I What exactly does quantum entanglement imply?

[stevendaryl]

I understand that if you remove realism from the situation then you can keep in touch with locality.
However my question is how does removing realism keep in touch with locality?

I think it's more that you can't PROVE nonlocality without assuming realism.

 

[bhobba]

The CLT is mathematical..

When applied it is more than mere mathematics and hinges on what an event is in your model.

This thread however is not a thread about the foundations of probability and the use of the Kolmogerov axioms in applications. It needs a thread of its own not in the QM section - but in the probability section. But the early pages of Feller's classic on probability discusses it.

Thanks
Bill

 

[bhobba]

However my question is how does removing realism keep in touch with locality?

One more time.

There is a theorem, called Bells theorem, I gave a link to that states: If QM is true and you want counterfactual definiteness then you need FTL. It also states if you don't insist on conterfactual definitiveness then you can keep locality.

How does it do that? Its a logical deduction like any theorem.

Thanks
Bill

 

[bhobba]

QM correlations really are "voodoo" in the sense that while we can see the results of the magic trick before us, we have no way of grasping how it took place.

I don't get this, I really don't. Why people get caught up in this dialectic of how does something happen, and what 'explains' means, has me beat. Its just useless philosophical mumbo jumbo of zero scientific value.

A logical deduction from the postulates of a theory, model, whatever you want to call it, explains how something occurs and is what science is all about. Quantum correlations are fully explained by QM. End of story. The rest is philosophical mumbo jumbo and IMHO a total waste of time. You achieve nothing in a fundamental sense expect maybe find a different explanation, that also by the nature of explanation, rests on other assumptions. You are simply looking for assumptions you like better. That isn't science.

QM is not 'voodoo' - it explains in a logical way its deductions. Some people just don't like what it says. That's fine - but don't try and make out there is some essential problem with QM like 'voodoo' - there isnt.

My old statistical modelling professor would occasionally touch on such things - his eyes would roll back - he would say - its like studying Niechie - pointless really. Guess what - he was right.

Thanks
Bill

 

[stevendaryl]

I don't get this, I really don't. Why people get caught up in this dialectic of how does something happen, and what 'explains' means, has me beat. Its just useless philosophical mumbo jumbo of zero scientific value.

I think that's completely unfair. QM really is a conceptual mess, and it seems uncharitable for you to criticize people for pointing out this truth.

Why is it a conceptual mess? If you say that QM is about making predictions about the results of measurements, that's a rule of thumb but it isn't actually a coherent thing to believe. A measurement is not a separate category. A measurement is just an interaction between one system (the thing being measured) and another (the thing doing the measuring). How can measurements have definite outcomes if other interactions don't? That doesn't really make any sense. Of course, you can just take it on faith, and get on with things, because for practical purposes, we can distinguish measurements from other interactions, but it doesn't actually make sense.

The other thing that doesn't make any sense is treating improper mixed states as if they are proper mixed states. They are very different things, because in the first case, you don't have uncertainty due to lack of knowledge, you just have a very complicated pure state. To treat it as a proper mixed state (where the weights represent lack of information) is to assume something that is false.

It's a complete mess, conceptually. Usually in science, the intuition that something is a mess is a good guide in searching for a better theory. In QM, that hasn't proved to be the case, because nobody has really made any progress. But it's not an analytic truth that it is impossible to make sense of QM.

 

[bhobba]

I think that's completely unfair. QM really is a conceptual mess, and it seems uncharitable for you to criticize people for pointing out this truth.

That, IMHO, is not true. I gave a link how the formalism comes from reasonable axioms. There is no 'voodo' involved, no conceptual mess. Its a theory about observations. Everyone has an intuitive idea of what observation means and that is what's used to start with - later it becomes more precise.

All there is, is people wanting to force a theory to conform to their intuition and present that as some kind of problem. There may have been issues in the early days of QM - there isn't these days.

Its OK to not like QM and want to put it on a foundation you like better. I get that. But to present it as more than that, to present that it contains some kind of 'voodoo' when all that going on is you don't like it is wrong.

The principles of QM fully explain entanglement - end of story.

