# Loopholes in optical Bell tests

In reading about Bell tests I found this link:http://www.abc.net.au/science/features/quantum/, which has discussion of numerous loopholes in both theory and experimental design of Bell tests, which when combined, appear to suggest that "local realist" explanations of all Bell test violations should exist. That is, no single Bell test does not have at least one of the following seven loopholes, and since it only takes one loophole (1 of 7) to invalidate test results as suggesting, "local realism" does not exist, in fact, there are no Bell tests that disprove "local realism". Second part of thread, is anyone aware of yet additional "loopholes" in the published literature ?

Bell Test Loopholes (no single Bell test does not contain at least one loophole)
1. Fair sampling loophole
2. Subtraction of accidentials loophole
3. Failure of rotational invariance loophole
4. Synchronization loophole
5. Enhancement loophole
6. Double detection loophole
7. Locality-light cone interpretation loophole

Staff Emeritus
In reading about Bell tests I found this link:http://www.abc.net.au/science/features/quantum/, which has discussion of numerous loopholes in both theory and experimental design of Bell tests, which when combined, appear to suggest that "local realist" explanations of all Bell test violations should exist. That is, no single Bell test does not have at least one of the following seven loopholes, and since it only takes one loophole (1 of 7) to invalidate test results as suggesting, "local realism" does not exist, in fact, there are no Bell tests that disprove "local realism". Second part of thread, is anyone aware of yet additional "loopholes" in the published literature ?

Bell Test Loopholes (no single Bell test does not contain at least one loophole)
1. Fair sampling loophole
2. Subtraction of accidentials loophole
3. Failure of rotational invariance loophole
4. Synchronization loophole
5. Enhancement loophole
6. Double detection loophole
7. Locality-light cone interpretation loophole

You appear to have gone one step too far. These so-called loopholes do NOT "invalidate" the results. That's like saying measurement of zero resistivity invalidates the existence of superconductors simply because superconductivity has other properties that resistivity measurement could not measure. On the contrary, each of the experiments, no matter which loopholes they have, clearly gives a strong indication that "local realism" isn't as convincing as QM description. Think about it. No matter which loophole an experiment has, none of them ever indicate a result that is not consistent with QM's predictions.

Furthermore, you seem to have ignored a more stringent test of local realism based on multipartite system, and the more stringent extension of Bell theorem in the CHSH theory. Again, collect ALL of the bell-type experiments that cover a huge amount of experimental variation and find those that claim an inconsistent result with QM. You make everyday decisions based on orders of magnitude LESS certainty than this.

Zz.

Gold Member
Bell Test Loopholes (no single Bell test does not contain at least one loophole)...

This is illogical. You can run any test and see that the presence or absence of any of the above "loopholes" makes no difference to the outcomes. All outcomes support QM and reject Local Reality. It is like asking: "Do you get the same results on Monday as you do on Tuesday?" Or "Do you get the same results in the Northern Hemisphere as in the Southern Hemisphere?"

Is it a loophole that every permutation of every experiment in science is not performed each and every time? No, of course not. The object is to determine what variables make a difference to the outcome. How is it that none of the 7 listed individually make any difference but several collectively do?

It is not generally accepted that these are loopholes anyway. The actual tests are very convincing and leave little room for doubt. I would spend more time reading the actual tests, and then you will see how ridiculous these claims really are.

ZapperZ said:
...you seem to have ignored a more stringent test of local realism based on multipartite system, and the more stringent extension of Bell theorem in the CHSH theory.
Not wishing to be argumentative, just passing on what I read. Thus, the CHSH-theory brings forth yet two more loopholes, as read at this link:http://www.math.uu.nl/people/gill/Preprints/poster.pdf [Broken]
8. Detection Loophole
9. Coincidence Loophole
and here is the conclusion of these authors: "Closing each separate loophole in different experiments is just not good enough. Maybe it is not a coincidence, that no one could close all loopholes in one experiment, despite 20 years progress".

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DrChinese said:
It is not generally accepted that these are loopholes anyway. The actual tests are very convincing and leave little room for doubt. I would spend more time reading the actual tests, and then you will see how ridiculous these claims really are.
OK, here is one manuscript where Bell tests were studied and they concluded that at least one loophole is very convincing indeed--I do not find their logic ridiculous, perhaps open to debate, but not ridiculous:
http://freespace.virgin.net/ch.thompson1/Papers/chaotic2.v4.pdf

I seem to have hit some sort of QM nerve with this thread--why so defensive of Bell tests ? This is an unscientific position--one must always show how results of experiments may be incorrect based on faulty experimental design and or false conclusions based on holding false premises (so-called loopholes). That fact that a number of physicists claim these loopholes are valid is for me a red flag--Bell tests are not all that convincing afterall that local realism does not exist.

It seems to me that this "loophole" problem is very valid--QM makes an extraordinary claim--that local realism at both the macroscopic and microscopic level does not exist--thus it requires extraordinary experimental evidence from "at least one experiment" that is free of any and all "loopholes". As far as I know, no such experiment exists, and at least one author has suggested that it is impossible for such an experiment to ever be conducted. It is clear from the numerous discussions about loopholes such as above internet link that QM does not prove that reality is non-local--it is only a hypothesis.

But I shall stop hitting nerve cells and let others that have detailed knowledge of Bell loopholes continue the dialog on this thread.

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Gold Member
Not wishing to be argumentative, ...

If you are asking: The opinions of those authors is not really mainstream science. Try a quote from something more authoritative. If you bother to actually check the experiments, instead of someone else's biased views, you will see that the evidence is very convincing. Try this, for example:

http://arxiv.org/PS_cache/quant-ph/pdf/9810/9810080.pdf [Broken]

This is peer-reviewed science. It is considered definitive by scientists, although I doubt that any Bell experiment can be constructed to satisify some people.

If you are telling: Some people think the world is flat - they see loopholes in the theory that the Earth is round. Meanwhile, the rest of us fly around the globe.

Ultimately, you are free to believe whatever you want.

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Gold Member
OK, here is one manuscript where Bell tests were studied and they concluded that at least one loophole is very convincing indeed...

You are quoting Caroline Thompson, an obnoxious crackpot who was banned from this board. I can tell you from personal experience that there is something seriously wrong with her. And besides denying the validity of Bell tests, she also denies the existence of photons. If you also question the existence of photons, please let us know now and save the rest of us the time we might otherwise spend discussing the subject with you.

As to the nerve you are hitting: this subject has been debated ad nauseum here. PhysicsForums is focused on mainstream and cutting edge science; those who deny accepted/published scientific experiments are neither. If you have something useful to add to the discussion, that is great. If you are planning to parrot the words of Caroline, you likely won't last long here.

As I said previously, it is instructive to read the actual experiments and understand Bell before you try to tear it down. This area has been studied at great depth by some pretty good scientists.

Now to the science itself:

I will tell you that there are in fact 2 separate issues: a) Bell's Theorem and b) tests of Bell's Inequality. Bell's Theorem says that QM and Local Reality are incompatible in some respects. Tests of Bell's Inequality uniformly show that Local Reality is violated. Separate tests of QM uniformly support it. So the problem is a lot bigger for the local realist than it might first appear. You must explain not only why LR is violated (that's the loopholes you mention); but you must also explain why the experimental error always takes you to the QM predicted value! That's a tall order, and one that not a single LR supporter has ever endeavored to address. On the other hand, if there is no significance to the purported loopholes, then the QM position is fine as is.

