A Criteria for a good quantum interpretation

[Demystifier]

Matt Leifer gives criteria that any good quantum interpretation should satisfy.
http://mattleifer.info/2006/06/28/p...doA53UMty42JP5dADhA5SDxRjaTMNPkq1i9Ag3WPCwB5Q

 

[vanhees71]

"At this stage, I am even prepared to allow you to say that only detector clicks exist in reality, so long as you are clear about this and are prepared to face the later challenges."

Well, I'm glad that he allows me to say this, because that's in fact what it is. What's real are in indeed the objective observations in nature. What else should I consider "real" as a physicist?

 

[Demystifier]

"At this stage, I am even prepared to allow you to say that only detector clicks exist in reality, so long as you are clear about this and are prepared to face the later challenges."

Well, I'm glad that he allows me to say this, because that's in fact what it is. What's real are in indeed the objective observations in nature. What else should I consider "real" as a physicist?

But as he warns, are you prepared to face the later challenges? In particular, are you prepared then to say that the Moon is not there when nobody observes it? If you are not prepared for that, then actually Matt would not allow you to say what you said above.

 

[PeroK]

In particular, are you prepared then to say that the Moon is not there when nobody observes it?

First, what does it mean to ask "is the Moon there"? In the above terms, it means that if you carry out a set of experiments you will definitely get a set of results (that set of experiments being whatever we agree we need to establish that the Moon is at a certain location at a certain time). In that sense, the Moon is there when no one observes it.

In addition, the Moon is continuously interacting with its environment, so it's practically impossible to stop observing the Moon, owing to the record of its continuous gravitational effect on the Earth.

Instead of considering the Moon, let's consider my bicycle. It's probably in my bike shed as I type. The probability that it is not there has nothing to do with QM. The probablity is (almost) entirely that someone has broken into my shed and stolen it since yesterday.

If you twist my arm, I will say: the probability that my bike is in the shed is $1- \epsilon_1 - \epsilon_2$, where $\epsilon_1 \approx 10^{-4}$ is the probability that it's been stolen since yesterday and $\epsilon_2$ is an inexpressibily small number and the probablity that the QM system that I yesterday identified as my bike is no longer recognisable as a bike.

Philosophically, of course, $\epsilon_2$ may have some significance; but, in my view, it is of no physical significance.

 

[martinbn]

I would add to his list the requirement that anyone who write about interpretations needs to clearly explain the terminology he uses. Especially if it is different from what the dictionary says or how it has been used so far.

 

[martinbn]

About the moon, I think there two things here, and one has to be clear which one he is talking about. The statement is "The moon isn't (or is) there when no one looks at it." One could be rejecting the existence of the moon when not observed or the existence of values of the position observable. In other words the moon may exists, but the position observable may not have a definite value unless measured. And I think it is the second one the is meant by the statement, at least it is what QM says. People shouldn't confuse it, or purposefully mislead, with the existence of the moon.

 

[EPR]

Instead of considering the Moon, let's consider my bicycle. It's probably in my bike shed as I type. The probability that it is not there has nothing to do with QM. The probablity is (almost) entirely that someone has broken into my shed and stolen it since yesterday.

The issue is that the mathematical formalism of QM doesn't claim that your bike is in your bike shed when nothing is measuring it. Everyone agrees that it's probably there but interpreting how that takes place is another matter.

 

[PeroK]

The issue is that the mathematical formalism of QM doesn't claim that your bike is in your bike shed when nothing is measuring it. Everyone agrees that it's probably there but interpreting how that takes place is another matter.

What does that even mean? You may be precise in terms of macroscopic concepts. Or, you may be precise in terms of QM. But, the fallacy is to mix the two in the same sentence or argument.

 

[martinbn]

The issue is that the mathematical formalism of QM doesn't claim that your bike is in your bike shed when nothing is measuring it. Everyone agrees that it's probably there but interpreting how that takes place is another matter.

