[JP]

And it all goes back to HUP (Heisenbergs Uncertainty Principle) which is a statement of indeterminacy to me, stating that you can't know all parameters/properties simultaneously.

How does entanglement go back to the uncertainty principle?

[yor_on]

This is how I see it JP. And there it had to do with how Einstein viewed HUP. He didn't like the view of how it was implicit in HUP that you couldn't get all parameters, simultaneously, from a particle, so he devised thought experiments in where he meant that you could get all parameters.

But he lost out to Bohrs reasoning, and so devised the EPR paradox to create another angle in where you could assume a underlying 'realism', guaranteeing no mysterious 'action at a distance' as the entanglement might be seen as, loosely speaking, even though we assume that information can't be transferred.

In his thought experiments (later ones, the one that's most famous was drafted by his companions without him, and he wasn't wholly happy with its statements as I understands it) he tried to envision situations in where you could say that all parameters was 'known' for a entangled particle. This is from memory though, no links to it, but I'm sure it is as correct as my memory, hopefully

And that's what I see as a more accurate description to how EPR came to be historically. Still, the debates they had are well known, looked on the internet and found Bohr–Einstein debates.

[yor_on]

This might give an insight in how Einstein thought. Einstein's Reply to Criticisms.


"The attempt to conceive the quantum-theoretical description as the complete description of the individual systems leads to unnatural theoretical interpretations, which become immediately unnecessary if one accepts the interpretation that the description refers to ensembles of systems and not to individual systems." - Albert Einstein:

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Btw: Isn't this the idea that 'weak measurements' resurrects? That you can 'know' all parameters? I'm not sure what I think of 'weak measurements' myself, I have some problems with accepting it.

[JP]

Ah, thanks.  That makes it clearer. 

I was (mis)reading your statement to mean that entanglement is a consequence of the uncertainty principle.  I don't think its fair to say that.  Entanglement is a consequence of QM, and as such it has to play by the rules of the uncertainty principle, as does every other quantum system. 

[yor_on]

No, that wasn't my intention. Although I find both HUP and entanglements as thought provoking, and so similar in 'intent' to me, as they both question 'reality'. And I guess Einstein saw it that way too. I think that what Einstein didn't agree on was the assumption that as causality disappear we replace it with statistics and probabilities, in some way defining that as 'real' as the causality chains we observe macroscopically.

Albert Einstein. “Physics and Reality.” Journal of the Franklin Institute 221 (1936), 349-82.

"Consider a mechanical system consisting of two partial systems A and B which interact with each other only during a limited time. Let the Ψ function before their interaction be given. Then the Schrödinger equation will furnish the Ψ function after the interaction has taken place. Let us now determine the physical state of the partial system A through a measurement which is as complete as possible. Then quantum mechanics allows us to determine the Ψ function of the partial system B from the measurements made, and from the Ψ function of the total system.

This determination, however, gives a result which depends upon which of the state variables of A have been measured (for instance, coordinates or momenta). Since there can be only one physical state of B after the interaction, which state cannot reasonably be considered to depend upon the kinds of measurements I carry out on the system A separated from B, it is thus shown that the Ψ function is not unambiguously correlated with the physical state. This correlation of several Ψ functions to the same physical state of system B shows again that the Ψ function cannot be interpreted as a (complete) description of a physical state (of an individual system)."

And

"Now it appears to me that one may speak of the real state of the partial system S2. To begin with, before performing the measurement on S₁, we know even less of this real state than we know of a system described by the Ψ-function. But on one assumption we should, in my opinion, unconditionally hold fast: The real situation (state) of system S₂ must be independent of what is done with system S₁, which is spatially separated from the former. According to the type of measurement I perform on S₁, I get, however, a very different Ψ₂ for the second partial system. (Ψ₂, Ψ'₂, . . .)  But now the real state of S₂ must be independent of what happens to S₁. Thus, different Ψ-functions can be found (depending on the choice of the measurement on S₁) for the same real state of S₂.

(One can only avoid this conclusion either by assuming that the measurement on S₁ changes (telepathically) the real state of S₂, or by generally denying independent real states to things which are spatially separated from one another. Both alternatives appear to me entirely unacceptable.)

If now the physicists A and B accept this reasoning as sound, then B will have to give up his position that the Ψ-function is a complete description of a real situation. For in this case it would be impossible that two different types of Ψ-functions could be correlated with the same situation (of S₂)."

