Why the Quantum | A Response to Wheeler's 1986 Paper - Comments

[PeterDonis]

the very fact that proponents of a quantum-classical cut always agree that the cut can shifted arbitrarily shows that the cut is as unnecessary as the aether in classical electromagnetics.

No, it doesn't, it shows that quantum mechanics as currently formulated is an incomplete theory. A cut is required to extract predictions from the theory, but the theory does not tell you where to put the cut; practically speaking, physicists put it wherever it works best in making predictions for a particular problem.

 

[atyy]

Well, the very fact that proponents of a quantum-classical cut always agree that the cut can shifted arbitrarily shows that the cut is as unnecessary as the aether in classical electromagnetics.

But you cannot shift it completely arbitrarily - it cannot be shifted such that the whole universe is quantum, unless perhaps one introduces hidden variables or MWI.

 

[vanhees71]

Hm, where do my experimental colleagues at CERN make some arbitrary cut when constructing their detectors?

 

[atyy]

Hm, where do my experimental colleagues at CERN make some arbitrary cut when constructing their detectors?

When they apply the Born rule :)

 

[PeterDonis]

where do my experimental colleagues at CERN make some arbitrary cut when constructing their detectors?

The cut isn't in the actual experimental devices; it's in the theory. As I said in post #176: "a cut is required to extract predictions from the theory". Extracting predictions from the theory is not the same as running experiments. And the fact that there is obviously no cut in the experimental devices, whereas you need one to extract predictions from the theory, is just another way of putting what I said in post #176, that QM as currently formulated is an incomplete theory.

 

[vanhees71]

But you cannot shift it completely arbitrarily - it cannot be shifted such that the whole universe is quantum, unless perhaps one introduces hidden variables or MWI.

That's true. The entire universe cannot be described quantum theortically within the minimal interpretation, because you cannot define what's observable about it, because for that you'd need to prepare many universes in the same initial state to investigate whether the probabilistic meaning of the state is correctly predicting its behavior.

 

[vanhees71]

The cut isn't in the actual experimental devices; it's in the theory. As I said in post #176: "a cut is required to extract predictions from the theory". Extracting predictions from the theory is not the same as running experiments. And the fact that there is obviously no cut in the experimental devices, whereas you need one to extract predictions from the theory, is just another way of putting what I said in post #176, that QM as currently formulated is an incomplete theory.

Hm, I don't need a cut to describe heavy-ion collisions and compare it to experiment. I just calculate the quantities, like dilepton invariant-mass and transverse-momentum spectra and compare them with the experimental results. Of course, these spectra are the collection of data from an ensemble ("the more statistics the better").

 

[atyy]

Hm, where do my experimental colleagues at CERN make some arbitrary cut when constructing their detectors?

Also, it must be admitted (von Neumann knew this, and it is discussed in the textbook by Wiseman and Milburn) the cut is not entirely arbitrary: https://arxiv.org/abs/quant-ph/9712044.

 

[vanhees71]

Ok, if the application of the Born rule is a cut, that's fine with me. But why should I call it a cut?

 

[atyy]

Ok, if the application of the Born rule is a cut, that's fine with me. But why should I call it a cut?

Because the point at which you decide to apply the Born rule comes from "outside" the quantum system - the initial quantum state and Schroedinger equation does not tell you when the Born rule is applied.

 

[vanhees71]

Also, it must be admitted (von Neumann knew this, and it is discussed in the textbook by Wiseman and Milburn) the cut is not entirely arbitrary: https://arxiv.org/abs/quant-ph/9712044.

I'd say von Neumann was very far away from a physical understanding of QT. His merit is in the proper mathematical formulation. His solipsistic Princeton Interpretation, however, is the worst flavor of the Copenhagen spirit ever.

I've to read the paper to comment it. As far as I see for usual measurements using a usual observable as a pointer there's really no problem with putting the cut anywhere, where a classical description is sensible.

 

[vanhees71]

Because the point at which you decide to apply the Born rule comes from "outside" the quantum system - the initial quantum state and Schroedinger equation does not tell you when the Born rule is applied.

No. Also Newton's equation of motion doesn't tell me when I look at the point particle I describe. Why should it?

 

[martinbn]

I thought that QT can make predictions without any cut. Say, if you smash these particles, then the probability to get those is so and so. No cut and a very spesific prediction. Or something along the lines a black hole will radiate and loose energy, no cut. Or is the cut somewhere implicit.

 

[atyy]

No. Also Newton's equation of motion doesn't tell me when I look at the point particle I describe. Why should it?

