[AndroidNeox]

Entanglement is a direct consequence of causality. In a quantum sense, every quantum of information acquired about something is an observation. Prior to observation, while the observer possesses no information about the state of a system, the system exists (for the observer) in a multiplicity of all allowed states. Bell's inequality, proven by experiment, shows that this multiplicity of states is real. When the observer makes the observation, causality requires that all future observations will be consistent. Based on the old concept of "universe", this would require that the act of observation alters the state of physically distant systems instantly. There is no way to reconcile this interpretation with Relativity because it violates non-simultaneity and the speed of light.

Fortunately, there are alternative interpretations that are consistent with Relativity and the observations of Quantum Mechanics. One can presume that all physics is local, as required by Relativity, and that observation doesn't alter the universe, but that observation alters the observer.

However, these require that we exist in a multiverse which many (most?) people aren't comfortable thinking about.

[Skyli]

Great discussion! Thank you to Alan and correspondents.

I cannot say by any means that I understand everything, but much has become clearer. You might describe me as an "interested layman" who has taken some (not enough) time to research QM; I am still far from being able to say "understand QM". In particular the reference to the Wikipedia article on Entanglement supplied by imatfaan was excellent and I wish I had the wherewithal to understand the mathematics of the second half of the article. There are, indeed, references to other Wiki articles that will, no doubt, help to furnish me with such but I would be very grateful to the forum if you could recommend a book or set of articles for the "interested layman" that could take me further.

I'm an old EDP guy - we didn't call it IT back in the 70's - with plenty of experience with structured techniques and concepts that, at first, appear counter-intuitive if that helps you with a recommendation.

[Pmb]

Entanglement is a direct consequence of causality.

In what sense? What is the cause and effect that you speak of here? Entanglement is a direct consequence of other things such as conservation of angular momentum. What did you think it was?

In a quantum sense, every quantum of information acquired about something is an observation.

That’s incorrect. An observation in this sense has a radically different meaning than it does throughout the quantum mechanics literature. Please give a concrete example of what you mean by this and in your example please show how the wave function collapses from such an observation because that is what must happen during any observation in the quantum mechanical sense.

Prior to observation, while the observer possesses no information about the state of a system, the system exists (for the observer) in a multiplicity of all allowed states.

I don’t understand why you’re not using standard terminology here but your own terminology. What you said is more simply said as the system exists in a superposition of states. However you’ve over generalized. There’s no reason to assume that the system exists in a “multiplicity of all allowed states.”  A quantum state can be a linear combination of a finite subset of the allowed states.

Bell's inequality, proven by experiment, shows that this multiplicity of states is real.

We didn’t need that equality to know that the principle of superposition was real.

When the observer makes the observation, causality requires that all future observations will be consistent.

You’re being extremely unclear. It’s almost as if you’re just tossing words together to make a word salad. First you say above “every quantum of information acquired about something is an observation.“ and here you say “the observer makes the observation” and never do you correlate the two. And you’re talking about causality only in the most vaguest of ways. I can’t see what it means in this context.

Based on the old concept of "universe", this would require that the act of observation alters the state of physically distant systems instantly. There is no way to reconcile this interpretation with Relativity because it violates non-simultaneity and the speed of light.

What is this “old concept of universe” that you speak of? And all light alters matter. Big deal. But nothing is instantaneous. Suppose I create pairs of photons that have opposite spin and fire them off in opposite directions, each pair having opposite spins but the direction of the pair is arbitrary. This results in the same effects on the universe as two photons in an entangled state. However in both cases nothing is moving faster than the speed of light. Nothing is instantaneous. I told you earlier to look this up in a text such as that of Griffith. Did you consider doing that? You can find it online. I can help you if you really want to learn about this.

However, these require that we exist in a multiverse which many (most?) people aren't comfortable thinking about.

Wrong. You’ve only come to this conclusion because you have some serious misunderstandings of quantum theory. The source of your mistake is confusing information traveling instantaneously with entangled states. It isn’t and therefore there is no causality traveling instantaneously either. Any book about this topic makes this abundantly clear so I can’t imagine where you’ve picked this up from. Some of the things you’re saying may be meaningful to you but not to the quantum mechanics community. Hence my “word salad” comment. Nothing personal.

