I Does a "physically real" quantum interpretation exist?

[timmdeeg]

There are several interpretations of quantum mechanics, most of which are in contradiction to each other, e.g. whether or not the wave function collapses, is ontic, has to be understood in the instrumentalist's view and so forth.

My question refers to the meaning of the term "interpretation". Do we think their predictions are physically real and we are yet just unable to figure out which one? Or do we think they are not physically testable in principle and thus a matter of taste?

If the latter is true why then such enormous efforts?

 

[pines-demon]

There are several interpretations of quantum mechanics, most of which are in contradiction to each other, e.g. whether or not the wave function collapses, is ontic, has to be understood in the instrumentalist's view and so forth.

My question refers to the meaning of the term "interpretation". Do we think their predictions are physically real and we are yet just unable to figure out which one? Or do we think they are not physically testable in principle and thus a matter of taste?

There are two kinds of proposals. First there are proposals that are true interpretations in the sense that we cannot tests their differences so far, examples include Copenhagen interpretation, many-worlds interpretation and Bohmian interpretation.

Then there are "interpretations" that are more like new hypotheses, in the sense that are already known to be falsifiable by experiments because make predictions beyond standard quantum mechanics. These proposals include things like objective-collapse models (Penrose model, GRW model) where there is a new mechanism for collapse that can be tested. These proposals are often called interpretations because make the same predictions up to certain experiments.

If the latter is true why then such enormous efforts?

Because we want to be able to make sense of it. This might be futile but it hasn't stopped anybody from trying.

 

[FactChecker]

I think that you need to be careful about saying an interpretation can not be verified. Bell's Theorem is a good example where something that was thought not to be verifiable turned out to be verifiable.

 

[Peter Morgan]

There are several interpretations of quantum mechanics, most of which are in contradiction to each other, e.g. whether or not the wave function collapses, is ontic, has to be understood in the instrumentalist's view and so forth.

My question refers to the meaning of the term "interpretation". Do we think their predictions are physically real and we are yet just unable to figure out which one? Or do we think they are not physically testable in principle and thus a matter of taste?

If the latter is true why then such enormous efforts?

There's a lot of philosophy of physics about this kind of question, but in an engineering perspective I think of different interpretations typically having different ideas about what experiments it would be interesting to do next.

Hypothesis: if an interpretation proves itself to be better at getting money and material for new experiments, people will adopt it. Can we call this kind of hypothesis, with physically real physicists as part of the experiment, "physically testable"? We could, for example, plan for a social sciences experiment that constructs a database of which experiments are performed and which interpretations the people who performed them prefer.

 

[pines-demon]

There's a lot of philosophy of physics about this kind of question, but in an engineering perspective I think of different interpretations typically having different ideas about what experiments it would be interesting to do next.

Hypothesis: if an interpretation proves itself to be better at getting money and material for new experiments, people will adopt it. Can we call this kind of hypothesis, with physically real physicists as part of the experiment, "physically testable"? We could, for example, plan for a social sciences experiment that constructs a database of which experiments are performed and which interpretations the people who performed them prefer.

That would test the popularity of an idea but physicists usually do not care about that. I think the idea is to either find experiments that can decide on one interpretation over the other, or simplicity arguments (simpler axioms that derive all the phenomenology and then use Occam's razor).

 

[PeterDonis]

do we think they are not physically testable in principle and thus a matter of taste?

This is true by defnition. By definition, all interpretations of QM make the same predictions for all experimental results. So you can't possibly test one against another by experiment.

If you have something that makes different predictions from other QM interpretations, it's not an interpretation of QM, it's a different theory. An example is the GRW stochastic collapse model. All such different theories that have been proposed so far have been falsified by experiment.

 

[PeterDonis]

If the latter is true why then such enormous efforts?

Because for most people, just being able to calculate predictions for the results of experiments, and test them against actual experiments, is not enough. Most people want to be able to tell some kind of story about what is "really happening". And no interpretation of QM gives a story that everyone can accept.

 

[timmdeeg]

Thanks to everybody for your contributions.

Then there are "interpretations" that are more like new hypotheses, in the sense that are already known to be falsifiable by experiments because make predictions beyond standard quantum mechanics. These proposals include things like objective-collapse models (Penrose model, GRW model) where there is a new mechanism for collapse that can be tested. These proposals are often called interpretations because make the same predictions up to certain experiments.

As I understand it the major issue concerns the nature of the wave function. Could you please elaborate on "objective-collapse models". Are at least in principle experiments thinkable which clarify if the wave function collapses during a measurement or not?

 

[haushofer]

I think that you need to be careful about saying an interpretation can not be verified. Bell's Theorem is a good example where something that was thought not to be verifiable turned out to be verifiable.

