Determinism, realism, hidden variables

[Ilja]

I don't see why probabilities only make sense that way. You can only confirm your expectation by doing a run of similar experiments. So everything happening - mostly in other worlds - doesn't affect your measurement of the distribution in *your* world. In other words the observed distribution is identical to that of a true probability distribution even though it's actually caused by deterministic branching.

There is no well-defined process of branching and no well-defined history - which would be equivalent to a Bohmian trajectory, even if splitted (which one would have in a stochastic interpretation too). There is nothing which clearly distinguishes a branch - except the completely vague idea that it is something like an isolated wave packet, but it is doubtful that real wave functions split into localized packages instead of something very smooth. Roughly, you have nothing. Except a wave function. And whatever else from common sense one decides to use for some particular purpose.

Since when does MWI reject Bell's theorem?

They claim to be a realistic and Einstein-causal interpretation. But Bell's theorem holds for realistic Einstein-causal theories. So, they have to use some form of creative naming conventions or so (many words interpretation) to avoid Bell's theorem.

 

[Paul Colby]

How on Earth does she know what Bob is going to measure in the future? Different polarizer angles are different bases. Bob hasn't chosen his yet.

Bob's polarizer will react as if the particle was in a state which is now known by Alice. Bob still thinks the state is in a general entangled one and is none the wiser. I get that you find this mysterious. I don't because I've accepted QM as fundamental. QM states possessing their very own unique properties that aren't classical properties. When I was first confronted with the fact that the speed of light is independent of frame I had similar (identical) conceptual problems. But, the logic which 100% based on observations, dictate that space-time has this property. I got over it. QM very well (and almost assuredly is) just as fundamental. When I ask why is accepting SR so different than QM the replies boil down to because it doesn't conform to the way we think (or in some cases define) the world should work. I would suggest that people just accept the rules as fundamental for a week just as an exercise. If you find yourself asking but how did bob's measurement know...the answer is this is how the known state reacts to that measuring device.

 

[Paul Colby]

It is this dependency on *both* angles (actually their difference) that makes it quite impossible to explain the violations of Bell's Inequality by local variables.

Okay.

 

[Ilja]

You say that quantum systems ( such as our EPR pairs ) can best be understood in a realistic and causal way;

I say that to understand requires to describe it in a realistic and causal way. Everything else is mysticism, not understanding.

What would it be like for me to be entangled with another quantum object ? If QT is both realistic and causal, then it should be possible and meaningful to ask this question, no ?

I don't know, and I don't think so. What would it be like to be attracted by Newtonian gravity for a particle? I don't know, even if I know Newtonian gravity IMHO good enough.

 

[Derek Potter]

There is no well-defined process of branching and no well-defined history - which would be equivalent to a Bohmian trajectory, even if splitted (which one would have in a stochastic interpretation too). There is nothing which clearly distinguishes a branch - except the completely vague idea that it is something like an isolated wave packet, but it is doubtful that real wave functions split into localized packages instead of something very smooth. Roughly, you have nothing. Except a wave function. And whatever else from common sense one decides to use for some particular purpose.

They claim to be a realistic and Einstein-causal interpretation. But Bell's theorem holds for realistic Einstein-causal theories. So, they have to use some form of creative naming conventions or so (many words interpretation) to avoid Bell's theorem.

I didn't say anything about actual branching. Can we backtrack a bit and make the distinction which Demystifier brought up, between relative states and Wheeler's reinterpretation? I am happy to agree to the term MWI being reserved for Wheeler's nonsense, but in that case everything I've just said applies to relative states. You still get different "worlds" in RSF, the difference being that you, the commentator, not the observer, decompose a state in whatever basis you fancy. Often it will be sensible to use whatever the experiment sets out to measure, or else use the emergent preferred basis, but fundamentally you can use anything you like.

I don't see that MWI (unless you still mean Wheeler's version) rejects Bell's theorem. It is after all a theorem. It would be like rejecting 2 + 2 = 4. However for the theorem to be applicable, the kind of realism required is that "When I see a dog, it's a dog, not a ****ing kangaroo!" - which is, to put it mildly, a big assumption when we are dealing with a theory a) of observations and b) in which superposition is fundamental. Do you not agree?

