Understanding the Fundamental Difference in Interpretations of QM

[Lynch101]

TL;DR Summary

Trying to understand what the different interpretations of QM say and don't say and which interpretations fall into which category.

I was originally going to start a thread on individual interpretations of quantum mechanics to try to understand them better, but after reading this article by @PeterDonis it might make more sense to use that as a starting point and try to understand how the different interpretations fit into the framework outlined by Peter. In the article, Peter outlines two fundamentally different interpretations:

Type 1 - Instrumentalist?

(1) The state is a tool that we use to predict the probabilities of different results for measurements we might choose to make of the system. Changes in the state represent changes in the predicted probabilities; for example, when we make a measurement and obtain a particular result, we update the state to reflect that observed result, so that our predictions of probabilities of future measurements change.

I think I am correct in stating that interpretations of this type are referred to as "Instrumentalist" interpretations because the mathematical machinery of QM is a tool or an instrument that allows us to make [probabilistic] predictions about macroscopic events. In such interpretations, the mathematical formalism is not taken to represent an underlying ontology or the physical state of the system. As such, interpretations of this type do not allow us to make deterministic predictions about the outcome of individual experiments, nor do they tell us what happens in individual experiments, or how the quantum state interacts with the macroscopic measuring device. For this reason, such interpretations are often criticised as being "incomplete". Is that much correct?

The article also mentions

For #1, the obviously true part is that we can never directly observe the state

Does this have something to do with the Heisenberg uncertainty principle, where we can never precisely know both the momentum and position of a particle, because there is a trade-off between the information that can be known about either? I thought I had read something about no-go theorems which suggest that incompleteness is not particular to QM, that it is not possible to develop a more complete theory. Am I correct in any of that, or am I misinterpreted some information along the way?

Particular Interpretations
Which particular interpretations fall into this category? Am I right in saying that QFT is such an instrumentalist interpretation? Is it possible for Copenhagen to be interpreted both instrumentally and non-instrumentally? I'm thinking where the superposition of the wave-function is taken to be a physical representation of the system. Is this interpretation what gives rise to the Schrodinger's cat problem?Type 2 - Deterministic?

2) The state describes the physically real state of the system; the state allows us to predict the probabilities of different results for measurements because it describes something physically real, and measurements do physically real things to it. Changes in the state represent physically real changes in the system; for example, when we make a measurement, the state of the measured system becomes entangled with the state of the measuring device, which is a physically real change in both of them.

Are interpretations of this type considered to be non-instrumentalist and deterministic? Under these interpretations, is the mathematical formalism taken to represent an underlying ontology i.e. it describes something physically real? Do interpretations of this type purport to tell us what is happening in individual experiments, although they are unable to offer deterministic predictions because there are hidden variables which are unaccounted for leaving us with only the ability to make probabilistic predictions?

Does the statement above, that we can never directly observe the state of the system, equally apply to interpretations of this type? As in, is there a fundamental limit to what we can know about the physical system that is simply unavoidable?

Particular Interpretations
Which particular interpretations fall into this category? Is de Broglie-Bohm Pilot Wave one such interpretation? Is this the same thing as Bohmian mechanics? Does the Many Worlds Interpretation fall into this category? Is it possible to interpret Copenhagen in this manner also?

I thought I had read somewhere that hidden variables theories, or the MWI, cannot reproduce all of the predictions of quantum mechanics. Is there any truth in that, or have I again misinterpreted something along the way?Other interpretations
I'm not too familiar with the other interpretations of QM. I've heard of QBism but I'm not too sure what it says. Which category would that fall into? Are there others?

 

[Demystifier]

I think I am correct in stating that interpretations of this type are referred to as "Instrumentalist" interpretations because the mathematical machinery of QM is a tool or an instrument that allows us to make [probabilistic] predictions about macroscopic events. In such interpretations, the mathematical formalism is not taken to represent an underlying ontology or the physical state of the system. As such, interpretations of this type do not allow us to make deterministic predictions about the outcome of individual experiments, nor do they tell us what happens in individual experiments, or how the quantum state interacts with the macroscopic measuring device. For this reason, such interpretations are often criticised as being "incomplete". Is that much correct?

Yes, that's correct.

Am I right in saying that QFT is such an instrumentalist interpretation?

No, QFT is definitely not an interpretation.

Is it possible for Copenhagen to be interpreted both instrumentally and non-instrumentally?

It's possible, because there are many variants of "Copenhagen".

Type 2 - Deterministic?
Are interpretations of this type considered to be non-instrumentalist and deterministic?

Yes.

Under these interpretations, is the mathematical formalism taken to represent an underlying ontology i.e. it describes something physically real? Do interpretations of this type purport to tell us what is happening in individual experiments, although they are unable to offer deterministic predictions because there are hidden variables which are unaccounted for leaving us with only the ability to make probabilistic predictions?

There is a lot of confusion about that in the literature. Ontological interpretations do not necessarily need to be deterministic. The notion of "hidden variables" sometimes refers to hidden determinism and sometimes to hidden ontology.

Is de Broglie-Bohm Pilot Wave one such interpretation? Is this the same thing as Bohmian mechanics? Does the Many Worlds Interpretation fall into this category? Is it possible to interpret Copenhagen in this manner also?

