An abstract long-distance correlation experiment

Click For Summary
The discussion centers on a generalized long-distance correlation experiment inspired by an EPR-like setting, aiming to abstract away distractions from reality and imagination to focus on the essentials of quantum mechanics. The experiment involves two devices operated by Alice and Bob, receiving signals from a source, Norbert, with specific timing and conditions for data collection. The goal is not to prove or disprove local realism but to explore the "weirdness" of quantum mechanics and how language influences this understanding. Participants are encouraged to engage with the experimental framework while avoiding discussions on other nonlocality settings. The thread emphasizes the importance of refining the experimental setup based on constructive criticism to ensure clarity and validity in the analysis.
  • #91
georgir said:
Remap the degree of correlations between different-setting measurements from the range of [0.6666, 0.75] to [0, 1].
You are assuming we actually get perfect correlations for identical settings. If we want to cover all possible results, we need to point out that if those correlations are not perfect then the weirdness is less, or zero, because LHV models (like edguy99's) can theoretically explain such results. Note that in the OP there was no specification about what Norbert sends! We need to consider "ordinary", entanglement-free results as well.
As for me, I still don't see any point of calling some results weirder than others. What is weird is the clash between how Nature operates and how we (at least I) are able to conceive of it. We only need one proof to establish that such a clash exists. Once that is true, we judge results only by how they accord with our theory, namely QM, and of course, they do. The discussion about weirdness/ understanding/ interpretation becomes independent of experiment, which is one reason it is so disliked. But for me something is definitely weird...
 
Physics news on Phys.org
  • #92
Maline, you seem to have missed point 3 in my post, allowing for somewhat less than perfect correlations.
 
  • #93
I just mean that Bell's reasoning starts with the point that to get definite results for any pair of equal settings, a local & realist model needs definite hypothetical results for all possible individual settings. A full "weirdness function" based on a Bell violation needs to depend explicitly on both matrices E & F to make sure the logic holds.
 
  • #94
If it's a Bell inequality using only the matrices defined in the beginning of this thread that you want, then a simple one depending only on ##E## and ##F## is $$- \bar{E} + 3 \bar{F} \leq 2 \,, \qquad (*)$$ with $$\begin{eqnarray*}
\bar{E} &=& E_{00} - E_{01} - E_{10} + E_{11} \,, \\
\bar{F} &=& F_{00} - F_{01} - F_{10} + F_{11} \,.
\end{eqnarray*}$$ This is taking results from the different measurement settings to contribute equally to ##E## and ##F##, i.e., in terms of the conditional probabilities, $$\begin{eqnarray*}
E_{ab} &=& \frac{1}{3} \sum_{x} P(ab \mid xx) \,, \\
F_{ab} &=& \frac{1}{6} \sum_{x \neq y} P(ab \mid xy)
\end{eqnarray*}$$ for ##a, b \in \{0, 1\}## and ##x, y \in \{0, 1, 2\}##. (The simplest way to ensure this is just to make this the definition of the matrices ##E## and ##F##.) Otherwise, the inequality holds under the same sort of assumptions as other Bell inequalities, e.g., if you don't do any postselection or you make the fair sampling hypothesis.

I'll skip the details on how (*) was derived unless someone asks. In any case, with (*) given it's not especially difficult to check that it must hold for any LHV. Using a state and measurements similar to those that maximally violate the 1964 Bell inequality it's possible to have ##\bar{E} = -1## and ##\bar{F} = 1/2##, so QM can attain at least ##- \bar{E} + 3 \bar{F} = 2.5##, which violates (*). Finally, since ##\lvert \bar{E} \rvert, \lvert \bar{F} \rvert \leq 1##, the algebraic bound is ##- \bar{E} + 3 \bar{F} \leq 4##, and there's a ##3 \times 3##-measurement version of the PR box that can attain this while still respecting the no-signalling principle. So if you want a function of value between 0 and 1 when it detects Bell-nonlocal correlations then one possibility is $$W = -1 + \frac{- \bar{E} + 3 \bar{F}}{2} \,.$$
 
  • Like
Likes maline and Mentz114
  • #95
wle said:
So if you want a function of value between 0 and 1 when it detects Bell-nonlocal correlations then one possibility is ##W = -1 + \frac{- \bar{E} + 3 \bar{F}}{2} \,.##
This can become negative (uniformly random output independent of input), hence is not yet good. What about ##W=\max(0,\min(6\bar F-2\bar E-4,1))##? This would satisfy my requirements, and makes the specific case you described completely weird while leaving local hidden variable results not weird at all.
 