As I said this is not the problem. Whether it is here or there, or somewhere else or it is meaningles to say that until you measure position is another question. Some people use the moon quote to claim that the according to some the moon/bike doesn't exist at all. I think this is misleading. If you prepare the bike in a state with exact momentum it does exist, after all it has that momentum. But you cannot say that it is there because the position doesn't have a value. So you say that it isn't there until you measure its position. But it doesn't mean that you say (although you may) that it(the bike) doesn't exist.

 

[vanhees71]

But as he warns, are you prepared to face the later challenges? In particular, are you prepared then to say that the Moon is not there when nobody observes it? If you are not prepared for that, then actually Matt would not allow you to say what you said above.

The moon is always there because there are many conservation laws telling me that it cannot simply puff into nothing. Once seen, the moon is where she is for a pretty long time and obeying good old Newtonian classical laws of motion. In a sense it's "always observed". The interaction with the CMBR is already sufficient to make its motion FAPP classical.

 

[PeroK]

As I said this is not the problem. Whether it is here or there, or somewhere else or it is meaningles to say that until you measure position is another question. Some people use the moon quote to claim that the according to some the moon/bike doesn't exist at all. I think this is misleading. If you prepare the bike in a state with exact momentum it does exist, after all it has that momentum. But you cannot say that it is there because the position doesn't have a value. So you say that it isn't there until you measure its position. But it doesn't mean that you say (although you may) that it(the bike) doesn't exist.

I don't agree with this. It's not meaningless for a macroscopic object. That is the fallacy. A statement about the macroscopic world is not meaningless because you can't pin it down precisely quantum-mechanically. QM does not have a monopoly on meaningful statements.

 

[Demystifier]

The moon is always there because there are many conservation laws telling me that it cannot simply puff into nothing.

By using this argument, you assume that conservation laws say something about reality beyond detector clicks. Hence you contradict yourself and therefore violate the Matt's criterion that you cited.

In addition, this argument is irrelevant, because the conservation laws do not forbid Moon to puff into something which is not the Moon but has the same mass, momentum and charge.

 

[EPR]

What does that even mean? You may be precise in terms of macroscopic concepts. Or, you may be precise in terms of QM. But, the fallacy is to mix the two in the same sentence or argument.

No. It's not a fallacy to mix the two. It's a range of possible interpretations of qm wrt to classical reality. Some interpretations deny the reality of unmeasured macroscopic objects, some are agnostic and some posit that your bike is there irrespective of whether anyone bothers to look.

Saying the bike is there already adopts a particular interpretation.

 

[martinbn]

I don't agree with this. It's not meaningless for a macroscopic object. That is the fallacy. A statement about the macroscopic world is not meaningless because you can't pin it down precisely quantum-mechanically. QM does not have a monopoly on meaningful statements.

Yes, that is true, but it is not the point. The "moon" here is just a colorful expression, it stands for a QM system. But I do agree with you. So the statement actually has three interpretations, not just two as I said earlier. The third is if you interpreted as a statement about macroscopic objects.

 

[Demystifier]

One could be rejecting the existence of the moon when not observed or the existence of values of the position observable. In other words the moon may exists, but the position observable may not have a definite value unless measured.

What do you mean by "position of the Moon"? Do you mean the position of its center of mass? Or do you mean the relative positions of all its parts?

 

[PeroK]

Saying the bike is there already adopts a particular interpretation.

Saying my bike is in the shed says nothing about QM! That's the point. You don't need an interpretation of QM to say something meaningful.

 

[PeroK]

By using this argument, you assume that conservation laws say something about reality beyond detector clicks. Hence you contradict yourself and therefore violate the Matt's criterion that you cited.

In addition, this argument is irrelevant, because the conservation laws do not forbid Moon to puff into something which is not the Moon but has the same mass, momentum and charge.

You don't need an interpretation of QM to have a consistent description of the solar system. The set of experiments are not just a few random detector clicks. The more precision you require about exactly what is where we think the Moon should be, then the more experimental tests we need to add.

You can still classically have an enormously complex biological system such a human being fully and consistently described. A medical text-book is not meaningless because it doesn't adhere to one or other interpretations of QM; nor does it imply a particular interpretation of QM. It is a meaningful scientific work, with meaningful classical and biological statements.

