some special cases where predicted probabilities approach 1)?

 

[vanhees71]

The commutator of local observables vanishes at space-like separated arguments identically. That's why actions at a distance (i.e., over space-like distances in Minkowski space) have probability 0, i.e., they don't happen with certainty!

 

[vanhees71]

The mystery has to do with ontology and ontology is always underdetermined by the formalism. Thus, more sophisticated experiments have not reduced the number of QM interpretations, indeed they have proliferated. Everyone picks their personal favorite and continues doing physics accordingly, i.e., everyone is using the same formalism and successfully fitting the same data, but offering different explanations afterwards. That's fine for the status quo, the book argues that to advance physics from the status quo (e.g., obtain quantum gravity and explain dark energy and dark matter data) we should consider moving to the "God's-eye view" per Wilczek. It's that simple.

For me the only mystery is why this is considered a mystery since the formalism tells us precisely what to expect, and that's what's empirically confirmed with high significance. You cannot expect more from any natural-science theory. It's about observable objective facts not some mysterious "ontology behind the phenomena".

 

[vanhees71]

The statement follows from the results of the cited papers (all of which I have checked). Nothing you've said here refutes that work.

The statement that the generators of boosts commute with the momentum operators (the generators of spatial translations) is wrong within standard non-relativistic QM, as shown already by Inönü and Wigner. For non-relativistic particles a vanishing mass doesn't make sense, because it doesn't lead to a physically interpretable quantum dynamics. For non-0 mass the commutator is
$$[\hat{K}_j,\hat{p}_k]=\mathrm{i} \delta_{jk} m \hat{1} \neq 0.$$
The group-representation theoretical reasons for this are quite obvious (see posting #78).

 

[Lord Jestocost]

The correlations are indeed there due to the preparation of the polarization-entangled biphoton state, and there's no assumption about causal effects between the later measurements necessary.

Maybe, as an experimental physicist, I don’t get the point when reasoning about “due to the preparation“. Does that mean that each entangled photon pair emerges from the source with, in effect, a set of pre-programmed, no longer modifiable, instructions for how each photon of the pair has to respond to a measurement on one of its observables?

 

[vanhees71]

Indeed, "due to preparation" means that the photon pairs have been created (today usually using parametric downconversion) in the polarization-entangled state. With appropriate "optical elements" you can produce the needed Bell states. I don't know, what you mean by "pre-programmed". The meaning of the state is very clearly given by Born's rule, and I don't know what you mean by "no longer modifiable". Of course, when the photons interact with something later, the state is modified. E.g., when the photon hits a detector usually it's absorbed and thus you have a state with this photon gone. So the state clearly changes through interactions. There are also no "instructions for how each photon of the pair has to respond to a measurement on one of its observables". The state of the single photon in the entangled pair is given according to the standard rules of QT by tracing out the other photon, i.e.,
$$\hat{\rho}_A=\mathrm{Tr}_{B} \hat{\rho}_{AB}=\sum_{j} |ij\rangle \langle ij |\hat{\rho}_{AB}|\langle kj \rangle \langle kj|.$$
For the "singlet state" you have
$$\hat{\rho}_{AB}=|\Psi \rangle \langle \Psi|, \quad |\Psi \rangle=\frac{1}{\sqrt{2}} (|HV \rangle-|VH \rangle),$$
and
$$\hat{\rho}_A=\frac{1}{2} \hat{1}.$$
I.e., the single photons in the entangled pair each are totally unpolarized, i.e., a polarizer in any direction let's the photon through with probability 1/2 and absorbs it with probability 1/2. There's nothing more known about the single photons than that.

 

[RUTA]

For me the only mystery is why this is considered a mystery since the formalism tells us precisely what to expect, and that's what's empirically confirmed with high significance. You cannot expect more from any natural-science theory. It's about observable objective facts not some mysterious "ontology behind the phenomena".

Do you understand why those who do wonder about the nature of reality (ontology) are mystified? Or is that why you're participating in such threads?

 

[RUTA]

The statement that the generators of boosts commute with the momentum operators (the generators of spatial translations) is wrong within standard non-relativistic QM, as shown already by Inönü and Wigner. For non-relativistic particles a vanishing mass doesn't make sense, because it doesn't lead to a physically interpretable quantum dynamics. For non-0 mass the commutator is
$$[\hat{K}_j,\hat{p}_k]=\mathrm{i} \delta_{jk} m \hat{1} \neq 0.$$
The group-representation theoretical reasons for this are quite obvious (see posting #78).

You're making the point for me. Clearly "Galilean boosts commute with spatial translations (time being absolute)" so in order to get the proper commutation relationship for QM, you need boosts that don't commute with spatial translations and for that you have to go to the Poincare group. That's Kaiser's point and yours.

 

[RUTA]

You attach physical meaning to convention. You can not blame reality for not respecting particular set of conventions.

The statement you quoted here is true and doesn't entail any interpretation. What you choose to believe ontologically about it can and does differ between different interpretations of QM.

 

[vanhees71]

You're making the point for me. Clearly "Galilean boosts commute with spatial translations (time being absolute)" so in order to get the proper commutation relationship for QM, you need boosts that don't commute with spatial translations and for that you have to go to the Poincare group. That's Kaiser's point and yours.

You don't need the Poincare group to get this result. You can get it from a careful analysis of the unitary ray representations of the Galilei group. Of course, it's equivalent to use the appropriate "deformation" of the Poincare group. I guess, I simply misunderstood your statement, and I think all in all we agree in this matter :-).

