A Smolin: Realistic and anti-realistic interpretations of QM

[charters]

Why should my statement above imply such obvious nonsense of non-existence of free electrons?

Because you reject all possible affirmative claims about where in spacetime the free electron is present. You don't accept it is unsharply, ontologically everywhere that it's quantum state has non-zero amplitude (or amplitude in excess of |0>). You don't accept the Bohmian idea that it is sharply at a single (epistemically uncertain) point. You claim quantum theory is just a theory of detector clicks.

Therefore, you don't believe free electrons are present in spacetime, but somehow pop into spacetime when forming macro objects, and you want to start using chemistry, materials science etc.

 

[PeterDonis]

you reject all possible affirmative claims about where in spacetime the free electron is present

No, he just rejected the two you describe:

You don't accept it is unsharply, ontologically everywhere that it's quantum state has non-zero amplitude (or amplitude in excess of |0>). You don't accept the Bohmian idea that it is sharply at a single (epistemically uncertain) point.

But these by no means exhaust the possibilities. As I understand @vanhees71's position, it is that "electron" is a name for a particular class of states of a particular quantum field (the charged lepton field in the Standard Model). Your description of "unsharply, ontologically everywhere that its quantum state has nonzero amplitude" is a non-relativistic description, so is not a valid description of a quantum field state; and Bohmian mechanics also is a non-relativistic model, so it has the same issue.

 

[charters]

No, he just rejected the two you describe:
But these by no means exhaust the possibilities. As I understand @vanhees71's position, it is that "electron" is a name for a particular class of states of a particular quantum field (the charged lepton field in the Standard Model). Your description of "unsharply, ontologically everywhere that its quantum state has nonzero amplitude" is a non-relativistic description, so is not a valid description of a quantum field state; and Bohmian mechanics also is a non-relativistic model, so it has the same issue.

The particles identified with the particle number eigenstates of free (or asymptotic interacting) QFT is very much an example of unsharp, ontologically extended entities. See the discussion here: https://arxiv.org/abs/quant-ph/0112149

Indeed, it is the impossibility of strict particle localization or even sub-Compton effective localization in QFT (due to Reeh-Schlieder) which partially motivates the picture of unsharp entities.

The options I offered are logically exhaustive. It is a tautology an entity must be either sharply somewhere, unsharply somewhere/everywhere, or nowhere. Each leads to a different aspect of the measurement problem, and I am just trying to walk vanhees to the version most salient to him.

 

[vanhees71]

Because you reject all possible affirmative claims about where in spacetime the free electron is present. You don't accept it is unsharply, ontologically everywhere that it's quantum state has non-zero amplitude (or amplitude in excess of |0>). You don't accept the Bohmian idea that it is sharply at a single (epistemically uncertain) point. You claim quantum theory is just a theory of detector clicks.

Therefore, you don't believe free electrons are present in spacetime, but somehow pop into spacetime when forming macro objects, and you want to start using chemistry, materials science etc.

Well, I like Bohmian mechanics as an alternative interpretation for non-relativistic quantum mechanics. What's still missing is a convincing Bohmian interpretation of relativistic QFT.

Of course, I believe free electrons are present at spacetime. They are well observed and in fact they were the first discovered free elementary particles (Wichert, Thomson 1897). They are described in the Standard model as spin-1/2 Dirac quantum fields. Since they carry no color charge they have asymptotic free states and thus are observable has free particles.

It doesn't make sense to think about them as classical point particles, because this contradicts a plethora of known empirical facts. Despite this, classical relativistic interacting point particles are a mathematical nuissance rather than a simplified description as in non-relativistic physics, but that's another story.

 

[charters]

Of course, I believe free electrons are present at spacetime

Then you have to answer my question of *where* in spacetime you think they are, between measurements. Everywhere/exactly where the quantum state says or somewhere more sharply defined? I have often found when people see the measurement problem as a non-issue, it is because they aren't asking all the relevant questions, such as this one, to fully vet the logical coherence of their views.

Well, I like Bohmian mechanics as an alternative interpretation

But your whole argument has been that the measurement problem is a non-issue, while the purpose of Bohmian mechanics is to try to deal with the measurement problem. What's there for you to like about it?

 

[vanhees71]

To the first point: An electron is described as a Dirac field. A free electron is described as a one-particle Fock state, and it's somewhere. Given the state (i.e., the statistical operator) it is prepared in, there's a probability distribution of its location, no more no less. It doesn't take a determined position, but there's only a probability distribution where it'll be found. I don't see a problem in this, because that's how electrons behave in the lab with high precision.

