# I What is the difference between cfd and realism?

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1. Dec 23, 2017

### entropy1

What is the difference between counterfactual definiteness on one side and realism on the other?

And what are their definitions? (references will be welcome if given).

2. Dec 23, 2017

### Greg Bernhardt

Have you searched for definitions on Google? Is there a specific point of confusion after researching both?

3. Dec 23, 2017

### entropy1

As far as I interpret it, CFD is the idea that one can do meaningful reasoning about experiments not (yet) performed, and realism the proposition that if we don't measure, we must still assert that the thing we could have measured is still there.

I can't put it to words very accurately, to my regret.

The notions as put in my words above seem to have much in common. I recall (though a PF search yields nothing) that there was some confusion between the two in the past.

4. Dec 23, 2017

### entropy1

So one of the things I was wondering about is, if superposition is a kind of non-realism, in the sense that a realistic entitity has an unambiquous reality?

5. Dec 23, 2017

### Staff: Mentor

I don't think this is quite correct; I would say CFD is the assumption that one can do meaningful reasoning about experiments that could have been performed, but were not--and therefore cannot now be performed, since the time at which one would have performed them is past. For example, if you measure the spin of an electron in the $z$ direction, CFD is the assumption that one can meaningfully talk about what the result would have been if one had instead measured its spin in the $x$ direction (or any other direction besides $z$). But that is not the same as talking about what could happen if one were to measure the spin in the $x$ direction at some future time.

I don't think this really captures the reason why debates about "realism" in QM never get anywhere; the problem is that seemingly innocuous phrase "the thing we could have measured".

If the unambiguous reality is the quantum state, then superposition is perfectly real. The problem is the implications you have to accept if you want to believe that the unambiguous reality is the quantum state. But those problems are problems of interpretation, not theory. In fact, that's probably true of any discussion of realism in a quantum context.

6. Dec 24, 2017

### entropy1

Suppose we 'trap' a particle. Could we speak of that the particle is 'really' in the trap, though we haven't (yet) measured it? (even if we did measure it)

7. Dec 24, 2017

### vanhees71

The reason is that the notion of "reality" is not well defined. It's some unsharp philosophical idea, with a meaning at least as numerous as people using the word. I think it shouldn't be used in physics discussions at all for this reason.

The culprit most probably is Podolski who is the one who in fact wrote the (in)famous Einstein-Podolski-Rosen paper. Einstein was quite unhappy with this paper, saying that it "buries the main point by erudition" (in a letter to Schrödinger, with whom he discussed these issues in person when they were colleagues in Berlin in the 1920ies and later on via mail). Einstein's criticism against the completeness of QT was rather about the inseparability implied by the formalism by the possibility of entanglement of system parts that are observed at far distant places. He was well aware of all the subtleties concerning this inseparability feature of QT on the one hand and the apparent tension with locality on the other hand. Unfortunately relativistic QFT wasn't well enough understood at the time and Bell's paper came about 10 years after Einstein's death and the first experimental observations of the violation of Bell's inequality only in the late 1970ies/early 1980ies. I'd really like to know how Einstein would have reacted to these ground-breaking results on the foundations of QT.

For a very thorough analysis about what Einstein really wanted to say in the EPR paper and what he indeed said later in the Dialectica article (quoted in this paper):

D. Howard, Einstein on locality and separability, Studies in History and Philosophy of Science 16, 171 (1985)
https://doi.org/10.1016/0039-3681(85)90001-9

8. Dec 24, 2017

### stevendaryl

Staff Emeritus
To me, it makes sense to call a theory realistic if there is a notion of the state of the universe, and every fact about the universe follows from that state. In contrast, a probabilistic theory is (usually) not realistic in this sense, because there are facts that are not captured by the probability distribution. If you have a probability distribution $\rho(x)$ giving the probability of a particle being at position $x$, then where the particle actually is is a fact above and beyond the probability distribution. The distribution $\rho$ either reflects our knowledge of the particle's position, or it reflects some kind of statistical fact about many systems that are similar to the one we are studying, but it does not reflect (just) facts about our system.

Quantum mechanics is definitely not a realistic theory, and the question was whether there is a realistic theory that gives rise to quantum mechanics in a similar way that Newtonian physics gives rise to statistical mechanics. The answer seems to be "no", if we demand that the realistic theory be local (in the sense of factorizability of the state of the universe into local states that evolve locally).

9. Dec 24, 2017

### entropy1

So perhaps the non-realism in QT is a translation of the conflict between two concepts in physics, that are locality and inseparability?

Last edited: Dec 24, 2017
10. Dec 24, 2017

### Staff: Mentor

No - eg Bell does not deny realism, we simply do not know if QM has realism or not. Bells theorem however does set bounds on it.

But first lets pin down your original question on CFD:
https://arxiv.org/pdf/1212.5214.pdf
Let us define “counterfactual-definite” [14, 15] a theory whose experiments uncover properties that are pre-existing. In other words, in a counterfactual-definite theory it is meaningful to assign a property to a system (e.g. the position of an electron) independently of whether the measurement of such property is carried out. [Sometime this counterfactual definiteness property is also called “realism”, but it is best to avoid such philosophically laden term to avoid misconceptions.]

Once that's understood you may wish to rephrase your question.

Thanks
Bill

11. Dec 24, 2017

### Staff: Mentor

Nice paper - reading it now.

