Realism in the vein of EPR and Bell

DrChinese

Morrobay suggested we split off this discussion from another thread.

I will post my summary of the EPR definition a bit later today.

DrChinese

EPR is a local realist's defense. EPR define "local realism"* by marrying 2 assumptions to the "elements of reality" criterion. We can show sufficiently that elements of reality exist for photon polarization (as our example) at angles 0, 120, 240 (same as -120). This is experimentally verifiable and has never been much in question. They ASSUME that the following are true (see last few paragraphs of EPR):

1. There are no FTL influences (i.e. no spooky action at a distance).
2. The individual physical quantities are considered to be simultaneous elements of reality. Otherwise, they state for the local realist position, reality here is dependent on the nature of a measurement there, and that would be unreasonable.

Bell showed that there are no datasets for angle settings 0/120/240** which also match the QM (and classical optics) predictions: cos^2(theta) in the case of photons. IF you consider the EPR criterion of element of reality to have been SUFFICIENTly satisfied for 0/120/240 (not much to quibble about that); AND you accept that experiments support the cos^2 relationship QM predicts****, THEN either 1. or 2. (or both) are unwarranted assumptions. QED.

So: There is one criterion, and 2 assumptions put forth by EPR. Aspect et al plus hundreds more have demonstrated that the QM predictions are accurate. This is the definition of local realism as envisioned by EPR and Bell. Basically, the elements of reality imply hidden variables. But the hidden variables, per Bell, cannot be local and traditional realistic. Where or what are they?

* The phrase is never used as such.
** Bell does not use these angles specifically, they are just one set of many possible. Makes no difference how many as long as there is one.
*** Notice that there is no need to simultaneouly measure 3 of anything to see this relationship. This is lost on many writers. This has been experimentally verified to well over 30 standard deviations.

DrChinese

There are several "escapes". The hidden variables cannot be located in the past light cone. That is what Bell teaches us.

But they can be located in the present. That is the Bohmian answer. In this view, the positions of other particles are participating in what is often called the "context".

The hidden variables can also include a future component. This means that the past and the future both form a context. This is consistent with time symmetric interpretations of QM.

Both of these views have the advantage of explaining why the hidden variables cannot exist in the past as predetermined quantities that exist independent of how the measurement is performed.

Loosely speaking:

Contextual <=> Observer dependent <=> Non-realistic

nanosiborg

DrChinese, I'm not sure what your point is.

mbd

Here are some good sources for definitions relevant to EPR and Bell:

Antonio Di Lorenzo defines clearly the assumptions of Bell's Theorem.
http://arxiv.org/abs/1105.1286

Andrei Khrennikov defines realism, locality, and other relevent terms such as value definiteness, contextuality, etc.
http://arxiv.org/abs/1108.0001

jk22

In QM treatment of EPRB it is seen from the onset that the measurement disturbs the system since suppose we measure 1st particle with + then
psi_before equals ((+-)-(-+))/sqrt2 and
psi_after equals* (+-)
since both differ we conclude there was a disturbance.
Hence we cannot use the EPR criterion of existence of elem. of phys. reality.
Bell showed that the supposition of such elements lead to a contradiction implying, if the criterion is correct, that either we disturb the system, which is already known, or we cannot predict with certainty, see * after, or both which seems to be the case

*normally the state of 2nd particle should remain undermined after the measurement of the 1st since 2nd was not measured and should be written as
psi_after equals (+,und) with und meaning any (parametrized by unknowns) state of 2nd system

thus we can't predict with certainty the outcome of a subsequent measurement of 2nd subsystem.

DrChinese

We split this off from another thread. Thought I would make a few statements to get things going.

DrChinese

He is a local realist. As I have said many times, if you deviate from the Bell definition of realism, then you may get a different result. So what in this paper do you think is relevant to the discussion of Bell realism?

mbd

Both papers to which I provided links define terms consistently with Bell.