To the second point: It's funny, how you cut my previous comment. I said, Bohmian mechanics is a nice alternative interpretation of non-relativistic quantum mechanis, but it's incomplete, because there's no convincing non-relativistic version of it and thus is incomplete compared to standard QT.

 

[charters]

It doesn't take a determined position, but there's only a probability distribution

This is a contradiction. A "probability distribution" means the electron has a determined but unknown location. So, I have to ask again, between measurements, do you claim the position is A) determined but unknown or B) objectively undetermined, such that the system is "smeared" across all possible positions?

Alternatively, I would say failing to recognize the significance of this nuance is why you resist the validity of the measurement problem.

To the second point: It's funny, how you cut my previous comment. I said, Bohmian mechanics is a nice alternative interpretation of non-relativistic quantum mechanis, but it's incomplete, because there's no convincing non-relativistic version of it and thus is incomplete compared to standard QT.

But why do you think anything at all is nice about BM/whatsoever care about its completeness, given you deny the measurement problem, which is all BM is about?

 

[PeterDonis]

A "probability distribution" means the electron has a determined but unknown location.

Not if the probability distribution is over measurement outcomes. Such a probability distribution makes no ontological claim at all about the state of the system prior to measurement.

 

[charters]

Not if the probability distribution is over measurement outcomes. Such a probability distribution makes no ontological claim at all about the state of the system prior to measurement.

Correct. This was explicitly anticipated above. It is also where I thought we were heading until vanhees affirmed the ontological claim that free electrons exist in spacetime, taking this option off the table.

 

[PeterDonis]

It is also where I thought we were heading until vanhees affirmed the ontological claim that free electrons exist in spacetime, taking this option off the table.

I don't see why. Saying that there is a probability distribution over measurement outcomes is perfectly consistent with saying free electrons exist in spacetime but don't have a determined position. The first statement, as you agree, makes no ontological claim at all about the state prior to measurement; the second makes an ontological claim about the state prior to measurement that rules out having a determined position, but since the first statement makes no ontological claim, it does not require the electron to have a determined position.

 

[charters]

I don't see why. Saying that there is a probability distribution over measurement outcomes is perfectly consistent with saying free electrons exist in spacetime but don't have a determined position. The first statement, as you agree, makes no ontological claim at all about the state prior to measurement; the second makes an ontological claim about the state prior to measurement that rules out having a determined position, but since the first statement makes no ontological claim, it does not require the electron to have a determined position.

But this isn't where the discussion between vanhees and I is currently focused. In #55 I was careful to ask about the nature of the free electron "between measurements." In #56, vanhees said "there's a probability distribution of its location" not a probability distribution of measurement outcomes. So, I worry this is going to un-focus a discussion in which we were already kind of struggling.

 

[vanhees71]

This is a contradiction. A "probability distribution" means the electron has a determined but unknown location. So, I have to ask again, between measurements, do you claim the position is A) determined but unknown or B) objectively undetermined, such that the system is "smeared" across all possible positions?

Alternatively, I would say failing to recognize the significance of this nuance is why you resist the validity of the measurement problem.
But why do you think anything at all is nice about BM/whatsoever care about its completeness, given you deny the measurement problem, which is all BM is about?

No, it's no contradiction. According to QT the electron doesn't take a determined position, no more no less. The state provides a probability distribution to find an electron at a given position. Where should there be a contradiction? According to QT neither A) nor B) is correct. According to B) the position is objectively determined, but whenever I register an electron it's registered as one electron not some smeared entity.

An interpretation like Bohmian mechanics which doesn't cover relativistic physics is incomplete in comparison to minimally interpreted relativistic QFT. So why should I bother about Bohm's interpretation, which doesn't provide anything except a funny deterministic interpretation of non-relativistic QM but doesn't provide anything more than minimally interpreted QT as far as the physics is concerned and is less complete than the minimally interpreted QT?

 

[Auto-Didact]

According to QT neither A) nor B) is correct. According to B) the position is objectively determined, but whenever I register an electron it's registered as one electron not some smeared entity.

This makes your claim about states in between measurements to either be 1) that such states are purely epistemic, or 2) you outright reject the use of standard logic w.r.t. physics i.e. you are claiming that discussing Nature explicitly requires a more exotic form of logic.

Option 1 is explicitly ruled out by the PBR theorem. Incidentally, Smolin discusses this in the book.

Option 2 has been tried before in a specific implementation known as quantum logic and the consensus is that this form of logic fails, but the issue is still somewhat open. For years I myself believed the need for an exotic logic to be the answer, but I have long since changed my mind; important to note is that such an extension to standard logic seems to be both unwarranted and unnecessary given that there actually are other interpretations which work perfectly well using standard logic and Occam's razor.