Last edited:
  • #96
georgir said:
I'm not sure which will suffice for you
Any that satisfies the criteria stated in post #49 and reflects your personal view of weirdness. But it must be an explicit formula that I can evaluate for any chocie of E and F.
 
  • #97
wle said:
If it's a Bell inequality using only the matrices defined in the beginning of this thread that you want, then a simple one depending only on ##E## and ##F## is $$- \bar{E} + 3 \bar{F} \leq 2 \,, \qquad (*)$$ with $$\begin{eqnarray*}
\bar{E} &=& E_{00} - E_{01} - E_{10} + E_{11} \,, \\
\bar{F} &=& F_{00} - F_{01} - F_{10} + F_{11} \,.
\end{eqnarray*}$$ This is taking results from the different measurement settings to contribute equally to ##E## and ##F##, i.e., in terms of the conditional probabilities, $$\begin{eqnarray*}
E_{ab} &=& \frac{1}{3} \sum_{x} P(ab \mid xx) \,, \\
F_{ab} &=& \frac{1}{6} \sum_{x \neq y} P(ab \mid xy)
\end{eqnarray*}$$ for ##a, b \in \{0, 1\}## and ##x, y \in \{0, 1, 2\}##. (The simplest way to ensure this is just to make this the definition of the matrices ##E## and ##F##.) Otherwise, the inequality holds under the same sort of assumptions as other Bell inequalities, e.g., if you don't do any postselection or you make the fair sampling hypothesis.

I'll skip the details on how (*) was derived unless someone asks. In any case, with (*) given it's not especially difficult to check that it must hold for any LHV. Using a state and measurements similar to those that maximally violate the 1964 Bell inequality it's possible to have ##\bar{E} = -1## and ##\bar{F} = 1/2##, so QM can attain at least ##- \bar{E} + 3 \bar{F} = 2.5##, which violates (*). Finally, since ##\lvert \bar{E} \rvert, \lvert \bar{F} \rvert \leq 1##, the algebraic bound is ##- \bar{E} + 3 \bar{F} \leq 4##, and there's a ##3 \times 3##-measurement version of the PR box that can attain this while still respecting the no-signalling principle. So if you want a function of value between 0 and 1 when it detects Bell-nonlocal correlations then one possibility is $$W = -1 + \frac{- \bar{E} + 3 \bar{F}}{2} \,.$$

Thank you for the post. Can you add a sample calculation with 2 matrices to help some of us that may be a little confused by the terminology. Regards.
 
  • #98
I'm not particularly comfortable with this "degree of weirdness" variable W. To me, what's weird is the lack of an answer to some basic questions about QM, particular to the EPR experiment (a variant of which is being discussed here).

Let's enumerate the critical events:
  • e_0 where the twin pair (or whatever it is) is created.
  • e_{a1} at which Alice picks her setting.
  • e_{a2} at which Alice gets her result (one of two possibilities)
  • e_{b1} at which Bob picks his setting.
  • e_{b2} at which Bob gets his result.
To simplify the discussion, let me first assume that Alice and Bob choose the same setting, and they both know ahead of time which setting that is. For definiteness, let's assume that in Alice's rest frame, e_{a2} takes place slightly before e_{b2}.

Before e_{a2}, Alice doesn't have any idea what result Bob will get at e_{b2}. Then suppose she gets spin-up at event e_2. Afterward, she knows exactly what he will get (because of the perfect anti-correlations): spin-down.