Indeed. Bell turned metaphysics into physics. That's why I never understand physicists who claim the ontology of QM is a fruitless exercise because "they are all empirically indistinguishable".

As a collegue of Bell put it (paraphrased): "Bell's theorem was not a victory of intellect, but of character".

 

[martinbn]

Indeed. Bell turned metaphysics into physics. That's why I never understand physicists who claim the ontology of QM is a fruitless exercise because "they are all empirically indistinguishable".

How does Bell make it fruitful?

 

[pines-demon]

Thanks to everybody for your contributions.


As I understand it the major issue concerns the nature of the wave function. Could you please elaborate on "objective-collapse models". Are at least in principle experiments thinkable which clarify if the wave function collapses during a measurement or not?

You can open a thread on it if you want. Check Objective-collapse theory, GRW theory, and Penrose model. The idea is that up particles behave quantum mechanically until some mechanism collapses the wave-function, usually happens only for a large number of particles.

In GRW theory, there is a random mechanism, for a single particle it does not happen often so it remains "quantum", but if you have many particles entangled like in a macroscopic body it suffices that one particle collapses under that mechanism in order for the whole system to collapse. So small number of particles behave quantum mechanically and macroscopic objects do not. In Penrose theory is basically the same but it is based on gravitation. Too many particles create an important gravitational well and is this gravitational effect that collapses the wavefunction.

In these theories energy is not quite conserved so the collapse causes heating or noise. They also predict a specific type of spontaneous emission.

 

[WernerQH]

As I understand it the major issue concerns the nature of the wave function.

Most physicists seem to think that interpreting quantum mechanics means interpreting the wave function. But there's more to quantum theory than the wave function. How do you arrive at numbers that can be checked in experiments? It is strange to think of something that evolves continuously and deterministically according to Schrödinger's equation should faithfully describe the discreteness and randomness that we observe in the real world. An individual carbon-14 atom, for example, does not turn gradually into a nitrogen atom over the course of thousands of years, but in a fraction of a second.

Could you please elaborate on "objective-collapse models".

If you don't focus on the wave function, then it need not be a wave function that collapses, but there should be physically real events. In the Transactional Interpretation Ruth Kastner prefers to speak of objective reduction. But it is not clear how these transactions come about. Then there are "flash" models that assume that quantum objects do not have a continuous existence. The wave function could then be just a statistical tool describing a stochastic process.

Are at least in principle experiments thinkable which clarify if the wave function collapses during a measurement or not?

Not only quantum theory, but also experiments require interpretation. If people like to read a collapsing wave function into a particular experiment, they are of course free to do so. But I can't conceive of an experiment that could be interpreted in that way only.

My question refers to the meaning of the term "interpretation". Do we think their predictions are physically real and we are yet just unable to figure out which one? Or do we think they are not physically testable in principle and thus a matter of taste?

I think it's not a matter of taste -- we just haven't yet found the "natural" interpretation of quantum (field) theory. At the end of the 19th century Maxwell's electrodynamics was successful, but theorists were unhappy with it, looking for a mechanical model of the ether. Was Maxwell's ether real? Nowadays most physicists would say that the Michelson-Morley experiment demonstrated that there is no ether, but it took several decades to rid Maxwell's theory of superfluous metaphysical baggage.

 

[A. Neumaier]

An individual carbon-14 atom, for example, does not turn gradually into a nitrogen atom over the course of thousands of years, but in a fraction of a second.

How do you know?

If people like to read a collapsing wave function into a particular experiment, they are of course free to do so. But I can't conceive of an experiment that could be interpreted in that way only.

Light in a superposition of a 0-photon and a 1-photon state moves towards a screen with a tiny hole. If it gets stuck, its wave function collapses to the 0-photon state. If it passed the hole, its wave function collapses to a spherical 1-photon state. That's the standard view.

 

[WernerQH]

How do you know?

The energy released in the decay is 155 keV. Now go and figure out the decoherence time scale.

Light in a superposition of a 0-photon and a 1-photon state moves towards a screen with a tiny hole. If it gets stuck, its wave function collapses to the 0-photon state. If it passed the hole, its wave function collapses to a spherical 1-photon state. That's the standard view.

That's a theorist's picture. How could an experimentalist verify that the radiation field is in 0-photon or a 1-photon state?

 

[A. Neumaier]

The energy released in the decay is 155 keV. Now go and figure out the decoherence time scale.

Decoherence is not the same as decay!

That's a theorist's picture. How could an experimentalist verify that the radiation field is in 0-photon or a 1-photon state?

