A question regarding the Copenhagen interpretation.

[atyy]

If QM is non local is very interpretation dependent - that's the import of Bells Theorem and Einsteins error. QM rules out naive-reality ie local realism. If you reject realism (ie properties do not exist independent of observation) then locality is saved. If you keep it then locality is gone. But SR is still saved since it can't be used to send information which is what's required to sync clocks.

Basically all Bell type 'experiments' are doing is observing systems with spatial extent, and because of how its arranged if one thing in the system has a property on observation, so does the other thing - but they are spatially separated.

I have two pieces of paper, one black, and one white and put them in envelopes. I randomly send one to one person, and another to a different person. If any of those people open their envelope they immediately know what the other person will get when they open their envelope. Their is nothing Earth shattering going on. Same with Bell type experiments, with the twist we can't say it has the property of blackness or whiteness until observation.

Griffiths book - Consistent Quantum Theory discusses it from this interesting perspective:
https://www.amazon.com/dp/0521539293/?tag=pfamazon01-20

Thanks
Bill

I think Griffiths's book indicates that the nonrealism has to be much stronger than just not having the properties before measurement. The nonrealism has to assume that "reality" can be described in various incompatible ways which cannot be combined http://quantum.phys.cmu.edu/CQT/chaps/cqt27.pdf. Griffiths says his interpretation is realistic and local, but if one wants to argue that it is not realistic, that seems plausible. According to an FAQ about consistent histories, "Colored slips of paper, one red and one green, are placed in two opaque envelopes, which are then mailed to scientists in Atlanta and Boston. The scientist who opens the envelope in Atlanta and finds a red slip of paper can immediately infer, given the experimental protocol, the color of the slip of paper contained in the envelope in Boston, whether or not it has already been opened. There is nothing peculiar going on, and in particular there is no mysterious influence of one "measurement" on the other slip of paper." http://quantum.phys.cmu.edu/CHS/quest.html#EPR. Similarly, Griffths's book says that measurements reveal properties of a system before the measurement took place, and further says there is an independent reality within the consistent histories framework. http://quantum.phys.cmu.edu/CQT/chaps/cqt27.pdf. So I don't think the twist is that the cards don't have the colour before the measurement, but that reality can be described in incompatible ways.

 

[bhobba]

So I don't think the twist is that the cards don't have the colour before the measurement, but that reality can be described in incompatible ways.

In consistent histories things are analysed in terms of frameworks. One must specify a framework to analyse a particular situation and can't use incompatible ones. The other aspect is a history which is a sequence of projection operators and its only when an observation is made does a particular history become real.

If you want to see the exact detail of how Consistent Histories analyses EPR then its probably best to get the book. I have a copy and have gone through it - and in that interpretation everything works out perfectly OK - but its not an interpretation that can be discussed superficially - although I will make a stab.

I like Consistent Histories, but my issue with it is it started out as a minimalist interpretation, but turned out to be a bit more complicated.

In many ways its Many Worlds without the worlds. Because of that it has to cater to some things that is trivial in MW eg all histories occur in MW, so its trivial why a particular history happens - not so in Consistent Histories. Of course the Consistent History guys would say its just what's necessary to avoid the weirdness of MW.

I dug up the following discussing EPR in terms of Consistent Histories:
http://www.siue.edu/~evailat/pdf/qm12.pdf

'Although CH allows a realist understanding of quantum mechanics, it does not follow EPR in attributing quantum mechanically incompatible properties to a system. Griffiths gives an instructive story about what happens if one insists that P intersection Q is defined even if P and Q are incompatible. Consider a spin-half particle, and to simplify the notation, let [Z +] stand for the projector associated with Sz =1, and similarly for other projectors. Suppose now that the composite property Sz =1 intersection Sx =1 existed. Then its corresponding projector would have to project onto a subspace P of the two dimensional Hilbert space H of the spin-half particle. However, no such subspace can exist.'

Basically in EPR there are two consistent frameworks - one with +1 and -1 and the converse. Others simply do not exist so no problems can arise.

One framework corresponds to one envelope getting black and the other white, and the other framework the converse.

Thanks
Bill

 

[DevilsAvocado]

The outcomes are anti-correlated with 100% certainty - nothing else is possible. Anti-correlated means only two outcomes are possible called + and -. If one measures + the other is automatically - and conversely.