 

[member 656954]

First I would recommend the book to anyone who reads or takes part in discussions on QM foundations.

Is it comprehensible for a non-QM expert, do you feel, @Auto-Didact?

 

[Auto-Didact]

Is it comprehensible for a non-QM expert, do you feel, @Auto-Didact?

Yes, I would argue it is. As Smolin makes clear in his talk linked in the first post, he is directly speaking to the public. The book is also written in a style that anyone should be able to read and understand. If you can follow his argument in the talk you should be able to follow his arguments in the book; a minor caveat is that fully understanding a few of the last chapters requires having read his previous books.

 

[member 656954]

Thanks, I'll give it a go

 

[vanhees71]

This makes your claim about states in between measurements to either be 1) that such states are purely epistemic, or 2) you outright reject the use of standard logic w.r.t. physics i.e. you are claiming that discussing Nature explicitly requires a more exotic form of logic.

Option 1 is explicitly ruled out by the PBR theorem. Incidentally, Smolin discusses this in the book.

Option 2 has been tried before in a specific implementation known as quantum logic and the consensus is that this form of logic fails, but the issue is still somewhat open. For years I myself believed the need for an exotic logic to be the answer, but I have long since changed my mind; important to note is that such an extension to standard logic seems to be both unwarranted and unnecessary given that there actually are other interpretations which work perfectly well using standard logic and Occam's razor.

Can you explain, why a purely epistemic interpretation is ruled out by the PBR theorem? PBR assume that there is an "ontic state" beyond the quantum mechanical state. Within QT there is no such thing, but only the quantum state, and its meaning is the probabilistic one given by Born's rule. Whether or not there is something beyond the quantum state or not, is not addressed by quantum theory. If you assume that there's an ontic state, the PBR theorem shows that then the epistemic interpretation of the quantum state leads to contradictions, but I don't assume any such thing as an "ontic state". There's not the slightest hint of something like this in any observation of nature, i.e., I don't see why I need the $\lambda$ of the PBR paper (Nat. Phys. 8, 475 (2012)).

 

[charters]

No, it's no contradiction. According to QT the electron doesn't take a determined position, no more no less. The state provides a probability distribution to find an electron at a given position. Where should there be a contradiction? According to QT neither A) nor B) is correct. According to B) the position is objectively determined, but whenever I register an electron it's registered as one electron not some smeared entity.

I specifically asked about *where* in spacetime you claim the free electron is *between measurements* to avoid this dodge. Saying you believe the ontological premise that the free electron exists in spacetime requires that you commit to a belief about this, independent of any talk about measurements or "finding." I am trying to show that your view is untenable under scrutiny, but you resist the application of this scrutiny by not directly answering plain questions that would lock you to a view and the consequences of it.

So why should I bother about Bohm's interpretation,

That's what I'm asking you. You said you like Bohm, not me.

But ok, I think I'm going to bow out of this discussion, not because you've shown the measurement problem to be trivial, but because there isn't enough recollection of our progress from day to day, so this is going in circles. It will probably be more effective for you to discuss this face to face, I think.

 

[vanhees71]

I don't claim "the electron is between measurements". It doesn't make sense at all. An electron is prepared somewhere in space. It's position is always indetermined due to the Heisenberg uncertainty relation. Thus its about its position there's always only a probability distribution to find it at a given position, no more no less. It doesn't make sense to talk about an electron or any other entity in physics, including macroscopic bodies without talking about their observability.

A measurement is also nothing than the interaction of the electron with other entities, following the fundamental laws of (quantum) physics. Thereby an electron may even be annihilated (e.g., if you let it collide with a positron and in the collision it's annihilated together with the positron into two photons). Thus you can only say an electron has been prepared in some state at time $t$. About its fate, i.e., whether it will still "be somewhere" (necessarily with a more or less uncertain position) or not, I cannot say anything, if I don't know the complete setup. As the example with the positron shows, it can even be annihilated. Then there's no electron left at all. That's all well-described by quantum theory, without any contradictions (neither intrinsic contradictions nor contradictions with any observation, so far).

 

[Auto-Didact]

Can you explain, why a purely epistemic interpretation is ruled out by the PBR theorem? PBR assume that there is an "ontic state" beyond the quantum mechanical state. Within QT there is no such thing, but only the quantum state, and its meaning is the probabilistic one given by Born's rule. Whether or not there is something beyond the quantum state or not, is not addressed by quantum theory. If you assume that there's an ontic state, the PBR theorem shows that then the epistemic interpretation of the quantum state leads to contradictions, but I don't assume any such thing as an "ontic state". There's not the slightest hint of something like this in any observation of nature, i.e., I don't see why I need the $\lambda$ of the PBR paper (Nat. Phys. 8, 475 (2012)).