LR predicted value + significant experimental error = observed value

The above is what a local realist asserts, but the problem is you have to come to grips with the following coincidence which cannot be denied:

QM predicted value = observed value

So I assert the simplest explanation of all: There is no experimental error. So you see, the local realist has a tremendous burden. If you do not follow this logic, then look at the LR predictions at various angles and you will quickly see how far off you are from observed results.

Staff Emeritus
OK, here is one manuscript where Bell tests were studied and they concluded that at least one loophole is very convincing indeed--I do not find their logic ridiculous, perhaps open to debate, but not ridiculous:
http://freespace.virgin.net/ch.thompson1/Papers/chaotic2.v4.pdf

Oh, now this is getting somewhere.

You are quoting someone who (i) cannot derive the wave equation from the Maxwell equation even though she's claiming that photons are waves and (ii) have been known to say that maxwell equations are nothing more than "just math". So this is the person you pay more attention to?

Here's something you might want to ponder. NO EXPERIMENT and no experimentalists have made any conclusion that the results of their experiment are inconsistent with QM's prediction. The ONLY people still clinging on to such possibilities are either a very small and obscure number of theorists, or crackpots like your hero. None of these people have actually gone out and DO the experiment themselves! Why? Because they can't, or do not understand the nature of detectors and thus, have zero understanding on how such measurements are done.

This important fact is totally ignored when someone comes here and claim these things. The amazingly large number of experiments that covers a large variation of techniques, but giving CONSISTENT conclusions is astounding! The logic you used would also cause you to claim that superconductivity experiments are "invalid" because they didn't measure everything at once. Would you like to go out on the limb and claim that too?

Zz.

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Sherlock
In reading about Bell tests I found this link:http://www.abc.net.au/science/features/quantum/, which has discussion of numerous loopholes in both theory and experimental design of Bell tests, which when combined, appear to suggest that "local realist" explanations of all Bell test violations should exist. That is, no single Bell test does not have at least one of the following seven loopholes, and since it only takes one loophole (1 of 7) to invalidate test results as suggesting, "local realism" does not exist, in fact, there are no Bell tests that disprove "local realism". Second part of thread, is anyone aware of yet additional "loopholes" in the published literature ?

Bell Test Loopholes (no single Bell test does not contain at least one loophole)
1. Fair sampling loophole
2. Subtraction of accidentials loophole
3. Failure of rotational invariance loophole
4. Synchronization loophole
5. Enhancement loophole
6. Double detection loophole
7. Locality-light cone interpretation loophole
8. Detection Loophole
9. Coincidence Loophole

That's a lot of loopholes. But, experimenters are quite careful
Bell tests are getting better. Instrumentation and detection is getting
better. I'm betting that when a test is done that everybody can agree
is loophole free, then that test will show a violation of the inequality
that was applied to it.

The only questions remaining to be answered will then have to do with
how the results are interpreted. What do experimental violations of the
inequalities mean? Apparently most physicists don't take
the idea that nature is nonlocal too seriously. Keep in mind that
"quantum nonlocality" doesn't mean superluminal propagation
of anything. I would also guess that most physicists think that
there is a real world out there full of phenomena that exist whether
we happen to be looking at them or not -- and that these phenomena
have certain definite properties at any instant, and function according
to certain rules

So, what exactly does it mean to say that experimental violations
of Bell inequalities contradict local realism, or that such violations
disallow local realistic models? I'm not sure.

Here's Wikipedia's definition of local realism:
-----------
In physics, the principle of locality is that distant objects cannot have direct influence on one another: an object is influenced directly only by its immediate surroundings. This was stated as follows by Albert Einstein in his article "Quantum Mechanics and Reality" ("Quanten-Mechanik und Wirklichkeit", Dialectica 2:320-324, 1948):

"The following idea characterises the relative independence of objects far apart in space (A and B): external influence on A has no direct influence on B; this is known as the Principle of Local Action, which is used consistently only in field theory. If this axiom were to be completely abolished, the idea of the existence of quasienclosed systems, and thereby the postulation of laws which can be checked empirically in the accepted sense, would become impossible."

Local realism is the combination of the principle of locality with the assumption that all objects must objectively have their properties already before these properties are observed. Einstein liked to say that the Moon is "out there" even when no one is observing it.
------------

According to this definition, locality means that the settings (or results or whatever) at A don't affect the results at B, and vice versa. There's nothing in the experiments themselves to indicate that A and B are affecting each other. Saying that A and B are affecting each other is a matter of interpretation. My current approach is to assume that the experimental results are more or less correct, and that nature is local (ie., that the speed of light is a limit) -- so there's something wrong with interpretations
that conclude that violations of the inequalities means that 'something' is propagating superluminally between A and B.

Realism here is defined as "the assumption that all objects must objectively have their properties already before these properties are observed." Well, nobody really knows what the term "photon", as defined in quantum theory, corresponds to in nature.
In optical Bell tests, the photons that are detected are transmitted by
the filters. It isn't known exactly what is actually happening 'in nature' at the level of the interaction between the emitted light and the filter. It isn't known exactly what is happening between emission and detection, or during the emission process. There's no qualitative apprehension of entities or events or processes at the 'quantum level' of nature. There are models which relate experimental preparations and results of course, but these don't give a clear qualitative picture of the 'quantum world'.

Einstein wanted a more realistic theory. Everybody would like to have
a more realistic theory -- ie., an accurate 'picture' of what's happening at
the level of the quantum. But there isn't one.

So, what does it mean to say that a photon has certain properties
independent of detection? Not much so far -- all attempts, afaik,
to model Bell tests in a 'realistic' way have been contradicted.
Of course that doesn't mean that the light incident on the filters
doesn't have any physical properties. It wouldn't make any
sense to say that. But the 'realistic' models are not up to speed yet.

The optical Bell tests that use atomic cascades (see, for example, Aspect et al. 1982) indicate that in order to get entanglement (ie., violation of an inequality), coincidental detections have to be causally linked to the same oscillator (the same atom). Quantum theory says that two photons emitted in opposite directions by the same atom will have identical polarization. This doesn't refer to any property of the emitted light prior to detection. Nevertheless, the tacit assumption is that light emitted in opposite directions by the same oscillator at the same instant has some common physical characteristics (which would correspond to Schroedinger's
meaning when he coined the term, entanglement) -- which is the thing that Bell's theorem, as interpreted by some, is supposed to disallow.

The whole thing is an intriguing mystery, a significant part of which is evaluating the physical meaning of Bell's theorem. My current understanding is that, while experimental violations of inequalities can be used as a fairly trustworthy indicator of entanglement, the Bell tests aren't necessarily telling us that nature is nonlocal or that a more realistic theory is, in principle, impossible.

Anyway, instead of worrying about the loopholes I would rather marvel at the promise of actually being able to do stuff with quantum entanglement.