You can never get ++ or --.

This is from the definition of the experiment. There is no ifs or buts about it - it's its very definition.

One framework corresponds to one envelope getting black and the other white, and the other framework the converse.

Errrrr.

What's your point?

I have been talking about EPR type experiments as proposed by Bell, Einstein etc

What have you been talking about?

Bill, you have helped me many times and you know a lot more than I do, about different issues. However, it's only human to do mistakes – just ask me – I've done terribly embarrassing mistakes many times. But this time, all facts available clearly shows that, any confusion/mistake is not on my behalf.

My experience is also that the longer one defends an obvious mistake – the worse things get...

Therefore, as a friend, it is not in my interest to prolong this "unfortunate situation". If you don't get it this time, I'm afraid there's nothing more I can do:

https://en.wikipedia.org/wiki/Bell's_theorem]The[/PLAIN] probability of *the same result* being obtained at the two locations varies, depending on the relative angles at which the two spin measurements are made, and is strictly between *zero* and *one* for all relative angles other than perfectly parallel alignments (0° or 180°). Bell's theorem is concerned with correlations defined in terms of averages taken over very many trials of the experiment. [...] if the pairs of outcomes are always the same, the correlation is +1, no matter which same value each pair of outcomes have. If the pairs of outcomes are always opposite, the correlation is -1. Finally, if the pairs of outcomes are perfectly balanced, being 50% of the times in accordance, and 50% of the times opposite, the correlation, being an average, is 0. [...]

Measuring the spin of these entangled particles along anti-parallel directions—i.e., along the same axis but in opposite directions, the set of all results is perfectly correlated. On the other hand, if measurements are performed along parallel directions they always yield opposite results, and the set of measurements shows perfect anti-correlation. Finally, measurement at perpendicular directions has a 50% chance of matching, and the total set of measurements is uncorrelated. These basic cases are illustrated in the table below.

 

[Ken G]

All I am saying, and all the people that invoke this analogy are saying, is its not this really weird thing some seem to think it is. Its very much like the envelope analogy with the twist it doesn't have the property until observed.

Yet it seems to me that what needs to be clarified, by any classical analogy, is that there are actually two twists here, one which is already quite a bit different from our daily experience, but also another which is so different it is regarded as "spooky." The first twist is just that the envelope doesn't hold a particular color, but this has nothing to do with entanglement, it is just the concept of complementarity (a pure state in regard to one measurement is not a pure state in regard to another). So we cannot say "the envelope contains a white piece of paper and not a red one" until we know if white/black or red/blue is the measurement. That is a strange enough state of affairs, but we might still wish to imagine the envelope is thinking "if Alice measures white/black, I'll be white, and if red/blue, I'll be blue", independently with anything else that could be going on elsewhere. But entanglement ruins even that-- the envelope cannot independently know what result it will give to any given measurement, because there need to be correlations with other measurements that the envelope does not know about. These two twists are often contrasted with "Bertlmann's socks", where there's a left sock and a right sock so the envelope "knows" the measurement in question is going to be about leftness/rightness and so the envelope can know which it will produce (and it will be opposite the other envelope), even if Alice does not. But note that Bertlmann's socks fails to include either of the two types of twists I mentioned above, so sometimes it is not clear there are two completely different twists there to appreciate, not just one.

 

[stevendaryl]

The "twist" that quantum mechanics places on correlated values makes all the difference in the world, it seems to me.

In the classical situation there are two envelopes, one containing a black piece of paper, and one containing a white piece of paper. Alice gets one envelope, and Bob gets the other. When Alice opens her envelope, she immediately knows what's in Bob's envelope. This is explained as purely a change in knowledge--the envelope already was in a definite, but unknown state, and opening it just revealed information about this pre-existing state.

The difference with the quantum EPR experiment is that it is not consistent (at least not without jumping through strange hoops) to assume that each particle had a definite, but unknown state prior to measuring its spin. So an explanation purely in terms of a change knowledge doesn't seem possible.

On the other hand, an explanation in terms of causal influence doesn't seem very plausible, either, since this influence would have to propagate faster-than-light (which means back-in-time for some observers). To me, it's a really tough nut to crack. Maybe retrocausality or super-determinism would explain it, or MWI.