 

[martinbn]

What do you mean by "position of the Moon"? Do you mean the position of its center of mass? Or do you mean the relative positions of all its parts?

If the "moon" is an electron, then I mean the position, if it is something else then it depends on the context.

 

[atyy]

You can still classically have an enormously complex biological system such a human being fully and consistently described.

But this human being is not made of protons and electrons. If it were, its atoms would be unstable with only classical physics.

 

[PeroK]

But this human being is not made of protons and electrons. If it were, its atoms would be unstable with only classical physics.

That doesn't render biology and medicine meaningless, unless they explicitly conform to a particular interpretation of QM. For example, I claim that:

The heart is a muscular organ about the size of a fist, located just behind and slightly left of the breastbone. The heart pumps blood through the network of arteries and veins called the cardiovascular system.

Is a meaningful statement, independent of QM or any interpretation thereof. It doesn't require any specific future measurement to render that statement meaningful.

Any imprecision or probabilistic variations on that statement are classical (and not QM) in nature: in terms of people with an abnormal heart.

 

[Morbert]

From the article:

An interpretation should have a well-defined ontology [...] At this stage, I am even prepared to allow you to say that only detector clicks exist in reality, so long as you are clear about this and are prepared to face the later challenges

This sounds like a common misunderstanding of anti-realist interpretations of quantum mechanics. These interpretations do not assert some things are not real. Instead they interpret QM as unconcerned with evaluating ontological claims. So e.g. a claim like "only detector clicks exist in reality" is not the kind of claim QM would evaluate.

No facticious sample spaces.

• OK this is a bit of a personal bugbear of mine. Some interpretations introduce [...] generalizations of the notion of a sample space (as in consistent histories). Quantum theory is then thought of as being a sort of probability theory over these spaces. Often, however, the “quantum states” on these sample spaces are a strict subset of the allowed measures on the sample space, and the question is why?

This objection seems peculiar to me. We construct sample spaces to describe what our experiment will resolve. We can of course construct irrelevant sample spaces, but why would we?

 

[Demystifier]

If the "moon" is an electron, then I mean the position, if it is something else then it depends on the context.

In this context, "Moon" is a macroscopic object, definitely not an electron.

 

[atyy]

That doesn't render biology and medicine meaningless, unless they explicitly conform to a particular interpretation of QM. For example, I claim that:

The heart is a muscular organ about the size of a fist, located just behind and slightly left of the breastbone. The heart pumps blood through the network of arteries and veins called the cardiovascular system.

Is a meaningful statement, independent of QM or any interpretation thereof. It doesn't require any specific future measurement to render that statement meaningful.

Any imprecision or probabilistic variations on that statement are classical (and not QM) in nature: in terms of people with an abnormal heart.

Well, I'm compelled by my faith to agree, as a believer in Copenhagen. These statements belong on the classical side of the Heisenberg cut.

 

[martinbn]

In this context, "Moon" is a macroscopic object, definitely not an electron.

Ok, what do you mean by "there"? The position of the center of mass, the relative positions of the parts? In any case by position i mean whatever observable can have the value "there".

 

[Demystifier]

Ok, what do you mean by "there"? The position of the center of mass, the relative positions of the parts?

I mean both. When I say that the Moon is there, I mean that I can point with my finger in the direction of the Moon (which more-or-less corresponds to the center of the mass), but also that the thing to which I point has a shape of the Moon (which is defined by relative positions of the parts).

 

[EPR]

Saying my bike is in the shed says nothing about QM! That's the point. You don't need an interpretation of QM to say something meaningful.

Only if you believe, as you seem to do, that your bike is somehow special and not subject to the rules of QM.

I believe QM has something meaningful to say about your bike. And about everything else that appears to be entirely quantum.

 

[vanhees71]

Well, at least you need QT to understand, why the stable matter around us is stable to begin with.

 

[PeroK]

Only if you believe, as you seem to do, that your bike is somehow special and not subject to the rules of QM.