 

[Derek Potter]

Bob's polarizer will react as if the particle was in a state which is now known by Alice. Bob still thinks the state is in a general entangled one and is none the wiser. I get that you find this mysterious. I don't because I've accepted QM as fundamental. QM states possessing their very own unique properties that aren't classical properties. When I was first confronted with the fact that the speed of light is independent of frame I had similar (identical) conceptual problems. But, the logic which 100% based on observations, dictate that space-time has this property. I got over it. QM very well (and almost assuredly is) just as fundamental. When I ask why is accepting SR so different than QM the replies boil down to because it doesn't conform to the way we think (or in some cases define) the world should work. I would suggest that people just accept the rules as fundamental for a week just as an exercise. If you find yourself asking but how did bob's measurement know...the answer is this is how the known state reacts to that measuring device.

I really don't know where you get the idea that I or anyone else here does not agree that QM is fundamental. It is a total waste of time explaining that we need to accept what we already accept.

 

[Ilja]

Can we backtrack a bit and make the distinction which Demystifier brought up, between relative states and Wheeler's reinterpretation? I am happy to agree to the term MWI being reserved for Wheeler's nonsense, but in that case everything I've just said applies to relative states.

Its hard, I have never seen an MWI variant which made sense for me, and so I have to acknowledge that I have not tried hard to distinguish the variants. My main criticism was that they need additional structure to make sense at all, but that I have never seen a variant which really specifies the additional structure(s) which are required.

I don't see that MWI (unless you still mean Wheeler's version) rejects Bell's theorem.

They claim "MWI is a realist, deterministic, local theory" even on the Wiki level. Its not my job to make their claims compatible with Bell's theorem.

But what I have seen, they can simply claim that Bell's theorem is not applicable, and it is hard to question this given that Bell's theorem requires a meaningful notion of probability theory, and this is nothing one can reasonably claim that it exists.

On the other hand, they use some common sense postulates to argue they have enough probability theory to prove the Born rule.

 

[Paul Colby]

It is a total waste of time explaining that we need to accept what we already accept.

If this is accepted then why do you ask,

How on Earth does she know what Bob is going to measure in the future? Different polarizer angles are different bases. Bob hasn't chosen his yet.

because in QM, which you claim to accept, she knows the STATE of bob's particle and therefore may answer all statistical questions regarding Bob's eventual choice of polarizer before Bob chooses. What's not to understand?

 

[Markus Hanke]

I don't know, and I don't think so. What would it be like to be attracted by Newtonian gravity for a particle? I don't know, even if I know Newtonian gravity IMHO good enough.

I think it is a perfectly reasonable question to ask what it would be like to "ride" a classical test particle under the influence of Newtonian gravity, and an easy to answer one too. If one assumes both realism and causality, it should likewise be possible to wonder what it would be like to "ride" a quantum particle ( never mind the entanglement thing for now ), and what the rest of the universe would look like from that perspective. I don't know the answer either, but it is an interesting question, though possibly not meaningful. I don't know.

 

[Derek Potter]

Its hard, I have never seen an MWI variant which made sense for me, and so I have to acknowledge that I have not tried hard to distinguish the variants. My main criticism was that they need additional structure to make sense at all, but that I have never seen a variant which really specifies the additional structure(s) which are required.

Okay. To my simple understanding of relative states it makes sense without any additional structure.

They claim "MWI is a realist, deterministic, local theory" even on the Wiki level. Its not my job to make their claims compatible with Bell's theorem.

But what I have seen, they can simply claim that Bell's theorem is not applicable, and it is hard to question this given that Bell's theorem requires a meaningful notion of probability theory, and this is nothing one can reasonably claim that it exists.

On the other hand, they use some common sense postulates to argue they have enough probability theory to prove the Born rule.

MWI has probability - the expectation of particular statistics. What it does not have is absolute outcomes - typically a measurement results in an entanglement. I don't see the problem with having emergent probability whilst asserting an ontic model which precludes applying Bell's Theorem. It's not as if it claims to violate the theorem, only that the necessary conditions are not met.

 

[Derek Potter]

If this is accepted then why do you ask,

because in QM, which you claim to accept, she knows the STATE of bob's particle and therefore may answer all statistical questions regarding Bob's eventual choice of polarizer before Bob chooses. What's not to understand?

Ah yes, my bad. She knows the STATE of Bob's particle. Oops.

But that does not alter anything. She *knows* the state, no mystery. But she also has had a causal influence on it. From a distance instantly. And that is a serious problem.

Saying "that's just the way QM is" does not get around the fact that the EPR correlations are impossible given local causal definite-realism. If you are happy that at least one of them has to go because "that's just the way QM is" then fine, let's leave it at that. I prefer to ask what does this tell us is going on a bit deeper than "shut up and let me calculate the probabilities" :)

 

[Ilja]

Okay. To my simple understanding of relative states it makes sense without any additional structure.

