What exactly does quantum entanglement imply?

[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.