It's not that experimental loopholes aren't an important consideration -- it's that statements about nature being nonlocal or realistic theories being impossible because of Bell's theorem aren't an important consideration.

Inequalities based on a Bell-type model of the prototypical optical Bell test will always be violated. What A and B are measuring individually is not the same as what A and be are measuring jointly. In the joint context (the Bell test context), the settings and results at one end aren't separable from (aren't independent of) the settings and results at the other. They're being considered *together*. Inequalities based on a model that separates them will always be violated.

Keep in mind that the only physical connection that A and B have is that they're jointly measuring something that came from the same oscillator at the same instant. The coincidence correlations are produced by the joint settings of the filters.

So, there's only two possibilities. Either the filters are jointly analyzing the same thing, or the two arms of the setup are 'communicating' or 'influencing each other' via some unknown and undetectable, superluminally propagating 'disturbance' that has no upper limit. My guess is that most working physicists are inclined to believe the former rather than the latter.

Staff Emeritus
Gold Member
I seem to have hit some sort of QM nerve with this thread--why so defensive of Bell tests ? This is an unscientific position--one must always show how results of experiments may be incorrect based on faulty experimental design and or false conclusions based on holding false premises (so-called loopholes). That fact that a number of physicists claim these loopholes are valid is for me a red flag--Bell tests are not all that convincing afterall that local realism does not exist.

It is true that there has not yet been a single experiment where raw data as such have demonstrated a violation of a Bell inequality (at least in 2-photon experiments, I'm less aware of others). In order to be able to do so with 2-photon systems, the individual photodetectors should have a quantum efficiency of the order of 86% and neglegible noise counts, and this is technologically not realized yet (I'm less well up in my reading concerning more than 2 photons). So experimental corrections are necessary.
As such, it has to be said, that the entire family of local realist theories has indeed not been definitely excluded BY RAW DATA. What has been done, however, each time, is that using the standard knowledge people have of 1) the experimental devices (such as quantum efficiency) and 2) of simple quantum mechanics, they found results IN AGREEMENT with these predictions. Now, each time, you could also think up A MODEL MADE UP ON PURPOSE that would also wiggle into explain the data, but these models have huge difficulties predicting OTHER things that are correct, and moreover have very peculiar views on how well-known apparatus (such as photodetectors) operate. Usually it is erroneously claimed that "nobody knows exactly how these detectors/polarizers... work".
For instance, one model is the classical field approach with some added vacuum noise. No electrons, no photons, just classical fields. I'm talking here about Stochastic Electrodynamics (SED). However, SED has difficulties explaining things like the helium atom.
The theory that fits ALL data well is quantum theory. This does not exclude of course that something else might not exist, but you have a hard time doing so. The idea is that if quantum theory had a serious problem, one would see DEVIATIONS from the predictions, using the usual properties of the apparatus.

It is a bit as if you are, in court, confronted with:
- witnesses have seen you 5 minutes before the crime near the house,
- you have a great financial motive to have killed the person
- your car got a parking ticket 20 meters from the place of the crime 10 minutes before the crime
- there were blood stains of the victim found in your car,
- a week before the crime, you said, when drunk, on a party that you were going to cut that bastard's throat
- a knife fitting the victim's wounds was found in your dustbin.

Each individual element can have another explanation:
- you were just having a walk there
- you are not really interested in the money
- you parked your car there just to have a walk
- you forgot to close the car with the key, and the true murderer tried to steal your car, hence the blood stains
- one should not take you seriously when you are drunk
- you simply happened to have a similar knife which you happened to throw away by coincidence that day.

Ok, this is indeed possible. But what will the jury decide, do you think ?

Local realist explanations sound a bit like this explanation. True, sometimes an innocent gets convicted...

Sherlock said:
... My current understanding is that, while experimental violations of inequalities can be used as a fairly trustworthy indicator of entanglement, the Bell tests aren't necessarily telling us that nature is nonlocal or that a more realistic theory is, in principle, impossible...AND...
It's not that experimental loopholes aren't an important consideration -- it's that statements about nature being nonlocal or realistic theories being impossible because of Bell's theorem aren't an important consideration.
Thank you Sherlock...your detailed explanation and the conclusions you reach above "make logical sense" to me, since I hold a realistic view of existence. Now, as you state, what Bell tests "test" has nothing to do with local realism, but has much to say about "entanglement", which is a very important topic. So, now I can fly around the world happy that the world really could care less that I (or any other human) exist.

Staff Emeritus
Thank you Sherlock...your detailed explanation and the conclusions you reach above "make logical sense" to me, since I hold a realistic view of existence. Now, as you state, what Bell tests "test" has nothing to do with local realism, but has much to say about "entanglement", which is a very important topic. So, now I can fly around the world happy that the world really could care less that I (or any other human) exist.

Just because that makes you sleep better at night doesn't mean that is what it does. There are OTHER experiments that make quantum entanglement even more pronounced than Bell-type tests. The experiment that shows 2 entangled photons beat the diffraction limit is MORE convincing. If all we need are tests of QM entanglement, we already have that without doing the Bell tests.

Yet, the Bell and EPR-type experiments are still being done AND refined. Why? Because each one of them points to a CONSISTENT evidence of the failure of LR. It isn't JUST an experiment on entanglement, because if it is, the "loopholes" would not matter a single bit!

Like I have said, YOU make decisions based on LESS convincing evidence than what we have seen about QM entanglement. Yet, these don't seem to bother you one bit.

Zz.

Homework Helper
I don't understand why some people are so determined to prove that Quantum Mechanics is "not the complete picture". The accusation has often been made that scientists are not being scientific in their "blind" or "dogmatic" adherence to the tenants of QM over local realism, but it seems to me that just the opposite is true. The irrational and persistent attempt to get around the results of QM by proposing all kinds of hidden variable theories is, it seems to me, the very definition of personal bias in science.

Do these more complicated hidden variable theories make any predictions that QM does not? It seems to me that these theories are tailored to give exactly the predictions of QM, and that they exist only so some researchers can put their minds at ease on a number of non-scientific philosophical points. When an experiment produces a result which QM fails to describe, then I will listen to competing theories. Until then I'll stick with the simple scientific theory of QM and justly call everything else philosophy.

Staff Emeritus
Physics Monkey said:
I don't understand why some people are so determined to prove that Quantum Mechanics is "not the complete picture". The accusation has often been made that scientists are not being scientific in their "blind" or "dogmatic" adherence to the tenants of QM over local realism, but it seems to me that just the opposite is true. The irrational and persistent attempt to get around the results of QM by proposing all kinds of hidden variable theories is, it seems to me, the very definition of personal bias in science.

And to provide evidence of what you have just said, the very fact that there CONTINUE to be more and more experimental tests of this phenomenon is clear evidence that physicists simply won't take things for granted on anything. If we all have already decided that this is a done deal, why all these more refined tests? That makes no sense.

On the other hand, these people who want to uphold Local Realism, NONE of them are doing ANY kinds of experiments! All they do is piggy-back on top of existing experiments (all of which claim failure of LR and supports QM's picture) and take the easy way out by pointing out the "loopholes". At best (and that itself is rather dubious), all they can say that these experiments cannot prove definitively that LR is dead. They certainly can't show anything that the results are inconsistent with QM's description. However, the OP of this thread took this an illogical step further by claiming that these experiments "invalidates" QM. Oy vey!

