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 probability 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 probability 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 probability 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