Talk:Counterfactual definiteness

message: I think the article is ambiguous, and the examples misleading. CFD in this article is related too much to quantum mechanics while this should also be understood independently. For example (I'm not sure), "CDF implies that all values in nature are definite whether they are measured or not. To assume that something unmeasured is definite is counterfactual." Correct me please.Mastertek (talk) 03:59, 25 October 2011 (UTC)
thread_title:  Ambiguity 

message: You know, I've yet to see a single comprehensible explanation of why it is that Bell's theorem assumes CFD, nor why many-worlds violates it. For that matter, I've only ever seen one good explanation of Bell's theorem and unfortunately it discusses neither CFD, nor many-worlds.
http://www.mathpages.com/rr/rrtoc.htm (sections 9.5-9.8)
Kevin Aylward:
According to Leslie Ballentine, Professor at Simon Fraser University, and writer of the text book "Quantum Mechanics, A Modern Development" ISBN981-02-4105-4., Bell's theorem dosnt.
He explains in the above book that, firstly, EPR, *derives* CFD, not assumes it. Second, he points out that H. Strap dispenses with CFD, and still shows that QM violates locality. 
Subsequently, other have argued that he still assumes CFD implicitly, however, H. P. Stap further answers these criticisms, but the debate is ongoing e.g http://adsabs.harvard.edu/abs/2004quant.ph..4121S, probably till the end of time.
:Eberhard showed that CFD and locality lead to Bell's theorem while Arthur Fine showed that any theory conforming to Bell's theorem has a local hidden variable model which implies that it must satisfy CFD. Thus locality and CFD are in fact equivalent to Bell's theorem. Kuratowski's Ghost (talk) 22:42, 4 August 2008 (UTC)
replies: text: The bomb-testing problem doesn't determine an answer to a counterfactual question, the bomb detector is present, the bomb detector performs factual measurements, there is nothing counterfactual here in the CFD sense, the whole bomb detector methodology relies on the factual measurement the bomb detector performs. Kuratowski's Ghost (talk) 23:03, 4 August 2008 (UTC)
thread_title:  Clarification 

message: The last statement on this page says: "Abandoning CFD allows one to claim that violation of Bell's inequalities does not necessarily imply a violation of the locality principle."   That's not true.  There is always nonlocal correlation (the point of Bell's Inequality) in the experimental setup.   Bell's Theorem says more about locality than it does CFD,  thus GLOBAL hidden variable theories like Bohmian mechanics.  I'm deleting that statement as "abandoning CFD" does not in any way allow for an accounting of the correlation between the detectors seen in experiments testing Bell's Theorem. Eluusive  —Preceding undated comment added 00:10, 10 June 2009 (UTC)
thread_title: CFD, Bell's Thorem, and Locality

message: The line: "Bell's Theorem actually proves that every quantum theory must violate either locality or CFD." may be meaningful to people who understand the subject matter, but nothing in the article makes it clear why it matters if CFD is violated.Janus Shadowsong | contribs 20:39, 25 October 2010 (UTC)
thread_title: Locality vs CFD

message: The article erroneously states the Heisenberg Uncertainty Principle (HUP). The article says that HUP implies that you cannot measure both the position and momentum of an electron. In fact, HUP has nothing to say about individual electrons. You can only derive the result if you use repeated measurements, so it relates either simultaneous measurement of many electrons or repeated measurement of the same electron (or a mixture of both). There is a well-known thought experiment for this (I first heard it in a physics lecture in 1976). Prepare a beam of electrons of known momentum, you can make this as precise as you like. The more precise the more spread out the beam will be (HUP). Measure the position of one electron in the beam, you can make this as precise as you like too. You have now measured the position and momentum to arbitrary precision. Of course, the electron subsequently will have neither.  — Preceding unsigned comment added by Jschlesinger (talk • contribs) 21:52, 20 November 2011 (UTC)
thread_title:  Accuracy 

message: The following is a really stupid analogy! It really has no connection to physics: blocking photon (potential) waves in one arm of an interferometer gives us 'unmeasured' knowledge of the photon received in the other arm. This article is unworthy of Wikipedia.
  "To use a macroscopic analogy, an interpretation which rejects counterfactual definiteness views measuring the position as akin to asking where in a room a person is located, while measuring the momentum is akin to asking whether the person's lap is empty or has something on it. If the person's position has changed by making him or her stand rather than sit, then that person has no lap and neither the statement "the person's lap is empty" nor "there is something on the person's lap" is true. Any statistical calculation based on values where the person is standing at some place in the room and simultaneously has a lap as if sitting would be meaningless."  — Preceding unsigned comment added by Chrisman62 (talk • contribs) 03:18, 14 August 2015 (UTC)
thread_title:  Macroscopic analogy or example 