Zz.

Gold Member
ZapperZ said:
And to provide evidence of what you have just said, the very fact that there CONTINUE to be more and more experimental tests of this phenomenon is clear evidence that physicists simply won't take things for granted on anything. If we all have already decided that this is a done deal, why all these more refined tests? That makes no sense.

On the other hand, these people who want to uphold Local Realism, NONE of them are doing ANY kinds of experiments! ...

Zz.

As usual, well said. And just to drive the point home: in all of science, it is normal and common to refine tests to get even more and more accurate results and limit further the possibility of any deviations from accepted theory. Examples: tests of the value of c; tests of general relativity; tests of special relativity; tests of the equivalence principle; tests of the mass of the electron; etc. etc. and this is just in physics.

So while I assert that the results of Bell tests are conclusive, in the same breath I want to see them performed with greater efficiency and accuracy. You never know what you might find!

Locrian
Locrian said:
If the optical tests bother you so much, try different tests.
OK, a very nice paper dealing with mesons---then in a few seconds looking on the internet I find this recently posted (2005):http://arxiv.org/PS_cache/quant-ph/pdf/0501/0501039.pdf [Broken], and yet another loophole is identified, now with attempt to use mesons for Bell tests.

I did not start this thread to start an argument of facts--not possible because I know little about Bell tests. My point of the thread is that I find it very interesting that every time someone puts forth a Bell Test that claims to prove that local realism does not exist, yet another paper follows that shows the claim to be suspect--not conclusive--to have an interpretation loophole or one of at least nine other loopholes. I know of no loophole arguments against theory of gravity, not a single paper in > 100 years published on loopholes of cell theory in biology, second law of thermodynamics--no loopholes. Yet, each and every one of so-called Bell tests have papers published that identify loopholes. Is there not a pattern here that at least causes a scientist to raise an eyebrow ?--that is all I am doing at this point--eyebrow raising.

Perhaps those that hold local realism to be valid in physics have no need to waste limited and expensive lab time attempting to prove the obvious, that reality exists independent of human observation. Perhaps it is non-sense to even attempt to test such a proposition if in fact it is an axiomatic given, because in philosophy, and mathematics, from which physics derives, an axiom cannot be falsified by logic or reason or experiment--it just must be accepted (I will call this Bell's laspe). If in fact local realism is such an axiom concept, then it must be predicted that no single test can ever be devised to falsify local realism that will not always be found to have some loophole. And this is exactly the state of affairs since Bell over the past 40+ years, as predicted if local realism is an axiomatic concept, not a single Bell test that does not contain at least one loophole (e.g., 100% experimental verification by all Bell's Tests in favor of the hypothesis that local realism is an axiomatic concept of both physics and philosophy).

How about if we ask an experimental local realist physicist to chime in on the thread as to why they do not spend time testing Bell Inequalities--does anyone know of such a person ?

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Staff Emeritus
OK, a very nice paper dealing with mesons---then in a few seconds looking on the internet I find this recently posted (2005):http://arxiv.org/PS_cache/quant-ph/pdf/0501/0501039.pdf [Broken], and yet another loophole is identified, now with attempt to use mesons for Bell tests.

I did not start this thread to start an argument of facts--not possible because I know little about Bell tests. My point of the thread is that I find it very interesting that every time someone puts forth a Bell Test that claims to prove that local realism does not exist, yet another paper follows that shows the claim to be suspect--not conclusive--to have an interpretation loophole or one of at least nine other loopholes.

This is where you get your "facts" wrong. Look at ALL the experimental paper on such test. Now count how many peer-reviewed papers that appear to counter those conclusions!

There is no such thing as "yet another paper follows". There is, however something like "yet another INTERNET WEBPAGE" production that make such claims. Do these people manage to even write a rebuttal to those papers (you do know that one CAN write a rebuttal to such things if one believes that those papers are wrong, don't you?) in the respective journals? Nope!

You really should seriously consider the credibility of your sources that you somehow are putting a lot of weight on.

How about if we ask an experimental local realist physicist to chime in on the thread as to why they do not spend time testing Bell Inequalities--does anyone know of such a person ?

Good luck in finding one. I have my own opinion on why such a person does not exist.

Zz.

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Homework Helper
If I may, I would draw everyone's attention to the second sentence of the introduction to the most recent "loophole" paper (http://arxiv.org/PS_cache/quant-ph/pdf/0501/0501039.pdf [Broken]).

The author states, "Even if now SQM is confirmed by a huge amount of data and represents one of the pillars or modern physics, the fundamental quest about a possible realistic theory reproducing its results remains open."

What are the key phrases here? First, the author admits a huge amount of data supports QM. Second, the author admits that QM is justly regarded as a pillar of modern physics. Third, the author states that the goal of a local realistic theory is to reproduce the results of QM.

I think the author's description of this search as a "fundamental quest" is particularly apt. For local realists, the fundamental desire is to describe the world in terms of local realistic theories. This is their overriding goal and it constitutes a severe personal bias.

We can debate all the day long the question of local realism, and local realists can continue to show that they can construct excessively complicated theories that reproduce QM. However, until the day an LR researcher can propose some experiment that will distinguish LR from QM, the question remains in the realm of metaphysics.

At least that's how I see it.

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ZapperZ said:
Look at ALL the experimental paper on such test. Now count how many peer-reviewed papers that appear to counter those conclusions! There is no such thing as "yet another paper follows".
You really should seriously consider the credibility of your sources that you somehow are putting a lot of weight on.
I agree with you 100%--it is to peer reviewed papers on loopholes that we must put greatest weight of evidence. Here is my initial count for tonight--I note that they range from 1970 to 2004, very much in line with the time period of Bell tests that attempt to prove that local realism does not exist, some of these papers are reviews that cover many different Bell tests, not just one.

1. P. M. Pearle, Phys. Rev. D 2, 1418 (1970)
2. J. F. Clauser and M. A. Horne, Phys. Rev. D, 10, 526 (1974)
3. T. W. Marshall, E. Santos and F. Selleri, Local realism has not been refuted by atomic-cascade experiments, Phys. Lett. A, 98, 5-9 (1983)
4. T. W. Marshall and E. Santos, Stochastic optics - a reaffirmation of the wave nature of light, Foundations of Physics, 18, 185-201 (1988).
5. T. W. Marshall and E. Santos, Stochastic optics: a local realist analysis of optical tests of the Bell inequalities, Phys. Rev. A, 39, 6271-83 (1989).
6. C. H. Thompson, Subtraction of ?accidentals? and the validity of Bell tests, Galilean Electrodynamics 14 (3), 43-50 (2003)
7. C. H. Thompson, The Chaotic Ball: An Intuitive Analogy for EPR Experiments, Found. Phys. Lett. 9, 357 (1996)
8. A. Cabello, E. Santos. Phys. Lett A. 214, 316 (1996)
9. E. Santos, Phys Rev A, 69, 022305 (2004)
10. O. A. Barut. The fallacy of arguments against local realism in quantum phenomenon. in Waves and particles in light and matter. A van der merwe et al. eds. Plenum (1993).