So my question is about Alice's change of knowledge about Bob. It seems to me that there are three possibilities:
  1. Bob's (future) result was already determined prior to Alice's measurement, and the only thing that changed by her measurement was her knowledge about that outcome.
  2. Bob's result becomes definite as a result of Alice's measurement.
  3. Something more exotic, such as Many-Worlds.
Choice number 1 seems to be a hidden-variables theory of the type that is ruled out by Bell's inequality (unless we get into loopholes such as retrocausality or superdeterminism). Choice number 2 seems to require a nonlocal interaction. Choice 3 is weird, for reasons that I won't get into here.

The complications allowing Alice and Bob to choose a setting in-flight doesn't really change the weirdness. It only serves to rule out possibility 1.

So if #1 is ruled out, then it would seem to me that the EPR experiment implies either nonlocality, or various exotically weird possibilities (retrocausality, superdeterminism, many-worlds).

But most people who deny that there is anything weird about QM seem to reject all the possibilities:
  1. QM is not retrocausal.
  2. QM is not superdeterministic.
  3. QM is not nonlocal.
  4. QM does not imply Many-Worlds.
--
Daryl McCullough
 
  • Like
Likes maline
  • #99
Choice 2's nonlocal interaction is ill-defined in special relativity. You have to pick a direction arbitrarily or invoke an preferred spacetime foliation. This makes it even weirder for me.
 
  • #100
stevendaryl said:
  1. Bob's (future) result was already determined prior to Alice's measurement, and the only thing that changed by her measurement was her knowledge about that outcome.
Choice number 1 seems to be a hidden-variables theory of the type that is ruled out by Bell's inequality
... ruled out only under the assumption of a local hidden variable theory with signals moving independently along the rays to Alice and Bob. But this assumption is too strong to have implications when the signal is a field rather than particles.

This is apparent from a (simpler) single-photon nonlocality experiments such as that discussed in my slides here (slides 46-59). The argument there doesn't extend to the setting under discussion here but shows that the assumptions of Bell are tied to an implicit particle assumption.

I'd appreciate if (in a new thread) you'd assess the weirdness of my setting in the slides according to your criteria. For I think the same concerns that you raise above apply to the single-photon nonlocality experiment, although the latter has a fully classical field explanation.
 
  • #101
A. Neumaier said:
... ruled out only under the assumption of a local hidden variable theory with signals moving independently along the rays to Alice and Bob. But this assumption is too strong to have implications when the signal is a field rather than particles.

Suppose the detectors are at the sides of the source, all on the same axis. Whatever the source is emitting makes the detectors show EPR correlations with light-speed timing. The field would have to instantly jump double the distance to "tell the other side" what choice of measurement was made so to make the correlations show up.
 
  • #102
ddd123 said:
Suppose the detectors are at the sides of the source, all on the same axis. Whatever the source is emitting makes the detectors show EPR correlations with light-speed timing. The field would have to instantly jump double the distance to "tell the other side" what choice of measurement was made so to make the correlations show up.
I know, but your observation is independent of what I was asserting. The speed of light is nowhere used in the description or analysis of the experiment, except to conclude ''nonlocality''.
 
  • #103
A nonlocal field reminds me of pilot wave. It's not less weird.
 
  • #104
A. Neumaier said:
For I think the same concerns that you raise above apply to the single-photon nonlocality experiment, although the latter has a fully classical field explanation.
The classical field explanation is precisely of the local "hidden" variable type. The light has a particular polarization at every point, which determines its behavior. There is no reason to assume that a signal hitting a beam splitter goes in only one direction, other than the weird quantum fact that there exist "single photons" that get fully absorbed at a single point!
 
Last edited:
  • #105
maline said:
The classical field explanation is precisely of the local hidden variable type. The light has a particular polarization at every point, which determines its behavior. There is no reason to assume that a signal hitting a beam splitter goes in only one direction, other than the weird quantum fact that there exist "single photons" that can only be detected at one point at most!
Did you read my slides?

