# Quantum Entanglement
**URL:** https://boards.straightdope.com/t/quantum-entanglement/821045
**Category:** Factual Questions
**Created:** [September 9, 2018, 10:26am UTC](https://boards.straightdope.com/t/quantum-entanglement/821045 "2018-09-09T10:26:00Z")
**Posts on this page:** 13
**Page:** 1
### Author:  [@Batano](https://boards.straightdope.com/u/Batano)
#### Post date: [September 9, 2018, 10:26am UTC](https://boards.straightdope.com/t/quantum-entanglement/821045/1 "2018-09-09T10:26:00Z")
You have two entangled particles flying off in different directions. You interact with one to discover its spin and know its pair has the opposite spin. How is this different from having one black and one white poker chip sealed in envelopes. You mail one to a friend and when he opens his envelope he knows the color of your chip?
---
### Author:  [@GreenWyvern](https://boards.straightdope.com/u/GreenWyvern)
#### Post date: [September 9, 2018, 10:46am UTC](https://boards.straightdope.com/t/quantum-entanglement/821045/2 "2018-09-09T10:46:35Z")
It’s different because in QM both poker chips are grey until you look at one. The colour is undefined - _not_ definite but unknown.
---
### Author:  [@Riemann](https://boards.straightdope.com/u/Riemann)
#### Post date: [September 9, 2018, 11:03am UTC](https://boards.straightdope.com/t/quantum-entanglement/821045/3 "2018-09-09T11:03:52Z")
This would be a Local Hidden Variable theory, a form of Local Realism. It has been ruled out by experimental proofs of Bell’s Theorem.
> **[Bell's theorem](https://en.wikipedia.org/wiki/Bell%27s_theorem)**
>
> Bell's theorem is a term encompassing a number of closely related results in physics, all of which determine that quantum mechanics is incompatible with local hidden-variable theories given some basic assumptions about the nature of measurement. "Local" here refers to the principle of locality, the idea that a particle can only be influenced by its immediate surroundings, and that interactions mediated by physical fields cannot propagate faster than the speed of light. "Hidden variables" are puta...
> **[Local hidden-variable theory](https://en.wikipedia.org/wiki/Local_hidden_variable_theory)**
>
> In the interpretation of quantum mechanics, a local hidden-variable theory is a hidden-variable theory that satisfies the condition of being consistent with local realism. This definition restricts all types of those theories that attempt to account for the probabilistic features of quantum mechanics via the mechanism of underlying inaccessible variables with the additional requirement that distant events be independent, ruling out instantaneous (that is, faster-than-light) interactions between s...
Bell’s Theorem shows that the results you get from observations of entangled particles are correlated, but cannot have been “preplanned” by carrying hidden data with the particles from the point of origin. The reasoning is subtle, but you can find numerous explanations that don’t require any math, e.g. this paper:
> **[Mark Alford](https://web.physics.wustl.edu/alford/)**
>
> Mark Alford's home page
Or many other briefer explanations/analogies to be found online, e.g. the one from Brian Greene’s book here:
> **[Bell's theorem](https://simple.wikipedia.org/wiki/Bell%27s_theorem)**
>
> Bell's theorem, also called "Bell's inequality," is a thought experiment. When joined with real experiments, it shows there are no hidden variables which can explain some of the consequences of quantum mechanics. This study, closely related to quantum mechanics, was done by John Stewart Bell.
> The following analogy has been provided by Brian Greene:
> Pairs of boxes have been prepared and sent to Earth and on Vulcan. There is something inside that shows a light when a door is opened. If the same d...
---
### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [September 9, 2018, 12:32pm UTC](https://boards.straightdope.com/t/quantum-entanglement/821045/4 "2018-09-09T12:32:38Z")
> [@](#):
>
> Quoth **GreenWyvern** :
>
> It’s different because in QM both poker chips are grey until you look at one. The colour is undefined - not definite but unknown.
