# Backward Causation in Quantum Physics
**URL:** https://boards.straightdope.com/t/backward-causation-in-quantum-physics/629531
**Category:** Factual Questions
**Created:** [July 27, 2012, 4:23am UTC](https://boards.straightdope.com/t/backward-causation-in-quantum-physics/629531 "2012-07-27T04:23:39Z")
**Posts on this page:** 12
**Page:** 3
### Author:  [@iamnotbatman](https://boards.straightdope.com/u/iamnotbatman)
#### Post date: [August 2, 2012, 8:25am UTC](https://boards.straightdope.com/t/backward-causation-in-quantum-physics/629531/41 "2012-08-02T08:25:42Z")
> [@Half\_Man\_Half\_Wit](#):
>
> I think we must be talking past each other… I have repeatedly emphasized that there is no measurement-independent reality, i.e. no ‘real value’, to be assigned to a state in superposition, so I’m not sure where the problem lies.
It seems you are positing that fact rather than showing it to be necessary in the case of a local description. Obviously in various HV models there _is_ a measurement-independent reality even for what in QM can be described by the word “superposition.” So your simply _defining_ that “superposition” = “lack of counterfactual definiteness” does not really address the question you were responding to.
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### Author:  [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)
#### Post date: [August 2, 2012, 8:38am UTC](https://boards.straightdope.com/t/backward-causation-in-quantum-physics/629531/42 "2012-08-02T08:38:15Z")
> [@iamnotbatman](#):
>
> It seems you are positing that fact rather than showing it to be necessary in the case of a local description.
Of course I’m positing that fact – that’s what the question was! (‘How would the behavior entangled particles be explained via non-reality (lack of counterfactual definiteness)…?’)
The work in showing it to be necessary in case of a local (realist) description was already carried out by Bell…
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### Author:  [@iamnotbatman](https://boards.straightdope.com/u/iamnotbatman)
#### Post date: [August 2, 2012, 8:47am UTC](https://boards.straightdope.com/t/backward-causation-in-quantum-physics/629531/43 "2012-08-02T08:47:49Z")
> [@Half\_Man\_Half\_Wit](#):
>
> Of course I’m positing that fact – that’s what the question was! (‘How would the behavior entangled particles be explained via non-reality (lack of counterfactual definiteness)…?’)
>
> The work in showing it to be necessary in case of a local (realist) description was already carried out by Bell…
Gah. It seemed to me the question was asking for an explanation rather than a “Bell already showed it” sort of comment, but whatever, this is not a productive dialog.
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### Author:  [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)
#### Post date: [August 2, 2012, 10:38am UTC](https://boards.straightdope.com/t/backward-causation-in-quantum-physics/629531/44 "2012-08-02T10:38:36Z")
> [@iamnotbatman](#):
>
> Gah. It seemed to me the question was asking for an explanation rather than a “Bell already showed it” sort of comment, but whatever, this is not a productive dialog.
Agreed (though in all fairness, that’s also not the question you seemed to be answering in your first response). Perhaps **allotrope** can clarify what he intended to ask.
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### Author:  [@allotrope](https://boards.straightdope.com/u/allotrope)
#### Post date: [August 2, 2012, 2:10pm UTC](https://boards.straightdope.com/t/backward-causation-in-quantum-physics/629531/45 "2012-08-02T14:10:13Z")
> [@Half\_Man\_Half\_Wit](#):
>
> Agreed (though in all fairness, that’s also not the question you seemed to be answering in your first response). Perhaps **allotrope** can clarify what he intended to ask.
Huh?
I asked what he meant by the sentence I quoted. 😕
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### Author:  [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)
#### Post date: [August 2, 2012, 2:19pm UTC](https://boards.straightdope.com/t/backward-causation-in-quantum-physics/629531/46 "2012-08-02T14:19:37Z")
I meant re your first post in this thread…
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### Author:  [@allotrope](https://boards.straightdope.com/u/allotrope)
#### Post date: [August 2, 2012, 3:10pm UTC](https://boards.straightdope.com/t/backward-causation-in-quantum-physics/629531/47 "2012-08-02T15:10:57Z")
> [@Half\_Man\_Half\_Wit](#):
>
> I meant re your first post in this thread…
Ah, OK.
For the viewers at home:
> [@allotrope](#):
>
> How would the behavior entangled particles be explained via non-reality (lack of counterfactual definiteness) as opposed to non-locality?
