The typical and the exceptional in physics

[vanhees71]

In this regard I disagree with you and agree with @atyy . But to explain why I disagree I would need to talk about ontology, which is something you don't really care about. So I can't explain my reasons for disagreement in a way you would care about.

Ok, but this is explicitly contradicting the very foundations of the model (QFT) you want to interpret. So there's an explicit contradiction between the mathematical structure of the model and the (ontological?) interpretation. What should this be good for?

 

[Demystifier]

Ok, but this is explicitly contradicting the very foundations of the model (QFT) you want to interpret. So there's an explicit contradiction between the mathematical structure of the model and the (ontological?) interpretation.

There is no contradiction at all. For instance, Bohmian formulation of QFT retains all the mathematical structure of standard QFT. However, this formulation does not stop there but makes one further step by adding one additional equation. It is this additional equation which makes the theory non-local. This additional equation does not contradict any of the previous local equations.

 

[zonde]

No, by construction the interactions are local in QED.

I do not deny that known interactions are local in QED. I say that there is unknown (hidden) interaction that does not appear explicitly in QED.

 

[stevendaryl]

Before QM came along I suspect that very few scientists would have held that it is the job of science just to predict stuff, and not to say anything about 'reality'. Of course after QM, when it became awkward (to say the least) to ascribe some 'reality' to the state, I get the impression it was as if there was some collective decision to 'redefine' what science is about.

Right. That's why I suggest that it is revisionism to say that science is not about ontology, but only about epistemology. Or maybe it's sour grapes---when you don't know how to get something, you pretend that you never wanted it in the first place.

 

[vanhees71]

There is no contradiction at all. For instance, Bohmian formulation of QFT retains all the mathematical structure of standard QFT. However, this formulation does not stop there but makes one further step by adding one additional equation. It is this additional equation which makes the theory non-local. This additional equation does not contradict any of the previous local equations.

I don't know enough about Bohmian QFT, but if provides nonlocal interactions it obviously contradicts QFT. So it's a new theory rather than just an interpretation of standard QFT. In other words, are there observable consequences of such nonlocal interactions and if so, have they been tested in real experiments?

 

[vanhees71]

I do not deny that known interactions are local in QED. I say that there is unknown (hidden) interaction that does not appear explicitly in QED.

Then it's a new theory and not QED. To discuss it (I'm not sure that this is allowed in the forum) you'd need to specify it clearly. I can't get what your theory looks like ;-)).

 

[zonde]

Then it's a new theory and not QED.

No, I am not talking about new theory. I talk about mathematical elements and operations in standard QED model.

 

[stevendaryl]

The big problem with ignoring ontology is that people who claim to ignore it actually promote it: eg. Ballentine, Peres and vanhees71.

If they truly did not care about ontology, they would have no problems with collapse.

There are well respected positions that ignore ontology, eg. Bohr, Landau and Lifshitz, Copenhagen as described by Weinberg - but all of these have things which are disavowed by vanhees71, eg. collapse and the Heisenberg cut.

It seems to me that the minimal interpretation--that measurements produce eigenvalues of the observable being measured, with probabilities given by the Born rule--does have an ontology. What's considered real is the observation or measurement. My issue with it is that if you go on to say that measurements/observations are explainable in terms of microscopic objects and processes, then you're explaining real things in terms of things that are not real. I suppose this applies to both modern adherents to the minimalist interpretation but also the original adherents of the Copenhagen interpretation.

So it seems to me that there is a conceptual circularity involved in the minimalist interpretation. You explain quantum mechanics in terms of macroscopic measurements and explain measurements in terms of quantum mechanics. I suppose that the circularity doesn't ruin the theory, though, because circularity can be handled in an iterative fashion: Start with a classical description of measurements and measurement devices, and then make quantum corrections to this description to get an improved description, and iterate until you reach some kind of fixed point.

 

[stevendaryl]

Then it's a new theory and not QED. To discuss it (I'm not sure that this is allowed in the forum) you'd need to specify it clearly. I can't get what your theory looks like ;-)).

I'm thinking that it has two kinds of processes: (1) smooth Hamiltonian evolution of microscopic systems, and (2) measurements always return an eigenvalue of the observable being measured, with probabilities given by the Born rule.

Since (2) is not derivable from (1), you need both types of processes. QED only describes (1).

 

[stevendaryl]

I'm thinking that it has two kinds of processes: (1) smooth Hamiltonian evolution of microscopic systems, and (2) measurements always return an eigenvalue of the observable being measured, with probabilities given by the Born rule.