I gave a local hidden variable argument of precisely the kind that was used by Bell and found a Bell-type inequality that was violated by the prediction of quantum mechanics. According to your criticism, there should be a fault in my formal reasoning since repeating the analysis using instead the Maxwell equations gives full agreement with the quantum predictions.

So please point out where my arguments are faulty instead of arguing in a roundabout way that is too vague to spot the problems! it was Bell's accomplishment to do this for the EPR problem and thus turn it from a philosophical issue into something that can be investigated in a scientific manner. So please argue on the level of equations rather than philosophy if you want to make a scientific point in the spirit of Bell!
 
  • #106
In your "inequality derivation" you assumed that the photon takes one path at the splitter. Bell's argument does not rely on any such assumption.
 
  • #107
maline said:
In your "inequality derivation" you assumed that the photon takes one path at the splitter. Bell's argument does not rely on any such assumption.
Isn't it already extremely weird to allow that a classical local hidden variable photon travels along several beams? I don't think that it is satisfying to explain away weirdness by basing the explanations on weird assumptions.
 
  • #108
A. Neumaier said:
This is apparent from a (simpler) single-photon nonlocality experiments such as that discussed in my slides here (slides 46-59). The argument there doesn't extend to the setting under discussion here but shows that the assumptions of Bell are tied to an implicit particle assumption.
Here is field type explanation of Bell inequality violation http://arxiv.org/abs/0906.1539. But it needs to exploit loophole to do that.
 
  • #109
A. Neumaier said:
Isn't it already extremely weird to allow that a classical local hidden variable photon travels along several beams?
Classically, there are no photons, only an EM wave. Of course it spreads through space along all possible paths. Nevertheless, its polarization is a local variable, because it propagates at light speed.
 
  • #110
maline said:
an EM wave. Of course it spreads through space along all possible paths
No. There are no paths in a field context. And as any experimenter in classical optics knows, if you input a polarized electromagnetic wave focussed in a beam (in the paraxial approximation) into a beam splitter, the output will be a polarized electromagnetic wave focussed just along two beams. And its polarization is bilocal, not local. This is why one gets the quantum mechanical results and not the local hidden variable results.
 
  • #111
A. Neumaier said:
There are no paths in a field context
"Along all possible paths" was just a way of saying "as per Maxwell's equations". The point is that classically the wave must travel both ways, and there is nothing weird about that. "Photon" is purely a quantum- or rather a QFT- concept (and to me is indeed quite weird).

A. Neumaier said:
And its polarization is bilocal, not local. This is why one gets the quantum mechanical results and not the local hidden variable results
The word "local" in "local hidden variable" does not mean "localized to a particular region". It means "respecting the principle of locality"- the variable can be described as a function on spacetime (including delta functions) and values at particular points depend only on the past light cone of those points. Classical polarization of a wave definitely qualifies.
 
  • #112
A. Neumaier said:
This can become negative (uniformly random output independent of input), hence is not yet good. What about ##W=\max(0,\min(6\bar F-2\bar E-4,1))##? This would satisfy my requirements, and makes the specific case you described completely weird while leaving local hidden variable results not weird at all.

If this fits with your requirements, sure. My opinion isn't really important here. Other participants in this thread have indicated that they want a measure of nonlocality and are apparently happy if they have one that's a function of only your matrices ##E## and ##F##, so I posted one. The only reasons I used the normalisation I did are a) I know that quantum physics can attain ##- \bar{E} + 3 \bar{F} = 2.5##, but I don't have a proof that this is the maximum that is consistent with QM, and b) it's easy to define a set of hypothetical conditional probabilities (almost certainly not allowed by QM) that attain the algebraic limit ##- \bar{E} + 3 \bar{F} = 4## without allowing instantaneous signalling (in the sense that the marginal statistics on Alice's side are independent of Bob's measurement choice and vice versa).
 