Yes to all.

I thought I had read somewhere that hidden variables theories, or the MWI, cannot reproduce all of the predictions of quantum mechanics.

I'm sure you did, but that's wrong.

Is there any truth in that, or have I again misinterpreted something along the way?

It's more likely that not you, but people who wrote that, misunderstood something.

Are there others?

Yes.

 

[Lynch101]

Thank you Demystifier.

No, QFT is definitely not an interpretation.

Is it possible to talk about QFT in the context of Peter's insight article?

Does QFT take an instrumentalist approach to QM, does it prescribe an underlying, deterministic ontology, or is it just not possible to talk about QFT in this way?

It's possible, because there are many variants of "Copenhagen".

Thank you, that is the impression I was getting.

There is a lot of confusion about that in the literature. Ontological interpretations do not necessarily need to be deterministic. The notion of "hidden variables" sometimes refers to hidden determinism and sometimes to hidden ontology.

Ah, OK. When you talk about hidden determinism as opposed to hidden ontology, what form might that take? Or more pointedly, what do you mean by that?

I'm sure you did, but that's wrong.
It's more likely that not you, but people who wrote that, misunderstood something.

Ah OK. I'm not even sure where I read that, so I might be mistaken. Thanks for the clarification.With regard to the instrumentalist interpretations, am I right in saying there are a few different schools of thought as far as what we can and can't say about the quantum state?

I'm thinking in terms of individual experiments. Do different instrumentalists say different things about what we can (or perhaps can't) say about the particle in the experiment, in the moments immediately prior to a Stern Gerlach plate registering the particle?

 

[PeterDonis]

Does this have something to do with the Heisenberg uncertainty principle,

Sort of. It has to do with the fact that, in general, there is no set of measurements you can make on a single quantum object that will tell you its exact quantum state. This is because, in general, the observables you would need to measure to completely specify the state do not all commute (which is what gives rise to the Heisenberg uncertainty principle), and also because, in general, measuring an observable on a quantum object changes its state, so future measurements are no longer measuring the original state but a different one.

 

[Lynch101]

Sort of. It has to do with the fact that, in general, there is no set of measurements you can make on a single quantum object that will tell you its exact quantum state. This is because, in general, the observables you would need to measure to completely specify the state do not all commute (which is what gives rise to the Heisenberg uncertainty principle), and also because, in general, measuring an observable on a quantum object changes its state, so future measurements are no longer measuring the original state but a different one.

Ah yes, that is what EPR set as their criterion for "reality" wasn't it, determining a property of the state without disturbing the system? This lead to Bell's theorem ruling out local hidden variables, is that correct?

Is this idea, that the exact quantum state can never be measured, interpreted differently under the instrumentalist paradigm? Am I right in getting the impression that some instrumentalists would say that we cannot talk about the particle prior to measurement because there is no particle prior to measurement, while others would say that there is something prior to measurement we just can't know its properties, while other would say "shut up and calculate"?

 

[Nugatory]

Ah yes, that is what EPR set as their criterion for "reality" wasn't it, determining a property of the state without disturbing the system?

Not quite. The EPR idea was a way of measuring two incompatible observables in spite of the inevitable disturbance of the system - this may be clearer in Bohm’s spin-based version of the argument than in the original EPR paper.

 

[Halc]

Does QFT take an instrumentalist approach to QM, does it prescribe an underlying, deterministic ontology, or is it just not possible to talk about QFT in this way?

QFT is all about what is known and making predictions, and not about going the extra step in attempting to explain the underlying metaphysical reality. Hence I would qualify it as an instrumental description of QM (not so much an approach since I'm unaware of another way to approach it).

All the actual interpretations do make some kind of metaphysical assertions about said underlying reality. As long as the interpretation is consistent with QFT, it cannot be falsified, hence is more philosophy than science. This is why many QM classes might touch on the various interpretations, but don't dwell on what lacks falsification tests.

Am I right in getting the impression that some instrumentalists would say that we cannot talk about the particle prior to measurement because there is no particle prior to measurement, while others would say that there is something prior to measurement we just can't know its properties, while other would say "shut up and calculate"?

Any statement about 'there is or is not a particle' seems to be a metaphysical assertion and beyond the instrumentalist restriction to what can be measured.

 

[PeterDonis]

Is this idea, that the exact quantum state can never be measured, interpreted differently under the instrumentalist paradigm?

Only in the sense that the quantum state itself is interpreted differently. Under interpretations of my type #1, the quantum state is just a way of calculating probabilities of future measurements. What future measurements? Well, if we have a device that claims to prepare a particular quantum state, the future measurements would be measurements of whatever it is that comes out of the device. For example, it could be a device that claims to prepare qubits in a particular quantum state, and the measurements would be Stern-Gerlach measurements of the qubits with various orientations of the measuring device. But you can only pass each qubit that comes out of the device through one measurement, because after that, its quantum state has changed--you will make different predictions about the probabilities of future measurements.

So under interpretations of type #1, saying the exact quantum state can never be measured on a single "quantum system" is just saying you can't figure out the complete, exact distribution of probabilities for all possible measurements of whatever it is that comes out of the preparation device (which is what you would need to specify the exact quantum state), on a single "run" of that device; you need a large number of runs, where you make different measurements on different runs.