Well, it's also subject to classical laws of motion, which an electron isn't. The bicycle is only subject to QM in such a way that the quantum mechanical effects are washed out and undetectable. For example, it cannot tunnel out of my shed - no matter how much you insist it is QM in nature. It's not that we are blind to the tunneling of bikes out of sheds, it's that bikes don't tunnel like electrons.

This is a difference: you cannot ascribe all electron-like properties to a bicycle. Nor vice versa.

And, in the same way that the atom is not a miniature solar system, the solar system is not a giant atom!

 

[Lord Jestocost]

Matt Leifer gives criteria that any good quantum interpretation should satisfy.

Within physics, there is no need for a quantum interpretation beyond the instrumentalist minimal interpretation (so to say “extreme operationalism”). Interpretations arise from psychological biases mirroring personal world views. As Richard Conn Henry puts it: “If you are not simply to be like a squirrel or a rabbit, you must choose some quantum mechanics interpretation (as it is called⎯ it is not really 'an interpretation,' of course; it is your theory of yourself and of your experience of observations).”

For example, does someone really care whether the moon is “there” or isn’t "there" when no one looks at it. Is such a question of any relevance regarding physics? No.

The kind of objectification which lies in the word “there” is simply impossible in quantum mechanics when regarding the situation between measurements. Quantum mechanics differs from classical physics because the assumption that something is “objectively" realized between observations is simply nonsensical.

 

[Fra]

A clean interpretations, usually make no difference as long as we discusse current theory. This is why this is not usually overly interesting, and discussions tend to run in circles. But when one is sniffing towards extending theories, or revising theories in order to solve open questions, and by definition engage in "speculation" then interpretations have another purpose:

They provide an conceptual ansatz that defines the research direction, and a source of intuitive insight. This is the main difference between us I think.

This is why i personally hold two interpretations, where one is a special case of the other one (when removing all speculation, and thus it gets minimal). This is close an instrumental Vanhees-version however we can label it :) This is because it appears minimal and faithful towards experimental contact.

But looking at conceptual problems of current theory, and open issues, I have a more extremal version of "agent-agent" interpretations, which is really an conceptual ansatz on a possible reconstruction of physical laws.

/Fredrik

 

[Fra]

What else should I consider "real" as a physicist?

All we need is a stable illusion. Actualy reality is grossly overrated, and an illusion itself.

In the speculative economy, one can start what is the real value of things and what's inflated expectations? The truth is, no one cares and it makes no difference. As long as everyone has the same illusion, we have a working game, and we make money. You just have to stay on top of whatever the illusion is.

As long as all agents has the same illusion of the rules of the game, they will coexist in harmony. The question is more from point of view of physics, how does stable illusions of law emerge? And how does illusions of law explain actions? And how does all this explain the population of agents encoding these illusions? Answer that and I am happy, even without "reality".

/Fredrik

 

[atyy]

Matt Leifer gives criteria that any good quantum interpretation should satisfy.
http://mattleifer.info/2006/06/28/p...doA53UMty42JP5dADhA5SDxRjaTMNPkq1i9Ag3WPCwB5Q

So the only interpretation surviving (especially after "No facticious sample spaces") is Valentini's version of dBB?

 

[PeterDonis]

I find it interesting that Leifer mentions Bell's Theorem and the Kochen-Specker Theorem, but not the PBR Theorem (IIRC Leifer is one of the authors of a review paper on this theorem that was discussed here on PF some time ago). Shouldn't that be on the list of theorems that an interpretation has to deal with?

 

[Quantumental]

I find it interesting that Leifer mentions Bell's Theorem and the Kochen-Specker Theorem, but not the PBR Theorem (IIRC Leifer is one of the authors of a review paper on this theorem that was discussed here on PF some time ago). Shouldn't that be on the list of theorems that an interpretation has to deal with?

PBR was not even conceived at the time (2006) when Leifer wrote this piece up; ought to explain the gap. I know, based on following his papers since then, it would certainly look quite a bit different anno 2021

 

[PeterDonis]

PBR was not even conceived at the time (2006) when Leifer wrote this piece up

Ah, I didn't see the date on Leifer's article. Yes, I expect there would be plenty of changes if he did an update.

