Curtains for locality in QM?

Foundations of physics and/or philosophy of physics, and in particular, posts on unresolved or controversial issues

Curtains for locality in QM?

Tried unsuccessfully, via both comment and private email, to get feedback from the presenter Derek Muller re the following vid at his YT Veritasium site:
Quantum Entanglement & Spooky Action at a Distance
In particular, that portion from ~ 1:06 - 3:10. Just that from ~ 2:20-2:40 is most relevant here. Fixed detector angles - each orthogonal to incident particle spin. It's clear such an experiment cannot involve photons as annihilation of such (e.g. orthogonally oriented polarizers) would not allow continued propagation. So e.g. spin 1/2 massive particles are assumed. Presumably something involving say a cascaded Mach-Zender setup can achieve such a result.

Whatever the physical apparatus details, the claimed result - perfect anti-correlation of measured spins, yet purely random measurements at each detector, is NOT simply a consequence of overall angular momentum conservation of the particle pair, as Derek wrongly asserted. That's because momentum conservation should be independently obeyed during angular momentum exchange i.e. particle-detector interaction at each detector INDEPENDENT of what happens at the other one. IF locality in QM holds!
Directly implying zero average correlation of spin measurements. Contrary to experiment which (sans inevitable non-ideal detector efficiencies), yields perfect anti-correlation of measured spins - every time. Which result while consistent with overall conserved particle-pair angular momentum, also clearly transcends it.

NO SUBTLE STATISTICAL ARGUMENTS TO INVOKE HERE!!
So, either Derek has his facts wrong re the setup and measurement results claimed, or it's true and imo any hope of locality holding in QM has vanished for good.
Anyone here aware of a preferably non-paywalled article detailing and experimentally confirming in particular 2:20-2:40 in above linked vid?
Q-reeus

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Re: Curtains for locality in QM?

It's just the standard quantum woo-woo nonsense. The bottom line is that no experiment has ever proved quantum nonlocality, despite all the claims.
local

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Re: Curtains for locality in QM?

Last edited by Q-reeus on Thu Oct 17, 2019 5:01 pm, edited 1 time in total.
Q-reeus

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Re: Curtains for locality in QM?

local wrote:It's just the standard quantum woo-woo nonsense. The bottom line is that no experiment has ever proved quantum nonlocality, despite all the claims.

Which is tantamount to claiming Derek Muller is at best badly misinformed, or outright deceptive. But then he is simply reiterating the QM correlation prediction
C = -a.b, which Joy Christian agrees is so. The spin singlet state measurements example between 2:20 - 2:40 coincides with C = -a.b = -1. Perfect anti-correlation - every joint measurement performed. How can locality possibly hold given there is also completely random measurements at either detector? To repeat - NO non-trivial statistics involved at all!
Q-reeus

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Re: Curtains for locality in QM?

It's all fantasy. No experiment has ever proved nonlocality. Go ahead, cite the one experiment you consider decisive. There must be one, right?
local

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Re: Curtains for locality in QM?

Q-reeus wrote:How can locality possibly hold given there is also completely random measurements at either detector?

What do you mean by completely random measurements at either detector? That statement is too vague. You will need to explain further.
minkwe

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Re: Curtains for locality in QM?

minkwe wrote:
Q-reeus wrote:How can locality possibly hold given there is also completely random measurements at either detector?

What do you mean by completely random measurements at either detector? That statement is too vague. You will need to explain further.

Just what is mentioned in that vid - given the orthogonality between detector angle and spin axes, there is a 50:50 chance of either spin up or spin down measurement, at any one detector. That much is intuitively obvious. It's the perfect anti-correlations that are troublesome for any local realist interpretation.
As covered in first post, appealing to conservation of angular momentum to 'explain' the results is false, and where Derek gets it wrong.
Q-reeus

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Re: Curtains for locality in QM?

Q-reeus wrote:Just what is mentioned in that vid - given the orthogonality between detector angle and spin axes, there is a 50:50 chance of either spin up or spin down measurement, at any one detector. That much is intuitively obvious. It's the perfect anti-correlations that are troublesome for any local realist interpretation.

I'm just not sure why you single out local realistic interpretations. What about QM? Do the bell-test experiments confirm the perfect anti-correlation? If not then why should it be a problem for local realism and not QM?