This post demonstrates a clear misunderstanding of what it means to have an ontology: having an ontology means having an actual existence and being ontic simply means actually existing.

Example: Unicorns (one-horned horses) don't have an ontology (or aren't ontic) in the science of biology.

Similarly, any state that actually exists in any literal sense is by definition an ontic state.

If you don't accept this explanation, you are implicitly committing to option 2 from post #63.

 

[PeterDonis]

In #56, vanhees said "there's a probability distribution of its location" not a probability distribution of measurement outcomes.

As his later posts make clear, by "probability distribution of its location" he meant "probability distribution of measurement outcomes when you measure its location". You need to take people's posts in context instead of fixating on a particular phrase.

I specifically asked about *where* in spacetime you claim the free electron is *between measurements* to avoid this dodge.

It's not a dodge, it's a refusal to accept your claim that your categories are logically exhaustive. Your categories are interpretation dependent.

 

[samalkhaiat]

According to QT the electron doesn't take a determined position, no more no less.

Exactly. And the uncertainty principle is the reason why probability enters QM, full stop. Why do need to participate in this kind discussions? Let me just tell you that if this Smolin guy gave that talk in London or Oxford/Cambridge, he would be, after 15 minutes, talking to an empty lecture theatre in Oxford/Cambridge or get booed in London.

 

[charters]

As his later posts make clear, by "probability distribution of its location" he meant "probability distribution of measurement outcomes when you measure its location". You need to take people's posts in context instead of fixating on a particular phrase.

It's not a dodge, it's a refusal to accept your claim that your categories are logically exhaustive. Your categories are interpretation dependent.

Then he should have simply continued to agree to this instrumentalist position we were focusing on in the #40s, instead of rejecting it in the #50s (this is the more complete context). The overall categories I gave are exhaustive, and I accept what you suggest here is a logically possible option - in these later posts I am merely assuming some winnowing has taken place. The problem we've had is as soon as I try to get a firm, particular commitment to move us along, they backtrack on the commitment in order to reject the implications of the commitment that would lead to the measurement problem.

I think this is just not an effective medium/format for the socratic approach I was trying, with the strictly linear comment thread.

 

[PeterDonis]

Then he should have simply continued to agree to this instrumentalist position we were focusing on in the #40s, instead of rejecting it in the #50s (this is the more complete context).

The term "instrumentalist" is also interpretation dependent (as are Smolin's terms "realist" and "anti-realist"). This kind of labeling game does not strike me as a good way to make progress.

The overall categories I gave are exhaustive

I don't think they are, and apparently @vanhees71 doesn't think they are either. The Aristotelian game of stating a set of categories, claiming they are logically exhaustive, and then trying to force every claim into one of them also does not strike me as a good way to make progress.

 

[charters]

I don't think they are, and apparently @vanhees71 doesn't think they are either. The Aristotelian game of stating a set of categories, claiming they are logically exhaustive, and then trying to force every claim into one of them also does not strike me as a good way to make progress.

It is widely agreed any interpretation of QM is a) psi-ontic, b) psi-epistemic, or c) instrumental, and that this is exhaustive. A pseudo-exception are d) dynamical modification a la GRW/Penrose, but this is no longer an interpretation of quantum theory per se, but instead a new (but no less problematic) theory in its own right. See for example Aaronson's classification at the end of this article for an endorsement of the classification scheme: https://www.nature.com/articles/nphys2325?draft=journal&platform=oscar

However, you or vanhees can easily change my mind by presenting a concrete counter-example - just summarily saying this quite standard classification scheme is incomplete is unfair. But so far, this thread has not shown a counter example. It has just been a sort of whack-a-mole game of shifting between the familiar views, though too quickly for me to hone in on the version of the measurement problem most salient to each (which is all I was hoping to do).

It is easy to deny the significance of the measurement problem when one picks and chooses only the good parts of different, incompatible philosophical stances at the necessary times. The hard thing is sticking to a firm commitment, being clear eyed about the weird or difficult implications of it, and then working on a solution.

 

[PeterDonis]

you or vanhees can easily change my mind by presenting a concrete counter-example - just summarily saying this quite standard classification scheme is incomplete is unfair.