Because in classical mechanics the particle is there whether you look at it or not. In quantum mechanics, the formalism does not assign the particle a position until you look at it.

In quantum mechanics, there is a fundamental difficulty with applying the quantum state to the whole universe including the observer. In classical mechanics, there is no equivalent difficulty (there is a difficulty to include the whole universe from the singularities of GR, but that is different from needing to exclude the observer).

 

[martinbn]

In classical mechanics, there is no equivalent difficulty (there is a difficulty from the singularities of GR, but that is different from needing to exclude the observer).

Why is that? I mean the part in the parentheses.

 

[Demystifier]

I thought that QT can make predictions without any cut. Say, if you smash these particles, then the probability to get those is so and so. No cut and a very spesific prediction. Or something along the lines a black hole will radiate and loose energy, no cut. Or is the cut somewhere implicit.

Yes, the cut is implicit in your expression "to get". This really means "to observe" or "to measure by macroscopic apparatus", so you need a cut in order to distinginsh observers from non-observers or macroscopic from non-macroscopic.

 

[martinbn]

Yes, the cut is implicit in your expression "to get". This really means "to observe" or "to measure by macroscopic apparatus", so you need a cut in order to distinginsh observers from non-observers or macroscopic from non-macroscopic.

I don't get it. To make it specific let's look at the following. The probability that a photon will decay to a proton is zero. What measurement is needed to make this prediction?

 

[Demystifier]

I don't get it. To make it specific let's look at the following. The probability that a photon will decay to a proton is zero. What measurement is needed to make this prediction?

It's not a good example. Give me an example in which probability is neither zero nor one.

 

[martinbn]

I didn't mean that all predictions can be made without a cut. I said that the theory can make predictions without a cut.

 

[Demystifier]

Also Newton's equation of motion doesn't tell me when I look at the point particle I describe. Why should it?

Newton's equation is a statement of the form "position of the particle is such and such". It is not a statement of the form "When position of the particle is measured, then position of the particle is such and such".

But quantum mechanics is different. QM does not state that "probability of the position of the particle is such and such". It states that "When position of the particle is measured, then probability of the position of the particle is such and such".

 

[Demystifier]

I didn't mean that all predictions can be made without a cut. I said that the theory can make predictions without a cut.

But if some predictions cannot be made without a cut, then QM as a whole needs a cut.

Just as real numbers, as a whole, need Dedekind cuts, despite the fact that some real numbers (the rational ones) don't need Dedekind cuts.

 

[martinbn]

But if some predictions cannot be made without a cut, then QM as a whole needs a cut.

Yes, but somewhere above it was said that the theory cannot make predictions without a cut.

 

[Demystifier]

Yes, but somewhere above it was said that the theory cannot make predictions without a cut.

That statement, taken literally, was wrong. But it's clear (at least to me) that atyy wanted to say that the theory cannot make some of its predictions without a cut.

 

[bhobba]

that QM as currently formulated is an incomplete theory.

You could put it that way, and I would not argue it. However it depends on how you look at it. I prefer to say right now we have some unresolved issues - progress definitely has been made and research is ongoing. You can say observation is a primitive and all theories have primitives. I am not sure it does resolve it - but is it physics? To me it may be a bit semantic and what philosophers argue about. Personally I tend to side with Dirac and think all theories have issues and we just keep progressing and chipping away - it's very hard to predict where it will lead. I have posted it before, but just for completeness in case someone has not seen it see:
http://philsci-archive.pitt.edu/1614/1/Open_or_Closed-preprint.pdf

Thanks
Bill

 

[RUTA]

I didn't realize how close to this "minimalist interpretation" our interpretation was. We're saying the fundamental explanation for the QM correlations is conservation (of whatever) on average, not trial by trial. What we mean by "fundamental explanation" is that there is nothing deeper to explain this conservation principle. I just updated the arXiv version of the paper (will appear Mon -- I keep it updated at users.etown.edu/s/stuckeym/TsirelsonBound.pdf) which contains this:

Thus, we see explicitly in this result how quantum mechanics conforms statistically to a conservation principle without need of a `causal influence' or hidden variables acting on a trial-by-trial basis to account for that conservation. There are many attempts to add such classical mechanisms, but they are superfluous as far as the physics is concerned. The light postulate of special relativity is a good analogy for our proposed constraint. That is, ``the speed of light c is the same in all reference frames'' explains time dilation, length contraction, etc., but there is nothing to explain the light postulate. Likewise, ``conservation per no preferred reference frame'' (the most general form of our constraint) explains the Tsirelson bound, but there is nothing to explain that conservation principle (constraint).