You should ask the question of QM. How is it possible to have perfect anti-correlations and yet random outcomes at each station. That is what QM says.
minkwe

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Re: Curtains for locality in QM?

minkwe wrote:I'm just not sure why you single out local realistic interpretations. What about QM? Do the bell-test experiments confirm the perfect anti-correlation? If not then why should it be a problem for local realism and not QM?

You should ask the question of QM. How is it possible to have perfect anti-correlations and yet random outcomes at each station. That is what QM says.

I haven't so far come across an actual Bell experiment coinciding with that example given. A simple spin filter would naively be expected to reject rather than randomly reorient and then transmit an incident spin 1/2 particle with orthogonal incident spin polarization. Which is why I asked for any links to actual experiments that can do what is claimed in the vid. I have little doubt the presenter is correctly drawing on known results, but annoyingly, he provides no references.

But then, apart from experimental confirmation, it should be asked what is Joy's own prediction for the settings given? He claims to reproduce all the QM predictions, hence will have to explain perfect anti-correlation along with individual detector random results.
It's contemplating the predictions for that one relative angular setting between detectors that doesn't seem to have been seriously done before at SPF. Forget all the convoluted stats involving a million runs over random angular settings etc. Try actually reconciling this one, fixed angles case, with your pet concept of reality.
Q-reeus

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Re: Curtains for locality in QM?

The perfect anti-correlation is easy to duplicate classically. The sawtooth and the cosine correspond at some points and aligned detectors is one of those points.

I can generate a pair of cards, one white and one black, with 50% probability for white-black and black-white. When I send one card to each of two sides they will always be anticorrelated but each side sees random black/white.
Last edited by local on Thu Oct 17, 2019 7:19 pm, edited 1 time in total.
local

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Re: Curtains for locality in QM?

Q-reeus wrote:I haven't so far come across an actual Bell experiment coinciding with that example given.

Have you come across any experiment which illustrates the point you are trying to make?

Note that there is nothing particularly troubling about what you are asking. Bell's outcome functions are

$A(a, \lambda) = -B(a, \lambda)$

You don't need a model to see this as it is obvious. For a specific $\lambda$, the outcomes at Bob and Alice are perfectly anti-correlated. However, Alice is not measuring the same particle repeatedly, nor is Bob. Different particle pairs arrive at Alice and Bob with different $\lambda$ values. Since the $\lambda$ values are random, the sequence of outcomes are random. The randomness refers to the sequence of values, not individual values. It is only when you compare two particles of the same pair that you get perfect anti-correlation. and if you calculate the average of the paired-product $$ over all the outcomes, you end up with -1. I encourage you to read the recent thread on the meaning of randomness.

So what exactly is it that you claim local realism can't do but QM can?
minkwe

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Re: Curtains for locality in QM?

Q-reeus wrote:
It's the perfect anti-correlations that are troublesome for any local realist interpretation.

There is nothing mysterious going on here. Anit-correations are not a problem for local realistic interpretation. I watched a few minutes of the video from 2:20. The guy does not understand even the most basic facts about the EPRB correlations. No experimental complications or randomness definitions are necessary to understand his mistake. There is perfect anti-correlation even in Dr. Bertlmann's socks, as Bell used to point out fondly.

Imagine you are walking from your home in winter and midway to your destination you reach out your pockets for hand-gloves, but you find only one of them. At that moment you instantly know that you forgot the other one at home. Not only do you know that, but you also instantly know that the one you forgot at home is a lefthand glove if the one you pulled out from your pocket happens to be a righthand glove. Note that this is true even if your home happens to be in the far corner of the Universe. You have instant information about the handedness of the glove you forgot at home by simply looking at the glove you just pulled out from your pocket. That is perfect anti-correlation and it has nothing to do with nonlocality of any sort. It is just simple classical physics. The same is true for the anti-correlation of spins in the EPRB setup.

***
Joy Christian
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Re: Curtains for locality in QM?

local wrote:The perfect anti-correlation is easy to duplicate classically. The sawtooth and the cosine correspond at some points and aligned detectors is one of those points....