 

[Jilang]

This paper from Studies in History and Philosophy of Modern Physics

http://www.lophisc.org/wp-content/uploads/Price.pdf

sums up very nicely the sort of choices we are faced with for the different interpretations. The gist of it seems to be that if we are any sort of realist other than an Everettian or a Bohmian there is a choice to be made between time-asymmetric ontology and retro-causality and that this is true in QM in a manner that is not true in classical physics.

 

[Ken G]

The question I ask is, why not just conclude the marriage of causation and local realism is a classical notion? Both Einstein and Bohm seemed to say we should take classical thinking as our basic paradigm, and use it whenever we can, no matter how badly we need to retrofit it. That seems kind of opposite to me-- we built classical notions from a set of experiences, now we are having new experiences, so we need to be ready to relax old notions and build new ones. Local realism, and causation, seem like they need to be chucked, and retrocausation seems to have simply not gotten the memo that the whole causation notion should have been let go, other than as a perfectly viable theory of information propagation between people doing experiments and asking questions. In that spirit, we just say we have an entangled system, so we get entangled correlations, and leave it at that. Let QM speak for itself, with classical analogs used only as devices to show how QM isn't classical, not to show what's wrong with how we think about QM that needs creative retrofitting.

 

[Jilang]

Ken, nice post. If I was a betting type I might put my money on retrocausality being able to be demonstrated by experiment. With MWI it would seem no chance would exist. If there is an accommodating universe there should always be a way to move forward. We live in interesting times!

 

[bohm2]

Ken, nice post. If I was a betting type I might put my money on retrocausality being able to be demonstrated by experiment. With MWI it would seem no chance would exist.

GRW and even Bohmian might also be experimentally distinguishable from OQM.

 

[DevilsAvocado]

Griffiths says his interpretation is realistic and local, but if one wants to argue that it is not realistic, that seems plausible. According to an FAQ about consistent histories, "Colored slips of paper, one red and one green, are placed in two opaque envelopes, [...]

I smell a rat.

On PF we've seen numerous cranky attempts with "African Doctors", "French Epidemics" and god knows what, "explaining" EPR-Bell experiments preserving local realism. Everybody has failed, catastrophically, and of course the "Slip/Envelope" case is no different.

The "Slip/Envelope" case is mentioned in the FAQ, explaining how the 1935 EPR paradox gets handled in Consistent Histories. Of course, Bell's 1964 theorem is not mentioned once, since this would ruin the case completely.

In the Brief Introduction to Consistent Histories we are served the same moldy dish:

Consistent histories can be used to analyze various quantum paradoxes, such as the interference produced by a particle passing through a double slit, or the correlated pair of particles considered by Einstein, Podolksy, and Rosen. This allows the paradox to be understood in quantum terms, without any need to invoke peculiar long-range influences or other ghostly effects.

But what does Griffiths say about this in his book Consistent Quantum Theory?

Well, the "Slip/Envelope" case gets introduced in chapter 23.4 about Stern-Gerlach and Measurements of One Spin, where we have the mutual exclusion outcome of Z⁺ or Z^-, in which the "Slip/Envelope" case naturally fit like a glove. If we combine this with the eccentric methodology of stopping time at 1935, and 'forgetting' everything about Bell's theorem, we're almost there...

Wow, Griffiths is a crackpot??

No, certainly not, because in chapter 24.4 about Bell Inequalities, the "Slip/Envelope" case is not mentioned once (only in 24.1 about the 1935 EPR Paradox). Instead we get this restriction:

Thus the point at which the derivation of (24.10) begins to deviate from quantum principles is in the assumption that a function \alpha(w_a ,λ) exists for different directions w_a . As long as only a single choice for w_a is under consideration there is no problem, for then the “hidden” variable λ can simply be the value of S_aw at some earlier time. But when two (excluding the trivial case of w_a and -w_a) or even more possibilities are allowed, the assumption that \alpha(w_a ,λ) exists is in conflict with basic quantum principles.

The phrase "the “hidden” variable λ can simply be the value of S_aw at some earlier time" is just a circumscription of the "Slip/Envelope" case, where this prerequisite is an absolute necessity, thus the "Slip/Envelope" case only works as long as "only a single choice for w_a is under consideration".