That looks very similar to what the minimalist interpretation is calling an ensemble interpretation of the wave function. Further, the measured values proper are what contribute directly to this conversation. That is, whether or not there is some underlying or hidden "true" value of the angular momentum giving rise to what is measured is irrelevant, it is the actual measured values that account for the conservation.

 

[atyy]

That statement, taken literally, was wrong. But it's clear (at least to me) that atyy wanted to say that the theory cannot make some of its predictions without a cut.

I meant the theory cannot make any predictions without a cut. If the observer is included in the wave function and all we have is the unitarily evolving quantum state, the theory makes no predictions.

 

[Boing3000]

That statement, taken literally, was wrong. But it's clear (at least to me) that atyy wanted to say that the theory cannot make some of its predictions without a cut.

I would have said: the theory cannot make verifiable prediction without a cut.
Is there some theoretical predictions that could lead to 0 or 1 without a the need of some previous measurement (based on known eigenvalue ?).

 

[Mentz114]

Because in classical mechanics the particle is there whether you look at it or not. In quantum mechanics, the formalism does not assign the particle a position until you look at it.

Even after measurement you still don't have a value - only a (hopefully) more precise probability !

Only projective measurements allow one to say what the state is. And in that case it is not a measurement because all information about the original state has been lost.

 

[Boing3000]

Again for me this is the very statement, I don't buy. There is no difference between interaction and measurement. This is vaguely formulated, so maybe I understand you and other proponents of this claim in this thread in a wrong way. For me this says that you and others claim that there's a difference in the interaction of the measured object with the measurement apparatus and all other interactions.

But how is it you don't buy your own preferred interpretation ?
But first things first, I don't (nor anybody else) think (let's say on a philosophical/ontological level) that the "stuff" of the laboratory (or the universe or whatnot) is made of two different "categories" of stuff, obeying different rule. For example I am quite confident that classical mechanics assume you can measure things of the theory (like force and mass and ...) with the same thing in the laboratory (force and mass) (in the same unit)
Also the discussion here is only about phenomenology, and differences between them (and their completeness)

This doesn't make sense to me since the same physical laws apply to interactions no matter whether it's the interaction with a measurement apparatus or not.

Maybe in the lab... but you seem to be doubting that... I don't. And again, it is not the "problem". The problem is to accurately analyse the theory itself.

What is uncontroversial, in that in the model/theory the physical law describe imaginary(hmmm complex) vector in arbitrary dimension. From what I understand the Schrodinger equation is deterministic and continuous.
Where I think you make a unconscious philosophical leap, is to believe that measurement apparatus (used to test QM) are "displaying" those imaginary pointer from other dimensions... they don't. Not because they are macroscopic, but because the unit don't even match those of the theory...
...because the complete minimal interpretation must add something fundamentally different to classical mechanic, in order to make it scientific (testable).
This process (the Born rule) is discrete, and only happens "on measurement" (not on interaction), and is probabilistic. But at least probabilities of "stuff" in the same unit as the laboratory (all classical).

Neither in classical nor in quantum theory is any dichotomy in the applicability of the rules to measurement apparati and other objects.

Ok, then my mistake. When do you use the Born rule inside the Schrodinger equation ?

Measurement apparati are made of the same stuff as anything else, and also all physical laws apply to measurement devices as to any other object. That's all I'm claiming.

That's an ontological claim (that I share btw).
But you don't claim that. You are claiming (as far as I understand) that the epistemology is not based on such a dichotomy.

Maybe we have to reformulate our claims, but I don't know, in which way I can reformulate mine.

But there is no need to. The minimalist interpretation is fine. You believe in ensemble, and the Born rule applied. Period.
You seem to believe that one day another interpretation will derive the Born Rule. Why not ? As far I can tel RUTA's one is a good start. It is even based on a classical axiom...
But as thing are currently, the current minimal interpretation does make such a distinction.

Both phase space in classical mechanics and the operators in Hilbert space are representing properties of observable facts about objects, described in an abstract mathematical way.

No, the units don't match in QM, they do in CM

In QT the description is explicitly probabilistic

As far as I known, probabilities are not complex numbers... even (0,0)

 

[RUTA]

Wow, it always amazes me how many human-IQ-hours have been invested trying to find a way to reinvent QM so as to rid it of the measurement problem (see Schlosshauer quote in #135). Accepting QM as supplying spatiotemporal constraints on the distribution of quantum events, rather than dynamical laws for the behavior of quantum systems, automatically rids us of the MP. Then, QM is seen as complete by simply accepting quantum-classical contextuality. There is nothing in Nature that demands we recover classical reality from a quantum reality in toto. Certainly not with any empirical consequences. That's just a reductive bias. If Weinberg tried and failed, it's certainly above my pay grade! But, it looks to be entertaining lots of brilliant mathematical minds, so by all means, enjoy :-)

 

[martinbn]

I meant the theory cannot make any predictions without a cut

What about my examples?