  • #113
maline said:
The word "local" in "local hidden variable" does not mean "localized to a particular region". It means "respecting the principle of locality"- the variable can be described as a function on spacetime (including delta functions) and values at particular points depend only on the past light cone of those points.
Local in local hidden variable theories cannot mean anything related to relativity theory - all of quantum mechanics is purely nonrelativistic!

Indeed, I have never seen a Bell-type argument where formal use was made of the the fact that values depend or do not depend on the past light cone. The arguments never involve space or time at all, only simultaneity, which is intrinsically nonrelativistic!
 
  • #114
wle said:
a) I know that quantum physics can attain ##- \bar{E} + 3 \bar{F} = 2.5##, but I don't have a proof that this is the maximum that is consistent with QM, and b) it's easy to define a set of hypothetical conditional probabilities (almost certainly not allowed by QM) that attain the algebraic limit [...] 4
Anything with ##- \bar{E} + 3 \bar{F} \ge 2.5## has degree of weirdness 1 according to my amended formula.
 
  • #115
edguy99 said:
Thank you for the post. Can you add a sample calculation with 2 matrices to help some of us that may be a little confused by the terminology. Regards.

I'll elaborate for the quantum example. The example I had in mind is where Alice and Bob each have a spin 1/2 particle entangled in the state ##\lvert \Psi^{-} \rangle = \bigl( \lvert 0 \rangle_{\mathrm{A}} \lvert 1 \rangle_{\mathrm{B}} - \lvert 1 \rangle_{\mathrm{A}} \lvert 0 \rangle_{\mathrm{B}} \bigr) / \sqrt{2}## and can measure the spin projections respectively along the angles ##\alpha_{x}## and ##\beta_{y}##, ##x, y \in \{0, 1, 2\}## on the ##\sigma_{z}-\sigma_{x}## plane. The well known prediction by QM is that, depending on the angles, Alice and Bob get the results (noted '0' and '1') with joint conditional probabilities $$\begin{eqnarray*}
P(00 \mid xy) &=& P(11 \mid xy) &=& \frac{1 - \cos(\alpha_{x} - \beta_{y})}{4} \,, \\
P(01 \mid xy) &=& P(10 \mid xy) &=& \frac{1 + \cos(\alpha_{x} - \beta_{y})}{4} \,.
\end{eqnarray*}$$ The result I described is obtained with the choices of angles (in degrees) $$\begin{eqnarray*}
\alpha_{0} &=& \beta_{0} &=& 0^{\circ} \,, \\
\alpha_{1} &=& \beta_{1} &=& 120^{\circ} \,, \\
\alpha_{2} &=& \beta_{2} &=& 240^{\circ} \,,
\end{eqnarray*}$$ so that the angular difference ##\alpha_{x} - \beta_{y}## is always ##0^{\circ}##, ##\pm 120^{\circ}##, or ##\pm 240^{\circ}##. Basic trigonometry says that ##\cos(0^{\circ}) = 0## and ##\cos(\pm 120^{\circ}) = \cos(\pm 240^{\circ}) = -1/2##, so that $$\cos(\alpha_{x} - \beta_{y}) = \begin{cases}
0 &\text{if } x = y \\
-1/2 &\text{if } x \neq y
\end{cases} \,.$$
Inserted into the probabilities above, this gives $$\begin{eqnarray*}
P(00 \mid xy) &=& P(11 \mid xy) &=& 0 \,, \\
P(01 \mid xy) &=& P(10 \mid xy) &=& 1/2
\end{eqnarray*}$$ if ##x = y## and $$\begin{eqnarray*}
P(00 \mid xy) &=& P(11 \mid xy) &=& 3/8 \,, \\
P(01 \mid xy) &=& P(10 \mid xy) &=& 1/8
\end{eqnarray*}$$ if ##x \neq y##. In other words, Alice and Bob get perfectly anticorrelated results if they choose the same angle settings (##x = y##) and partially correlated results if they choose different angle settings (##x \neq y##). The elements ##E_{ab}## and ##F_{ab}##, ##a, b \in \{0, 1\}##, of the matrices ##E## and ##F## are just the probabilities ##P(ab \mid xy)## averaged over the cases ##x = y## and ##x \neq y##, i.e., $$\begin{eqnarray*}
E_{ab} &=& \frac{1}{3} \sum_{x} P(ab \mid xx) \,, \\
F_{ab} &=& \frac{1}{6} \sum_{x \neq y} P(ab \mid xy) \,.
\end{eqnarray*}$$ Here each of the contributing probabilities is the same in both cases, so the matrices are just $$E = \begin{bmatrix} 0 & 1/2 \\ 1/2 & 0 \end{bmatrix}$$ and $$F = \begin{bmatrix} 3/4 & 1/4 \\ 1/4 & 3/4 \end{bmatrix} \,.$$ Finally, using the definition from my earlier post, this gives ##\bar{E} = 0 - 1/2 - 1/2 + 0 = -1## and ##\bar{F} = 3/8 - 1/8 - 1/8 + 3/8 = 1/2##, which produces ##- \bar{E} + 3 \bar{F} = 2.5##.