Hence, Griffiths himself, in chapter 24.4 points out that the "Slip/Envelope" case in not compatible with Bell Inequalities, and he sums up the chapter with the following:

[my bolding]

In summary, the basic lesson to be learned from the Bell inequalities is that it is difficult to construct a plausible hidden variable theory which will mimic the sorts of correlations predicted by quantum theory and confirmed by experiment. Such a theory must either exhibit peculiar nonlocalities which violate relativity theory, or else incorporate influences which travel backwards in time, in contrast to everyday experience.

Everybody, including Einstein, understands that the "Slip/Envelope" case is a hidden variable theory. End of discussion.

What's the problem?

The problem is when guys like postdoctoral fellow Vlad Gheorghiu packages the whole thing into an unrecognizable quicksand of delusion, capable of making an erudite SA swallow it hook, line, and sinker – into the hole of scientific deception.

 

[bohm2]

I think that Bohmian mechanics has some sort of "real explanation" (do you know?), but not in detail how the non-locality is 'implemented', and anyhow, there are serious trouble with RoS as soon as you make "real stuff" being there and influencing other distant "real stuff".

This is one possibility and is sort of based on Couder's experimental stuff:

We refer neither to potentials nor to a "quantum force" or some other dynamics, but show that a "systemic nonlocality" may be obtained as a phenomenon that emerges from an assumed sub-quantum kinematics, which is manipulated only by changing its constraints as determined by the changes of the apparatus.

"Systemic nonlocality" from changing constraints on sub-quantum kinematics
http://iopscience.iop.org/1742-6596/442/1/012012/pdf/1742-6596_442_1_012012.pdf

A Classical Framework for Nonlocality and Entanglement
http://arxiv.org/pdf/1210.4406.pdf

 

[atyy]

I smell a rat.

On PF we've seen numerous cranky attempts with "African Doctors", "French Epidemics" and god knows what, "explaining" EPR-Bell experiments preserving local realism. Everybody has failed, catastrophically, and of course the "Slip/Envelope" case is no different.

The "Slip/Envelope" case is mentioned in the FAQ, explaining how the 1935 EPR paradox gets handled in Consistent Histories. Of course, Bell's 1964 theorem is not mentioned once, since this would ruin the case completely.

In the Brief Introduction to Consistent Histories we are served the same moldy dish:
But what does Griffiths say about this in his book Consistent Quantum Theory?

Well, the "Slip/Envelope" case gets introduced in chapter 23.4 about Stern-Gerlach and Measurements of One Spin, where we have the mutual exclusion outcome of Z⁺ or Z^-, in which the "Slip/Envelope" case naturally fit like a glove. If we combine this with the eccentric methodology of stopping time at 1935, and 'forgetting' everything about Bell's theorem, we're almost there...

Wow, Griffiths is a crackpot??

No, certainly not, because in chapter 24.4 about Bell Inequalities, the "Slip/Envelope" case is not mentioned once (only in 24.1 about the 1935 EPR Paradox). Instead we get this restriction:
The phrase "the “hidden” variable λ can simply be the value of S_aw at some earlier time" is just a circumscription of the "Slip/Envelope" case, where this prerequisite is an absolute necessity, thus the "Slip/Envelope" case only works as long as "only a single choice for w_a is under consideration".

Hence, Griffiths himself, in chapter 24.4 points out that the "Slip/Envelope" case in not compatible with Bell Inequalities, and he sums up the chapter with the following:

[my bolding]Everybody, including Einstein, understands that the "Slip/Envelope" case is a hidden variable theory. End of discussion.

What's the problem?

The problem is when guys like postdoctoral fellow Vlad Gheorghiu packages the whole thing into an unrecognizable quicksand of delusion, capable of making an erudite SA swallow it hook, line, and sinker – into the hole of scientific deception.

Just to make clear, I do agree that one cannot have a local realistic interpretation of quantum mechanics unless one violates the assumptions of the Bell theorem. For example, one can violate the assumptions of the Bell theorem by assuming that a measurement does not yield a unique outcome, but rather all outcomes occur - even then - it is not clear if the interpretation is local - but I'll certainly grant that its nonlocality is not assured by violation of the Bell inequalities. I also agree that it is not enough to say that the measurement results don't exist before measurement in order to violate the assumptions. One has to do something drastic like saying that there are not two observers, but only one.