message: This article is an incredibly badly written impenetrable wall of text, and I say that as a physicist. A jumbled up mess of half-explanations, terms for which isn't explained why they are called that (and that includes the subject) and misleading and inappropriate examples. I think if a layperson would start reading this, his eyes would glaze over near the end of the first sentence or so.  — Preceding unsigned comment added by 82.139.81.0 (talk) 07:12, 9 May 2012 (UTC)
thread_title:  Badly written 

message: The definition of CFD in the article does not coincide with the meaning the term has in the modern literature on Bell's theorem, interpretation of QM, and so on. The article also lacks decent modern references. I corrected the text but did not yet support this with references. Richard Gill (talk) 17:05, 5 June 2012 (UTC)
thread_title:  ... and incorrect 

message: If counterfactual definiteness is a concept that arose from the analysis of experiments such as EPR, I wonder if we can make a macroscopic analogy that would make sense. I like that example about the caller's telephone number mentioned above, but I see one problem with it, which I'll explain: 
Counterfactual defineteness seems to apply to a variable which is "complementary" to the one that has been measured. Being the fact that in the macroscopic world ( which is governed by classical mechanics ) we don't have complementary variables, I don't think we can find a suitable set of variables to make an analogy. Well, perhaps we could think of some example where measurement of one variable spoils the other, but that would not say much about the state of this other variable before measurement was made. If on the other hand, we "defined" counterfactual definiteness, as applying to any measurement or observation which has not been made (without any reference to a complementary variable), then the meaning of this expresion would certainly apply to the macroscopic world. 
Of course we could suggest changing the meaning of this expression and extending it as I just mentioned, but the purpose of Wikipedia is to describe the meaning of words and expresions as they are currently used and not to introduce new ideas. It would be interesting to look at how this expression has been used in different published articles and specially to see who was the first to use it. Doing a quick search I found an article by Vaidman (http://www.tau.ac.il/~vaidman/lvhp/m105.pdf) But Vaidman talks about "counterfactuals" only and not together with "definiteness"  
Somewere else, I found "counterfactual definiteness" considered to be equivalent to "realism". I think these things may not be the same. 
Alexepascual (talk) 23:07, 25 July 2012 (UTC)
thread_title:  Macroscopic application  

message: The explanation is simple, also classical. In 4a/b the photons are not detected because of destructive interfering of both waves from the upper and lower branch. In 4 c/d the new mirror blocks the wave from the lower path, so no destructive interfering anymore, so photons can be detected again in A. Nothing counterfactual about that. DParlevliet (talk) 22:17, 21 February 2014 (UTC)
thread_title:  Wrong example of Counterfactual measurement in Mach-Zehnder interferometer 