[Pmb]

Can Causality be violated by say instantaneous quantum entanglement?

No.

Spooky action as Einstein called it or quantum non-locality and entanglement have shown fundamental particles can interact instantaneously regardless of the distance between them. Alter the spin of one particle alters the spin of its twin in the other direction instantaneously.

That is a misinterpretation. They do not interact instantaneously regardless of the distance between them. That’s not what happens in quantum entanglement.

Causality is fundamental to how the universe works, entropy for instant is intertwined with causality etc.

How so? When you make assertions like this you really need to state exactly what you mean. Just throwing two concepts together and saying they’re intertwined with causality helps nobody. I’m not even sure that it’s meaningful

Can Causality be violated by say instantaneous quantum entanglement?

That’s a confusing way to state that. Those two concepts don’t belong side by side like that in a sentence. What you meant to ask was whether quantum entanglement can cause two events to occur instantaneously even when a finite distance separates them. If so then it will result in a violation of causality.

Alter the spin of one particle alters the spin of its twin in the other direction instantaneously.

This is where you keep getting yourself into trouble. You seem to think that when the spin of a particle is measured then its altered by that measurement and as a result cause the spin of the other particle to take on a particular spin. This too is incorrect.

Let’s get this straight once and for all. In what follows, if you attempt to rephrase this in any other way, trying to force your beliefs on how causality works, then you will be making a serious error. Quantum mechanics is hard and it’s hard because of things exactly like this.
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Quantum entanglement is a physical phenomenon that occurs when pairs (or groups) of particles are generated or interact in ways such that the quantum state of each member must subsequently be described relative to each other.

When a measurement is made on one member of an entangled pair and the outcome is thus known (e.g., clockwise spin), the other member of the pair is at any subsequent time always found (when measured) to have taken the appropriately correlated value (e.g., counterclockwise spin). Thus, there is a correlation between the results of measurements performed on entangled pairs, and this correlation is observed even though the entangled pair may be separated by arbitrarily large distances.
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When you alter or rephrase anything above you will be making a mistake. Do not read more into what is going on in the funky world of quantum mechanics than is really there.

You keep mistaking this for action at a distance and it’s not. You also keep taking this to mean that the spin of the first particle alters the spin of the other particle. This is very wrong too.

[Pmb]

I am still far from being able to say "understand QM".

Don't worry about that. Richard Feynman is famous for saying that he didn't understand quantum mechanics and that nobody does. What he meant was that it shreds your basic understanding of the world around you to peices. What there is to learn is what we can and what we can't know. If it doesn't seem very odd to you then you've missed something.

[dlorde]

Wrong. You’ve only come to this conclusion because you have some serious misunderstandings of quantum theory. The source of your mistake is confusing information traveling instantaneously with entangled states. It isn’t and therefore there is no causality traveling instantaneously either. Any book about this topic makes this abundantly clear so I can’t imagine where you’ve picked this up from. Some of the things you’re saying may be meaningful to you but not to the quantum mechanics community. Hence my “word salad” comment. Nothing personal.

I'm glad it's not just me who thought it sounded a bit hokey...

[CPT ArkAngel]

What the latest experiments seem to demonstrate is that there is a correlation which is faster than light but the only way we have, at least for now, is to verify it with a light signal. So in the end, we can't really observe the correlation faster than the speed of light.

It doesn't mean there is no causality faster than the speed of light, what it means is we have not find it yet and maybe it is just a trick of nature. I must say that it is not true that QM explains the mechanism of entanglement, it just tries to describe the possible outcomes. A spin has not really only two possible outcomes, up or down, it has a 360^o in 3D... QM does not say much about entanglement, so the doors are still wide opened in both ways, Einstein's way and his opponents' way... Though great experiments have been done, there is still much to do to clear the confusion. On the other hand, we still don't know what mediates gravity: nothing can stop gravity... How and why?

If causality would be violated, this would be the end of the road for science. I am sure there is still a long road forward...

[distimpson]

Spooky action as Einstein called it or quantum non-locality and entanglement have shown fundamental particles can interact instantaneously regardless of the distance between them. Alter the spin of one particle alters the spin of its twin in the other direction instantaneously.

That is a misinterpretation. They do not interact instantaneously regardless of the distance between them. That’s not what happens in quantum entanglement.