Since (2) is not derivable from (1), you need both types of processes. QED only describes (1).

Many-worlds advocates claim that (2) is derivable from (1), but others disagree.

 

[Demystifier]

I don't know enough about Bohmian QFT,

I agree.

but if provides nonlocal interactions

True.

it obviously contradicts QFT.

Not true. Standard QFT makes a list of fundamental interactions, all of which are local. But standard QFT does not contain a statement of the form "there are no any other interactions except the listed ones". Standard QFT is agnostic on that.

(In addition, let me remind you that some effective actions in standard QFT do have a nonlocal form.)

So it's a new theory rather than just an interpretation of standard QFT.

It that was true, that would be actually good, wouldn't it?

In other words, are there observable consequences of such nonlocal interactions and if so, have they been tested in real experiments?

So far nobody found such a new observable consequence.

 

[dextercioby]

As far as I know there is no properly described "measurement problem" in relativistic QFT, or am I wrong ?
@Demystifier Common sense is that relativistic QFT in flat spacetime is only The Standard Model.

 

[vanhees71]

I agree.True.Not true. Standard QFT makes a list of fundamental interactions, all of which are local. But standard QFT does not contain a statement of the form "there are no any other interactions except the listed ones". Standard QFT is agnostic on that.

(In addition, let me remind you that some effective actions in standard QFT do have a nonlocal form.)It that was true, that would be actually good, wouldn't it?So far nobody found such a new observable consequence.

Well, then it's empty, and I can use good old relativistic QFT with the same physically relevant result :-).

 

[vanhees71]

As far as I know there is no properly described "measurement problem" in relativistic QFT, or am I wrong ?
@Demystifier Common sense is that relativistic QFT in flat spacetime is only The Standard Model.

Indeed, there is no measurement problem as is proven by the fact that physicists perform measurements all the time and explain the measured observations successfully with relativistic QFT.

 

[A. Neumaier]

Now you are changing my terminology which I fixed in post #154.

It is not my fault that you define your personal terminology in a way that already heavily loads the dice by introducing mental acts into the discussion.

Physics has not the slightest connection to mental acts done by conscious beings (apart from the trivial fact that the latter are likely conscious when doing physics). Wave functions have a meaning once a model is specified together with the information available for prediction.

 

[Demystifier]

As far as I know there is no properly described "measurement problem" in relativistic QFT, or am I wrong ?

The measurement problem in relativistic QFT is not less "properly" described than in non-relativistic QM. Measurement problem is rarely discussed in QFT only because QFT does not have much new to say about it.

 

[Demystifier]

I can use good old relativistic QFT with the same physically relevant result :-).

I have no objections to this. I only object when you categorically claim that something beyond that does not even exist.

 

[vanhees71]

Well, there's no measurement problem in non-relativistic QT either. So what?

 

[David Lewis]

What I mean that "science doesn't do ontology" goes well beyond the strawman argument that science only approximates, it questions what an approximation even is.

Or what the approximations are approximating.

 

[vanhees71]

I have no objections to this. I only object when you categorically claim that something beyond that does not even exist.

This I don't claim. However, before I change from a very successful theory to something else, I'd like to know, whether it provides any advantages in terms of observable consequences or if it's just another interpretation as is the case with Bohmian mechanics already in non-relativistic QT. I don't see any merit of Bohmian mechanics in non-relativistic QM compared to conventional non-relativistic QM.

 

[Demystifier]

This I don't claim. However, before I change from a very successful theory to something else, I'd like to know, whether it provides any advantages in terms of observable consequences or if it's just another interpretation as is the case with Bohmian mechanics already in non-relativistic QT. I don't see any merit of Bohmian mechanics in non-relativistic QM compared to conventional non-relativistic QM.

Fair enough! You don't see any merit because you don't care about ontology. I do care about ontology, so I do see a merit.

 

[ddd123]

Common sense is that relativistic QFT in flat spacetime is only The Standard Model.

Why is that? You don't think, say, GUTs are likely?

 

[secur]

The collapse means taking probabilities conditioned on the known observations. Thus it is included in my axioms. But it amounts to a change of the modeling assumptions rather than to a change in the system. Weirdness appears only when one mistakenly ascribes the collapse to the system rather than to the change in the model.

Fair enough. Given that view (with which I don't agree), your axiom system is complete as it stands.