Even a subdivision into system and observer is already additional structure.

MWI has probability - the expectation of particular statistics.

I see no base for such expectations. Maybe I'm blind, but I don't see it.

I don't see the problem with having emergent probability whilst asserting an ontic model which precludes applying Bell's Theorem.

I don't see any probability emerging.

It's not as if it claims to violate the theorem, only that the necessary conditions are not met.

Claiming they are somehow realist and have a probability, but the EPR criterion of reality is somehow not applicable.

 

[Paul Colby]

But that does not alter anything. She *knows* the state, no mystery. But she also has had a causal influence on it. From a distance instantly. And that is a serious problem.

Alice's measurement is on the entangled state. The result of her measurement produces a pair in a product state. At this the measurement process is the exactly the same as measuring a $z$-polarized particle along $x$. Now, a common assumption is that the $x$-measurement device physically kicks or "disturbs" each spin along $z$ into a spin along $\pm x$ kind of at random. That this is a deeply flawed classical imagining of the measurement process is amply underscored by exactly the class of argument you reference. For practical reasons one cannot measure things without some form of interaction with the thing being measured. However, I do not believe it is this unavoidable measurement interaction causes the selection of an individual eigenvalue.

 

[Derek Potter]

Even a subdivision into system and observer is already additional structure.

I see no base for such expectations. Maybe I'm blind, but I don't see it.

I don't see any probability emerging.

Claiming they are somehow realist and have a probability, but the EPR criterion of reality is somehow not applicable.

You mentioned the factorization problem once before in terms of state space. I didn't get it then and I still don't. Which could easily be because I am not mathematical though I am trying to follow the principles that people mention! So let's cut to the chase. If we can take an arbitrary subspace of the global system's state space, what structure is needed to justify the assumption that at least one such (not every such) subspace is the state space of an observer? What is it about observer state spaces that makes it necessary to add something to QM in order for them to exist?

I do appreciate that a quantitative derivation of the Born Rule is controversial. But a qualitative emergence of probability is trivial. A sequence of observation yields a frequency for each outcome. That's all the probability one needs. Isn't it?

 

[Derek Potter]

Alice's measurement is on the entangled state. The result of her measurement produces a pair in a product state. .

I don't understand you. If the two electron state is |u>|d> + |d>|u> then Alice's measurement creates an entanglement of exactly the same form except that the first ket is now Alice's state not her electron's state. That's not a product. Neither can it be made into one without changing the basis from local variables like spin to non-local ones like two-electron entangled spins!

What you seem to be saying is that |u>|d> + |d>|u> appears to Alice to have collapsed to one or other of the products. But if the collapse is merely an appearence then there is no way you can say Bob's particle state has collapsed. It is in fact still entangled with the Alice system. But if you say the collapse is real and applies to Bob, then you have FTL propagation or causality.

 

[Ilja]

If we can take an arbitrary subspace of the global system's state space, what structure is needed to justify the assumption that at least one such (not every such) subspace is the state space of an observer?

The question is not that one can take some arbitrary subspace or not. The point is that one has to take it. And this choice of a subspace is necessary to define something which is claimed to have some status of reality - a split into different worlds. So, this choice of a subspace cannot be simply an arbitrary subjective and otherwise irrelevant thing, it has to be something real.

What is it about observer state spaces that makes it necessary to add something to QM in order for them to exist?

This is not the point. The point is that without additional structure nor observer state spaces nor system state spaces exist, but only a single global space without any structure.

The Copenhagen interpretation has, instead, a lot of additional structure, in the classical part it has a whole classical world full of it.

I do appreciate that a quantitative derivation of the Born Rule is controversial. But a qualitative emergence of probability is trivial. A sequence of observation yields a frequency for each outcome. That's all the probability one needs. Isn't it?

Which sequence? There are no sequences in a universe where everything always exists.

 

[Paul Colby]

I don't understand you. If the two electron state is |u>|d> + |d>|u> then Alice's measurement creates an entanglement of exactly the same form except that the first ket is now Alice's state not her electron's state. That's not a product.

If the state is $\vert u d\rangle+\vert d u\rangle$ and Alice performs a measurement on particle 1 obtaining a $u$ eigenvalue, then the resulting state for the pair is $\vert u d\rangle$ which is very much a product.