---

Finally, although I know you will object, I include what I believe to be a recent balanced and fair defense of the local realist position about Bell tests by professor E. Santos. I personally would put weight on this internet post because Santos has published in recognized peer reviewed journals on this issue:

http://arxiv.org/PS_cache/quant-ph/pdf/0410/0410193.pdf [Broken]

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Staff Emeritus
Gold Member
I did not start this thread to start an argument of facts--not possible because I know little about Bell tests. My point of the thread is that I find it very interesting that every time someone puts forth a Bell Test that claims to prove that local realism does not exist, yet another paper follows that shows the claim to be suspect--not conclusive--to have an interpretation loophole or one of at least nine other loopholes. I know of no loophole arguments against theory of gravity, not a single paper in > 100 years published on loopholes of cell theory in biology, second law of thermodynamics--no loopholes.

Darwinism and relativity did have some papers against them. It is probably because the theory of (Newtonian) gravity doesn't shock any fundamental belief, while these others do.

Perhaps those that hold local realism to be valid in physics have no need to waste limited and expensive lab time attempting to prove the obvious, that reality exists independent of human observation.

Well, first of all you should know that this is a hypothesis, but a very well working one: reality exists and is independent of human observation. Locality is slightly less "fundamental" but only slightly so. If you can live with a violation of locality, try Bohmian mechanics: it is a totally realistic theory, but (of course, thanks to Bell) non-local, meaning: there's action at a distance (and hence its inner workings violate relativity, although its predictions don't). Btw, "standard" quantum mechanics with the projection postulate suffers from exactly the same problem: the projection of the wavefunction gives rise to "action at a distance" in the inner workings of the theory, if that wavefunction is considered to be "an objective state" of the world - but worse, this projection postulate has no physical grounding the way it is set up (no precise mechanism tells us, in QM, what exactly "projects out" the wave function).
On the other hand, there is a view of quantum mechanics which IS completely LOCAL and which assigns a reality to the world INDEPENDENT of human interaction, and that is many worlds. The problem there is that "reality" is quite different from what you might think it is, and the whole issue is translated to *what aspect of reality you consciously observe* (namely one term in the wavefunction). Although myself I'm a proponent of this view of quantum theory I'll tell you immediately that there are also problems with it, my hope being that this is only a "temporary way to view things" until we get a better grasp on the precise fitting together of quantum theory and gravity.

Sherlock
OK, a very nice paper dealing with mesons---then in a few seconds looking on the internet I find this recently posted (2005):http://arxiv.org/PS_cache/quant-ph/pdf/0501/0501039.pdf [Broken], and yet another loophole is identified, now with attempt to use mesons for Bell tests.

I did not start this thread to start an argument of facts--not possible because I know little about Bell tests. My point of the thread is that I find it very interesting that every time someone puts forth a Bell Test that claims to prove that local realism does not exist, yet another paper follows that shows the claim to be suspect--not conclusive--to have an interpretation loophole or one of at least nine other loopholes. I know of no loophole arguments against theory of gravity, not a single paper in > 100 years published on loopholes of cell theory in biology, second law of thermodynamics--no loopholes. Yet, each and every one of so-called Bell tests have papers published that identify loopholes. Is there not a pattern here that at least causes a scientist to raise an eyebrow ?--that is all I am doing at this point--eyebrow raising.

Perhaps those that hold local realism to be valid in physics have no need to waste limited and expensive lab time attempting to prove the obvious, that reality exists independent of human observation.
I think you're on the wrong track here. The tests aren't about whether or not reality exists independent of human observation. Nobody that I know doubts that. Or maybe I should say that we *behave* as if there is a reality that exists independent of human observation. (So, even though the qm predictions are very likely the correct ones, you can still rest easy, with full confidence that the world will continue to function whether anyone happens to be observing it -- and anyway, just in case, keep in mind that we aren't all asleep at the same time. At least I don't think we are. :-) ).

You said that you hold a realistic view of existence. I would guess that most people hold a realistic view of existence. I don't know what it would mean, or what value it would have to hold an unrealistic view of existence. :-)

However, the Bell tests are comparing the efficacy of a factorizable formulation (interpreted to represent all possible local hidden variable, or local realistic, theories) of the entangled state with a non-factorizable one (quantum theory).

Assuming that the experimental results have been correctly processed and reported and that the qm predictions are correct (and there really are very good reasons for assuming this), then it would seem that the qm formulation is, in a very important sense, more realistic than the so-called realistic theories.

Maybe a truly realistic description of the entangled state isn't separable as simply some combination of individual results at A and B. (This doesn't necessarily entail inventing some sort of superluminal, non-info-transmitting, ie., undetectable, disturbance traveling between A and B, that presumably would have to be propagating in a medium more fundamental than the electromagnetic one.) In which case, assuming the speed of light is a limit, the *form* of the local realistic description simply doesn't apply to the Bell test contexts -- so that Bell test results aren't telling us what some (many?) people think they're telling us.

One thing that everybody agrees on, however, is that experimental violation of a Bell-type inequality is a pretty good indicator of entanglement.

As I mentioned in another post, I think that everybody would like a more qualitatively realistic fundamental theory. But, it's premature to hope for that to happen in the foreseeable future. Maybe the qualitative nature of the 'quantum realm' will eventually be fleshed out. Maybe not. There are some good reasons to think that it can't be. In any case, quantum theory is the most realistic description of the 'quantum realm' that exists.

If I might presume to suggest a text: David Bohm's "Quantum Theory" is a good one to start with, and cheap (Dover). He discusses hidden variables and how the concepts of quantum theory are related (and also how they're not related) to our ordinary concepts. EPR is discussed. Lots of good stuff. He writes sort of like a philosopher, and I get the impression that you might like that. However, it is outdated (1950) so you'll also want a more modern text. Maybe some of the advisors/mentors here can recommend one.

Having said all that, your point that loopholes exist and that therefore the Bell tests are not *definitively* conclusive is correct (albeit unnecessarily, I think, nitpicky). However, as you learn how to read the experimental papers you'll understand why the experiments, considered together, are taken as *overwhelming* evidence that the inequalities are, in reality, being violated.

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Staff Emeritus
I agree with you 100%--it is to peer reviewed papers on loopholes that we must put greatest weight of evidence. Here is my initial count for tonight--I note that they range from 1970 to 2004, very much in line with the time period of Bell tests that attempt to prove that local realism does not exist, some of these papers are reviews that cover many different Bell tests, not just one.