On interpretations of type #2, the basic facts are the same, but the interpretation is that the device is preparing actual, individual quantum systems, each of which have the same quantum state, instead of just interpreting the state as a representation of the probability distribution for all possible measurements, without making any claims about the individual thingies that are coming out of the preparation device.

 

[Lynch101]

Not quite. The EPR idea was a way of measuring two incompatible observables in spite of the inevitable disturbance of the system - this may be clearer in Bohm’s spin-based version of the argument than in the original EPR paper.

Were they were proposing to leverage this idea:

If, without in any way disturbing a system, we can predict with certainty (i.e., with probability equal to unity) the value of a physical quantity, then there exists an element of reality corresponding to that quantity

as a way of measuring those two incompatible observables? As in, they proposed measuring the momentum of one particle and the position of the other and by doing so they suggested that they would have complete information about both.

Am I correct in that?

 

[Lynch101]

All of the below makes sense to me, but I struggle to reconcile it with some of the conclusions that I've heard. Again, it might be my misinterpretation of the information I've encountered. I'll try to pinpoint what I mean below.

Only in the sense that the quantum state itself is interpreted differently. Under interpretations of my type #1, the quantum state is just a way of calculating probabilities of future measurements. What future measurements? Well, if we have a device that claims to prepare a particular quantum state, the future measurements would be measurements of whatever it is that comes out of the device. For example, it could be a device that claims to prepare qubits in a particular quantum state, and the measurements would be Stern-Gerlach measurements of the qubits with various orientations of the measuring device. But you can only pass each qubit that comes out of the device through one measurement, because after that, its quantum state has changed--you will make different predictions about the probabilities of future measurements.

Please forgive my butchering of this description but, to my mind, if we have a device that claims to prepare a particular quantum state and then sends that quantum state in the direction of a measuring device, which then registers a measurement of that quantum state, this would seem to imply a deterministic process. Even if we cannot make deterministic predictions.

So under interpretations of type #1, saying the exact quantum state can never be measured on a single "quantum system" is just saying you can't figure out the complete, exact distribution of probabilities for all possible measurements of whatever it is that comes out of the preparation device (which is what you would need to specify the exact quantum state), on a single "run" of that device; you need a large number of runs, where you make different measurements on different runs.

I don't really understand the part I have emboldened. Is that the same as saying that we cannot make deterministic predictions for single "runs" of the device? I'm guessing there is more nuance to it than I am recognising.

On interpretations of type #2, the basic facts are the same, but the interpretation is that the device is preparing actual, individual quantum systems, each of which have the same quantum state, instead of just interpreting the state as a representation of the probability distribution for all possible measurements, without making any claims about the individual thingies that are coming out of the preparation device.

Can the emboldened statement be known for definite? I was thinking that it couldn't be measured but maybe there is something in the preparation that guarantees this?

 

[PeterDonis]

if we have a device that claims to prepare a particular quantum state and then sends that quantum state in the direction of a measuring device, which then registers a measurement of that quantum state, this would seem to imply a deterministic process.

Only if you think every process is a deterministic process. I don't see anything in particular that would single this process out as having to be deterministic if you don't already believe all processes are.

Is that the same as saying that we cannot make deterministic predictions for single "runs" of the device?

It's saying even more than that. It's saying that, even once you have the actual result of the measurement, it doesn't tell you the complete quantum state; that is, it doesn't tell you the complete probability distribution for all possible measurements that you could make.

Can the emboldened statement be known for definite?

What the emboldened statement means, operationally, is that we use the same preparation device, with the same settings, every time. In fact, operationally, the preparation process--what preparation device you use and what settings you select--defines the quantum state. For example, if I say the quantum state of a qubit tha I am about to put through a Stern-Gerlach measurement is z-spin up, what I mean is that it came out of a preparation device that prepares qubits in spin eigenstates, and I have selected the setting "prepare z-spin up".

What the statement of mine that you bolded previously means is that, if I have a preparation device but I don't tell you what its settings are, you have no way of figuring out what its settings are from one measurement. For example, if I have handed you a device that prepares qubits in spin eigenstates, with settings locked on some particular spin eigenstate, but I don't tell you which spin eigenstate it is, there is no way for you to figure out which spin eigenstate the device is preparing from one Stern-Gerlach measurement. You would have to make a large number of runs, holding the settings fixed, and make Stern-Gerlach measurements of spin in different directions.

 

[Demystifier]

Is it possible to talk about QFT in the context of Peter's insight article?

Yes.

Does QFT take an instrumentalist approach to QM, does it prescribe an underlying, deterministic ontology, or is it just not possible to talk about QFT in this way?

All interpretations used in QM can also be used (with a small adjustment) in QFT. In essence, QFT is just QM with an infinite number of degrees of freedom.

Ah, OK. When you talk about hidden determinism as opposed to hidden ontology, what form might that take? Or more pointedly, what do you mean by that?

What I really wanted to say is that there are ontological views of QM which are not deterministic. E.g. Nelson
https://en.wikipedia.org/wiki/Stochastic_quantum_mechanics
and GRW
https://en.wikipedia.org/wiki/Ghirardi–Rimini–Weber_theory

With regard to the instrumentalist interpretations, am I right in saying there are a few different schools of thought as far as what we can and can't say about the quantum state?