message: Assuming that the counterfactual definiteness business has something to do with counterfactual conditions and also with Bell Inequalities that deal with what would happen if researchers had not done something, I'll summarize what I know about the logical side of things. Maybe it will help us all look for useful materials, or otherwise define what we are looking for in a better way.
Researchers do some experiments in which momentum is detected and then they take further measurements in each run of the experiment to try to look at position. Perhaps they prepare photons identically, measure them for momentum using the identical physical operations, and then catch the photons on a distant detection screen. It may turn out that the photons land all over the place. Now some other researcher comes along after the fact and says, "If you hadn't measured Momentum then you could always measure and even predict Position." I'll symbolize that statement as: ¬M → P
 M P | M → P | ¬M  P |  ¬M → P  Logical predictions:
 1 1 |   1   | 0  1  |    1    (R does measure momentum and does get position) (1) XXX
 1 0 |   0   | 0  0  |    1    (R does measure momentum and does fail to get position) (2) lab: as expected
 0 1 |   1   | 1  1  |    1    (R does not measure momentum and does get position) (3) lab: as expected
 0 0 |   1   | 1  0  |    0    (R does not measure momentum and does not get position) (4) lab: as expected.
However, the predictions are wrong, so we can't accept the last researcher's position. (1) is logically true according to the premise offered by the last researcher, but it must fail in the laboratory. It fails because while if... then... statements are treated as logically true when the "if" part is false but the "then" part is true, logicians just mean that if something doesn't happen for the reason stated in the logical reason, but happens for some other reason, that doesn't make the original argument a lie. Saying "You'll die if you drink that poison," does not become false simply because you die of a heart attack instead. But in the physics lab case the individual was asserting that giving up measuring momentum was necessary and sufficient to getting good position readings, but we've kept the momentum measurements and logic says we could get position measurements. What logic "means" is that there is no reason why the position measurements should not be available somehow or "once in a while," but what happens in the physics lab shows that getting a good position measurement after the same photon was tested for momentum would be a very dicey thing indeed. (2) is true logically and it also is true in the physics lab. (To do the logic carefully, we should have supplied something like, "For all x runs of an experiment, if you don't measure Momentum then you can always measure position.") (3) is a good prediction in the lab. (Occasionally there will be experimental error, so the "for all x" part would be too optimistic.) (4) says that if the researchers do not measure momentum and they also do not get good position readings, then there is proof that the logical argument is wrong. The lab is going to support the theory offered because they aren't going to find this evidence. 
The physics problem is more like a situation in which someone who has embezzled money claims: "If you hadn't caught me, then I would have replaced the money the next day." We cannot test the alternative offered. The best we could hope to do would be to recreate the original "experiment" and wait one additional day to see if the second accountant replaced the stolen money. The two parts are, strangely, true, if by "true" you mean "not falsified." Like an untested theory, we can't prove that that man would not have replaced the money, and we can't know that "in another universe" the investigators would not have waited a couple of days. So the whole thing stands in the real of "not yet falsified." 
I think that anybody familiar with the twists and turns of contrafactual conditionals would have avoided just the word "contrafactual" unless they actually wanted to jump into this particular briar patch. 
If they did mean to use this word in all its social context, then the accompanying lesson must be that whenever you start talking about alternate pasts, or "what would have happened if only you hadn't let the cat out" situations, you risk getting into a lot of unintended complications. 
So what was happening with Bell researches that brought in assertions about what might have been? Anything? If Bell said something like, "If they had not measured the position, then they could have gotten a good measurement of momentum," are people possibly objecting, "How do you know?" Is Bell then saying, "Because that's what always happens in the lab?" That would be my guess from reading between the lines of some of the things I've seen thusfar. But at this point it is just a guess. P0M (talk) 23:13, 1 March 2014 (UTC)
replies: text: I did a reality check on contrafactual conditionals and it is even worse than I remembered. The philosophers can't agree on how to handle them. P0M (talk) 02:45, 2 March 2014 (UTC)
thread_title: Counterfactuals in the laboratory and in logic 