I’ve always hated this answer, because it’s just a “because I said so”, and doesn’t address the _reason_ why we say they’re unknowable, and so you could just as well say it about poker chips in envelopes.
First key point: You can’t measure a particle’s total spin. You can only measure one component of its spin, and see whether it’s positive or negative along that axis. So, for instance, you could measure one electron’s spin along the X axis, and find it to be positive. In that case, if you measure the other one’s spin along the X axis, you’re guaranteed to find that it’s negative. But once you measure one component of spin, you’ve changed the state of the system.
Second key point: You don’t have to measure both particles along the same axis. If you measure one particle along the X axis, and then measure the other one along the Y axis, the X measurement of the first will tell you absolutely nothing about the Y measurement of the second: It’s equally likely to be positive or negative. This is still consistent with a hidden-variables system; you just have to make your hidden variables slightly more sophisticated.
But the third key point: The angle between your measurement axes doesn’t have to be 90º (or a multiple thereof). You could, for instance, make one of your measurements along the X axis, and the other 45º between the X and Y axes. In this case, the results you get won’t match exactly, but will be correlated: The closer the two measurement axes are to each other, the greater the correlation. And even this is still consistent with hidden-variable models.
Finally, though, how _much_ correlation is there? Bell was able to mathematically prove that, in any hidden-variable theory (with some other constraints which seem reasonable, like locality), there must be an upper bound for the correlation at any given angle. And at angles which aren’t a multiple of 90º, quantum mechanics predicts a greater correlation, outside of the bounds of Bell’s Inequality. Thus, quantum mechanics cannot be a local hidden variable theory. And the experiment has been done, with results as predicted by quantum mechanics, not as predicted by Bell’s Inequality.
---
### Author:  [@Saffer](https://boards.straightdope.com/u/Saffer)
#### Post date: [September 9, 2018, 1:08pm UTC](https://boards.straightdope.com/t/quantum-entanglement/821045/5 "2018-09-09T13:08:49Z")
> [@Chronos](#):
>
> I’ve always hated this answer, because it’s just a “because I said so”, and doesn’t address the _reason_ why we say they’re unknowable, and so you could just as well say it about poker chips in envelopes.
>
> First key point: You can’t measure a particle’s total spin. You can only measure one component of its spin, and see whether it’s positive or negative along that axis. So, for instance, you could measure one electron’s spin along the X axis, and find it to be positive. In that case, if you measure the other one’s spin along the X axis, you’re guaranteed to find that it’s negative. But once you measure one component of spin, you’ve changed the state of the system.
>
> Second key point: You don’t have to measure both particles along the same axis. If you measure one particle along the X axis, and then measure the other one along the Y axis, the X measurement of the first will tell you absolutely nothing about the Y measurement of the second: It’s equally likely to be positive or negative. This is still consistent with a hidden-variables system; you just have to make your hidden variables slightly more sophisticated.
>
> But the third key point: The angle between your measurement axes doesn’t have to be 90º (or a multiple thereof). You could, for instance, make one of your measurements along the X axis, and the other 45º between the X and Y axes. In this case, the results you get won’t match exactly, but will be correlated: The closer the two measurement axes are to each other, the greater the correlation. And even this is still consistent with hidden-variable models.
>
> Finally, though, how _much_ correlation is there? Bell was able to mathematically prove that, in any hidden-variable theory (with some other constraints which seem reasonable, like locality), there must be an upper bound for the correlation at any given angle. And at angles which aren’t a multiple of 90º, quantum mechanics predicts a greater correlation, outside of the bounds of Bell’s Inequality. Thus, quantum mechanics cannot be a local hidden variable theory. And the experiment has been done, with results as predicted by quantum mechanics, not as predicted by Bell’s Inequality.
Can you elaborate on the comment about locality? What exactly is locality? What happens to your reasoning if we don’t assume this? Is this the same kind of locality that (the absence of) is mentioned as a downside of the pilot wave interpretation?