In this regard I disagree with you and agree with @atyy . But to explain why I disagree I would need to talk about ontology, which is something you don't really care about.

That's a problem talking with people to whom the wavefunction is mere subjective knowledge. To them the psi-ontic stance ((using a term from Leifer, http://mattleifer.info/wordpress/wp-content/uploads/2008/10/quanta-pbr.pdf) is like believing in ghosts. They don't want to talk about silly delusions.

That's why I suggest that it is revisionism to say that science is not about ontology, but only about epistemology.

You can reasonably call it revisionism but, IMHO, it's correcting a misunderstanding that's lasted for a few centuries.

I may appear inconsistent: above I assert the reality of the wavefunction but here I'm saying, with Ken G, that all of science is about epistemology. In a fundamental sense, even the scientific model of a rock is epistemological. Science deals only with numbers (position, momentum, etc) that we imagine are related to a "rock". It cranks those numbers through a calculation recipe and predicts where the so-called "rock" will be in the future. It can't prove that rocks are real, and has no need to do so. OTOH, in practical terms, scientists (and everybody else) assume - "know" - the rock is real. At this practical level, I think the wavefunction (or some related, more-or-less equivalent QM entity, such as Bohm's beable) is as real as a rock.

If lucky, now that I've clarified this distinction, you'll say we're on the same page - but for some reason I doubt it.

This is exactly the same what people handling stocks do - they use probabilities based on the most recently available information to make predictions, hence collapse their model probability distributions each time new information comes in.

The "collapse" of classical probability distribution is not the same as collapse of wavefunction. Seems so obvious I'm not sure where to start defending the statement.

But I have never heard of a financial analyst complain about the weirdness of classical stochastic modeling.

Then you've never worked as a "quant" consultant to an old-fashioned stockbroker!

 

[ddd123]

Yeah, there are even stock models that use quantum formalisms.

 

[Demystifier]

The "collapse" of classical probability distribution is not the same as collapse of wavefunction. Seems so obvious I'm not sure where to start defending the statement.

Wherever you start to defend this, sooner or later it will reduce to a discussion of ontology.

 

[stevendaryl]

I may appear inconsistent: above I assert the reality of the wavefunction but here I'm saying, with Ken G, that all of science is about epistemology. In a fundamental sense, even the scientific model of a rock is epistemological. Science deals only with numbers (position, momentum, etc) that we imagine are related to a "rock". It cranks those numbers through a calculation recipe and predicts where the so-called "rock" will be in the future.

I understand that point of view, I just disagree with it. To me, science is about an iterative process:

1. Trying to understand our observations.
2. Making new observations to test our understanding.

But the goal is understanding the phenomenal world. The point about making predictions is not that predictions define science, but that confirming or falsifying predictions is our way of improving our understanding. So I would say that science is about using observation (both passive and active, in the form of experiments) to improve our understanding of the world. Saying that science is about predictions is akin to saying that education is about passing exams. Presumably, the exams give feedback about the quality of the education received, but it isn't the goal of education.

 

[Demystifier]

I understand that point of view, I just disagree with it.

Your quote above has not been said by me.

 

[stevendaryl]

Your quote above has not been said by me.

Sorry, I made an error in editing a post with multiple quotes.

 

[atyy]

You cannot even describe the LHC by classical mechanics of each atom it consists of, and even if you could, it's a total overkill to do so. Nevertheless the classical theory of macroscopic observables is just an effective theory of QT for the relevant degrees of freedom to describe (even construct) the LHC.

OK, so you do believe that there is a wave function of the LHC.

 

[Simon Phoenix]

But the goal is understanding the phenomenal world.

Yes I would agree with that (at least it's true for me)

But I think if scientists were genuinely uninterested in 'ontology' and only cared about making predictions then we wouldn't see so much passionate argument over the meaning of a quantum state and measurement - or perhaps it's only the ontobots who are so passionate in the first place and the epistobots get involved to point out how silly they're being :-)

[I'm definitely an ontobot - although I certainly don't possesses anything like a consistent 'ontology' for QM - I just hope that one day we'll have one; one that doesn't just say all this rather beautiful formalism is merely a way to describe our state of knowledge and one that says the 'updating' of that knowledge is something more than stuff that happens in our minds and in the quantum states we write down but is linked to a real physical process]

What I do find quite wonderful is that we have a theory, QM, that admits so many weird and wonderful interpretations - that all make the same predictions!

Who'd have thunk it?