message: It has become clear to me that this article must have been written to fill out a "red link" in the article on Bell Inequalities. So we need to look at articles on Bell Inequalities that mention "counterfactual." 
One of them gives a good clue about how researchers are using "counterfactual."
As the Bell’s inequalities are derived within LR [local realism], counterfactual reasoning is acceptable and no additional hypotheses are necessary to derive the eq.13. But, there is still the problem of assigning values to the counterfactual terms. In order to solve this problem, a “possible world” must be defined to ensure logical consistency [7]. There is no mystery in this situation, but simply lack of information. Let see an example of everyday life: let suppose that when I go to the pub and find my friend Alice there, the result of measuring the variable A ≡ “I find Alice in the pub” is 1, and 0 when I do not find her there. After many visits to the pub, I measure the expectation value 〈A〉=0.3. Now, let consider the question: “what is the expectation value of A when I don’t go to the pub?” If it is assumed that Alice and the pub have a well defined existence even when I do not go there (roughly speaking, if Realism is assumed), the expectation value of A is some well defined number, say, q. But, the value of q cannot be known with the information available at this point. More information is needed, regarding the behavior of Alice and the properties of the pub when I am not observing them (i.e.: a “possible world” must be defined) to assign a numerical value to q.
Once a possible world is defined, the counterfactual terms can be calculated. Depending on the possible world chosen, the eq.1 is retrieved, or not [6]. The important point here is that the definition of a possible world unavoidably involves one assumption additional to LR. This weakens the consequences of the violation of the Bell’s inequalities reported in the experiments, for the violation can be interpreted as a refutation of the additional hypothesis, not necessarily of LR. Note that this weakening does not arise from an experimental imperfection (as in the case of the loopholes [1]). The setup is assumed ideally perfect. The weakening arises from the fact that real measurements are performed during time, and that it is impossible to measure with two different angle settings at the same time. For, time is a magnitude that takes any given value only once.
These quotations are taken from: "On the meaning of an additional hypothesis in the Bell’s inequalities."
Alejandro A. Hnilo. http://arxiv.org/pdf/1402.6177%E2%80%8E
A useful secondary source says:
Finally, one subtle assumption of the Bell inequalities is counterfactual definiteness. The derivation refers to several objective properties that cannot all be measured for any given particle, since the act of taking the measurement changes the state. Under local realism the difficulty is readily overcome, so long as we can assume that the source is stable, producing the same statistical distribution of states for all the subexperiments. If this assumption is felt to be unjustifiable, though, one can argue that Bell's inequality is unproven. In the Everett many-worlds interpretation, the assumption of counterfactual definiteness is abandoned, this interpretation assuming that the universe branches into many different observers, each of whom measures a different observation.
http://www.quantiki.org/wiki/Bell%27s_theorem
Here is the basic problem, as I conceive of it from the sources I've read. Sorry, but at this point I can't prove any conclusions.
A major finding of quantum mechanics is:
For all particlex measurements, accurate measurementx of position means that accurate measurementx of momentum is impossible.
For all particlex measurements, accurate measurementx of momentum means that accurate measurementx of position is impossible.
For any particlex measurement of a given quantum characteristic, EPR claim that the particlex of the covariant condition would fall within range A.
For any particlex measurement of a given quantum characteristic, Bell claims that the particlex of the covariant condition would fall within range B where B > A. 
However, particlex having been measured for, e.g., position, it is impossible to know any true value for the momentum of particlex. So it is impossible on the basis of empirical measurements to know the value of B. 
Is it, nevertheless, possible to know an accurate range of values for B after investigating a wide range of cases?
It's too late at night to dig out specifics on Bell inequalities. Does this much look right?P0M (talk) 08:02, 2 March 2014 (UTC)
thread_title:  The way forward 

message: Here is something I wrote for an earlier discussion:
I don't have a lot of time right now to go through The Fabric of the Cosmos and pick up the points that Brian Greene makes. Basically he says that something happens and that what happens is very hard to reconcile with any attempt that tries to limit the reasons for some changes to physical processes that proceed at the speed of light. For right now, let me show you the chart that results when you follow through his argument/analogy about correlations. His way is the reverse of what Tercer says, as I have remarked before. I think this difference is a function of the way the experiments are set up, not due to any error on Greene's part.
He sets up  (starting on page 107) the following analogy or thought experiment.  There are two "boxes" linked to receptors for two sets of entangled particles. Each box gives a measurement of three different characteristics of the incoming particles by opening doors (labeled G(rey), W(hite), and B(lack) in the chart below). Opening each door gives a different measurement, and the measurements are reported in terms of red and blue lights (labeled r and b). If the two boxes give the same response (color) when a door on each is opened, then that is counted as "+1", and if one shows red and the other shows blue, then that result is counted as "-1."  
Look at the second and third lines  in the chart below (Preset and Match).  If the boxes are pre-programmed (i.e., if the experiment is controlled by hidden variables) then when the G doors of the two boxes are opened, both see blue, if G is opened on one box and W is opened on the other box then blue is seen, when G is opened on one box and B is opened on the other box, then blue is seen on one and red is seen on the other (so no match). So if you go through and look at all possible connections, you win 55% of the time. 
What he concludes is that if you are measuring pairs (canonical conjugates) of phenomena in the same system of entangled particles, pure chance will let you "win the shell game" 55% of the time if there are hidden variables, but that Copenhagen quantum mechanics, if true, means that you can only win 50% of the time.  Here is a picture of his two "boxes" and the chart I made:
      	G-W-B	Choose	Choose	Choose	Choose	Choose	Choose	Choose	Choose	Choose	SUM
Preset	b-b-r	G-G   	G-W   	G-B   	W-G   	W-W   	W-B   	B-G   	B-W   	B-B   	
Match?	     	+1    	+1    	-1    	+1    	+1    	-1    	-1    	-1    	+1    	55%
Preset	b-r-b	G-G   	G-W   	G-B   	W-G   	W-W   	W-B   	B-G   	B-W   	B-B   	
Match 	     	+1    	-1    	+1    	-1    	+1    	-1    	+1    	-1    	+1    	55%
Preset	b-r-r	G-G   	G-W   	G-B   	W-G   	W-W   	W-B   	B-G   	B-W   	B-B   	
Match 	     	+1    	-1    	-1    	-1    	+1    	+1    	-1    	+1    	+1    	55%
Preset	r-b-b	G-G   	G-W   	G-B   	W-G   	W-W   	W-B   	B-G   	B-W   	B-B   	
Match 	     	+1    	-1    	-1    	-1    	+1    	+1    	-1    	+1    	+1    	55%
Preset	r-b-r	G-G   	G-W   	G-B   	W-G   	W-W   	W-B   	B-G   	B-W   	B-B   	
Match 	     	+1    	-1    	+1    	-1    	+1    	-1    	+1    	-1    	+1    	55%
Preset	r-r-b	G-G   	G-W   	G-B   	W-G   	W-W   	W-B   	B-G   	B-W   	B-B   	
Match 	     	+1    	+1    	-1    	+1    	+1    	-1    	-1    	-1    	+1    	55%
If Bell is right, i.e., if there are no "gotcha" factors that will mess up his analysis, then experiments would either show a true 50% figure when QM predicts 50%, or not. If not, then QM is missing some factor of reality and is incomplete.
End of copied material.
The reference to Tercer above was, I think, because most examples show quantum mechanical range of variables greater than classical mechanics range of variables, but the way Greene has set up his example it is classical physics that would provide a larger range (i.e. 55% rather than 50%). 
If I'm following the general argument correctly, then we should be looking at the empirical basis for the results that Bell predicts. I think people are claiming that he is depending on an argument that says that the "machine" always kicks out the same range of answers regardless of any other factors, so the results cited to establish Greene's inequalities should be reliable even though they are not empirical measures that pertain to the same photons or whatever quantum entities were being empirically measured. They are probably right, but is there any real problem with doing so? Technically it is "counterfactual" if I say it rains ten days this year on Xero Island but I haven't actually set out measuring devices and monitored them this year and am instead depending on weather records taken over the last 100 years that show that rain days have a maximum of 10.9 and a minimum of 10.0. If somebody's weather prediction says it ought to have rained on 100 days this year, do I have to interview the last resident of Xero Island to feel justified in questioning the new climate model? How much statistical information on weather and climate do I need before I can be reasonably assured that 100 days a year indicates a problem with the theory that predicts it? P0M (talk) 09:08, 2 March 2014 (UTC)
thread_title:  Greene on Bell 