You already gave a counterexample: dynamical modification. In other words, not believing that QM as it currently exists is a final theory. (The specific example you give is just one particular case of this.) The classification scheme you describe assumes that it is. Calling the contrary belief a "pseudo-exception" seems just as unfair to me as summarily saying that the classification scheme you describe is incomplete seems to you.

 

[charters]

You already gave a counterexample: dynamical modification. In other words, not believing that QM as it currently exists is a final theory. (The specific example you give is just one particular case of this.) The classification scheme you describe assumes that it is. Calling the contrary belief a "pseudo-exception" seems just as unfair to me as summarily saying that the classification scheme you describe is incomplete seems to you.

Dynamical modification exists in order to solve the measurement problem in textbook quantum theory. It is not an option for someone whose stance is to deny the significance of the measurement problem in textbook QT, which is where we began back in #20 and #22. And I've seen no indication in this thread that anyone here actually wants to advocate something like this, which requires replacing the Schrodinger equation with something non-unitary.

So, I think this is a misrepresentation of vanhees position, and these approaches are irrelevant here. But if it somehow is what he meant, then it is a concession that my only point (the measurement problem is real) has been correct all along.

 

[PeterDonis]

Dynamical modification exists in order to solve the measurement problem in textbook quantum theory.

Only in the trivial sense that, if you have a different theory, it obviously doesn't have to share whatever problem you think you see in textbook QM.

It is not an option for someone whose stance is to deny the significance of the measurement problem in textbook QT

Why not? Doesn't saying "QT is not a fundamental theory, so there's no point in even worrying about any measurement problem it might have" (which is basically what I read @vanhees71 as saying) count as denying the significance of the measurement problem in textbook QT?

 

[charters]

Why not? Doesn't saying "QT is not a fundamental theory, so there's no point in even worrying about any measurement problem it might have" (which is basically what I read @vanhees71 as saying) count as denying the significance of the measurement problem in textbook QT?

Sure, you can say you don't *care* about the issue because you think QT will be replaced (though in reality there is no good reason to believe this as a serious possibility given the nature of quantum gravity research, where no popular and closely studied approach tries to replace QT as the overarching framework).

But that's not what vanhees is saying. In #22: "There's no contradiction in the sense of logic nor in the empirical evidence for this probabilistic interpretation of the formalism." They clearly think QT *as is* has no logical/conceptual inconsistencies, in particular not the inconsistency as set up in the Wallace quote I began with.

The argument has been QT is fine, not that we shouldn't care whether or not it is fine.

 

[PeterDonis]

that's not what vanhees is saying. In #22: "There's no contradiction in the sense of logic nor in the empirical evidence for this probabilistic interpretation of the formalism." They clearly think QT *as is* has no logical/conceptual inconsistencies, in particular not the inconsistency as set up in the Wallace quote I began with.

The Wallace quote you began with talks about a problem with QT if it is a fundamental theory. It says so right there in the quote. So his argument obviously doesn't apply to any interpretation of QT that does not treat it as a fundamental theory. I read @vanhees71 as saying that if textbook QT is treated as an effective theory that makes correct predictions in its domain but nothing more, then there are no logical/conceptual inconsistencies.

 

[charters]

The Wallace quote you began with talks about a problem with QT if it is a fundamental theory. It says so right there in the quote. So his argument obviously doesn't apply to any interpretation of QT that does not treat it as a fundamental theory. I read @vanhees71 as saying that if textbook QT is treated as an effective theory that makes correct predictions in its domain but nothing more, then there are no logical/conceptual inconsistencies.

Well, I don't read the thread that way, and you're also misreading Wallace, who certainly does not think the measurement problem is only relevant to strictly fundamental theories. You can look into his many papers and find this out for yourself.

But at this point I'm tired of this. So all I will say is, if your presentation of vanhees's argument is correct, it's quite a bit of wishful thinking to assume or expect quantum theory writ large will be replaced by anything, let alone by something that happens to magically make the measurement problem moot.

 

[PeterDonis]

you're also misreading Wallace, who certainly does not think the measurement problem is only relevant to strictly fundamental theories

If I am, it's certainly not evident from what you quoted. When I have time I'll take a look at his papers to get a more complete view of what he is saying.

 

[PeterDonis]

it's quite a bit of wishful thinking to assume or expect quantum theory writ large will be replaced by anything, let alone by something that happens to magically make the measurement problem moot.

It seems to me to be wishful thinking to assume that a framework for thinking about quantum theory, the one that points up the measurement problem as being the fundamental issue, will suddenly turn out to solve the problem after making no progress on it for many decades.