Certainly there are. For instance, I advocate a Bohmian version of instrumentalism in the paper linked in my signature below.

 

[Lynch101]

Only if you think every process is a deterministic process. I don't see anything in particular that would single this process out as having to be deterministic if you don't already believe all processes are.

Again, it might be down to my [mis]interpretation of key ideas and perhaps drawing conclusions that cannot be drawn. It's probably down to my own classical level intuition, which I know doesn't always work when it comes to QM.

Determinism
Reading through some of the other threads on QM interpretations I read this post by a poster named @Allday. It puts into words a thought or a question I have had myself

In David Griffiths book on quantum mechanics he breaks people into three groups, and I paraphrase:
1. The realists. Where was a quantum particle just before you measured it and collapsed the wave function? An infinitesimal distance away from where you measured it.

If we interpret this in terms of the following ideas about causal determinsim:

Causal determinism is, roughly speaking, the idea that every event is necessitated by antecedent events and conditions together with the laws of nature.
...
As the following famous expression of determinism by Laplace shows, however, the two are also easy to commingle:
We ought to regard the present state of the universe as the effect of its antecedent state and as the cause of the state that is to follow.

In this broad sense, determinism is simply the idea that present states of a system are caused or necessitated by their antecedent states. A fallen domino, for example, is the result of the previous domino falling into it. In this broad sense, such realist interpretations of QM would be causally deterministic.

Determinism: The world is governed by (or is under the sway of) determinism if and only if, given a specified way things are at a time t, the way things go thereafter is fixed as a matter of natural law.

The same source gives a stricter definition of determinism, which might be interpreted to mean that prior to the outcome of an experiment, there is only one possible outcome. In this stricter sense, it might be argued that where QM interpretations allow for more than one outcome, they are not strictly deterministic.

There are deterministic interpretations of QM, though, aren't there? Would they say that - to use the crude analogy - if we were able to follow the row of dominos from the Stern Gerlach plate back to the detector, we would find a causally deterministic chain?

I have read in places that such deterministic interpretations of QM cannot reproduce the exact results of quantum mechanics (or perhaps it was QFT they were referring to?). @Demystifier has already suggested that this is incorrect, however.My Understanding of Instrumentalism
From what I can gather, instrumentalist interpretations seem to either say something along the following lines (I refer to Allday's paraphrasing of David Griffiths again because it is concise and it reflects the impression I have gotten myself):

2. The orthodox position. The particle really wasn't anywhere. It was the act of measuring that produced its exact location. There are various definitions of "measurement", but the main idea is the particle didn't have a definite position before the wavefunction collapsed.

3. Agnostic. Doesn't make sense to ask a question, that you can't know the answer to. In other words, asking where the particle was BEFORE the measurement is just not a question worth spending time on.

To try to summarise my own interpretation, it seems as though the instrumentalist interpretation says something along the lines of:
a) We cannot know the location of the particle just before it is measured, so we cannot talk about it.
or
b) We cannot talk about the particle in this way because there is no particle just before it is measured
or
c) Such spatio-temporal notions as location and "before the measurement" don't apply to the particle.

Intuitively, I can grasp the ideas above but I would question them in the following ways:
a) Even without knowing the location of the particle prior to being measured, we can talk in much broader terms and still make certain deductions.
or
b) There may not have been what we usually call a "particle" prior to measurement, but there must have been "something". Again, we can talk in very broad terms about this "something" and make certain deductions.
or
c) Such spatio-temporal notions may not apply to the quantum particle but we can still talk about the classical level in those terms and, in very broad terms, we can still make certain deductions; such as, there must have been "something" going through the experimental set-up to cause the exposure event on the Stern Gerlach plate.

To my mind, it seems entirely reasonable to talk in these very broad terms and to draw certain additional conclusions from the experimental evidence.

It's saying even more than that. It's saying that, even once you have the actual result of the measurement, it doesn't tell you the complete quantum state; that is, it doesn't tell you the complete probability distribution for all possible measurements that you could make.

What the statement of mine that you bolded previously means is that, if I have a preparation device but I don't tell you what its settings are, you have no way of figuring out what its settings are from one measurement. For example, if I have handed you a device that prepares qubits in spin eigenstates, with settings locked on some particular spin eigenstate, but I don't tell you which spin eigenstate it is, there is no way for you to figure out which spin eigenstate the device is preparing from one Stern-Gerlach measurement. You would have to make a large number of runs, holding the settings fixed, and make Stern-Gerlach measurements of spin in different directions.

Ah OK. I see that now, thank you.

What the emboldened statement means, operationally, is that we use the same preparation device, with the same settings, every time. In fact, operationally, the preparation process--what preparation device you use and what settings you select--defines the quantum state. For example, if I say the quantum state of a qubit tha I am about to put through a Stern-Gerlach measurement is z-spin up, what I mean is that it came out of a preparation device that prepares qubits in spin eigenstates, and I have selected the setting "prepare z-spin up".

Yes, thank you. This is what I was thinking when I said that there might be something in the preparation that guarantees that the exact same state is prepared every time. Is there any possibility that there might be slight variations among different particles prepared in a z-spin up sate?