There is perfect anti-correlation and no conflict between QM and classical for the case |a.b| = 1, AND perfect alignment between spins and detector orientations. Easy to correctly invoke conservation of angular momentum as explanation there too. But that easy situation fails for the orthogonal spin-detector orientations discussed here.
I can generate a pair of cards, one white and one black, with 50% probability for white-black and black-white. When I send one card to each of two sides they will always be anticorrelated but each side sees random black/white.

Stacking the decks like that is entirely artificial and ignores the inherent randomness of spin-up vs spin-down for orthogonal spin-detector orientation.
To correspond to the actual situation referred to in vid, your example should have been of cards each with one face white and the opposite face black. Part the cards in opposite directions, but with a perfect initial anti-correlation such that say a white face on one mates to a white face on the other card. But when detected re black or white face showing, each detector is vibrating such that it randomly flips each received card 50:50 black face or white face 'detected'. Yet the actual QM result has perfect anti-correlation for each joint measurement. No stacking the decks out allowed.
Again - it's reconciling inherent, fundamentally physical measurement randomness at each detector with perfect anti-correlation between detectors that needs an actual, physical explanation.

Hopefully this deals with Minkwe's and Joy Christian's latest posts also. Invoking Bell's Bertlmann's socks etc. is skew of the mark and imo not what's at stake.
Q-reeus

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Re: Curtains for locality in QM?

I'm not stacking any decks. The outcome streams at each side are fully random because the source pairs are randomly generated as white-black or black-white. Nevertheless, anticorrelation is obviously achieved.

inherent, fundamentally physical measurement randomness at each detector

That's metaphysical mumbo-jumbo. How could you define that and distinguish that from the randomness in my example?

And now for some reason you are talking about orthogonal settings when QM requires aligned detectors to get perfect anticorrelation.
Last edited by local on Thu Oct 17, 2019 8:36 pm, edited 2 times in total.
local

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Re: Curtains for locality in QM?

Q-reeus wrote:Stacking the decks like that is entirely artificial and ignores the inherent randomness of spin-up vs spin-down for orthogonal spin-detector orientation.

What you are saying here does not make sense.

To correspond to the actual situation referred to in vid, your example should have been of cards each with one face white and the opposite face black. Part the cards in opposite directions, but with a perfect initial anti-correlation such that say a white face on one mates to a white face on the other card. But when detected re black or white face showing, each detector is vibrating such that it randomly flips each received card 50:50 black face or white face 'detected'.

This also makes no sense. You are religiously stuck to some fake animation in a youtube video and refusing to listen to what is being explained here. There is no issue with perfect anti-correlation and random outcomes. For the last time, perfect anti-correlation originates from conservation of angular momentum between two particles of the same pair. While randomness of outcomes originates from different and random hidden variables between one pair and the next pair in a sequence. There is no conflict that needs to be explained. This is common sense. Just because you call it stacking the decks does not make it so.
minkwe

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Re: Curtains for locality in QM?

Joy Christian wrote:
Q-reeus wrote:
It's the perfect anti-correlations that are troublesome for any local realist interpretation.

There is nothing mysterious going on here. Anit-correations are not a problem for local realistic interpretation. I watched a few minutes of the video from 2:20. The guy does not understand even the most basic facts about the EPRB correlations. No experimental complications or randomness definitions are necessary to understand his mistake. There is perfect anti-correlation even in Dr. Bertlmann's socks, as Bell used to point out fondly.

Imagine you are walking from your home in winter and midway to your destination you reach out your pockets for hand-gloves, but you find only one of them. At that moment you instantly know that you forgot the other one at home. Not only do you know that, but you also instantly know that the one you forgot at home is a lefthand glove if the one you pulled out from your pocket happens to be a righthand glove. Note that this is true even if your home happens to be in the far corner of the Universe. You have instant information about the handedness of the glove you forgot at home by simply looking at the glove you just pulled out from your pocket. That is perfect anti-correlation and it has nothing to do with nonlocality of any sort. It is just simple classical physics. The same is true for the anti-correlation of spins in the EPRB setup.

***

Lovely example!

And Bertlmann confirmed that his friend John Bell’s story about himself was completely true. He deliberately wore a randomly allocated pink and blue sock on each foot, in open sandals, as a kind of hippy protest against The Establishment. That was in the days of flower power, man. Despite that, he had a position at CERN and did a lot of very important work on particle physics.
gill1109
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Re: Curtains for locality in QM?