However, it does seem that Griffiths claims consistent histories to be "realistic" and local. You can find the claim of locality in http://quantum.phys.cmu.edu/CQT/chaps/cqt24.pdf and the claim of "realism" in http://quantum.phys.cmu.edu/CQT/chaps/cqt27.pdf. In fact, both claims are made as points 2 and 3 on p318 of the second link. I don't know enough about consistent histories to comment on the claim, but my impression is that his definition of reality isn't "common sense realism", so if his claim is correct, my bet would be that his definition of reality is actually a form of nonrealism.

 

[bhobba]

my bet would be that his definition of reality is actually a form of nonrealism.

I have studied Consistent Histories and most definitely do NOT agree its a realistic interpretation in any usual sense.

In fact why it is described that way has me beat - but people do:
http://www.siue.edu/~evailat/pdf/qm12.pdf

This semantic nit picking of calling something 'weak property realism' leaves me cold. That's probably because philosophy in general leaves me cold.

Basically the idea seems to be only one history actually exists, and is in that sense real, but we do not know it and can only predict probabilities. Like I said before its MW without the MW's, and wanting that IMHO leads to an unnecessary complication. Still its a nice interpretation - just not my favorite.

Thanks
Bill

 

[bhobba]

I smell a rat.

Well its the other thing I often say about Consistent Histories - its defining your way out of problems.

Want a realistic theory - no problem - simply define realism the way you like.

Its a 'rat' all right - but not one I particularly worry about.

Thanks
Bill

 

[DevilsAvocado]

This is one possibility and is sort of based on Couder's experimental stuff:

Thanks bohm, that's interesting.

 

[DevilsAvocado]

Just to make clear, I do agree that one cannot have a local realistic interpretation of quantum mechanics unless one violates the assumptions of the Bell theorem. For example, one can violate the assumptions of the Bell theorem by assuming that a measurement does not yield a unique outcome, but rather all outcomes occur - even then - it is not clear if the interpretation is local - but I'll certainly grant that its nonlocality is not assured by violation of the Bell inequalities.

Okay, that must be MWI, right?

I also agree that it is not enough to say that the measurement results don't exist before measurement in order to violate the assumptions.

Agreed 100%, it especially goes baloney if the outcome assumption is always [1, 0] or [0, 1] ...

One has to do something drastic like saying that there are not two observers, but only one.

Wow, that's a new one, isn't it?

However, it does seem that Griffiths claims consistent histories to be "realistic" and local. You can find the claim of locality in http://quantum.phys.cmu.edu/CQT/chaps/cqt24.pdf and the claim of "realism" in http://quantum.phys.cmu.edu/CQT/chaps/cqt27.pdf. In fact, both claims are made as points 2 and 3 on p318 of the second link. I don't know enough about consistent histories to comment on the claim, but my impression is that his definition of reality isn't "common sense realism", so if his claim is correct, my bet would be that his definition of reality is actually a form of nonrealism.

Yeah I know... it's quite strange... he writes:

"By contrast, a consistent application of quantum principles provides a positive demonstration of the absence of nonlocal influences, as in the example discussed in Sec. 23.4."​

But chapter 23.4 is about Stern-Gerlach and Measurements of One Spin, not about Bell. And in chapter 24.4 about Bell inequalities, he instead admits that:

"Such a theory must either exhibit peculiar nonlocalities which violate relativity theory, or else incorporate influences which travel backwards in time, in contrast to everyday experience."​

And finish chapter 24.4 with:

"This seems a rather high price to pay just to have a theory which is more “classical” than ordinary quantum mechanics."​

And there it all ends... no explanation, just a 'statement'.

A quite inconsistent story, if you ask me.

 

[DevilsAvocado]

Well its the other thing I often say about Consistent Histories - its defining your way out of problems.

Okay, then there's some serious defining ahead (after post #93).

 

[atyy]

Wow, that's a new one, isn't it?

Old one :) http://arxiv.org/abs/quant-ph/0509061, see the discussion on p9-10, which includes "I suggest that Einstein was quite deliberately not assuming the existence of more than one observer"

 

[Dale]

Closed for moderation