My impression and I’m pretty sure it’s accurate, is that the ‘spooky action at a distance’ of entangled particles is generally regarded as a de facto if not de jure violation of locality. Sure, you can’t actually use that violation to do anything interesting so meh, no one gets a case of bunched up panties, but even so, it is what it is.
If on some level we happen to be worried about Bell breathing down our necks in addition to Albert, then we can tell him to bugger off as well since we’ve duly gored ourselves on one of his dilemma’s horns.
IOW, all is right with the world, or at least as right as things get round these parts.
But heaven forbid that there should be no controversy, so I was just wondering, ‘well, what if we thought it would be more fun to impale ourselves on the other horn?’ Let’s say we wanted to hang on to the notion of locality and were willing to give up the idea of counterfactual definiteness. How would that play out? How would we still get something like the [quantum eraser experiment](http://grad.physics.sunysb.edu/%7Eamarch/) to work in precisely the same way as it does now by throwing CFD to the wolves instead of locality?
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [August 2, 2012, 4:57pm UTC](https://boards.straightdope.com/t/backward-causation-in-quantum-physics/629531/48 "2012-08-02T16:57:20Z")
> [@](#):
>
> Quoth **Half Man Half Wit** :
>
> Yes, I’m not questioning that. But there is no non-local influence at work in Bell correlations if you do not assume counterfactual definiteness, or whatever else you want to call it. There’s no ‘action at a distance’. (There’s not even any interaction Hamiltonian!) What you have is non-factorizability: you can’t describe those experiments using product distributions. This may be considered some kind of ‘holism’ or ‘non-separability’, but that’s not the same as nonlocality in, for instance, Bohmian mechanics, where the quantum potential actually and causally changes its value in a non-local fashion.
Yeah, that’s why I specified “non-locality (or anything else equivalently weird)”. You have to do _something_ weird, whether it be non-locality, or lack of definiteness, or “holism”, or whatever. There are many different choices one can make for the “something weird” that are all consistent with quantum mechanics.
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### Author:  [@Indistinguishable](https://boards.straightdope.com/u/Indistinguishable)
#### Post date: [August 2, 2012, 6:40pm UTC](https://boards.straightdope.com/t/backward-causation-in-quantum-physics/629531/49 "2012-08-02T18:40:24Z")
> [@iamnotbatman](#):
>
> Everything in that paragraph is correct except the first sentence. The first sentence is just plain wrong. Completely wrong. Read the paper yourself if you are so inclined (it is fantastic):
> [http://www.univer.omsk.su/omsk/Sci/Everett/paper1957.html](http://www.univer.omsk.su/omsk/Sci/Everett/paper1957.html)
That link seems riddled with (minor, but distracting) typos. A better link is [here](http://www.nd.edu/~dhoward1/Everett%20Relative%20State.pdf) (though, warning, PDF, etc.)
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### Author:  [@iamnotbatman](https://boards.straightdope.com/u/iamnotbatman)
#### Post date: [August 2, 2012, 9:47pm UTC](https://boards.straightdope.com/t/backward-causation-in-quantum-physics/629531/50 "2012-08-02T21:47:28Z")
> [@Indistinguishable](#):
>
> That link seems riddled with (minor, but distracting) typos. A better link is [here](http://www.nd.edu/~dhoward1/Everett%20Relative%20State.pdf) (though, warning, PDF, etc.)
Thanks for finding it!
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### Author:  [@Frylock](https://boards.straightdope.com/u/Frylock)
#### Post date: [August 5, 2012, 8:40pm UTC](https://boards.straightdope.com/t/backward-causation-in-quantum-physics/629531/51 "2012-08-05T20:40:36Z")
[Possibly related.](http://physicsworld.com/cws/article/news/2012/aug/03/can-the-future-affect-the-past)
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### Author:  [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)
#### Post date: [August 5, 2012, 9:09pm UTC](https://boards.straightdope.com/t/backward-causation-in-quantum-physics/629531/52 "2012-08-05T21:09:32Z")
Heh, we actually discussed the paper in our group when it came out (that was the reason the two-state vector formalism was on my mind when I wrote my first reply). Basically, the consensus was rather skeptical – it doesn’t seem all that obvious that they’re really saying anything that should be surprising. As an analogy, the weak measurements give you information about the probability distribution, while the outcomes of the subsequent strong measurements tell you what to condition that information on; that both are consistent is probably not too surprising. But to be honest, I’m not sure I have too firm a grasp on all this ‘weak measurement’-stuff…
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