 

[Derek Potter]

The question is not that one can take some arbitrary subspace or not. The point is that one has to take it. And this choice of a subspace is necessary to define something which is claimed to have some status of reality - a split into different worlds. So, this choice of a subspace cannot be simply an arbitrary subjective and otherwise irrelevant thing, it has to be something real.

This is not the point. The point is that without additional structure nor observer state spaces nor system state spaces exist, but only a single global space without any structure.

The Copenhagen interpretation has, instead, a lot of additional structure, in the classical part it has a whole classical world full of it.

Which sequence? There are no sequences in a universe where everything always exists.

Okay, I haven't the faintest idea what any of that means. Thanks for trying.

 

[Derek Potter]

If the state is $\vert u d\rangle+\vert d u\rangle$ and Alice performs a measurement on particle 1 obtaining a $u$ eigenvalue, then the resulting state for the pair is $\vert u d\rangle$ which is very much a product.

Yes that theory(!) assumes that Alice's measurement collapses the wavefunction non-locally.

 

[Paul Colby]

What you seem to be saying is that |u>|d> + |d>|u> appears to Alice to have collapsed to one or other of the products. But if the collapse is merely an appearence then there is no way you can say Bob's particle state has collapsed. It is in fact still entangled with the Alice system. But if you say the collapse is real and applies to Bob, then you have FTL propagation or causality.

You bring much to what is being said that really hasn't been said by me. The EPR measurement (FTL propagation problem as you put it) assumes a causal connection that just isn't there. Viewing QM measurement as some form of random interaction is a flawed concept unsupported by experiment.

 

[Paul Colby]

Yes that theory(!) assumes that Alice's measurement collapses the wavefunction non-locally.

Yes, and I may owe the people here an apology. My interpretation of QM is quite "standard" at least from a 1960's view. I happen to hold the view that QM is quite strange, however, is quite consistent and doesn't need any help from additional interpretation. Reading the discussions here convince me even more of this position.

 

[Derek Potter]

You bring much to what is being said that really hasn't been said by me. The EPR measurement (FTL propagation problem as you put it) assumes a causal connection that just isn't there.

I cannot see how you can deny a causal connection when Bob's probabilities depend on Alice's basis and value.

Viewing QM measurement as some form of random interaction is a flawed concept unsupported by experiment.

Well we agree on something then.

 

[Derek Potter]

Yes, and I may owe the people here an apology. My interpretation of QM is quite "standard" at least from a 1960's view. I happen to hold the view that QM is quite strange, however, is quite consistent and doesn't need any help from additional interpretation. Reading the discussions here convince me even more of this position.

Depends what you mean by "need". If you want to know what happens when you fire electrons through a double slit you can "shut up and calculate" - the formalism will give you everything you need. If you want to make sense of what's going on you need interpretation. That's why Bell's contribution is so profound. It places severe contraints on how you can interpret QM.

 

[Paul Colby]

If you want to make sense of what's going on you need interpretation.

Well, nature doesn't have to make sense in the way you define sense. There are other examples in physics where nature doesn't make sense. People have over the years redefined what makes sense means for these cases. People did it with both relativities and with electromagnetic fields requiring a medium for propagation. You could say, well these make sense to me, and I would accept your assessment without question. Even to this day there are people who dispute the relativities and those that still flog the ether concept. What these people miss is they need to view the observed rules of nature as fundamental and move on. So, have you thought about why relativity makes sense to you but QM doesn't? The non-local nature of some QM states may seem spooky but I think this is just a refusal to accept the vector nature of QM state and what measurements mean operationally.

 

[Derek Potter]

So, have you thought about why relativity makes sense to you but QM doesn't? The non-local nature of some QM states may seem spooky but I think this is just a refusal to accept the vector nature of QM state and what measurements mean operationally.

I have never said QM doesn't make sense to me. Not for several years anyway and certainly not here. What I say is that non-locality does not make sense. Ergo, you may deduce, I believe QM is local. Not classical but local.

 

[Paul Colby]

I have never said QM doesn't make sense to me. Not for several years anyway and certainly not here. What I say is that non-locality does not make sense. Ergo, yopu may deduce, I believe QM is local. Not classical but local.

We,agree on even more. So, electrons are fermions, photons bosons etc. How much of this seeming non-locality is really due to shabby treatment of the problem. After all we are assuming distinguishable particles in all that's written here.