Sent from my iPhone using Tapatalk
---
### Author:  [@Riemann](https://boards.straightdope.com/u/Riemann)
#### Post date: [September 9, 2018, 1:20pm UTC](https://boards.straightdope.com/t/quantum-entanglement/821045/6 "2018-09-09T13:20:30Z")
> [@Saffer](#):
>
> Can you elaborate on the comment about locality? What exactly is locality? What happens to your reasoning if we don’t assume this? Is this the same kind of locality that (the absence of) is mentioned as a downside of the pilot wave interpretation?
Locality means no instantaneous action at a distance (violation of SR). And yes, DeBroglie-Bohm (pilot wave) is a non-local hidden variable theory, the only kind of hidden variable theory that’s consistent with experimental results given Bell’s Theorem.
> **[Principle of locality](https://en.wikipedia.org/wiki/Principle_of_locality)**
>
> In physics, the principle of locality states that an object is influenced directly only by its immediate surroundings. A theory that includes the principle of locality is said to be a "local theory". This is an alternative to the concept of instantaneous, or "non-local" action at a distance. Locality evolved out of the field theories of classical physics. The idea is that for a cause at one point to have an effect at another point, something in the space between those points must mediate the a...
---
### Author:  [@Saffer](https://boards.straightdope.com/u/Saffer)
#### Post date: [September 9, 2018, 2:08pm UTC](https://boards.straightdope.com/t/quantum-entanglement/821045/7 "2018-09-09T14:08:55Z")
> [@Riemann](#):
>
> Locality means no instantaneous action at a distance (violation of SR). And yes, DeBroglie-Bohm (pilot wave) is a non-local hidden variable theory, the only kind of hidden variable theory that’s consistent with experimental results given Bell’s Theorem.
>
> [Principle of locality - Wikipedia](https://en.wikipedia.org/wiki/Principle_of_locality)
Understood. However I’m a bit confused as to how some other interpretation can claim to be local. If the spins are at an indeterminate state, and measuring one leads to an instantaneous effect on the range of possible states of the other, then how can this be considered as not involving instantaneous action?
Sent from my iPhone using Tapatalk
---
### Author:  [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)
#### Post date: [September 9, 2018, 3:57pm UTC](https://boards.straightdope.com/t/quantum-entanglement/821045/8 "2018-09-09T15:57:25Z")
> [@Saffer](#):
>
> Understood. However I’m a bit confused as to how some other interpretation can claim to be local. If the spins are at an indeterminate state, and measuring one leads to an instantaneous effect on the range of possible states of the other, then how can this be considered as not involving instantaneous action?
> Sent from my iPhone using Tapatalk
Nobody is willing to give up the impossibility of FTL signalling so the search for some way around invoking it continues. A hidden variable might exist that would explain it or something else nobody has yet found or postulated. The only thing everybody agrees upon is that no instantaneous anything happens.
[What Is Real?: The Unfinished Quest for the Meaning of Quantum Physics](https://www.amazon.com/What-Real-Unfinished-Meaning-Quantum/dp/0465096050) by Adam Becker is a fascinating history of the various interpretations of QM. Each interpretation has a different way of attacking issues like these because the Copenhagen “shut up and calculate” mindset leaves questions that others feel itchy without having answers to. Becker takes sides in the controversy so a grain of salt is needed. Read it more for the questions than for answers.
---
### Author:  [@Riemann](https://boards.straightdope.com/u/Riemann)
#### Post date: [September 9, 2018, 7:37pm UTC](https://boards.straightdope.com/t/quantum-entanglement/821045/9 "2018-09-09T19:37:05Z")
> [@Saffer](#):
>
> Understood. However I’m a bit confused as to how some other interpretation can claim to be local. If the spins are at an indeterminate state, and measuring one leads to an instantaneous effect on the range of possible states of the other, then how can this be considered as not involving instantaneous action?