Antonio Di Lorenzo's paper (peer reviewed and recently published in Physical Review A) clearly defines the assumptions of Bell.

These are "measurement independence", "outcome independence", and "setting independence". See section III of the paper.

It's essential to state definitions independently of conclusions regarding theoretical or experimental evidence. These authors have succeed at this.

Previously I posted a link to a paper of a team that you'd probably characterize as non-localists (although the science of their paper is untainted with bias), and it too had clear definitions and a uniquely clear exposition of the implications of the assumptions due to Bayes' Rule. Here is that link again:

http://dx.doi.org/10.1073/pnas.1002780107

DrChinese

This paper features a straightforward definition: "Realism is a world view 'according to which external reality is assumed to exist and have definite properties, whether or not they are observed by someone'."

I believe the inner quote is from Shimony & Clauser, 1978. I would call that standard, well-stated description from some of the masters in the field. I would not call them non-localists myself, as I follow their ideas fairly closely. I would say they believe in quantum non-locality (as I do). That is: a quantum context is not limited by a classical light cone.

DrChinese

Not exactly. Not that I dispute the definition below, but I would hardly call that something
consistent with either EPR or Bell. These ideas are never mentioned in either paper.

From Appendix B:
"[Realism] is but counterfactual-definiteness, ... . It is well known [14, 21] that the hypothesis of counterfactual-definiteness alone is sufficient in order to derive Bell-type inequalities."

I say that both EPR and Bell explicitly state that locality is an assumption required to obtain their results. Again, I am not disputing the statement itself, just wondering how it ties in. As I keep repeating, when you deviate from the definitions of EPR/Bell, you may or may not get the Bell result. If you don't, that would not be a failure in Bell - it would be a semantic issue.

DrChinese

OK, since you won't bring in a quote, I will. I always enjoy Khrennikov's papers, and would enjoy watching him and Norsen debate the matter. From your reference:

"(R) Realism: The possibility of assigning to a quantum system the values
of observable quantities before measurement2 and these values are confirmed
by this measurement.

(L) Locality: No action at a distance.

Therefore, anyone who accepts that experiments are strong signs that
local realism has to be rejected has to make the choice between:

(NONL) Realism, but nonlocality (Bell’s position).
(NR) No realism (nonobjectivity) and locality (Bohr’s position).*"

I thoroughly agree with the above definitions of locality and realism, they are fully in keeping with both EPR and Bell.


*A third option is also mentioned, one in which both realism AND locality are rejected. This is often mentioned as well.

lugita15

Here's an old post of mine that's relevant to this:
And then from there, it's a fairly trivial matter to derive Bell's inequality. See this excellent explanation of Bell's theorem by Nick Herbert.

mbd

Here's a simple hypothetical counter-example to the conclusion that local realism is all but ruled out. I am not suggesting this to be physically plausible, nor am I proposing a theory. It is only an illuminating counter-example. It exploits two, if not more, known loopholes, that have not been closed experimentally. It has no backward-in-time effects, and no action at a distance, and results in CHSH correlations of 4.0.

Suppose that the measuring apparatuses are spewing, toward the emitter, a continuous, speed-of-light, stream of messenger particles that report the angle of the apparatus at the moment of messenger emission? Suppose that the emitter emits a pair of photon-precursor particles, one toward each measuring apparatus and that the photon pre-cursors remember the angle reported by the messenger particle arriving from their respective destinations at the moment of emission? Also, imbue the photon precursors with identical values if the measuring angles are pi/8 apart, and opposite values if the measuring angles are 3pi/8 apart. Now, also suppose that the photon pre-cursor observes the messenger particles it encounters along the way, and pauses whenever the messenger particle does not match the one seen on photon-precursor emission? Then, the pair of emitted photon pre-cursors can only arrive at the detectors coincidentally if both measuring apparatuses are at the same angles as they were when the photon pre-cursors were emitted. Upon arrival at the detector, the photon-precursors then produce a detection having the value imbued upon their emission.