(Corrected the text slightly, missed a 'sub')

[yor_on]

What is really weird, is that some complaining over the theory of relativity seems to accept QM logic on face value. They don't seem to get that Einstein was a realist after all. He wanted stuff to make sense, and causality chains to exist and be accounted for. He didn't like spooky actions at a distance as entanglements, he wanted an accounting for all we see. Not theoretical, but understandable in classical terms.

And that's what's his 'ensembles' try to do, as I see it. In it he accepts what we experimentally see but he does not try to define QM as causality chains from a classic perspective. Whether he thought of it as existing a 'hidden causality' or not is not important there.  I call it the 'rules of the game', including 'c' and Planck constants as one and the same (constants) even though one is classical, the other QM. And that goes for the conservation laws too, because that's what we build on.

[JP]

. . . he wanted an accounting for all we see. . . in classical terms.

Yep.  He didn't accept many features of QM that make it distinct from classical mechanics.  If you think of two entangled particles classically, then they're two separate particles that either can send information faster than light or have some extra internal variables that they each carry which specify how they're correlated.  If you think of them as a quantum state, however, they are no longer separate particles, but rather are two particles described by one quantum state, so they can be correlated through that state, without having to send information or carry around extra variables.

[yor_on]

Yeah, and that's one of the hardest nuts there are to crack to me. Because it invalidates 'distance', although, depending on how you define information, not 'c' as the constant defining what is 'useful/understandable information' to us.

But if 'c' was considered a pure clock beat, instead of something signifying the combination of a 'clock' creating a distance? That as any distance needs a 'clock' to exist for us, on any conceptual level I can think of.

distances are weird, both in relativity and QM.

[JP]

It doesn't invalidate distance.  It invalidates our classical idea that two entangled particles can be described as two separate physical objects.  The two measurements are still separated by a physical distance, which behaves in the usual way. 

[yor_on]

well yeah, that is how I see it.

It is two definitions, one macroscopic and one quantum mechanical, but if you want something leading from one to the other? I don't find my assumption weirder than to assume they are connected through a waveform ignoring SpaceTime distances and 'c'.

[JP]

You can see it how you like, but that doesn't change the fact that they're still separated by a distance.  QM may be weird, but it follows rules.  Throwing out distance because they're entangled breaks the rules of QM and relativity!

[yor_on]

I'm not throwing it out JP, I should have wrote 'question' instead of invalidate. Bad choice of words there. And it also depends on from where you look at it. If I look at it from Relativity you find Lorentz contractions, if you look at it from QM you find tunnelings and entanglements. But we have distances, and they work.
=

And that's weird

[JP]

Ah, but if you look at it from QM, you find tunneling, entanglement AND Lorentz contractions.  Time and space still exist in QM just as they do in SR.  Two particles undergoing "spooky action at a distance" doesn't change anything about space-time any more than two non-entangled particles do.

[yor_on]

Are you telling me that Lorentz contraction is a quantum mechanical effect? That was a new one to me? Not that I mind that much, physics huh
=
Or maybe not so new thinking again? Although I haven't thought of it that way.
I've thought of it as a macroscopic effect, coupled to mass, although some might argue there, and relativistic 'speeds/velocities'.

I better get some sleep here
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Had to look again and.

I do have thought of it that way. Strange that I forgot, probably it's because it was you saying that JP not me. And as you said it, it became new to me. But I've only considered it as a very weak possibility, in that it should be so incredibly hard to define experimentally. But it makes very good sense existing on a Quantum mechanical scale too, well, as I see it. Although depending on your choice of measuring there, assumably.

Not so hard measuring in 'motion', maybe? Or maybe I'm grossly bicycling in the blue younder there:)

But as I also expect it to be a constant opposite 'mirror' to time dilations, a sort of 'symmetry', I've wondered if it may exist even 'at rest', as in a particle of rest-mass able to be defined as being at rest. But there indeterminacy comes in, and?

So I do hope that was what you meant..

[JP]

What I meant was that QM is a model telling us how particles behave in space and time.  Another model has to define space and time, so you can have norelativistic QM for slow moving particles or relativistic QM for fast-moving particles.  In the latter, Lorentz transformations are obeyed.  If you have entangled particles, it doesn't change anything about the distance because distance is defined by another theory, not by QM.

[yor_on]

JP, is there experiments done on relativistically moving particles, proving a Lorentz contraction? That would be a very nice experiment to me, or is it 'time dilations' we can see.

[JP]

Anything done in particle accelerators indirectly tests Lorentz contractions.  I don't think anyone's specifically tested for only length contraction in QM, but the standard model requires Lorentz transformations in order to work.