 

[atyy]

Wow, it always amazes me how many human-IQ-hours have been invested trying to find a way to reinvent QM so as to rid it of the measurement problem (see Schlosshauer quote in #135). Accepting QM as supplying spatiotemporal constraints on the distribution of quantum events, rather than dynamical laws for the behavior of quantum systems, automatically rids us of the MP. Then, QM is seen as complete by simply accepting quantum-classical contextuality. There is nothing in Nature that demands we recover classical reality from a quantum reality in toto. Certainly not with any empirical consequences. That's just a reductive bias. If Weinberg tried and failed, it's certainly above my pay grade! But, it looks to be entertaining lots of brilliant mathematical minds, so by all means, enjoy :-)

I guess your interpretation is not an "interpretation" in traditional quantum terminology since it retains the cut and doesn't attempt to solve the measurement problem.

Especially since you frame it with Wheeler's question, which was not about solving the measurement problem, I guess your programme is more like trying to provide alternative axioms for quantum theory, like the odl quantum logic thinking of von Neumann, Birkhoff, Mackey, etc, and the more recent ones of Lucien Hardy https://arxiv.org/abs/quant-ph/0101012 or of Chiribella and colleagues https://arxiv.org/abs/1011.6451 ?

 

[vanhees71]

Because in classical mechanics the particle is there whether you look at it or not. In quantum mechanics, the formalism does not assign the particle a position until you look at it.

In quantum mechanics, there is a fundamental difficulty with applying the quantum state to the whole universe including the observer. In classical mechanics, there is no equivalent difficulty (there is a difficulty to include the whole universe from the singularities of GR, but that is different from needing to exclude the observer).

In quantum mechanics a particle is there too, provided there's a conservation law ensuring this. If there is no conservation law there's a certain probability that the particle vanishes by interaction with other particles. The only difference is that the position of a particle is never determined, and thus in any state the probability distribution has a finite width around a point (if you consider a state where the particle is pretty well localized) or it might even be a very broad distribution or the distribution might peak around different locations (if the particle is not so well localized).

As I already wrote yesterday, indeed the notion of the "quantum state of the entire universe" doesn't make sense within the minimal interpretation. However "the entire universe" is a pretty abstract and unapproachable fiction. If you believe in inflation, and there are good reasons to believe in the cosmological standard model with inflation (whatever the "mechanism" behind it might be), then it's clear that "the entire universe" isn't even in principle observable. In this sense we always deal with open systems.

 

[Fra]

Of course when speaking about probabilities you have different views on that - Vanhees and myself take the Frequentest view - as many people in areas that apply probability do - but it is far from the only one. The frequentest view naturally leads to the Ensemble interpretation. As John Baez says much of the argument about QM interpretations is the same as arguments about what probability means:
http://math.ucr.edu/home/baez/bayes.html

Me and Vanhees do not ascribe to the Bayesian view - but really its just philosophy and in applying it makes no difference in practice - well most of the time anyway.

Thanks
Bill

Bhobbas perspective (especially on symmetry) has been quite different than mine in past discussions on here, but I fully agree here that the above is indeed at the heart of the discussions! So we probably agree roughly where the core of the issues are but not on the resolution.

QM foundations is certainly (in one way or the other) about connecting the foundations of inference using probability, statistics or what framework you prefer - to the foundations of physics and measurement and science.

My own perspective is that of inference with a mix between frequentist and bayesian, as i argue that the process of actually counting and computing (in the frequentist view) is subjective (hence the bayesian angle). This is because i conjecture that the process of arriving at the expectations from "counting, datareducing and storing" data from history is a physical process, that are encoded in the microstructure of matter. But this perspective also makes it clear what current formulation of QM need relaxation and revisiion. But its equally clear to me at least why - until then - the original Bohr view of the classical measurement device is required for formulating quantum theory in a physically meaningful way (not talking about math realm where you can of course have not constraint on your fantasies)

/Fredrik

 

[vanhees71]

But how is it you don't buy your own preferred interpretation ?
But first things first, I don't (nor anybody else) think (let's say on a philosophical/ontological level) that the "stuff" of the laboratory (or the universe or whatnot) is made of two different "categories" of stuff, obeying different rule. For example I am quite confident that classical mechanics assume you can measure things of the theory (like force and mass and ...) with the same thing in the laboratory (force and mass) (in the same unit)
Also the discussion here is only about phenomenology, and differences between them (and their completeness)

Maybe in the lab... but you seem to be doubting that... I don't. And again, it is not the "problem". The problem is to accurately analyse the theory itself.