Does this help? I didn't know which part of the notation you weren't following so I aimed to explain the example thoroughly.
 
Last edited:
  • #116
A. Neumaier said:
So please point out where my arguments are faulty instead of arguing in a roundabout way that is too vague to spot the problems!

The experiment depicted on p. 47 of your slides doesn't fit the format of a Bell experiment. In particular, you have only one detector and it is possible for it to be influenced by both filters ##F(B_{1})## and ##F(B_{2})## without any faster-than-light communication. This -- and this is a big difference from Bell -- makes it impossible to derive constraints on what the detector in your setup can register based on relativistic causality alone. In Bell's theorem the point was to derive constraints on possible correlations assuming relativistic causality and very little else. This is why there are at least two spatially separated detectors in a Bell experiment and why there's a lot of importance put on having the measurements performed and results recorded nearly simultaneously: those are minimum conditions necessary in order for slower-than-light causation to actually become a constraint on anything.
A. Neumaier said:
Indeed, I have never seen a Bell-type argument where formal use was made of the the fact that values depend or do not depend on the past light cone.

Here's some by Bell himself:
  1. J. S. Bell, "The theory of local beables", CERN-TH-2053 (1975).
  2. J. S. Bell, "Bertlmann's socks and the nature of reality", CERN-TH-2926 (1981).
  3. J. S. Bell, "La nouvelle cuisine", doi:10.1017/CBO9780511815676.026 (1990).
Of these, 1. and 3. are very explicitly grounded in relativistic causality; in both, Minkowski diagrams depicting light cones are used as aides to the argument. 2. is also worth a read. It isn't so explicit about the role of relativity but the presentation is still kept very generic and Bell emphasises that the argument does not depend on, say, some specific model of "particles".

I linked to scans of 1. and 2. that are freely available online, though it's probably possible to find nicer reprints elsewhere. The link for 3. is unfortunately a paywall. All three are included in the second edition of "Speakable and Unspeakable in Quantum Mechanics", printed in 2004.

Some more recently written introductions which draw heavily on Bell's writings, including the above three essays:
  1. T. Norsen, "John S. Bell’s concept of local causality", Am. J. Phys. 79, 1261 (2011), arXiv:0707.0401 [quant-ph].
  2. S. Goldstein et. al., "Bell's theorem", Scholarpedia 6, 8378 (2011).
 
Last edited:
  • Like
Likes maline
  • #117
wle said:
in both, Minkowski diagrams depicting light cones are used as aides to the argument.
I know that Bell (like earlier Einstein) used causality to motivate the experiment and to deduce nonlocality, but my emphasis was on ''formal use made of'' it. No formula involves anything relativistic - only the talk around it does. But the relevant physics is always in the formulas only, the accompanying talk is only interpretation. That's why multiple interpretations abound, while the formalism is universally agreed upon.