 

[Derek Potter]

We,agree on even more. So, electrons are fermions, photons bosons etc. How much of this seeming non-locality is really due to shabby treatment of the problem. After all we are assuming distinguishable particles in all that's written here.

Well one of the particles is a detector called Alice with a big red label on it and it lives in the West wing of the building and the other is Bob with a big blue label and it lives in the East wing. That should be distinguishable enough.

 

[Paul Colby]

Well one of the particles is a detector called Alice with a big red label on it and it lives in the West wing of the building and the other is Bob with a big blue label and it lives in the East wing. That should be distinguishable enough.

Okay, what ever. Alice and Bob hear clicks and these are associated by time of flight coincidence. There are many details kind of glossed over that I usually find helpful in understanding what is actually observed. Bottom line is they are looking at excitations of field modes in highly correlated states. Does this help? Probably not. However, field theory is manifestly local as far as I understand. I'm guessing how this locality gets lost is likely in the skipped details.

 

[Derek Potter]

Okay, what ever. Alice and Bob hear clicks and these are associated by time of flight coincidence. There are many details kind of glossed over that I usually find helpful in understanding what is actually observed. Bottom line is they are looking at excitations of field modes in highly correlated states. Does this help? Probably not. However, field theory is manifestly local as far as I understand. I'm guessing how this locality gets lost is likely in the skipped details.

Strange. I am reliably informed that QFT is notoriously non-local.

In any case, as I have said before, the *observed* violations of Bell's Inequality do not depend on quantum theory. (Bell's Theorem says that the BI will be violated under QM, the BI itself is classical statistics of observed events.)

If QFT could explain the correlations locally, the field excitations would be local hidden variables so one of the other BI criteria would have to be wrong - causality, or reality. For the probability of an interaction at Bob to depend on what Alice does but not be caused by it would be odd to say the least, but perhaps no odder than Bob making reliable observations when there is nothing there to observe.

 

[Demystifier]

However, field theory is manifestly local as far as I understand.

Strange. I am reliably informed that QFT is notoriously non-local.

There are different kinds of locality, and people should distinguish them. The two most important kinds are signal locality and Bell locality. QFT obeys signal locality, but not Bell locality. In other words, you are both right and both wrong. Or more correctly, you are both vague unless you specify what kind of locality you have in mind.

 

[Derek Potter]

There are different kinds of locality, and people should distinguish them. The two most important kinds are signal locality and Bell locality. QFT obeys signal locality, but not Bell locality. In other words, you are both right and both wrong. Or more correctly, you are both vague unless you specify what kind of locality you have in mind.

We're talking exclusively about Bell locality. Signalling doesn't come into it - not least because the violations of the BI that are observed as well as predicted can't be used for signalling.

 

[Demystifier]

We're talking exclusively about Bell locality. Signalling doesn't come into it - not least because the violations of the BI that are observed as well as predicted can't be used for signalling.

Good, but some people simply don't realize the fact that there are different kinds of locality. Especially those who use the QFT-is-local argument in the context of Bell inequalities and hidden variables.

 

[Paul Colby]

Good, but some people simply don't realize the fact that there are different kinds of locality. Especially those who use the QFT-is-local argument in the context of Bell inequalities and hidden variables.

Agreed, I was confused.

 

[Derek Potter]

Good, but some people simply don't realize the fact that there are different kinds of locality. Especially those who use the QFT-is-local argument in the context of Bell inequalities and hidden variables.

Whereas it simply wouldn't occur to others that anyone would confuse the two :) [edit - sorry Paul, just seen your comment. Ho ho.]

To be honest I've never seen the no-signalling theorem as asserting a kind of locality. It seems to me to be something that can be cited as a caveat to non-locality along the lines of "entanglements are non-local but you cannot signal with them". The in-ability to influence remote events at will doesn't suggest anything about where the variables are located. The other way round would make sense. If there were a theory which did allow signalling, then it would be non-local. Or conversely, a local theory would not allow signalling. Here, though, we have a non-local theory that does not allow signalling so I don't see why it should be called local at all.

 

[Paul Colby]

Depends what you mean by "need". If you want to know what happens when you fire electrons through a double slit you can "shut up and calculate" - the formalism will give you everything you need. If you want to make sense of what's going on you need interpretation.

For photons the interference is evident prior to quantization, same for electrons. This doesn't counter the shock an awe of Bell non-locality but the interference pattern is a result of classical waves. The wave amplitudes are then quantized and people dance gleefully about the particle picture. When viewed as a quantum filed slit interference doesn't leave me reeling.