 

[atyy]

PBR was not even conceived at the time (2006) when Leifer wrote this piece up; ought to explain the gap. I know, based on following his papers since then, it would certainly look quite a bit different anno 2021

He knew about it, but left it out to rule out retrocausation, thereby proving it.

 

[bhobba]

But as he warns, are you prepared to face the later challenges? In particular, are you prepared then to say that the Moon is not there when nobody observes it? If you are not prepared for that, then actually Matt would not allow you to say what you said above.

I think that issue is at least very close to being solved, if it has not already, with detailed accounts of how the classical world emerges from coarse graining as detailed by Gell-Mann, Hartle and others. I have read just before he died Feynman was converted to the view as well eg:
http://spkurdyumov.ru/uploads/2013/08/gellman.pdf
'The place of classical physics in a quantum universe is correctly understood as a property of a particular class of sets of decoherent coarse-grained alternative histories, the quasiclassical realms. In particular, the limits of a quasiclassical description can be explored. Dechoherence may fail if the graining is too fine. Predictability is limited by quantum noise and by the major branchings that arise from the amplification of quantum phenomena as in a measurement situation. Finally, we cannot expect a quasiclassical description of the universe in its earliest moments where the very geometry of spacetime may be undergoing large quantum fluctuations'

In the classical world things do behave in a common-sense way - the moon is really there when you are not looking etc.

I, these days, think QM, like all physical theories, is a model - but a peculiar one in that we do not have direct experience with what it is modeling - we only have direct experience with the classical world. Certainly I believe, as most people do, that the classical world is objectively real and external from us. I have no reason to believe the quantum world is not the same - but since we have no direct experience of it such is not as certain. But we do have a mathematical model of it grounded in what we do have direct experience with - the classical world. For example experimental observations are very real and QM allows us to predict those - but the moon and other macro objects also are, using coarse raining. I do think while we have the broad outlines of such a view, more work needs to be done. Omnes in his book Understanding Quantum Mechanics mentions we are missing the proof of a few key theorems for example. What I do firmly believe is progress on the issue is being made and eventually will be fully resolved - but there may indeed be surprises along the way.

I did find an interesting overview article on the issues involved:
https://www.researchgate.net/publication/335869303_Understanding_quantum_mechanics_a_review_and_synthesis_in_precise_language

Thanks
Bill

 

[bhobba]

Within physics, there is no need for a quantum interpretation beyond the instrumentalist minimal interpretation (so to say “extreme operationalism”)

Of course there is no need - and indeed a hero of many here (including me), Dirac, would hold a similar view. But it leaves open an interesting question - how does the classical world that we experience everyday emerge from QM? Operationalism simply assumes it does, and we interpret QM by 'operationalism' in that world. Coarse graining research by Gell-Mann, Hartel and others has given us insights into that - and may have even resolved it. With that in hand we can analyse what is going on in 'extreme operationalism' to gain deeper insights into QM. Surely that is a worthwhile undertaking? As I mentioned in another post, QM, like all physical theories, is a mathematical model - but of a realm we do not have direct experience with. We do have direct experience with the world around us - and the operationalist view gives the link - but just assumes there is a link - no detail of it. If we can understand how that world emerges, and what is happening in that 'link' purely in terms of the mathematical model that is QM, then I contend progress has been made in making QM less mysterious. We do not need to do it to understand QM, but it will shed light on it.

I gave one link to the 'program' previously - here is another:
https://arxiv.org/pdf/1905.05859.pdf

Thanks
Bill

 

[Demystifier]

I find it interesting that Leifer mentions Bell's Theorem and the Kochen-Specker Theorem, but not the PBR Theorem (IIRC Leifer is one of the authors of a review paper on this theorem that was discussed here on PF some time ago). Shouldn't that be on the list of theorems that an interpretation has to deal with?

He wrote it before the PBR theorem existed. The P from PBR was a teenager at that time.

EDIT: Ah, now I've seen that @Quantumental already answered it.

 

[Demystifier]

Within physics, there is no need for a quantum interpretation beyond the instrumentalist minimal interpretation

Maybe physics doesn't need it, but physicists obviously do.