I've seen this misinterpretation more than once. Just my opinion here, some of the technical papers use jargon that is defined in that context but can be misunderstood or misused in the magazines and TV of pop culture science where sales are top priority, teleportation and ftl always generate interest. But on the bright side, at least it gets people talking about science.

For what it's worth, the examples at this site helped make things much less spooky for me: http://www.askamathematician.com/2013/01/q-what-is-quantum-teleportation-why-cant-we-use-it-to-communicate-faster-than-light/comment-page-1/#comment-269796. I'm still wading through the math for Bell's <= for these cases.

[CPT ArkAngel]

Regarding the article, it is wrong to say it is teleportation because it is not proved that it has exactly the same alignment of the spin. They just know it has a spin up or down, not the precise direction of the spin. I would never go in their teleportation machine... )

[Skyli]

Great answer! I must admit, what I have read and tried to understand probably hasn't done my sanity much good to date, and I have no illusions about ever being anything other than an interested spectator, but it's hard to describe the field as anything but fascinating; the mother of all Mindbogglings. As luck would have it, I was extolling the site to my nephew last night and he had a couple of "Teach Yourself QM" type books. Between that and NSF I should be drooling by Christmas. Thanks.

[Pmb]

What the latest experiments ..

What "latest experiments"?

..seem to demonstrate is that there is a correlation which is faster than light but the only way we have, at least for now, is to verify it with a light signal.

That's incorrect. Correlation has been demonstrated when entanglement was demonstrated. And one doesn't need faster than light signals to verfify it. One need only record the time, location and state of measurements of particles and then later correlate the data.

So in the end, we can't really observe the correlation faster than the speed of light.

That's incorrect for the same reason. Where are you getting these hokey ideas from?

[CPT ArkAngel]

Prof Zeilinger 's team, Chinese physicists and now others have demonstrate entanglement at long distances, switching the states of detectors during the flight path of photons. They showed clearly that correlations appeared faster than the speed of light. The limitation is in the validation between the two locations where they have no way to do it faster than light. But there is still possible loopholes...

I am entirely correct!

Are you giving up causality or faster than light correlations or the possibility Einstein was right and there is still loopholes?

[Pmb]

Prof Zeilinger 's team, Chinese physicists and now others have demonstrate entanglement at long distances, switching the states of detectors during the flight path of photons. They showed clearly that correlations appeared faster than the speed of light. The limitation is in the validation between the two locations where they have no way to do it faster than light. But there is still possible loopholes...

I am entirely correct!

Nope. You're mistaken. You've mistaken the interpretation of this and you continue to do so. And I have no intention to keep repeating myself if you're not going to make an attempt to demonstrate that your assertions are correct rather than merely state it.

[CPT ArkAngel]

Open your mind a bit... I gave you 3 choices...

I just give up non causality. That's it!

[Pmb]

Open your mind a bit...

My mind is more open than you'll ever be able to understand.

[CPT ArkAngel]

You seem to adopt the usual "shut up and calculate" interpretation of QM that doesn't care about the causality of the possible faster than light correlations. You say no information travel faster than light, you say there is faster than light correlations. How to reconcile both point of view? QM has absolutely no answer to that!

[Pmb]

You seem to adopt the usual "shut up and calculate" interpretation of QM that doesn't care about the causality of the possible faster than light correlations. You say no information travel faster than light, you say there is faster than light correlations. How to reconcile both point of view? QM has absolutely no answer to that!

Your mind reading skills have a lot to be desired. lol!

[CPT ArkAngel]

I have no pretention that is the way you think for all physics subjects and I know you have many things on your mind.

[CPT ArkAngel]

http://www.pnas.org/content/110/4/1221.full

http://phys.org/news/2013-01-einstein-entanglement-quantum-erasure-deconstructs.html

[CPT ArkAngel]

They pretend to have closed all loopholes.

If it is the case, I see 3 possibilities:

1) information can travel faster than light.

2) information can travel no faster than light but can travel backward in time, from the future to the past, pilot wave, transactional interpretation or others...

3) many worlds interpretation (multiverse).

For me, the third one is clearly science-fiction and it is equivalent to giving up causality. And I don't see how it can explain the causal relationships of these experiments.

And I just wonder what Einstein would have preferred between the first and the second one...