To put it another way, I would describe the fundamental problem not as "the measurement problem" but as "the quantum foundations problem"--is QM a fundamental theory or not? If it is, then nobody knows how to make it a consistent fundamental theory. If it isn't, then nobody knows what could possibly replace it. Talking about "the measurement problem" basically means you've chosen the first path--QM is a fundamental theory, the problem is how to make it a consistent one. But that problem doesn't even show up on the radar if you choose the second path--QM is not a fundamental theory, the problem is what to replace it with.

 

[charters]

To put it another way, I would describe the fundamental problem not as "the measurement problem" but as "the quantum foundations problem"--is QM a fundamental theory or not? If it is, then nobody knows how to make it a consistent fundamental theory

This is a topic for another thread, but I think at least one philosophically consistent/acceptable version of QT exists for both fundamental, non-fundamental, and non-physics (i.e., quantum information) applications of QT, so the situation is less bleak than you suggest. However, this comes through reckoning with the measurement problem and biting some bullets, not wishing the problem away and hoping the future will offer some return to classicality. Even if I'm wrong, and the situation is totally bleak, it's still unfair to equate kicking the can with actively trying to do our best with the theory we have right now. Its more honest and responsible to keep trying to make sense of QT, given the highly likely case it doesn't get supplanted. So I don't see these two approaches you outline as equally meritorious.

 

[PeterDonis]

This is a topic for another thread, but I think at least one philosophically consistent/acceptable version of QT exists for both fundamental, non-fundamental, and non-physics (i.e., quantum information) applications of QT

Yes, this would be a topic for another thread.

 

[vanhees71]

This post demonstrates a clear misunderstanding of what it means to have an ontology: having an ontology means having an actual existence and being ontic simply means actually existing.

Example: Unicorns (one-horned horses) don't have an ontology (or aren't ontic) in the science of biology.

Similarly, any state that actually exists in any literal sense is by definition an ontic state.

If you don't accept this explanation, you are implicitly committing to option 2 from post #63.

Quantum theory provides clear ontics. The actual existence of, e.g., elementary particles is not in question by the probabilistic description of QT. An electron actually exists in the description of relativistic QFT and is described by a quantum field. It doesn't exist as visualized by classical physics as a "point particle" of course, but that's because there's progress in science going way beyond a naive picture based on our experience with macroscopic objects which behave, under the circumstances of everyday life, to an excellent approximation as descxribed by classical physics.

 

[vanhees71]

Then he should have simply continued to agree to this instrumentalist position we were focusing on in the #40s, instead of rejecting it in the #50s (this is the more complete context). The overall categories I gave are exhaustive, and I accept what you suggest here is a logically possible option - in these later posts I am merely assuming some winnowing has taken place. The problem we've had is as soon as I try to get a firm, particular commitment to move us along, they backtrack on the commitment in order to reject the implications of the commitment that would lead to the measurement problem.

I think this is just not an effective medium/format for the socratic approach I was trying, with the strictly linear comment thread.

Well, you must allow me to have my point of view. It doesn't need to fit into one of your isms. Philosophy is indeed utmost inappropriate to shed light on the modern findings of the natural (and also structural) sciences. I'll take @samalkhaiat 's advice, not to participate in such fruitless discussions anymore. It's useless.

 

[vanhees71]

Exactly. And the uncertainty principle is the reason why probability enters QM, full stop. Why do need to participate in this kind discussions? Let me just tell you that if this Smolin guy gave that talk in London or Oxford/Cambridge, he would be, after 15 minutes, talking to an empty lecture theatre in Oxford/Cambridge or get booed in London.

Well, I think you are right. I shouldn't waste my time anymore to discuss philosophical issues in this forum. It's kind of fighting against religious beliefs rather than having a constructive scientific discussion.

 

[Auto-Didact]

I'll take @samalkhaiat 's advice, not to participate in such fruitless discussions anymore. It's useless.

You might enjoy pretending that the practice of physics and science at large, is free from this kind of foundational disagreement that we have as with foundations of QT; nothing could be further from the truth.

The best known case in the history of physics, where problems and paradoxes in the theory led to as much confusion as they do in QM foundations today, was in the 18th and 19th century in fluid mechanics, d’Alembert’s paradox; in fact, this problem can be restated as a problem of the interpretation of the ontological versus epistemological status of a central object in the theory, namely boundary layers - exactly like the problem with $\psi$ in QM foundations.