That’s the whole point - there is no plausible non-local account. When [Einstein et al wrote their paper](https://en.wikipedia.org/wiki/EPR_paradox), they asserted that quantum theory must be incomplete, because it involved paradoxical “spooky action at a distance”. Einstein coined this expression sardonically, as something he thought cannot be true. But, given Bell’s Theorem, all experimental results have since shown that EPR was wrong - the experiments do seem to imply spooky action at a distance. In other words, the data show that prior to observation the superposition is all there is to reality, there is no hidden underlying reality; only upon measurement of one particle does the other entangled particle “know” its know its correlated state; and it knows it instantaneously.
But it’s theories that are both Local and Realistic that are ruled out. This is usually interpreted as requiring a non-Local interpretation. I must say I’m quite unclear myself on what it could plausibly mean to retain Locality but instead violate Realism. Bell’s Theorem is based on correlations, there is no superluminal transfer of information. And inference from statistics depends upon the assumption that the observations are truly random, which is related to the notion of counterfactual definiteness and the no-conspiracy assumption. See here:
> **[Counterfactual definiteness](https://en.wikipedia.org/wiki/Counterfactual_definiteness)**
>
> In quantum mechanics, counterfactual definiteness (CFD) is the ability to speak "meaningfully" of the definiteness of the results of measurements that have not been performed (i.e., the ability to assume the existence of objects, and properties of objects, even when they have not been measured). The term "counterfactual definiteness" is used in discussions of physics calculations, especially those related to the phenomenon called quantum entanglement and those related to the Bell inequaliti The s...
We can, in principle, explain the results by claiming that the statistical inference is invalid, see Superdeterminism.
> **[Superdeterminism](https://en.wikipedia.org/wiki/Superdeterminism)**
>
> In quantum mechanics, superdeterminism is a loophole in Bell's theorem. By postulating that all systems being measured are correlated with the choices of which measurements to make on them, the assumptions of the theorem are no longer fulfilled. A hidden variables theory which is superdeterministic can thus fulfill Bell's notion of local causality and still violate the inequalities derived from Bell's theorem. This makes it possible to construct a local hidden-variable theory that reproduces t Be...
But this amounts to asserting that the entire universe is like a vast conspiracy. I don’t think anyone takes it seriously. If it were true, all statistical inference would be invalid, so science wouldn’t work at all. (The John Bell quote in that article talks about “free will”, which I think is misleading. It’s nothing to do with free will, it’s a question of whether statistical inference is valid, which depends upon the randomization of measurements, not free will.)
---
### Author:  [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)
#### Post date: [September 9, 2018, 8:17pm UTC](https://boards.straightdope.com/t/quantum-entanglement/821045/10 "2018-09-09T20:17:43Z")
> [@Saffer](#):
>
> Understood. However I’m a bit confused as to how some other interpretation can claim to be local. If the spins are at an indeterminate state, and measuring one leads to an instantaneous effect on the range of possible states of the other, then how can this be considered as not involving instantaneous action?
> Sent from my iPhone using Tapatalk
Your confusion is understandable, the measurements in your example are always anticorrelated, which is perfectly consistent with a local classical explanation: i.e. the outcomes are already fixed. However Bell’s theorem, which uses a set-up where the measurements are not perfectly correlated, shows that, overall, the probabilities predicted by quantum theory do not match a classical local explanation.
---
### Author:  [@Riemann](https://boards.straightdope.com/u/Riemann)
#### Post date: [September 9, 2018, 9:59pm UTC](https://boards.straightdope.com/t/quantum-entanglement/821045/11 "2018-09-09T21:59:07Z")
ETA: I think Many Worlds would be considered Local, and violates conterfactual definiteness. I’m not really sure if that means it’s correct to say that it’s Local but not Realistic?