I had a slightly different idea in mind but down to my broad strokes interpretation: I had heard of an experimental set-up that prepares silver atoms in an oven and then "fires" them into the experimental set-up and the outcome is an exposure event on a Stern Gerlach plate (I've probably butchered that explanation, but hopefully it makes sense). In my mind, I could imagine that there would be slight variations in each silver atom that is sent through the experimental set-up, such as being in a slightly different location in the oven and perhaps miniscule fluctuations in the energy of the particle. Again, I've probably butchered that, but are such variations possible and would they even make a difference?

 

[martinbn]

l am not sure how to understand the first post. For example the two boxed statements seem to me to apply to all interpretations.

(1) The state is a tool that we use to predict the probabilities of different results for measurements we might choose to make of the system. Changes in the state represent changes in the predicted probabilities; for example, when we make a measurement and obtain a particular result, we update the state to reflect that observed result, so that our predictions of probabilities of future measurements change.

2) The state describes the physically real state of the system; the state allows us to predict the probabilities of different results for measurements because it describes something physically real, and measurements do physically real things to it. Changes in the state represent physically real changes in the system; for example, when we make a measurement, the state of the measured system becomes entangled with the state of the measuring device, which is a physically real change in both of them.

 

[Demystifier]

For example the two boxed statements seem to me to apply to all interpretations.

No. For instance, (1) does not apply to GRW and (2) does not apply to QBism.

 

[PeterDonis]

There are deterministic interpretations of QM, though, aren't there?

Yes. For example, the Bohmian interpretation, as @Demystifier has already mentioned.

a) We cannot know the location of the particle just before it is measured, so we cannot talk about it.

Yes.

b) We cannot talk about the particle in this way because there is no particle just before it is measured

The instrumentalist interpretation does not say this.

c) Such spatio-temporal notions as location and "before the measurement" don't apply to the particle.

The instrumentalist interpretation does not say this either.

a) Even without knowing the location of the particle prior to being measured, we can talk in much broader terms and still make certain deductions.

Not according to the instrumentalist interpretation, at least not if you are talking about deductions about position when we haven't measured position.

Is there any possibility that there might be slight variations among different particles prepared in a z-spin up sate?

In any real source, the preparation will not be 100% exact. For example, if we have a source that we have set up to prepare qubits in a z-spin up state as best we can, and we pass each qubit through a z-spin measurement device, we will not see 100% of them register the "up" result. There will be some unavoidable error.

However, if we only take the qubits that do in fact register the "up" result, what we then have, considering the whole setup of source plus z-spin measurement device as a single "device", is a preparation device that does prepare qubits exactly in the z-spin up state (since we discarded all the qubits that weren't exactly in that state). And then the answer to your question is, if there are any slight variations among these qubits, they are not variations that we have any way of detecting experimentally. As best we can tell, all of these qubits are the same.

And yet, if we pass all of those z-spin up qubits through, say, an x-spin measuring device, half of them will register "x spin up" and half will register "x spin down". That does invite the hypothesis that there must be something that varies between them. But if there is, it can't be anything as simple as "there is some kind of internal variation in the qubits that we can't directly detect". That hypothesis is called "local hidden variables" in the literature, and it is disproved by the existence of correlations between entangled qubits that violate the Bell inequalities.

I had heard of an experimental set-up that prepares silver atoms in an oven and then "fires" them into the experimental set-up and the outcome is an exposure event on a Stern Gerlach plate

That is a description of the original Stern-Gerlach experiment, except that Stern and Gerlach had no way of having the oven emit only one atom at a time; it emitted a beam of atoms that was split into two beams by the magnetic field in their device.

 

[PeterDonis]

the two boxed statements seem to me to apply to all interpretations.

To add to what @Demystifier said, (1) does not apply to the MWI, and (2) does not apply to ensemble interpretations.

 

[martinbn]

No. For instance, (1) does not apply to GRW and (2) does not apply to QBism.

To add to what @Demystifier said, (1) does not apply to the MWI, and (2) does not apply to ensemble interpretations.

Why not? Isn't the way to compute probabilities the same in all these. Thus the wave function is a tool to calculate probabilities, which is what (1) says. I don't see why (2) doesn't apply to QBism. For the ensemble interpretation, there is indeed a difference, the state doesn't represent the system, but the ensemble of equally prepared systems. But by a little abuse of language it says the same. Like saying that an element of $L^2(\mathbb R)$ is a function, when it really is an equivalence class.

 

[PeterDonis]

the wave function is a tool to calculate probabilities, which is what (1) says

Ah, I see the problem. I just edited the article to add the one word bolded below:

(1) The state is only a tool that we use to predict the probabilities of different results for measurements we might choose to make of the system.

Does that make it clearer what (1) is trying to say?

 

[PeterDonis]

For the ensemble interpretation, there is indeed a difference, the state doesn't represent the system, but the ensemble of equally prepared systems. But by a little abuse of language it says the same.

Ok, then check out this second change I just made to the article:

(2) The state describes the physically real state of the individual quantum system

Does that make it clearer what (2) is trying to say?