 

[vanhees71]

The statement you quoted here is true and doesn't entail any interpretation. What you choose to believe ontologically about it can and does differ between different interpretations of QM.

Of course, it's of high interest to understand all aspects of QT, including also the philosophical ones. It took me years to come to the conclusion that the "mystic approach" is simply due to an old-fashioned philosophical balast of the founding fathers, mostly Bohr and Heisenberg. Just taking the Born rule seriously for me resolves all problems, particularly the EPR problem (although the EPR paper is not reflecting Einstein's philosophical quibbles with QT very well; he's written a much clearer paper (Dialectica article in German):

http://onlinelibrary.wiley.com/doi/10.1111/j.1746-8361.1948.tb00704.x/abstract

I wonder, why this is still not accepted by many physicists and why modern textbooks still teach the confusing collapse hypothesis of some Copenhagen-interpretation flavors (admittedly that's not Bohr's fault, who, AFAIK, didn't include collapse in his version of the Copenhagen interpretation).

 

[Lord Jestocost]

It took me years to come to the conclusion that the "mystic approach" is simply due to an old-fashioned philosophical balast of the founding fathers, mostly Bohr and Heisenberg.

What the heck is that "mystic approach"? "Mystic" related to what? Related to 19th century physics? Related to some not pronounced implicit assumptions about "reality" which might merely based on your personal psychological predispositions? In case you don’t grasp Bohr’s and Heisenberg’s thinking, that’s your problem, but don’t coin the term “mystic” as a “killer argument”. Science should be based on arguments.

Listen, what Wheeler writes about Bohr:

“You tell me what isn’t the plan of physics, our friend rejoins. If you understand quantum mechanics so well, why don’t you tell we what is the plan of physics?
No one knows, we reply. We have clues, clues most of all in the writings of Bohr [23-25], but no answer. That he did not propose an answer, not philosophize, not go an inch beyond the soundest fullest statement of the inescapable lessons of quantum mechanics, was his way to build a clean pier for some later day’s bridge to the future.”

J. A. Wheeler in „Quantum Theory and Measurement“ (edited by John Archibald Wheeler and Wojciech Hubert Zurek), Princeton, New Jersey 1983, page 201

 

[bhobba]

What the heck is that "mystic approach"? "Mystic" related to what?

Related to someone like Dirac and even Einstein. Einstein and Bohr were good friends but in QM their approaches were different. Einstein would accuse Bohr of being 'mystical' - it was two different approaches. Actually Bohr wasn't too bad - he was more agnostic to it than a believer - others like Pauli, yes the great and rational Pauli, were worse. Worse still was the great Von-Neumann and Wigner that advocated the so called Princeton school based on the, it turns out to be wrong, but for reasons not known at the time, Von-Neumann's argument consciousness caused collapse:

See the attached document

As the above argues, correctly IMHO, even today we are still feeling the effects of those early years and it has been detrimental to understanding QM, especially at the beginner and even intermediate level. Fortunately books like Ballentine banish it, not that I agree 100% with Ballentine on every issue, but it is way more rational than others.

Thanks
Bill

 

[bhobba]

why don’t you tell we what is the plan of physics?

I am surprised his old student and friend, Feynman, didn't see the fallacy of that - first you have to show physics has a plan.

Thanks
Bill

 

[.Scott]

I believe Hawking is exaggerating in telling us (page 83):
«...the Universe doesn't have just a single history, but every possible history, each with its own probability; and our observations of the current state affect its past and determine the different histories of the Universe, just as the observations of the particles in the double slit experiment affect the particles' past.»
Do you agree?

I may come closer to agreeing with this than most. If you follow the Multi World Interpretation (MWI) - in the sense that the universe is continuously spawning slightly different copies of itself that do not further interact, then I have no counter argument for you. Otherwise, consider this:

The first problem with "changing the past" is the notion of choosing an observation - acting independently of "predestination". Let's say that we have two copies of the universe, one where you (Alice) observe something at noon, the other where you observe something different at noon. Would it be safe to say that these universes were the same up until noon? That's certainly what we had in mind when we set up the experiment (notwithstanding that this experiment cannot actually be set up).

But when we test this in the only ways we can, we sometimes find a violation of the Bell inequality. Alice makes measurements "randomly", and when her choices and results are compared to those of Bobs, it appears that Alice mucked with the past. Of course, the results are symmetric, Alice can accuse Bob of mucking with the past as well.

But since we apparently only end up with a single copy of the past, whose to say that it has changed? And we have an equal problem with the future. Just as we can't duplicate the universe, we can't really inject information into it from "outside" either.

So if we allow ourselves to claim that either the present or the future are not specific, why should we treat the past any differently?

To a large extent, this is a matter of semantics. We can all agree on what restrictions QM places on experimental results. But when we go from the Math to the English language, we discover that English has ambiguities that are well tolerated in common affairs - but work poorly in describing the Physics.

Let me describe the Bell Inequality by moving "cause" into the future. Remember that the only purpose of this is to change the English semantics describing the QM results. I am not claiming that this is a "better" description - only a consistent one:

Both Alice and Bob make their measurement choices of the entangled particle independently. Since Alice did this based on a telescope facing the Northern sky and Bob did it based on the Southern sky, we presume that no information has yet been exchanged between them. But we will allow for an indefinite present, so Alice may not have measured the result, and if she did, it may be + or -. Similarly, Bob's may be any of the three as well. When they compare results, certain Alice/Bob combinations are inconsistent with QM and will not persist. They can be made to violate Bell's inequality without any FTL issue.