Well someone(s) has wrong thinking on this. Instead of addressing either of the last few posts individually, I'll just refer respondents back to the middle para of first post.
To repeat yet again - overall conservation of angular momentum is not an answer. If locality in QM holds, conservative exchanges of angular momenta between each particle and each detector should be an entirely local affair quite independent of any initial correlations between the particle pair. With 50:50 chance of spin-up vs spin-down detection - at each detector separately.
Consequently, assuming QM obeys locality, spin measurement at detector A should have no statistical correlation with the measurement at detector B. Given the specified settings.
Viz - parallel or anti-parallel detector orientations, coupled with orthogonal orientations between each detector and incident spin 1/2 particle. Yet actually perfect anti-correlations result according to QM.
Is that hard to grasp?
Q-reeus

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Re: Curtains for locality in QM?

Q-reeus wrote:Well someone(s) has wrong thinking on this. Instead of addressing either of the last few posts individually, I'll just refer respondents back to the middle para of first post.
To repeat yet again - overall conservation of angular momentum is not an answer. If locality in QM holds, conservative exchanges of angular momenta between each particle and each detector should be an entirely local affair quite independent of any initial correlations between the particle pair. With 50:50 chance of spin-up vs spin-down detection - at each detector separately.
Consequently, assuming QM obeys locality, spin measurement at detector A should have no statistical correlation with the measurement at detector B. Given the specified settings.
Viz - parallel or anti-parallel detector orientations, coupled with orthogonal orientations between each detector and incident spin 1/2 particle. Yet actually perfect anti-correlations result according to QM.
Is that hard to grasp?

Conservation of angular momentum is exactly the answer. Here are a couple different treatments of how QM is local for the EPR-Bohm scenario.

viewtopic.php?f=6&t=409

viewtopic.php?f=6&t=412
.
FrediFizzx
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Re: Curtains for locality in QM?

FrediFizzx wrote:Conservation of angular momentum is exactly the answer. Here are a couple different treatments of how QM is local for the EPR-Bohm scenario.

viewtopic.php?f=6&t=409

viewtopic.php?f=6&t=412
.

Well Fred, I won't question the legitimacy of the mathematical underpinnings of that nice cosine result shown in
viewtopic.php?f=6&t=409#p10150
Merely point out that it coincides with the QM predictions obviously. Now, apply it to the scenario I have referred to. We all agree, given a spin singlet initial state, perfect anti-correlation for parallel detector orientations. Independent of relative orientations between detector and incident particle spin at each detector. Now, explain how locality makes sense when the relative detector/particle spin orientations are orthogonal. You can sensibly reconcile 50:50 probabilities spin-up vs spin-down at each detector with that perfect anti-correlation shown?
All consistent with locality? How exactly?
Q-reeus

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Re: Curtains for locality in QM?

Q-reeus wrote:Well someone(s) has wrong thinking on this. Instead of addressing either of the last few posts individually, I'll just refer respondents back to the middle para of first post.
To repeat yet again - overall conservation of angular momentum is not an answer. If locality in QM holds, conservative exchanges of angular momenta between each particle and each detector should be an entirely local affair quite independent of any initial correlations between the particle pair. With 50:50 chance of spin-up vs spin-down detection - at each detector separately.
Consequently, assuming QM obeys locality, spin measurement at detector A should have no statistical correlation with the measurement at detector B. Given the specified settings.
Viz - parallel or anti-parallel detector orientations, coupled with orthogonal orientations between each detector and incident spin 1/2 particle. Yet actually perfect anti-correlations result according to QM.
Is that hard to grasp?

Since Copenhagen QM is an intrinsically stochastic theory, conservation of angular momentum does not hold in individual histories or worlds. It only holds on average, or collectively. For instance, in the Many Worlds theory it holds when we refer to all worlds together - ie we refer to the actual multiverse. Individual observers following experiences in one particular world (universe) do not experience conservation of angular momentum. MWT says all observers in all worlds do exist as one object in the Multuverse. At that level, the multiverse of many branching worlds seen as one undivided thing is local, deterministic, and satisfies all the conservation laws which physicists hold dear.

I think that Many Worlds Theory is just Many Words Religion. Opium for the masses. (The masses of busy research physicists who have no time or inclination to think).
gill1109
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