 

[Lynch101]

All interpretations used in QM can also be used (with a small adjustment) in QFT. In essence, QFT is just QM with an infinite number of degrees of freedom.

Ah, OK. I think I have a better understanding of this point now. Thank you Demystifier.

What I really wanted to say is that there are ontological views of QM which are not deterministic. E.g. Nelson
https://en.wikipedia.org/wiki/Stochastic_quantum_mechanics
and GRW
https://en.wikipedia.org/wiki/Ghirardi–Rimini–Weber_theory

Thank you. I've heard of those. I'll give them a further look.

Certainly there are. For instance, I advocate a Bohmian version of instrumentalism in the paper linked in my signature below.

Oh, interesting! I've seen the link in your sig. I'll take a read of it and see what I can understand.

I was quite interested in the idea of Bohmian mechanics when I first heard of it, for the obvious reason that it seemed a bit more intuitive. What would you say the biggest criticisms of Bohmian mechanics would be?

I'm probably incorrect in how I imagine the Pilot Wave theory (am I right in saying you do use the term "theory" in this case?), but in my mind I liken it somewhat to lightning moving through a cloud - a phenomenon I am equally fuzzy on . Does the possible path of lightning change with every "bolt" due to the charge in different parts of the cloud? (I imagine that is likely quite wide of the mark). Would there be something similar in Bohmian mechanics where the possible path a wave can take changes based on the previous wave and its affect on the field through which it travels?

I imagine the latter is even more garbled than the first, but that is kind of the image I have in my head. Is there even the basis there for an understanding or should I try to forget that kind of analogy altogether?

 

[martinbn]

Ah, I see the problem. I just edited the article to add the one word bolded below:

(1) The state is only a tool that we use to predict the probabilities of different results for measurements we might choose to make of the system.

Does that make it clearer what (1) is trying to say?

Not quite. What does it mean to be only a tool? How can it not be more than that?

Ok, then check out this second change I just made to the article:

(2) The state describes the physically real state of the individual quantum system

Does that make it clearer what (2) is trying to say?

Yes, then there are two types of interpretations according to this one. Ensemble and others.

 

[Lynch101]

Yes. For example, the Bohmian interpretation, as @Demystifier has already mentioned.

Thank you. The Bohmian interpretation and MWI would be the ones I am most familiar with, in that category.

The instrumentalist interpretation does not say this.
The instrumentalist interpretation does not say this either.

Sorry, that's probably my loose paraphrasing.

It might be easier to explore the point on the basis of the question: where was a quantum particle just before it was measured? It is the answers I hear to this question that I usually get stuck on.

It's probably just my classical intuition, but I struggle to get my head around why we don't conclude that the underlying process must be deterministic. I know we can't make deterministic predictions but it seems reasonable, to my mind, to conclude that there must be a chain of causal determinism, leading to the Stern Gerlach exposure event. Even if we can never fully account for every detail of it, it seems [to my mind] that we can imply it.

Not according to the instrumentalist interpretation, at least not if you are talking about deductions about position when we haven't measured position.

That much I understand, that we cannot talk about the location of the particle prior to measuring it. I'm talking about a much broader sense which pertains to the idea of determinism.

As mentioned above, to the question: where was a quantum particle just before you measured it and collapsed the wave function?
Deterministic interpretations would answer: an infinitesimal distance away from where you measured it.

This doesn't seem to be the case with instrumentalist interpretations, however, which is where I run into difficulty with the instrumentalist position. I'm not sure how they would answer the above question. I've heard a variety of answers including those outlined by David Griffiths, as paraphrased by @Allday.

I can understand that we might not be able to give the answer "an infinitesimal distance away" because we cannot know its location until we measure it. But, speaking in the broadest possible terms, I feel like we should be able to say that it was "somwhere" in the Universe at the very least, if not somewhere in the experimental set-up.

If we can say even this much, it seems to me as though the underlying process must be deterministic because the exposure event on the SG plate is caused by the antecedent state. If we cannot use terms such as "state" or "particle" in this specific sense, then I think we can again speak in the broadest possible terms and say that it was caused by "something", which likely came from the device we used in the experiment.

Again, this might simply be my classical intuition but that's where I seem to "butt heads" with instrumentalism.

But if there is, it can't be anything as simple as "there is some kind of internal variation in the qubits that we can't directly detect". That hypothesis is called "local hidden variables" in the literature, and it is disproved by the existence of correlations between entangled qubits that violate the Bell inequalities.

Ah yes! Of course. Thank you Peter.

That is a description of the original Stern-Gerlach experiment, except that Stern and Gerlach had no way of having the oven emit only one atom at a time; it emitted a beam of atoms that was split into two beams by the magnetic field in their device.

Thank you for the clarification.

 

[PeterDonis]

What does it mean to be only a tool?

That it doesn't describe the physically real state of the individual quantum system. On this kind of interpretation, if I have a device that prepares qubits in the quantum state "spin-z up", that does not mean that state describes the physically real state of each individual qubit. The state only allows me to make claims about the statistics of the results I will get if I measure a stream of qubits from the device with Stern-Gerlach measurements oriented in various directions.

How can it not be more than that?

Um, by not being more than that according to that particular interpretation?