1. P. M. Pearle, Phys. Rev. D 2, 1418 (1970)
2. J. F. Clauser and M. A. Horne, Phys. Rev. D, 10, 526 (1974)
3. T. W. Marshall, E. Santos and F. Selleri, Local realism has not been refuted by atomic-cascade experiments, Phys. Lett. A, 98, 5-9 (1983)
4. T. W. Marshall and E. Santos, Stochastic optics - a reaffirmation of the wave nature of light, Foundations of Physics, 18, 185-201 (1988).
5. T. W. Marshall and E. Santos, Stochastic optics: a local realist analysis of optical tests of the Bell inequalities, Phys. Rev. A, 39, 6271-83 (1989).
6. C. H. Thompson, Subtraction of ?accidentals? and the validity of Bell tests, Galilean Electrodynamics 14 (3), 43-50 (2003)
7. C. H. Thompson, The Chaotic Ball: An Intuitive Analogy for EPR Experiments, Found. Phys. Lett. 9, 357 (1996)
8. A. Cabello, E. Santos. Phys. Lett A. 214, 316 (1996)
9. E. Santos, Phys Rev A, 69, 022305 (2004)
10. O. A. Barut. The fallacy of arguments against local realism in quantum phenomenon. in Waves and particles in light and matter. A van der merwe et al. eds. Plenum (1993).

And then, compare this to the AMOUNT of experimental paper, and note that these experimental papers appear in Nature, Science, and PRL. C.H. Thompson could not even get her "chaotic ball" paper published in Am. J. Phys.,and we all know what kind of papers are typically published in Found. Phys. Lett. (most institutions don't even carry this journal). And I haven't even counted the number of theoretical papers supporting the QM view.

And oh, for your information, if you browse the QM2 yahoo group, at least a couple of people have pointed out the errors in that chaotic ball paper to caroline. I still can't believe you are citing this quack even after you were told who she is. She has a problem even with the classical Maxwell equations and thinks those are also wrong!

I still want to know if you think the evidence for superconductivity is "invalid" because if we apply your logic, each of the experimental evidence has "loopholes". I strongly suggest you look at various physical description that we already know, and then apply your logic to them. See if it is beginning to crumble.

Zz.

Staff Emeritus
BTW, the "stochastic" argument of using semiclassical theories (see Rade's list) has serious credibility problem especially when they argued that it can mimmick QM's result. Mermin has addressed this several times, and some of the strongest argument against this has been put forth by P. Kinsler[1]. The higher-order N-particle correlation between QM and the stochastic picture will clearly diverge and will not produce the identical result. It is why the GHZ-type tests are a more stringent test of the Bell-type scenarios.

Zz.

[1] P. Kinsler Phys. Rev. A v.53, p.2000 (1996).

Locrian
My point of the thread is that I find it very interesting that every time someone puts forth a Bell Test that claims to prove that local realism does not exist, yet another paper follows that shows the claim to be suspect--not conclusive--to have an interpretation loophole or one of at least nine other loopholes.

In the papers I've read, the authors do not claim to prove local realism does not exist. Some claim that they have performed an experiment which provides evidence that there may not be one, but this is not the same thing.

As for your experimental local realist, every experiment that's been listed here on the subject has had the opportunity to provide evidence for a local realist theory, even if they aren't capable of proving one doesn't exist. Therefore, I contend that you've already heard from countless local realist experimentalists - they've just never provided the tiniest bit of evidence the philosophy has any grounds in reality.

As Zz has pointed out, that's what this topic really boils down to. Dozens of experimentalists failing repeatedly to provide the slightest shred of evidence for local realism, and theoretical physicists turning up their noses and pronouncing the tests imperfect.

Gold Member
There is a HUGE difference in the claims of the folks who say the Bell tests are conclusive in support of QM, and the claims of the supporters of local reality (LR).

1. In all cases, we have each group agreeing as to the following:

Actual results - Predicted Results = Experimental Error

2. For the QM side we have:

Actual - Predicted = nearly zero

No problem here. If QM is correct, you would expect that you would see this. In fact, as experimental efficiency increases, you would expect that the difference would get closer and closer to zero. That is in fact the direction that experiments are going. (The current difference amounts to about 30 standard deviations of accuracy i.e. very close to zero.)

3. For the LR side we have:

Actual - Predicted... OOPS.

There is no predicted! That's right, there is NO predictions whatsoever made by those supporting LR. I have personally discussed this subject at length with Caroline Thompson. She won't... can't... provide such a prediction. Why is that?

There is a simple reason. Malus' Law (1807) is the associated classical theory, and it exactly matches QM. The formula is: cos^2. Caroline hand waves around this point (see http://freespace.virgin.net/ch.thompson1/Against/vigier.htm) unconvincingly.

4. In fact, you can apply Malus' law using LR concepts (observer independence) to get predictions for coincidences. The formula is (1/4)+( cos^2/2; see Wiki). The reason you don't see this put forth by supporters of LR is simple: Actual - Predicted is HUGE. You get values that vary from the observed results by a lot more than Bell's Inequality would indicate. In other words:

Coincidences at 0 degrees:
Actual = near 100%
QM predicted = 100%
Bell's Inequality = 100%
LR predicted (applying Malus) = 75%

Coincidences at 22.5 degrees:
Actual = near 85%
QM predicted = 85%
Bell's Inequality = 75%
LR predicted (applying Malus) = 68%

Coincidences at 45 degrees:
Actual = near 50%
QM predicted = 50%
Bell's Inequality = 50%
LR predicted (applying Malus) = 50%

Note that the differences between the actual and LR predicted varies widely:

At 0 degrees = 25%
At 22.5 degrees = 7%
At 45 degrees = 0%

On the other hand, the differences between the actual and QM predicted values are immaterial:

At 0 degrees = 0%
At 22.5 degrees = 0%
At 45 degrees = 0%

If you buy the ideas of LR, you now have to explain why the "experimental error" of all existing Bell test experiments just happens to be the above values, and that the experimental error is very nearly zero relative to the QM predicted value.

5. To the supporters of LR, I ask:

a) If QM is wrong, why do all experiments yield the QM predicted values?
b) If LR is right, why does the experimental error vary so widely at some angles while approaching zero at others?
c) And lastly, why are the experimental results biased towards correlation when you would expect exactly the opposite if there were no quantum entanglement?

Or maybe there are no loopholes after all. :rofl:

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Sherlock said:
I think you're on the wrong track here. The tests [Bell tests] aren't about whether or not reality exists independent of human observation. Nobody that I know doubts that.
Thank you Sherlock, having heard this, this thread is closed as far as I am concerned--Bell's tests say nothing about the existence of reality independent of human observation, and that is all that I have been trying to say about "loopholes". Sorry to all for all of the confusion and heat under colllars this thread has caused.

Gold Member
Sherlock said:
The tests aren't about whether or not reality exists independent of human observation. Nobody that I know doubts that.

We agree that Bell's Theorem says nothing about the role of humans, and that is the interpretation of most. Bell's Theorem also does not mention the actual existence of fundamental particles depending on observation, something I am sure we agree upon.

Bell's Theorem only addresses whether particles have simultaneous well-defined local values for all quantum observables. They don't, and Bell tests prove this.

By all standards of what consists of knowledge, there are no loopholes. As ZapperZ pointed out previously, we are more sure about the results of Bell tests than we are about many things. If you dismiss Bell tests because of purported loopholes, you must dismiss much of what we know about physics. Some, like Caroline Thompson, would like to roll back the scientific clock about 100 years - to when there were no photons, no big bang, etc. etc.

object2
DrChinese said:
a) If QM is wrong, why do all experiments yield the QM predicted values?
b) If LR is right, why does the experimental error vary so widely at some angles while approaching zero at others?
c) And lastly, why are the experimental results biased towards correlation when you would expect exactly the opposite if there were no quantum entanglement?