It is therefore nothing short of a tragedy that this tale isn't universally known among physicists; here a brief retelling is quoted from (Bush, 2015):

And lest the longevity of the quantum paradoxes be mistaken for their insurmountability, fluid mechanics has a cautionary tale to tell. In 1749, d’Alembert’s paradox indicated that an object moving through an inviscid fluid experiences no drag, a prediction that was clearly at odds with experiments on high–Reynolds number gas flows. The result was a longstanding rift between experimentalists and theorists: For much of the nineteenth century, the former worked on phenomena that could not be explained, and the latter on those that could not be observed (Lighthill 1956). D’Alembert’s paradox stood for over 150 years, until Prandtl’s developments (Anderson 2005) allowed for the resolution of the dynamics on the hitherto hidden scale of the viscous boundary layer.

 

[Auto-Didact]

Quantum theory provides clear ontics.

You are outright disagreeing with all the experts in the world on this matter, but keep telling yourself that if it makes you sleep better at night.

The actual existence of, e.g., elementary particles is not in question by the probabilistic description of QT. An electron actually exists in the description of relativistic QFT and is described by a quantum field. It doesn't exist as visualized by classical physics as a "point particle" of course, but that's because there's progress in science going way beyond a naive picture based on our experience with macroscopic objects which behave, under the circumstances of everyday life, to an excellent approximation as descxribed by classical physics.

This isn't under question at all. I don't see the need to keep bringing it up. Maybe it would be instructive to state that 'practical physics' purely focussed on applications (i.e. physics as an extension of engineering) is pretty much the opposite of 'foundational physics', wherein everything that has been swept under the rug is exposed so that it can be fixed. See the above post #89.

Why do need to participate in this kind discussions? Let me just tell you that if this Smolin guy gave that talk in London or Oxford/Cambridge, he would be, after 15 minutes, talking to an empty lecture theatre in Oxford/Cambridge or get booed in London.

Again, just because you don't find fundamental physics important, doesn't mean it isn't important.

Smolin actually devotes an entire chapter to discussing what the experts over at Oxford (Deutsch, Greaves, Myrvold, Sauders, Wallace et al.) think about the matter; he refers to their collective stance as critical realism, or more specifically the Oxford interpretation. Oxfordians - like Copenhagenists before them - have the same core belief but disagree to differing degrees on different specific points.

Simply stated, Oxfordians believe that decoherence, a irreversible statistical concept, is completely sufficient to solve the measurement problem. Smolin - like Bell, Shimony, Penrose and @A. Neumaier before him - keenly demonstrates that this argument is actually insufficient because it introduces observers into the foundations of the theory.

The problem with decoherence as a solution to the measurement problem is that if unitary evolution is fundamental to QT, then complete decoherence is impossible because decohered states will recohere if we wait long enough due to the Poincaré recurrence theorem; this is literally the same reason why entropy can increase.

Now if we are only interested in times shorter than it takes to recohere - that is if we are only interested in an approximate description of measurements for all practical purposes (FAPP) - then decoherence works, but as a matter of principle - i.e. as a question of foundational and mathematical physics - decoherence outright fails as a complete explanation.

 

[stevendaryl]

To put it another way, I would describe the fundamental problem not as "the measurement problem" but as "the quantum foundations problem"--is QM a fundamental theory or not? If it is, then nobody knows how to make it a consistent fundamental theory. If it isn't, then nobody knows what could possibly replace it.

That's a very good way to summarize the situation.

 

[stevendaryl]

The problem with decoherence as a solution to the measurement problem is that if unitary evolution is fundamental to QT, then complete decoherence is impossible because decohered states will recohere if we wait long enough due to the Poincaré recurrence theorem; this is literally the same reason why entropy can increase.

Now if we are only interested in times shorter than it takes to recohere - that is if we are only interested in an approximate description of measurements for all practical purposes (FAPP) - then decoherence works, but as a matter of principle - i.e. as a question of foundational and mathematical physics - decoherence outright fails as a complete explanation.

To me, there is possibly another problem with decoherence, and that is that, as I understand it, decoherence involves splitting the universe into three parts:

1. The system of interest, which might be a single electron
2. The measuring device
3. Everything else (the "environment")

After making such a split, you can trace out the environmental degrees of freedom, and what you find for the reduced density matrix is that it rapidly evolves into a mixed state. That mixed state can be interpreted as the situation: The measuring device nondeterministically goes into a definite "pointer" state, with probabilities given by the Born rule. So decoherence seems to give the same result as a "measurement collapses the wave function" interpretation without introducing a separate collapse event.

However, it seems subjective to me to split the world into the three parts that way. And it seems inconsistent to interpret a state that you know is an improper mixed state (due to tracing out environmental degrees of freedom) as if it were a proper mixed state (due to ignorance of the actual state).