Basically, it seems like you are simply denying that interpretations in category (1) are even possible. That does not seem to me to be a viable position. You might think such interpretations are wrong, but I don't see how you can say they're impossible.

there are two types of interpretations according to this one. Ensemble and others.

No, ensemble is not the only type of interpretation that is not in category (2). @Demystifier gave QBism as another example. At least some versions of Copenhagen are also not in category (2) (I say "some versions" because the term "Copenhagen" doesn't really name one particular interpretation).

 

[PeterDonis]

it seems reasonable, to my mind, to conclude that there must be a chain of causal determinism, leading to the Stern Gerlach exposure event. Even if we can never fully account for every detail of it, it seems [to my mind] that we can imply it.

We have already agreed that there are interpretations of QM which are deterministic, so I don't see your point here. If you're just saying you don't like the instrumentalist interpretation because it isn't deterministic, that's fine, but then the obvious thing for you to do would be to not use the instrumentalist interpretation, but to adopt one of the deterministic ones instead. And then you get to deal with all the highly counterintuitive and difficult features of whichever deterministic interpretation you adopt, instead of the problems you see with the instrumentalist interpretation. There is no interpretation which tells a nice simple story that accords with all of our intuitions; any interpretation of QM you adopt will force you to confront difficulties. That's one of my main points in the Insights article.

 

[RUTA]

I've heard of QBism but I'm not too sure what it says.

Here is Mermin’s take on QBism. https://arxiv.org/abs/1809.01639 It’s an essay accessible to the general reader.

 

[Lynch101]

We have already agreed that there are interpretations of QM which are deterministic, so I don't see your point here.
then you get to deal with all the highly counterintuitive and difficult features of whichever deterministic interpretation you adopt, instead of the problems you see with the instrumentalist interpretation.

The point I am trying to get at is that when probing the features of instrumentalist interpretations, it seems to me like we should conclude that the underlying process is deterministic, even if we cannot access all the information about the system or make deterministic predictions.

If we take the exposure event of the Stern Gerlach plate, is it fair to conclude that the event must have been caused by the particle? If we can't use the term "particle" in this sense then we would have to talk in the broadest possible terms and say that "something" caused the SG exposure. In this sense, the SG event could be said to be caused by antecedent events or states. If we were to work backwards from the exposure event, we could trace a deterministically causal chain back to the device we used to prepare the particle.

 

[PeterDonis]

when probing the features of instrumentalist interpretations, it seems to me like we should conclude that the underlying process is deterministic

You're not "probing the features of instrumentalist interpretations". You're just describing the actual process of measurement, which is the same for all QM interpretations, and putting your preferred interpretation on it.

If we take the exposure event of the Stern Gerlach plate, is it fair to conclude that the event must have been caused by the particle?

Sure, but that doesn't mean it was deterministically caused by the particle.

Suppose I have a device that prepares particles in the spin-z up state, and I make a spin-x Stern-Gerlach measurement on one such particle. There is no way to predict in advance whether the result will be spin-x up or spin-x down; there is an equal probability of each. But that in no way prevents me from saying that, once the result is observed (a spot on the detector in either the "up" location or the "down" location), that result was caused by the particle.

The lack of predictability could be because the fundamental process involved is truly indeterministic--it is literally impossible, even in principle, to know which way the result will come out until it happens; the result is truly not determined by the previous state.

Or the lack of predictability could be only because we do not have a sufficiently exact knowledge of the initial conditions: for example, if we are using the Bohmian interpretation, we don't know the initial position of the particle exactly enough to know which way the quantum potential will push it during the interaction with the Stern-Gerlach device. But on the Bohmian interpretation, which is deterministic, the particle has some exact initial position, even if we don't know it, and that exact initial position completely determines the result of the measurement--i.e., at which location, the "up" one or the "down" one, the particle ends up.

Which of the above possibilities you think is true depends on your interpretation of QM.

 

[EPR]

If we take the exposure event of the Stern Gerlach plate, is it fair to conclude that the event must have been caused by the particle? If we can't use the term "particle" in this sense then we would have to talk in the broadest possible terms and say that "something" caused the SG exposure. In this sense, the SG event could be said to be caused by antecedent events or states. If we were to work backwards from the exposure event, we could trace a deterministically causal chain back to the device we used to prepare the particle.

Particles don't exist as such as far as we know and you won't get very far with it. This outdated concept is only useful for picturing a somewhat approximate condition imposed by a particular measurement setup.
Look at the individual atoms represented in this astonishing video. This is the only meaningful represenation of 'particles'. This is why everything is quantum mechanics. Even Covid-19 which is in reality a field with field strengths marked as particles.

 

[PeterDonis]

Particles don't exist as such as far as we know and you won't get very far with it. This outdated concept is only useful for picturing a somewhat approximate condition imposed by a particular measurement setup.

I think this is a bit strong. It isn't just one "particular measurement setup" that is usefully described using the particle concept. It is lots and lots of them. Including those used by an entire subfield that calls itself "particle physics" for a reason, even though its underlying theoretical model is ultimately quantum field theory. A concept can hardly be "outdated" under those circumstances.

 

[Lynch101]

You're not "probing the features of instrumentalist interpretations". You're just describing the actual process of measurement, which is the same for all QM interpretations, and putting your preferred interpretation on it.