Or maybe there are no loopholes after all. :rofl:

Actually, if you properly calculate the correlation of the signals reaching the two detectors, a classical treatment also yields Malus' law for the angular dependence of the coincidence rate (see my page [link deleted] ), so it has hard to see what the whole point of the Bell test experiments is in the first place.

Thomas

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Staff Emeritus
object2 said:
Actually, if you properly calculate the correlation of the signals reaching the two detectors, a classical treatment also yields Malus' law for the angular dependence of the coincidence rate (see my page [link deleted] ), so it has hard to see what the whole point of the Bell test experiments is in the first place.

Thomas

BTW, shouldn't you be worried that you are confusing "photoionization" with "photoelectric"? Your whole treatment on photoelectric effect deals with "isolated atom", when this is really photoionization and NOT the photoelectric effect? We have already progressed beyond the naive photoelectric effect (i.e. using a metallic cathode with no preferred direction) to more sophisticated understanding of photon-solid interactions. While the photoelectric effect by itself cannot be used as the definitive prove of the photon picture, it is damn consistent. The "wave" picture still cannot reconcile with our advances in photoemission spectroscopy.

Furthermore, you have also neglected ALL of the experimental work in angled-resolved photoemission AND multiphoton photoemission spectroscopy, where the latter clearly shows the slope of the photoelectron current versus photon intensity changing in discrete units of the number of photon absorption. I have not seen any wave picture even being tried to agree with such observations.

Zz.

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Gold Member
object2 said:
Actually, if you properly calculate the correlation of the signals reaching the two detectors, a classical treatment also yields Malus' law for the angular dependence of the coincidence rate (see my page [link deleted] ), so it has hard to see what the whole point of the Bell test experiments is in the first place.

Thomas

I read your page. If you believe in Malus' Law, then you are saying that you also agree that - per Bell's Theorem - there is NO LOCAL REALITY. It doesn't matter whether the QM predicted value is reached by your derivation or any other. If the QM value (which is of course also Malus' cos^2 theta) is accurate, LR is ruled out.

So the point of the Bell test is to rule out LR predictions which deviate from Malus. Don't be fooled that your classical derivation is somehow local realistic. It isn't! Bell forbids it because there are additional requirements you don't address.

So I hope your comment is not intended to imply that you have shown that classical treatment can lead to a local realistic outcome.

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Staff Emeritus
Gold Member
DrChinese said:
If you believe in Malus' Law, then you are saying that you also agree that - per Bell's Theorem - there is NO LOCAL REALITY.

There always seems to be a confusion when people say that "QM predictions are in agreement with Malus' law". The trick being of course, that Malus' law needs a re-interpretation in the context of a corpuscular theory (photons).

Classically, Malus' law simply says how much intensity gets through a polarizer when we know the angle between the polarizer and the plane of polarization of the light: I(through) = I(in) x cos^2 (theta) where theta is this famous angle of course.
But as such, this doesn't say anything about correlations between individual particle detections, as is the case in these Bell experiments. We need an extra hypothesis that links the particle detection with "classical intensity", and of course the standard way of doing this is by assigning a probability of detection somehow proportional to the intensity of the beam (at low intensities).
But then I don't understand the claim that the QM predictions would be in agreement with Malus' law. Take the two polarizers to be perfectly parallel (say, parallel to the Y axis). We know from quantum mechanics that a singlet state will HAVE 100% CORRELATION (meaning, whenever the Bob detector clicks, Alice's detector clicks too and whenever Bob's detector doesn't click, Alice's doesn't, either).

But that is NOT what Malus' law predicts ! Taking that the incoming polarization of the photon can be just any orientation, the probability of clicking at Bob's is proportional to cos^2theta, and the probability of clicking at Alice's is also proportional to cos^2theta. BUT theta IS UNIFORMLY DISTRIBUTED !
Indeed, for theta 0 (the classical light is also parallel to Y), we have 100% efficiency to click at Bob, and 100% efficienty to click at Alice, so the correlation is indeed 100%. But for theta = 45 degrees, we have 50% chance to click at Bob, and INDEPENDENTLY 50% chance to click at Alice, which means that we only have 50% correlation in this case.

The overall correlation is given by the number of clicks AT BOB AND AT ALICE, over the number of clicks AT BOB.

clicks at Bob : $$\frac{1}{2 Pi}\int cos^2(\theta) d\theta = 1/2$$

clicks at Bob and Alice: $$\frac{1}{2 Pi}\int cos^4(\theta) d\theta = 3/8$$

which comes down to only 75%.

This is not the 100% (with identically aligned polarizers) QM predicts. So what does it mean that "QM has the same predictions as Malus' law" ?

Or is this a "perverse" application of Malus' law, where it is applied ONCE THROUGH THE PROJECTION POSTULATE we have jumped the polarization of Alice's photon into the polarisation given by Bob's polarizer when it clicks ?

Sherlock
DrChinese said:

We agree that Bell's Theorem says nothing about the role of humans, and that is the interpretation of most. Bell's Theorem also does not mention the actual existence of fundamental particles depending on observation, something I am sure we agree upon.
Wrt Rade's last post, I'm not sure that he understands all that's involved here. I'm quite sure that I don't. :-)
In any case, I agree with all your statements, cited above and below, but I feel like elaborating. :-)

Talking about what exists independent of observation is necessarily less clear when dealing with quantum phenomena than when dealing with macroscopic phenomena. The EPR idea that we can take (at least wrt certain instrumental configurations) a measured value-property as being in one-to-one correspondence with something existing in nature independent of observation has been shown to be false when applied (via Bell and Bell tests) to most instrumental settings. But even before Bell and Bell tests it was evident that there's no well-defined (unambiguous and consistent) qualitative characterization of what a photon, *is* in nature. All we have is our macroscopic apprehension of instrumental results. So, the way that we talk about the photon (ie., the way that the results are modeled) depends in large part on how the results correspond to phenomena of our normal sensory experience (eg., waves and particles, which we can abstract and manipulate unambiguously and consistently). Wrt to some setups a wave charactarization is adequate, and wrt others a particle characterization is necessary.

Wrt Rade's concern, none of this means that reality, even at the level of quantum phenomena, doesn't exist when it's not being observed. It's just not too clear what reality at the level of quantum phenomena *is*. The term, photon, as defined in quantum theory, doesn't refer to something that exists in nature independent of observation. On the other hand, the moon for example does refer to something that exists in nature independent of observation.

Einstein wanted a theory of quantum phenomena that talked *directly* about those phenomena as they exist in nature. He regarded quantum theory as incomplete because it doesn't do that -- and quantum theory *is* incomplete in that sense. However, in learning about the development of the theory, I've come to the conclusion that it has been constructed in as complete and realistic a way as any theory could have been considering what it's dealing with.