A laser is to a very good accuracy a source of light in a coherent state. If the laser is very dim, this state is to a very good accuracy a superposition of a 0-photon and a 1-photon state. Thus it is easy to produce experimentally.
It results in a Poisson process of detection events on the screen with the hole, and in a much dimmer Poisson process of detection events on a second screen behind the hole. This is also easy to realize experimentally.

 

[Demystifier]

If the latter is true why then such enormous efforts?

Because without an interpretation, as Feynman famously said, nobody understands quantum mechanics. People want to understand it. Or at least try to understand it.

 

[timmdeeg]

Because without an interpretation, as Feynman famously said, nobody understands quantum mechanics. People want to understand it. Or at least try to understand it.

But to me if I follow discussions people seem to defend their interpretation as the real one.

 

[martinbn]

Because without an interpretation, as Feynman famously said, nobody understands quantum mechanics. People want to understand it. Or at least try to understand it.

Feynman never said that without an interpretation nobody understands QM!

 

[Demystifier]

Feynman never said that without an interpretation nobody understands QM!

I should have put quotation marks. Because without an interpretation, as Feynman famously said, "nobody understands quantum mechanics".

 

[Demystifier]

But to me if I follow discussions people seem to defend their interpretation as the real one.

Because they forget that map is not the territory.

 

[martinbn]

I should have put quotation marks. Because without an interpretation, as Feynman famously said, "nobody understands quantum mechanics".

I understood that. So you are not using his name as an argument in favour of your statement, that without an interpretation one cannot understand QM? Then why do you think so?

 

[Demystifier]

I understood that. So you are not using his name as an argument in favour of your statement, that without an interpretation one cannot understand QM? Then why do you think so?

You are nitpicking.

 

[javisot20]

I understood that Feynman's phrase meant that, if QM is fundamentally based on the uncertainty principle, and the uncertainity principle does not allow to speak of absolute certainty, never say that you understand "all" QM, because you contradict their principles.

A QM interpretation is a possible solution to Schrödinger's cat paradox and the measurement problem. (in addition to solving other problems)

 

[pines-demon]

I understood that Feynman's phrase meant that, if QM is fundamentally based on the uncertainty principle, and the uncertainity principle does not allow to speak of absolute certainty, never say that you understand "all" QM, because you contradict their principles.

That is, if I can say it, an... interpretation.

 

[weirdoguy]

if QM is fundamentally based on the uncertainty principle,

What does that even mean? UP is one of the 'theorems', if you will, you derive from the framework. If I were to say QM is based on something, I would say that it is Hilbert spaces and (generalized) probability.

But I guess when one reads only pop-sci books, one does not know that.

 

[javisot20]

What does that even mean? UP is one of the 'theorems', if you will, you derive from the framework. If I were to say QM is based on something, I would say that it is Hilbert spaces and (generalized) probability.

But I guess when one reads only pop-sci books, one does not know that.

If you deny the uncertainty principle and still want to explain the set of experimental data that make up QM, the result is a theory of hidden variables. The uncertainty principle is fundamental in QM.

 

[weirdoguy]

If you deny the uncertainty principle

You can't deny a mathematical theorem. Without changing underlying assumptions of course. But anyways, I was not talking about that - UP is one of the consequences of QM mathematical basis, not a basis in itself. But ok, that's still off-topic, so I'll leave it at that.

 

[Nugatory]

All such different theories that have been proposed so far have been falsified by experiment.

Is that "all", or "all that have been tested"? Last time I looked (which was a while back) GRW was in the category of proposed but not yet falsified, because testing is infeasible with current techniques.

 

[Lord Jestocost]

I understood that Feynman's phrase meant that, if QM is fundamentally based on the uncertainty principle, and the uncertainity principle does not allow to speak of absolute certainty, never say that you understand "all" QM, because you contradict their principles.

A QM interpretation is a possible solution to Schrödinger's cat paradox and the measurement problem. (in addition to solving other problems)

As concerns the orthodox ‘interpretation’ of QM and its minimalist program, there exists neither a Schrödinger's cat ‘paradox’ nor a measurement ‘problem’. Such putative ‘paradoxes’ or ‘problems’ arise merely from preconceived philosophical views which are superimposed on physics; one should not assume anything in physics one cannot strictly talk about.

 

[javisot20]

As concerns the orthodox ‘interpretation’ of QM and its minimalist program, there exists neither a Schrödinger's cat ‘paradox’ nor a measurement ‘problem’. Such putative ‘paradoxes’ or ‘problems’ arise merely from preconceived philosophical views which are superimposed on physics; one should not assume anything in physics one cannot strictly talk about.

I was trying to answer one of the main questions in the thread,

My question refers to the meaning of the term "interpretation". Do we think their predictions are physically real and we are yet just unable to figure out which one? Or do we think they are not physically testable in principle and thus a matter of taste?