 

[martinbn]

Maybe physics doesn't need it, but physicists obviously do.

Physicists or philosophers?

 

[Lord Jestocost]

But it leaves open an interesting question - how does the classical world that we experience everyday emerge from QM?

Is there a “classical” world? Following the purely quantum mechanical von Neumann measurement chain, nothing emerges as long as we regard all chain links as pure physical systems.

 

[Cobul]

I don't agree with this. It's not meaningless for a macroscopic object. That is the fallacy. A statement about the macroscopic world is not meaningless because you can't pin it down precisely quantum-mechanically. QM does not have a monopoly on meaningful statements.

And what quantum interpretations deny the reality of unmeasured macroscopic objects? How do you make the wall unmeasured so it can be uncreated?

 

[WernerQH]

A good theory shouldn't need an interpretation. Do we have an interpretation of electrodynamics? Or, for those who have been told that every theory needs an interpretation: the interpretation should be obvious. Which means that there should be a direct correspondence between the basic concepts of the theory and the real world.

Most physicists today wouldn't hesitate to call electric fields real. But for Maxwell an electric field was a theoretical construct describing mechanical stresses in the ether. Since then the ether has vanished, but the electromagnetic field persists like the grin of the Cheshire cat.

Until 1905 electrodynamics was a rather peculiar, not to say weird, theory describing an hypothetical ether with contradictory properties: it was both solid, to allow transverse waves, and fluid, to allow vortex lines to form. Sticking to Maxwell's equations one could safely ignore the conceptual problems. Today, QT is routinely applied without giving a damn about backwaters like the "measurement problem".

In the case of Maxwell's theory it took more than four decades to jettison its superfluous metaphysical baggage. My feeling is that, after more than nine decades, QT still carries a similar unnecessary burden. We still talk of "quantum objects" with conflicting wave and particle properties. A good theory should clearly say what it is about. Saying "QM is about quantum particles and measurements on them" is like saying "Electrodynamics is the theory of the ether".

 

[PeroK]

We still talk of "quantum objects" with conflicting wave and particle properties.

Quantum objects are governed by their wavefunction leading to a resolution of what were previously (before QT) seen as conflicting wave and particle properties. QT resolved this pre-existing conflict.

 

[AlexCaledin]

Let's say, QM (Feynman variant) is simply the math underlying the least action principle, extending itself to the particles behavior.

 

[Demystifier]

Physicists or philosophers?

Yes.

 

[Demystifier]

In the case of Maxwell's theory it took more than four decades to jettison its superfluous metaphysical baggage. My feeling is that, after more than nine decades, QT still carries a similar unnecessary burden. We still talk of "quantum objects" with conflicting wave and particle properties. A good theory should clearly say what it is about. Saying "QM is about quantum particles and measurements on them" is like saying "Electrodynamics is the theory of the ether".

So what is quantum theory really about?

 

[bhobba]

Is there a “classical” world? Following the purely quantum mechanical von Neumann measurement chain, nothing emerges as long as we regard all chain links as pure physical systems.

Well we certainly experience one day to day. The idea is not to challenge that chain, but to show using coarse graining a 'quasi classical' world can be defined that acts like that classical world. I have not kept up with the technical details of that program in recent years, but when I investigated it last there were still issues in showing that world is the classical world we experience. Certainly I think it has illuminated the so called quantum classical cut. In the 'extreme instrumentalist' view it must be accepted the instruments are themselves quantum. What exactly makes a quantum object an instrument? Of course that view simply accepts they exist and we can in practice tell what they are. That is simple and easy to conceptually grasp. But it is reasonable to want to be more precise. You are correct in that in order to have a viable interpretation you do not need to - but there is nothing wrong with seeing where such investigations lead.

Thanks
Bill

 

[bhobba]

So what is quantum theory really about?

We have no direct experience with the quantum world. So I would say it is a mathematical model about that world that has connections to the world we day to day experience. We can experimentally check the validity of the theory by checking it is accord with what happens in our everyday world.

Thanks
Bill