DrChinese said:
Bell's Theorem only addresses whether particles have simultaneous well-defined local values for all quantum observables. They don't, and Bell tests prove this.
Ok, and it's good to remember that it's we humans who are doing the defining. We might conclude therefore that our idealized conception of polarization for example is not telling us all the relevant aspects of the behavior of the light incident on, and transmitted by, the polarizers -- or of the details of the relationship, if any, due to emission from a common source, between the light, associated with joint measurments, incident on A and B.

Another way of framing it is that a certain general form embodying the separability (assuming this to be the only allowable form consistent with a limiting c) of events at A and B is seen to not apply to most joint (A,B) measurements in Bell setups. So, either c isn't a limit, or the joint measurement context is a nonseparable one for some other reason. Maybe, in a universe where all transmissions are limited by c, situations in which two particles are emitted at the same time from the same atom, and are being analyzed jointly by the same sort of device, are nonseparable -- in which case, the defining characteristic of the form of a proposed lhv description would be nonseparability.

It does, I think anyone would have to admit, make sense to think of the combined motions of two separated particles which have interacted or have a common source as being related (ie., nonseparable) in some way. This is, after all, the basis for conservation equations. It's the basis for the qm treatment of such biparticle situations in terms of a single wave function.

But, the problem is the same as when dealing with any quantum process. There's no qualitative apprehension of what's happening at that level. As has been demonstrated, just drawing a line in a unit circle and saying that that represents a common polarization value doesn't quite cut it.

So, as I see it, whether there are nonlocal transmissions in our universe, and whether lhv theories are in principle possible are still open questions.

DrChinese said:
By all standards of what consists of knowledge, there are no loopholes. As ZapperZ pointed out previously, we are more sure about the results of Bell tests than we are about many things. If you dismiss Bell tests because of purported loopholes, you must dismiss much of what we know about physics.

There are loopholes and they're important for metrological reasons. But I agree with you that the Bell tests are pretty conclusive evidence that the inequalities are being violated and that the qm predictions are correct.

Sherlock
vanesch said:
There always seems to be a confusion when people say that "QM predictions are in agreement with Malus' law". The trick being of course, that Malus' law needs a re-interpretation in the context of a corpuscular theory (photons).

Classically, Malus' law simply says how much intensity gets through a polarizer when we know the angle between the polarizer and the plane of polarization of the light: I(through) = I(in) x cos^2 (theta) where theta is this famous angle of course.
But as such, this doesn't say anything about correlations between individual particle detections, as is the case in these Bell experiments. We need an extra hypothesis that links the particle detection with "classical intensity", and of course the standard way of doing this is by assigning a probability of detection somehow proportional to the intensity of the beam (at low intensities).
But then I don't understand the claim that the QM predictions would be in agreement with Malus' law. Take the two polarizers to be perfectly parallel (say, parallel to the Y axis). We know from quantum mechanics that a singlet state will HAVE 100% CORRELATION (meaning, whenever the Bob detector clicks, Alice's detector clicks too and whenever Bob's detector doesn't click, Alice's doesn't, either).

But that is NOT what Malus' law predicts ! Taking that the incoming polarization of the photon can be just any orientation, the probability of clicking at Bob's is proportional to cos^2theta, and the probability of clicking at Alice's is also proportional to cos^2theta. BUT theta IS UNIFORMLY DISTRIBUTED !
Indeed, for theta 0 (the classical light is also parallel to Y), we have 100% efficiency to click at Bob, and 100% efficienty to click at Alice, so the correlation is indeed 100%. But for theta = 45 degrees, we have 50% chance to click at Bob, and INDEPENDENTLY 50% chance to click at Alice, which means that we only have 50% correlation in this case.

The overall correlation is given by the number of clicks AT BOB AND AT ALICE, over the number of clicks AT BOB.

clicks at Bob : $$\frac{1}{2 Pi}\int cos^2(\theta) d\theta = 1/2$$

clicks at Bob and Alice: $$\frac{1}{2 Pi}\int cos^4(\theta) d\theta = 3/8$$

which comes down to only 75%.

This is not the 100% (with identically aligned polarizers) QM predicts. So what does it mean that "QM has the same predictions as Malus' law" ?

Or is this a "perverse" application of Malus' law, where it is applied ONCE THROUGH THE PROJECTION POSTULATE we have jumped the polarization of Alice's photon into the polarisation given by Bob's polarizer when it clicks ?

Would a better way to show the difference between a characteristic lhv formulation and the qm formulation be in terms of expectation values between +1 (perfect correlation between results at A and B, Theta = 0) and -1 (perfect anti-correlation, Theta = 90 degrees), where the qm values plot a curve and the lhv values plot a straight line?

As for the applicability of Malus' Law -- just for fun:
Take the joint (A,B) measurement as a single, non-analyzable dependent variable.
Take Theta, associated with (A,B), as a single, non-analyzable independent variable.

Assume that A and B are measuring the same light.

Take 'intensity' to refer to the (A,B) coincidence count.

recorded 'intensity' = maximum 'intensity' x cos^2 Theta.

Ok, that was pretty perverse. :-)

But, I do think that part of the interpretational problem (wrt Bell's Theorem and a general form for lhv theories) lies in assuming that lhv theories must be constructed in a separable form.

Gold Member
vanesch said:
There always seems to be a confusion when people say that "QM predictions are in agreement with Malus' law". The trick being of course, that Malus' law needs a re-interpretation in the context of a corpuscular theory (photons).

Classically, Malus' law simply says how much intensity gets through a polarizer when we know the angle between the polarizer and the plane of polarization of the light: I(through) = I(in) x cos^2 (theta) where theta is this famous angle of course.

...

which comes down to only 75%.

This is not the 100% (with identically aligned polarizers) QM predicts. So what does it mean that "QM has the same predictions as Malus' law" ?

Or is this a "perverse" application of Malus' law, where it is applied ONCE THROUGH THE PROJECTION POSTULATE we have jumped the polarization of Alice's photon into the polarisation given by Bob's polarizer when it clicks ?

The point is that for those who want to roll back the clock BEFORE QM arrived on the scene, you have to have Malus as your backstop. Otherwise, you are really in trouble because you are ignoring the classical experiments. (The same thing happens with those who deny special or general relativity, they must go back to Newton.)

So if you apply Malus with a local realistic bent, as you calc'd, you get perverse values for correlations. They are perverse, of course, because they are much further away from any value that you would ever measure in nature.

On the other hand, if you apply Malus with a quantum mechanical bent (which I don't consider perverse at all), i.e. as applying on a particle by particle basis, you get exactly the values that are measured in experiment. I.e. once you know Alice's polarization, Bob's matches the application of Malus. Why wouldn't it? After all, such application also matches the results for an ensemble of particles as well.

For the local realist, all this is a big problem. They now have Bell's Inequality to consider, and they have the QM predictions (which must be argued as being wrong even though they match experiment), and yet they have Malus (applied local realistically) which is really far off. So now the local realist must concoct yet another prediction which is much closer to Bell's Inequality if it is to be considered seriously (because the experimental error/loophole idea otherwise makes no sense because the deviation is way too large). Yet not a single local realist will advance such a prediction because it is absurd on the face of it.

Certainly the local realists know of the function you describe (which varies between .25 and .75), but they never stand by it. So once again I ask the local realist: what is the function that describes entangled photon correlations?