Thanks
Bill

 

[stevendaryl]

That, IMHO, is not true. I gave a link how the formalism comes from reasonable axioms. There is no 'voodo' involved, no conceptual mess.

I don't agree. The standard way of presenting QM relies on making a distinction between "measurements" and other interactions. That distinction is incoherent, because a measurement is just a complicated collection of the same sort of interactions that govern electrons and photons. Any formalism for QM that makes measurements into a separate category is relying on voodoo.

 

[stevendaryl]

All there is, is people wanting to force a theory to conform to their intuition and present that as some kind of problem.

That is not an accurate description of people's complaints with QM.

 

[bhobba]

I don't agree. The standard way of presenting QM relies on making a distinction between "measurements" and other interactions..

See the link I gave previously:
http://arxiv.org/pdf/quant-ph/0101012.pdf

See figure 1.

That's the conceptual model its based on and gets generalised and elucidated further as the theory is developed.

Thanks
Bill

 

[bhobba]

That is not an accurate description of people's complaints with QM.

What's not accurate? Saying QM correlations are 'voodoo' when they are fully explainable by the principles of QM. Sorry - but that's just plain WRONG.

Thanks
Bill

 

[stevendaryl]

See the link I gave previously:
http://arxiv.org/pdf/quant-ph/0101012.pdf

That's exactly the sort of thing that I was talking about. If you separate a "measurement event" from other kinds of physical interactions, then what is written in that paper sounds sensible, but a measurement event is not a different category. Measurements are complicated interactions involving macroscopic systems. They aren't "positive operators on states". I consider that aspect of the paper to be voodoo, since a necessary assumption (the distinction between measurements and other kinds of interactions) is actually false.

 

[stevendaryl]

What's not accurate?

The claim that "All there is, is people wanting to force a theory to conform to their intuition"

I quoted that line, so I'm not sure why you would ask what I was referring to.

 

[stevendaryl]

I'm going to bow out before the thread is closed.

 

[bhobba]

I'm going to bow out before the thread is closed.

Same here.

Thanks
Bill

 

[Weddgyr]

QM is not 'voodoo' - it explains in a logical way its deductions. Some people just don't like what it says. That's fine - but don't try and make out there is some essential problem with QM like 'voodoo' - there isnt.

My chocie of the word "voodoo" was off the cuff, but not entirely unserious.

I don't dispute that the predictions of QM are arrived by by a logical, computable process. However what I would emphasise is that the physical events themselves(detector blips), on which the correlations are based, do not arise from a physical or logical process. If non-localism is forbidden, as SR would suggest, then no process involving elements of physical reality, or any simulation of such a process, can account for why the detectors individually beep or not. QM only accounts for the correllations of the beeps after the fact.

I feel obliged to elaborate on the "voodoo" remark, so I'll put forward the following deliberately unphysical thought experiment.

Two Voodoo Kings (L and R) meet at dawn, each bringing with them a pot containing an equal number of black and white beads. They perform some ritual or incantation on the pots, then leave, returning to their respective clinics having sworn an oath -- or hexed themselves-- not to communicate with each other in any way.

During the day, until dusk, an experimenter or experimenters can visit each king(L or R), as many times as they like, and ask for a bead drawing ritual to be performed. In the ritual, the voodoo king's cane is placed on the ground/table near the pot, and the tip moved so that it makes a certain angle between the pot and the cane. The king (L or R) then draws a bead from the pot. The color c_i of the bead B/W is recorded, along with the angle \alpha_i of the cane, and the current number i of rituals this king has performed on that day. The king then replaces the bead in the pot.

The experimenters requests generate an ordered set of drawing data from each king. i \quad c_{Li} \quad \alpha_{Li} and i \quad c_{Ri} \quad \alpha_{Ri} When draws with the same count number i are compared, it is found(*) that the drawn beads have the same color with probability \cos(\alpha_{Li} -\alpha_{Ri})^2. Nevertheless individually each King is found to have an equal chance in any given ritual of drawing a black or white bead.

It is clear that the above thought experiment corresponds exactly to an EPRB experiment with photons. We can alter the settings by having the experimenter choose the cane angles, randomly or from a fixed set. We can have one experimenter, or two, or multiples. We can have the experimenter obtain draws in any order, or even all from one king and then the other. We can have the kings moved to opposite ends of the earth, or universe, or any other typical settings applied in thought or actual EPRB experiments.