And indeed its very important to realize Einsteins arguments are not easily dismissed even today - although I am basically with Weinberg that both Bohr and Einstein were, in light of modern knowledge, wrong, there is a tendancy, I have even seen posted on this forum, to say Bohr was more right that Einstein. I do not agree with that - I think its the other way around. Bohr had a bit of a 'wishy washy' side IMHO, with ideas I never actually got like complementarity, however Einstein was always, to my mind, clear and concise in his objections. Many are still valid today, but I think Copenhagen as formulated by Bohr, Heisenberg etc has now morphed a bit in light of current knowledge. For example wave particle duality, which it states as a basic principle, isn't quite thought of the same way today. We know everything is really quantum stuff - its not particle or wave and thinking this way is in fact a myth:
https://arxiv.org/abs/quant-ph/0609163

Understandable in Bohr's day of course - but a lot of water has passed under the bridge since then.

Finished reading it. Einstein had an absolute conviction in the existence of a reality out there independent of us. To him the wave-function had to be real or the theory, almost by definition, was incomplete. Clearly and exactly stated - I don't necessarily agree with it, but wont go into what I think here, the point is its unambiguous. Then we have his actual EPR type objection - its based on separability. Kind of reminds me of Weinberg statement of the Cluster Decomposition property:

Not exactly the same of course - but you get the feeling Einsteins famous intuition was still there pointing to future developments.

Thanks
Bill

Last edited: Dec 24, 2017
12. Dec 24, 2017

### DrChinese

The details of the definitions will ultimately control the answer to your first question. But in any context in which they are mentioned together, there is no practical difference.

13. Dec 25, 2017

### vanhees71

To be honest, I never understood Bohr's writings on the subject, particularly his response to the EPR paper (with the same title) is a complete enigma to me. I wonder, what he really wanted to say.

In contradistinction to that, Einstein was absolutely clear about what he thought to be a "complete physical theory", and indeed for him QT (and also modern relativistic QFT) is not complete because of the inseparability, described by entanglement. In this respect, I think he was wrong, as is confirmed by all the fancy Bell experiments successfully done in the recent few decades. I can only speculate about what Einstein would have to say about it, but since he was very close to experiment (he even did experiments himself sometimes, but not with too much success, as the famous example of the Einstein-de Haas effect shows) I think, at least he would have become more convinced about the fact that QT is not incomplete because of inseparability, but that inseparability is a successful prediction of the theory.

Of course QT is not complete as long as nobody has found a satisfactory way to incorporate gravitation.

Concerning the socalled "interpretational problems", I think QT in the minimal interpretation is what all physical theories are, namely a mathematical description of what we can observe in nature by objective quantitative measurements, no more no less. There is no necessity for any "interpretation" in an ontic sense beyond the minimal one since I don't even know, where should be a problem. As long as QT describes all observed facts we can objectively measure, there is no problem.

Last but not least: If you want religion, go to church; you won't find it in any physics lecture hall, and in fact you shouldn't find it there!

14. Dec 25, 2017

### Staff: Mentor

I am pretty sure I was not Robinson Crusoe there. Einstein and Bohr were friends so I think he understood what Bohr was getting at, but I sure didn't.

Thanks
Bill

15. Dec 25, 2017

### vanhees71

Since I still don't know, what the term "reality" means in clearly defined operational or theoretical way, I say only that much: For me reality is what can be objectively observed and quantitatively measured, and in this regards, there's not a single reproducible observation that disproves QT.

There is also no conflict between locality and inseparability. Any relativistic local and microcausal QFT is, as the name says, both local and at the same time allows for entanglement ob observables between far-distant observations on quantum systems. In this sense QFT includes, as any QT, inseparability. E.g., a polarization-entangled biphoton as produced by parametric downconversion is one quantum system, no matter how far distant the photons are registered. Not long ago there was a big press hype about photons being entangled over astronomical distances. There's no problem with that concerning locality. QED by construction has only local interactions, and all observations are in accordance with it. Also note that the linked-cluster theorem by construction holds true for all these theories.

I recently got a copy of

A. Duncan, The conceptual framework of quantum field theory, Oxford University Press, Oxford, 2012.

It's the best textbook on QFT after Weinberg's masterpiece, and it's complementary, discussing many subjects not mentioned by Weinberg! There you can find a very clear discussion on locality, the linked-cluster theorem, causality, and all that.

BTW: Oxford University Press has a seasonal sale with 50% off (code: HOLIDAY17).

16. Dec 25, 2017

### entropy1

Is that "The Quantum Theory of Fields", of Steven Weinberg?

17. Dec 25, 2017

### vanhees71

Yes, it's

S. Weinberg, Quantum Theory of Fields, 3 vols., Cambridge University Press

18. Dec 27, 2017

### DrChinese

I never understood that either. Always seems a bit hand-wavy as a response to EPR.

19. Dec 27, 2017

### zonde

Hard to say about difference but there certainly is similarity - different people attach different meanings to both terms.

https://arxiv.org/abs/quant-ph/0607057 - here are the list of possible definitions of "realism"
https://en.wikipedia.org/wiki/Counterfactual_definiteness - wikipedia gives two different meanings in first sentence and if you take the meaning of the term "counterfactual" from Counterfactual thinking you have even the third meaning.

20. Dec 29, 2017

### physika

CFD = Values = object property.
no object -> no values -> no reality.

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