I had simplified the setting since the nonlocality part is still present and invites essentially the same weirdness considerations: There is no intuitively natural way of explaining how the detector can respond as it does - except for having the universe conspire to collect in a mysterious way the nonlocal information and turn it into the appropriate statistical output.

From my point of view, nothing more mysterious happens in Bell's experiment, since exactly the same algebra is used, and the speed of light argument is extreaneous to the whole formal setting. The quantum mechanics is nonrelativistic since simultaneity is essential, nowhere in the algebra the unity of space and time characteristic for relativity appear, and nowhere the Lorentz group figures.
 
  • #118
A. Neumaier said:
I gave a local hidden variable argument of precisely the kind that was used by Bell and found a Bell-type inequality that was violated by the prediction of quantum mechanics. According to your criticism, there should be a fault in my formal reasoning since repeating the analysis using instead the Maxwell equations gives full agreement with the quantum predictions.

A. Neumaier said:
Isn't it already extremely weird to allow that a classical local hidden variable photon travels along several beams? I don't think that it is satisfying to explain away weirdness by basing the explanations on weird assumptions.
You didn't take Bell assumptions. If you modify Bell assumptions because you think they are weird does not change the fact that the assumptions used by you are not the ones used by Bell.
 
  • #119
Let us return to the main topic.

Concerning Stage 2 (post #49), we now have one qualifying degree of
weirdness (post #112, fully justified in post #115) and an expression
of dissatisfaction about trying to quantify weirdness at all (post #98).

Leaving Stage 2 still open for a while, I'll begin with the next stage,
where I promised to state my own interpretation of the experiment,
and why I think the results are not weirder than what one finds
classically in other situations. My interpretation will extend over
two stages: Stage 3, where I make some general remarks that are
independent of what is discussed in Stage 2, and (when Stage 2
is completed) Stage 4, where I use the results of Stages 2 and 3 to
complete my view on the matter.

(Note: Stages 2 and 3 are now also closed for discussion.
Stage 4 begins in post #187.)

Stage 3 is opened with the following observations, whose discussion
is invited. My observations at this stage are completed in post #174,
with a discussion of how weirdness and knowledge are related.

In our setting, assume for the moment that the nature of Norbert's
signals are known to everyone, and are of the kind consistent with
quantum mechanics but inconsistent with Bell-type assumptions.
Assume also that there is a human Alice behind the dumb machine Alice.

Under these conditions I want to discuss what the human Alice
knows about Bob's results after she has completed her experiments.

My claim is that she knows nothing definite at all.

For the results Bob gets depend on what he is doing, and she is not
informed about the latter. At best she can draw conditional inferences
''If Bob's pointer position was set to ... then his results were ...''.
This is closer to guesswork of the form we use in medical diagnostics
when decisive facts are absent than to scientific knowledge of the kind
we can find in standard textbooks, and to engineering knowledge encoded
in properly working machines.

The knowledge that Alice has feels more like what we know about an
(ideal) pendulum when its initial conditions are unknown - we know the
general structure of the possible configurations, but we don't know
anything about the configuation itself. If we take the analogy seriously
we conclude that [given Norbert's fixed signalling strategy]
Nature solves an initial-value problem with two inputs
(pointer settings) and two outputs (color of response) - that on Alice's
side and that on Bob's side. The joint output depends on both inputs.

This dependence is Bell's form of nonlocality - demonstrated by this
kind of experiments and quite obvious from this way of thinking about
it, even without an experimental proof by the violation of corresponding
Bell inequalities.

Remarkably, Bell's findings wouldn't have seemed weird at the end of the
19th century - classical field equations such as the heat equation also show
this kind of nonlocality!
Nonlocality is classically intrinsic even to Newtonian mechanics in its
original form where celestial bodies act instantaneously over
arbitraily large distances. It is a standard part of nonrelativistic
classical mechanics. So why should nonlocality count as weird?