---
### Author:  [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)
#### Post date: [September 10, 2018, 5:25am UTC](https://boards.straightdope.com/t/quantum-entanglement/821045/12 "2018-09-10T05:25:47Z")
> [@Riemann](#):
>
> ETA: I think Many Worlds would be considered Local, and violates conterfactual definiteness. I’m not really sure if that means it’s correct to say that it’s Local but not Realistic?
Many-worlds is _both_ local and realistic, because it violates another, most often unstated, assumption of Bell’s reasoning: that experiment outcomes are single-valued, i. e. that you only either get a ‘spin-up’ or ‘spin-down’ result.
---
### Author:  [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)
#### Post date: [September 10, 2018, 11:22am UTC](https://boards.straightdope.com/t/quantum-entanglement/821045/13 "2018-09-10T11:22:39Z")
> [@Batano](#):
>
> You have two entangled particles flying off in different directions. You interact with one to discover its spin and know its pair has the opposite spin. How is this different from having one black and one white poker chip sealed in envelopes. You mail one to a friend and when he opens his envelope he knows the color of your chip?
There’s indeed nothing special about discovering a property of one system, and immediately knowing the property of another—that’s just ordinary correlation. Entanglement, however, is correlation across different, and most importantly, mutually incompatible properties.
You know Heisenberg’s uncertainty relation: basically, it asserts that a quantum system can’t simultaneously have a definite position and momentum. Thus, position and momentum are incompatible properties. Another example of such incompatible properties is spin in x- and y-direction (equivalently, in the z-direction, but we only need two to illustrate the point). Thus, a quantum system can’t simultaneously have a definite x- and y-spin.
However, if you set a system of two particles up in the proper, entangled (Bell-) state, what you’ll find is that not only do they show perfect correlation along the x-axis, but likewise, that correlation exists along the y-axis, as well–despite the fact that neither of these can have both a definite x- and y-spin! That’s where the strangeness of entanglement comes from: if you have a single particle, and you measure its x- and then y-spin, then it’s easy to say that the particle just randomly decided on a particular value for the y-spin. But if that happens in entanglement, then if a particle is measured along the y-axis, and gives out a random answer made up on the spot, then in order to reproduce the fact that we will find its entangled partner having correlated y-spin, it must somehow tell its partner what it came up with—which would seem to necessitate some form of instantaneous communication.
This is what prompted Einstein, Podolsky and Rosen to set up their attack on quantum mechanics (the famous EPR-argument). They argued that the fact that since (counterfactually) if we measured the x-spin (they actually used position and momentum, with the spin-example being due to Bohm) of particle 1, we would immediately know the x-spin of particle 2, and if we measured the y-spin of particle 1, we would immediately know the y-spin of particle 2, there must be some pre-arranged agreement between the two particles. However, quantum mechanics knows nothing of this agreement: hence, they concluded, quantum mechanics must be incomplete.
That’s where Bell comes in (a little afterwards): he showed that every theory that includes such agreements (hidden variables), and furthermore, where there are no superluminal influences, must obey certain bounds, known as Bell inequalities. Quantum mechanics, however, violates these bounds (albeit not as much as it could, and why QM doesn’t violate these inequalities maximally is an interesting research programme these days). Consequently, either there must be some non-local influence, in order to reproduce the observed phenomena, or there can’t be any hidden variables.
Recently, people have brought up the option that the second requirement, actually, is moot: surely, there must be some recipe for coming up with measurement outcomes anyway; so one can’t just reject the assumption of ‘realism’ (hidden variables, what have you), or it’s not really an extra assumption.
I think everybody making this argument essentially sneaks in some extra assumption, about what a ‘reasonable’ theory should be like, which basically is equivalent to assuming a classical framework, but the assumption is somewhat technical to state. So giving up realism, to me, still is a live option. As for how such a theory would look like, well, just look at quantum mechanics: not every observable has a definite value at all times, and there are no non-local influences. Everything else, to me, seems to boil down to adding certain extra structure to QM.