Then please precisely explain to me what you mean when you say "measurements are special" (within quantum mechanics). I have no clue what that should mean if you admit that measurement devices are usual "stuff" and thus behaves according to the generally valid physical laws. Indeed, measuring a force with a balance invokes the very laws the concept of force is based on within the theory (necessarily Newtonian mechanics, because the force concept only makes sense within Newtonian mechanics). I'm not doubting that, but you do, if I understand the statement "measurements are special". I'm arguing against this claim of the Copenhagen-like interpretation all the time.

All of physics is about phenomenogy. Theory aims at ever more comprehensive and ever more precise description of phenomena that are objectively observable in Nature. This does not imply a positivistic view on physical theories. QT is the prime example that the formalism is in very abstract terms which are not directly observable. QT is rather a mathematical formalism to predict probabilities for the outcome of measurements, and these probabilities are observables on ensembles via statistical evaluation methods.

What is uncontroversial, in that in the model/theory the physical law describe imaginary(hmmm complex) vector in arbitrary dimension. From what I understand the Schrodinger equation is deterministic and continuous.
Where I think you make a unconscious philosophical leap, is to believe that measurement apparatus (used to test QM) are "displaying" those imaginary pointer from other dimensions... they don't. Not because they are macroscopic, but because the unit don't even match those of the theory...
...because the complete minimal interpretation must add something fundamentally different to classical mechanic, in order to make it scientific (testable).
This process (the Born rule) is discrete, and only happens "on measurement" (not on interaction), and is probabilistic. But at least probabilities of "stuff" in the same unit as the laboratory (all classical).

That's exactly what I meant above: The wave function, which is a way to describe the quantum state for a special case, i.e., for systems of a fixed set of stable particles that can be described non-relativisticall, is not directly observable, but with the Hamiltonian of the it provides position or momentum probablity distributions (depending on whether you work in the position or momentum representation, but you can always convert from one to the other) given an initial condition. This time evolution is, for a closed system, described by a unitary time-evolution operator, and of coarse QT is causal (and even in a narrower sense causal, because it's also local in time, i.e., you need to know the initial condition just at one initial time, not the entire history of the wave function in the past). QT is, however, not deterministic (within the minimal interpretation and most other interpretations too). One has to distinguish between causality (knowing the state in the past tells you precisely the state in the future) and determinism (all observables of a system always have determined values, no matter in which state this system is in). Again: QT is causal but not deterministic.

Ok, then my mistake. When do you use the Born rule inside the Schrodinger equation ?That's an ontological claim (that I share btw).
But you don't claim that. You are claiming (as far as I understand) that the epistemology is not based on such a dichotomy.

What do you mean by that? I don't use Born's rule inside the Schrödinger equation. For me Born's rule is an independent postulate, necessary to give an interpretation to the wave function (in this very special case of systems, where a wave function is a sufficient description of the (pure) quantum states of the system) usable in the lab. The wave function and the Schrödinger equation is just a means to calculate these probabilities. There's no (direct) ontic meaning of the states (in the general case represented statistical operators) and obserables (represented by essentially self-adjoint operators). These are only tools to calculate the probabilities, which can be observed (on ensembles of equally prepared systems).

But there is no need to. The minimalist interpretation is fine. You believe in ensemble, and the Born rule applied. Period.

Exactly. So far there's nothing else.

You seem to believe that one day another interpretation will derive the Born Rule. Why not ? As far I can tel RUTA's one is a good start. It is even based on a classical axiom...

Well, it might well be that one day we'll find another more comprehensive theory, where QT turns out to be an effective theory with the Born rule derived from the more comprehensive theory. It's, maybe, even likely, when we understand quantum gravity better than we do now. So far the Born rule seems to be an independent postulate, necessary to give a minimal interpretation needed to apply the QT formalism to real-world observations.

But as thing are currently, the current minimal interpretation does make such a distinction.

Which "distinction"?

No, the units don't match in QM, they do in CMAs far as I known, probabilities are not complex numbers... even (0,0)

I have no clue, what you want to tell by this statements. The same units are used in QT as in classical physics. Already now many units are based on QT, because that's much more precise than using the historical original definitions based on classical physics. It is almost certain that the entire SI will be based on QT already next year, 2019.

Probabilities are of course numbers between 0 and 1. I've no clue, why you think probabilities might be complex numbers.