Being already manifestly present in nonrelativistic multiparticle
classical mechanics, it is no surprise at all that it is also present in
nonrelativistic multiparticle quantum mechanics such as Bell-type
experiments! Note that essentially all analysis of Bell nonlocality is
done in a nonrelativistic framework! Plus lip service paid to relativity,
in a form that doesn't enter at all into the formulas...

To impose weirdness by invoking arguments involving the speed of light
in an otherwise nonrelativistic framework also makes the heat equation
seem weird since a change in temperature at one place immediately
affects the temperature everywhere else.
... and Newton's celestial mechanics since the change in position of one
celestial body immediately affect the positions everywhere else.

The quibbles with this form of nonlocality are caused by a superficial
understanding of relativity theory and the use of superficial relativity
arguments in an explictly nonrelativistic classical or quantum setting.
What seems to be unnatural or weird is solely due to mixing two
incompatible settings.

If one attempts to disentangle the two settings interesting things happen:

On the purely nonrelativistic level, all weirdness has disappeared;
things are no worse in quantum mechanics than in classical celestial
mechanics or fluid mechanics.

On the other hand, one can try to see what happens when one looks
at classical relativistic multiparticle theories. Once one starts looking for
these (I challenge you to do such a search yourself) one finds that from
the outset, they are plagued with tremendous weirdness!

Clearly, it is the particle picture that - classically! - introduces this
weirdness into relativity theory since classical relativistic field theories
have no problem at all as long as one doesn't introduce point particles
into them. It thus appears that in quantum mechanics of point particles
the classical weirdness is even softened since it appears only in situations
that take a lot of effort to prepare, and disappears completely once one
consistently stays in the realm of quantum field theory.
 
Last edited:
  • #120
A. Neumaier said:
... ruled out only under the assumption of a local hidden variable theory with signals moving independently along the rays to Alice and Bob. But this assumption is too strong to have implications when the signal is a field rather than particles.

The sense of "local" that is important to Bell's analysis is the issue of whether something taking place at Alice's detector has an effect on Bob, or vice-versa.

So let me grant the possibility that the entire environment--the whole rest of the universe--works together in a nonlocal way to establish the two outcomes at Alice and Bob. Then the question becomes whether it is possible for Alice and Bob to make up their minds at the last minute as to which detector setting to choose. This is sometimes called the "free will assumption", but it doesn't actually need to rely on anything mystical about consciousness. It's just that Alice and Bob can base their choice on absolutely anything, such as a radioactive decay of a uranium atom, or some characteristic of the light from a distant star, etc.

Let's split up the universe into three parts:
  1. The part \lambda relevant to the production of the twin-pair.
  2. The part \alpha relevant to Alice's choice of her detector setting.
  3. The part \beta relevant to Bob's choice of his detector setting.
So, if you are claiming that the details of the whole universe make the outcomes deterministic, then that would seem to me to imply to me that there is a pair of functions determining the outcomes:
  • F_{Alice}(\alpha,\beta,\lambda) = the probability of Alice getting spin-up, given \alpha, \beta, \lambda
  • F_{Bob}(\alpha,\beta,\lambda) = the probability of Bob getting spin-up, given \alpha, \beta, \lambda
The key question relevant to Bell's argument is really about whether the three parameters \alpha, \beta and \lambda can be varied independently. If they cannot be, that's pretty weird. If they can be, then Bell's analysis goes through, showing that for any such functions F_{Alice} and F_{Bob}, Alice's result must depend, in a FTL way, on conditions at Bob, or Bob's result must depend, in an FTL way, on conditions at Alice.
 

Similar threads

Replies
58
Views
4K
Replies
11
Views
2K
  • · Replies 47 ·
2
Replies
47
Views
5K
  • · Replies 140 ·
5
Replies
140
Views
12K
Replies
97
Views
8K
  • · Replies 1 ·
Replies
1
Views
1K
  • · Replies 4 ·
Replies
4
Views
1K
Replies
23
Views
6K
  • · Replies 54 ·
2
Replies
54
Views
6K
  • · Replies 100 ·
4
Replies
100
Views
11K