message: counterfactual "Mach-Zehnder interferometer"
There are many references. The main idea seems to be that "had the bomb been 'measured' it would have exploded, but it wasn't 'measured' and yet we know what would have happened, so we can salvage this bomb." P0M (talk) 18:59, 2 March 2014 (UTC)
thread_title:  useful to Google for: 

message: If physics gives up locality, that amounts to admitting the possibility of sudden "magical" intervention in human affairs from unknown and possibly unknowable sources of action.
I removed the above sentence for two reasons. First, it was in the wrong place; locality was treated earlier in the section. Second, the reference to magic and unknowable things is somewhat over-the-top; the debate actually occurs between more down-to-earth alternatives, such as: the heat equation with infinite propagation speed, the incompressible Euler equation with a global equation for pressure fields; various well-defined mathematical theories where determinism fails, such as Markov processes or evolution equations that are not well-posed with respect to initial conditions.
178.38.122.45 (talk) 12:12, 10 April 2015 (UTC)
replies: text: I agree. Boris Tsirelson (talk) 13:03, 10 April 2015 (UTC)
thread_title:  Removed sentence about locality 

message: The account as it was before I made a couple of edits just now was very, very old-fashioned. Referring to Stap is going back to pre-history. Counterfactual definiteness is an uncontroversial property of classical physics. We can, in classical physics, talk about what would happen to the tea in my tea-cup if the moon were suddenly removed. Surely it would effect the gravitational pull on that body of fluid, and it would start to change shape and move towards a slightly different position, even if only a tiny bit. Just because the “measurement” is impossible does not mean that we cannot do calculations with it. Quantum mechanics does not *forbid* certain simultaneous measurements. It just disdains from talking about them. For practical purposes, that is no problem, since nobody can imagine how such a measurement could be performed. The story about disturbances is a classical, intuitive, attempt to rationalise the situation. It makes students feel comfortable with the situation. Richard Gill (talk) 15:03, 15 April 2022 (UTC)
replies: text: The usual measurement postulates actually forbids certain simultaneous measurements. Perarve (talk) 08:52, 16 April 2022 (UTC)
thread_title:  Rather old-fashioned account