 

[Auto-Didact]

To me, there is possibly another problem with decoherence, and that is that, as I understand it, decoherence involves splitting the universe into three parts:

1. The system of interest, which might be a single electron
2. The measuring device
3. Everything else (the "environment")

After making such a split, you can trace out the environmental degrees of freedom, and what you find for the reduced density matrix is that it rapidly evolves into a mixed state. That mixed state can be interpreted as the situation: The measuring device nondeterministically goes into a definite "pointer" state, with probabilities given by the Born rule. So decoherence seems to give the same result as a "measurement collapses the wave function" interpretation without introducing a separate collapse event.

However, it seems subjective to me to split the world into the three parts that way. And it seems inconsistent to interpret a state that you know is an improper mixed state (due to tracing out environmental degrees of freedom) as if it were a proper mixed state (due to ignorance of the actual state).

Agreed. Incidentally, that is more or less the same argument Penrose made 20 years ago against decoherence in The Road To Reality.

 

[akvadrako]

The problem with decoherence as a solution to the measurement problem is that if unitary evolution is fundamental to QT, then complete decoherence is impossible because decohered states will recohere if we wait long enough due to the Poincaré recurrence theorem; this is literally the same reason why entropy can increase.

Now if we are only interested in times shorter than it takes to recohere - that is if we are only interested in an approximate description of measurements for all practical purposes (FAPP) - then decoherence works, but as a matter of principle - i.e. as a question of foundational and mathematical physics - decoherence outright fails as a complete explanation.

Why, what's wrong with recoherence as part of the complete story? Presumably for a system like our universe the recoherance time could be far enough in the future that all structure would have long ago been lost due to heat death.

 

[Auto-Didact]

Why, what's wrong with recoherence as part of the complete story?

If decoherence solves the measurement problem per the Born rule, then it should be effectively completely irreversible; the fact is that decoherence is always incomplete and therefore, per Poincaré recurrence, reversible. Ergo, it cannot solve the measurement problem.

In orthodox QM, the act of measurement is de facto irreversible. Upon measurement, unitary evolution restarts again with effectively 'new initial conditions'; this process is not reversible even if unitary evolution itself up to this point is reversible. In other words, decoherence is patently insufficient to solve the measurement problem.

 

[A. Neumaier]

Well, you must allow me to have my point of view. It doesn't need to fit into one of your isms. Philosophy is indeed utmost inappropriate to shed light on the modern findings of the natural (and also structural) sciences. I'll take @samalkhaiat 's advice, not to participate in such fruitless discussions anymore. It's useless.

Its fruitless because you insist on your own rules. You preserve a sort of effective consistency by the same sort of vagueness of language that you criticize in Bohr's writing. You adhere to a conceptual uncertainty principle, with no attempt to match your concepts to those used by the community that discusses foundational questions in a more precise way. This makes foundational discussions with you frustrating for all participants.

 

[vanhees71]

It's also very frustrating for me, because I never get comprehensible explanations from the philosophical side. And it's not "my rules", but the way modern Q(F)T is successfully applied for nearly 100 years now, including the most modern applications in quantum optics and quantum information physics, which are both closest to these fundamental topics as it can be as far as the physics is concerned.

I think it's just impossible to discuss the issue strictly staying in the realm of physics without distorting the subject by philosophical arguments which are completely irrelevant for the scientific side of the matter, which could be interesting, but I give up these discussions from now on.

 

[A. Neumaier]

it's not "my rules"

Everyone but you (even Ballentine) allows for collapse, and everyone but you (even Peres) acknowledges that there is a problem applying the statistical interpretation of QM to large systems such as the solar system, whose preparation cannot be replicated multiple times.

I give up these discussions from now on.

Yes, it is fruitless.

 

[akvadrako]

If decoherence solves the measurement problem per the Born rule, then it should be effectively completely irreversible; the fact is that decoherence is always incomplete and therefore, per Poincaré recurrence, reversible. Ergo, it cannot solve the measurement problem.

Why do you define the "measurement problem" this way? I think most people consider it to be something different; it just needs to describe our universe. What unitary QM predicts, at least considering the other assumptions of Poincaré recurrence (maybe finite dimensionality) is that eventually recoherance will happen. You seem to be a priori ruling that out.

 

[A. Neumaier]

What unitary QM predicts, at least considering the other assumptions of Poincaré recurrence (maybe finite dimensionality) is that eventually recoherance will happen.

Can you point to a theorem proving a recurrence theorem in the quantum case?

Poincaré recurrence is for finite-dimensional bounded dynamical systems only. Already a single hydrogen atom violates both assumptions, let alone the universe.