It is probably fair to say that I am not "probing the features of instrumentalist interpretations", and equally so that I am describing the actual process of measurement, but I think when we probe the actual process of measurement we are - I would say - forced down the road of determinism - which I will try to outline below.

Sure, but that doesn't mean it was deterministically caused by the particle.
...
The lack of predictability could be because the fundamental process involved is truly indeterministic--it is literally impossible, even in principle, to know which way the result will come out until it happens;

I think how we analyse the notion of determinism is a key factor here. We often tend to think about it in terms of predictability, moving from cause to effect, and saying that given the state of the system at time t, there can only be one possible outcome. In practice, of course, we can't always make such predictions with a probability of 1.

Another way to think about it is going the other way, from cause to effect, and establishing a causally deterministic chain. In this sense we think about states of the world being caused by antecedent events. It is when we apply this to quantum experiments that, I believe, we are forced down (or back up) the road of determinism.

the result is truly not determined by the previous state.

It is this idea that the result is not truly determined by the previous state which I can't make sense of. To my mind, an event which is not caused by antecedent events happens completely spontaneously, for no reason whatsoever, and requires that something (the event) appears out of absolutely nothing, completely unconnected to anything else in the Universe.

If we take the exposure on the SG plate, if the exposure event is not caused by an antecedent event, then the exposure happens completely spontaneously and which would seem to mean that it was only a matter of coincidence that it occurs during the experiment.

If we say that the exposure was caused by the particle, then the particle interacting with the SG plate is the antecedent event. This is where things seem to get murky, because there is very little we can say about the particle, in terms of position and momentum, prior to its interaction with the plate. I fell, however, that we can talk about it in the broadest possible terms saying that the particle (or whatever it is that interacts with the SG plate) is in the Universe and probably within the region of space where we are conducting the experiment.

In these very broad terms we could, I think, establish a causally deterministic chain from the effect - the exposure event - back to the cause - the device we use to prepare the particle.

Or the lack of predictability could be only because we do not have a sufficiently exact knowledge of the initial conditions:

That we do not have sufficient knowledge of initial conditions is a given from the outset isn't it? Would the Heisenberg uncertainty principle not necessitate this?

 

[Lynch101]

Here is Mermin’s take on QBism. https://arxiv.org/abs/1809.01639 It’s an essay accessible to the general reader.

Thanks Ruta. I'll certainly give this a read.

 

[PeterDonis]

We often tend to think about it in terms of predictability, moving from cause to effect

Another way to think about it is going the other way, from cause to effect, and establishing a causally deterministic chain.

Determinism as it is usually understood implies both of these.

It is this idea that the result is not truly determined by the previous state which I can't make sense of.

Yes, obviously. But this is not an argument for us being "forced" to determinism. It is simply a statement that you can't make sense of the alternative. So what? The fact that you can't make sense of it is not an argument for anything. It's just a statement of your state of mind.

an event which is not caused by antecedent events

I did not say the result of the S-G measurement was not caused by an antecedent event. In fact I explicitly said the opposite:

that in no way prevents me from saying that, once the result is observed (a spot on the detector in either the "up" location or the "down" location), that result was caused by the particle.

You are simply conflating "caused by an antecedent event" with "determined by the previous state", which is not valid; it is possible for the first to be true while the second is not true. The fact that you are unable to conceive of such a possibility does not mean the possibility does not exist.

That we do not have sufficient knowledge of initial conditions is a given from the outset isn't it?

It's a given if we accept the limitations of standard QM. But a deterministic interpretation of QM like the Bohmian interpretation is perfectly consistent with the view that, at some time in the future, we might figure out how to gain more information about the initial conditions than standard QM allows us to. Or, to put it another way, that standard QM is not the final fundamental theory, so its limitations are not fundamental limitations.

 

[Demystifier]

What would you say the biggest criticisms of Bohmian mechanics would be?

There is no simple extension to relativistic QFT. There are various ways to make that extension (even though many wrongly claim that there isn't any), but neither of them is simple.

 

[martinbn]

That it doesn't describe the physically real state of the individual quantum system. On this kind of interpretation, if I have a device that prepares qubits in the quantum state "spin-z up", that does not mean that state describes the physically real state of each individual qubit. The state only allows me to make claims about the statistics of the results I will get if I measure a stream of qubits from the device with Stern-Gerlach measurements oriented in various directions.

What I don't understand is the following. If it doesn't describe the physically real state of the system, there must be a difference between the two, and yet in terms of predictions this tool only state gives the best possible predictions. Why isn't there an experiment where the results don't match the predictions? After all if the real state is different then the results must differ too.

Um, by not being more than that according to that particular interpretation?

Basically, it seems like you are simply denying that interpretations in category (1) are even possible. That does not seem to me to be a viable position. You might think such interpretations are wrong, but I don't see how you can say they're impossible.

I know that there are interpretations of this type. I just don't understand them to the point where it seems that they are impossible.

No, ensemble is not the only type of interpretation that is not in category (2). @Demystifier gave QBism as another example. At least some versions of Copenhagen are also not in category (2) (I say "some versions" because the term "Copenhagen" doesn't really name one particular interpretation).

This is related to my misunderstanding of (1), because to me it seems that they all belong to (2).