The result in CHSH? 4.0

To make it do Malus' Law (to confirm measurements are consistent with classical photons), the particles need to, instead, slow or reverse course in proportion to how different the messenger is from the messenger seen at photon-precursor emission. It's less clear why this achieves a CHSH > 2 as well, thus it's left out in the simplified example above.

In any case, it passes the realism test, it violates Bell's Inequality, but it is not counterfactual definite.

DrChinese

I don't know where to start here. When you put forth a counter-example to local realism, it necessarily must adhere to several additional requirements of EPR and Bell. You don’t run through an actual example, so I necessarily must make some assumptions. First, does it give the same answer when Alice and Bob have identical settings. The answer to this is YES, so that is a good start. I know this is true because you have a formula which always gives the same answer when the inputs are the same (I will give you the benefit of any doubt on this point). This is the perfect correlations requirement which is to say: it meets the EPR test for elements of reality.

Second: Does it reproduce Malus? Sadly, no. That is because being deterministic, there is no way to get the random element going we need. So when the angles are only a few degrees apart, say 10 degrees, you will get the same results as when the angle settings are the same.

Lastly: There is NO explanation of how Alice and Bob’s communication occurs. I get that there is some mechanism for the handshaking between Alice and the emitter, and Bob and the emitter. But for the example to work, the emitter needs to know BOTH Alice’s and Bob’s settings. That mechanism is ruled out when the locality loophole is closed in an experiment such as that of Weihs et al (1998). You can have all the “precursors” you like, but that changes nothing when you review the light cones in that experiment. This is what Bell’s Theorem is ultimately all about: how are the relative settings of Alice and Bob communicated to each other or to the source since classical means are ruled out?

mbd

Malus' Law only requires "randomness" if you assume the completeness, or correctness, of quantum mechanics. In the absolute mechanism I described, of course Malus' Law is not honored. In order for Malus' Law to be honored, the rate of emission and/or the velocity of the hypothetical "photon precursor" needs to be modulated according to the most recently received messenger particle. This would be consistent with classical conservation of angular momentum. But, if you insist on randomness, then it is in fact easier to simply put local randomness in each "photon precursor" and have it create a photon event upon impact with the polarizer conditioned by a probability distribution that will result in compliance with Malus' Law. (so, now we're using the detection loophole too!)

There is no communication between Alice and Bob. No need to explain something that doesn't happen. All interactions depend only on locally available information.

Note: The original formulation used the memory loophole, time-coincidence loophole, freedom-of-choice loophole (Zeilinger 2010 closed freedom-of-choice conditional on certain assumptions). To comply with Malus' law, the refined formulation uses the detection loophole.

DrChinese

1. Since Malus is not respected, the Bell premise is preserved. You can have local realistic mechanisms that do not match the predictions of QM. And, as here, they will not match experiment either.

2. True, there is no communication between Alice and Bob in your example. That is another reason why you can never achieve a violation of a Bell Inequality. You will need that to have that happen.

PS Just saying you can prove "my non-mainstream scientific idea works" does not fly around here. You cannot publish your own personal theories here and say stuff like "CHSH=4" is the result unless you can prove it upon challenge. You will be reported if you continue this line of reasoning. Please return to the subject of this thread without asserting you have a counterexample unless you intend to demonstrate its successful operation in specific. Keep in mind that I will expose any flaw I find, and believe me, I have seen a few over the years in this area.

So the DrChinese challenge for your example:

a) Must yield perfect correlations at identical angles (zero degrees difference). Your current example succeeds here.

b) Must yield results consistent with experiment (Malus). Your example fails here. You will not have a 25% correlation rate when the angles are 120 degrees apart in your example.

c) Must pass the strict locality test. Your example fails here because the communication you describe is excluded in the experiments of Weihs et al (1998) which closed the locality loophole.