And, in particular, we can assert that the data which results from these Voodoo rituals cannot be explained by any local hidden variable model. And if we add special relativity -- to my understanding at least -- we can assert that no element of physical reality, or physical process, can be invoked to explain why a bead is drawn as either black or white. The advantage of this Voodoo king thought experiment is that we have done away with detectors, photons and experimental setups, and here it is to some way of thinking "intuitively clear" that there is no underlying physical explanation for why each bead is drawn black or white. The Kings will claim that it is "magic", "voodoo", "ghosts", etc, and in particular beyond the scientific/mathematical/intellectual/rational ability of human beings to explain, and in physics we have a term for this thesis and it is called "Bell's Theorem".

Of course if you allow non-locality these difficulties disappear immediately. It's just that relativity seems to be getting in the way of anyone doing that.

My old statistical modelling professor would occasionally touch on such things - his eyes would roll back - he would say - its like studying Niechie - pointless really. Guess what - he was right.

I'd caution your old professor to be a little more careful in what he considers it pointless to discuss. Because under certain interpretations of EPRB experiments, philosophical mumbo jumbo is a very real element of our universe.

(*) I am probably not being entirely statistically precise here, but I will say that the data on agreement / disagreement of the beads is consistent with a dice with probabilities cos(a-b)^2 / sin(a-b)^2 being rolled for each paired draw. The setup could be modified so that the experimenter goes to each king with a fixed angle or set of angles, but I've kept things deliberately general here.

 

[Jeff Rosenbury]

In most of science, there's a good story behind the mathematical models. Planets circle the sun, animals descend from simpler ancestors, etc.

These stories are usually somewhat wrong. (Planets form affine paths. Species are as likely to revert as advance -- and what is "advance" anyway?) But the stories inform and help develop the mathematical model.

This doesn't seem to be as true for entanglement. The explanation "It just is" while accurate, seems a cop out.

Searching for a story is not a waste of time because if one can be found it could provide new insights into how the universe works. But it is important to understand that a lot of very smart people have searched before us. There is a potential for doing lots of searching and finding nothing new. Understanding why previous searchers found what they found (or mostly didn't find) will help to avoid dead ends.

Also understand there's not likely a paycheck in the search.

 

[David Byrden]

I have a vague idea for a "story" that would help people understand entanglement, or at least, not be surprised by it. Let me spell out an example.

Imagine a sheet of graph paper. The horizontal axis is Space (just one dimension of it) and the vertical is Time.

On the paper, draw a large "V". This represents the life-paths of two entangled particles. At the bottom, the base of the "V" is where they are interacting intimately and where they become entangled. At the top, each end of the "V" represents a particle hitting a detector, which erases the entanglement. The gap at the top of the "V" is the distance between the detectors.Since Time is an axis of the graph, you are outside of Time. There is no concept of things happening "before" or "after" for you. The "V" is the entire extent of the entangled system.

Is it surprising to you now, that the two measurements are correlated? They're touching both ends of a single V-shaped object. There is no need for "nonlocal instant communication" because both ends of the "V" are in contact, through the "V" itself. No need to move information across the gap at the top of the "V".

I think that our notions of "collapsing waveforms" and "multiple universes" are consequences of our false assumption that the future hasn't happened yet. We assume there's choice, free will. And then, at the point where the Nature shows us differently, we have to explain it away with magical, outrageous concepts.

In this "story", the future and the past coexist. It's not how we perceive reality. But it makes entanglement plain to understand.

I'm not saying that the mathematics of QM corresponds with this "story", or that it could be extended to more complex interactions with multiple particles. But if it stops one person beating their head against the wall of "nonlocality", or wondering what happens if "Alice" measures after "Bob", then I count it as useful.

David

 

[entropy1]

My two cents: I think that everyone has their favorite interpretation of non-locality... It's like painting a triangle: paint two sides blue, and the third turns out white; the whole triangle never is entirely blue.

 

[Nugatory]

OK, this thread has exhausted its modest allowance of mentorly patience. Closed.