The question is "what is an interpretation of quantum mechanics?" and not "what different interpretations are there?" (I say this because that question is usually answered by mentioning different interpretations, but not by answering what a QM interpretation is)

 

[PeterDonis]

people seem to defend their interpretation as the real one.

This often happens in the literature, yes. But such disputes are not resolvable, because there is no way to test interpretations against each other by experiment. Hence, such disputes are out of bounds here. (The guidelines for this subforum point this out.)

 

[RUTA]

Because for most people, just being able to calculate predictions for the results of experiments, and test them against actual experiments, is not enough. Most people want to be able to tell some kind of story about what is "really happening". And no interpretation of QM gives a story that everyone can accept.

Here is an excerpt from "Einstein's Entanglement: Bell Inequalities, Relativity, and the Qubit" (Oxford UP, 2024):

Quantum information theorist Chris Fuchs writes:

"What is the cause of this year-after-year sacrifice to the 'great mystery?' Whatever it is, it cannot be for want of a self-ordained solution: Go to any meeting, and it is like being in a holy city in great tumult. You will find all the religions with all their priests pitted in holy war ... . They all declare to see the light, the ultimate light. Each tells us that if we will accept their solution as our savior, then we too will see the light."

As Fuchs noted, the story from each "religion" is always, "Just give up [blank] and the mystery disappears!" Unfortunately for its advocates, everyone else considers [blank] essential to causal explanation. Maudlin agrees. Concerning what each "religion" says about the other, he writes:

"They may correctly note that according to every one of their rival theories, God was malicious, and having thus eliminated every other possibility, claim their own theory the victor. The problem is that *every* partisan can argue in this way since *every* theory posits some funny business on the part of the Deity."

 

[timmdeeg]

I think it's not a matter of taste -- we just haven't yet found the "natural" interpretation of quantum (field) theory.

There is one point, I didn't get yet. The Schrödinger equation describes the evolution of the wave function over time. But if the wave function is a mathematical description of a quantum state it can't be the quantum state of say an Elektron itself. So, if correct wouldn't it be natural to say that the wave function can't collapse because math usually doesn't collapse?

 

[WernerQH]

So, if correct wouldn't it be natural to say that the wave function can't collapse because math usually doesn't collapse?

Yes, that was Dyson's point (discussed in another thread):

A physical object can collapse when it bumps into an obstacle. But a wave-function cannot be a physical object. A wave-function is a description of a probability, and a probability is a statement of ignorance. Ignorance is not a physical object, and neither is a wave-function. When new knowledge displaces ignorance, the wave-function does not collapse; it merely becomes irrelevant.

 

[PeterDonis]

that was Dyson's point

Dyson's point is not true for all interpretations of QM. In the MWI, for example, the wave function is a direct mathematical description of the physical reality. In the Bohmian interpretation, the wave function is a direct mathematical description of part of the physical reality, the "pilot wave" that steers the particles.

 

[PeterDonis]

if the wave function is a mathematical description of a quantum state it can't be the quantum state of say an Elektron itself

It can in some interpretations--the MWI, for example.

 

[WernerQH]

As Fuchs noted, the story from each "religion" is always, "Just give up [blank] and the mystery disappears!" Unfortunately for its advocates, everyone else considers [blank] essential to causal explanation.

Does Fuchs reveal "the cause of this year-after-year sacrifice to the 'great mystery'"?

The religious analogy is perhaps amusing, but creating sects is not what scientists are striving for. And the number of interpretations (or what is passed off as such) has increased dramatically since I learnt quantum mechanics (half a century ago)! For me this is a symptom of an underlying unsolved problem. And it's only rationalization, or resignation to say that the interpretations are all equally "reasonable". It's unclear what quantum theory is about. Many people would like a better answer to the question than that it is about "wave functions", "quantum objects", or "measurements". (Of course there are also those who have given up and claim that the mathematical formalism is all that's needed.)

 

[timmdeeg]

It can in some interpretations--the MWI, for example.

So its not decidable whether the wave function describes a quantum state or just probability distributions.

If it describes a quantum state what does this mean if things like position, a definite path ... aren't describable prior to measurement?

 

[timmdeeg]

In the MWI, for example, the wave function is a direct mathematical description of the physical reality

What is the meaning of a mathematically describable "physical reality?

 

[Lord Jestocost]

What is the meaning of a mathematically describable "physical reality?

‘Physical reality’ is that what remains if the observer as a component can be eliminated from the process of knowing.

 

[timmdeeg]

‘Physical reality’ is that what remains if the observer as a component can be eliminated from the process of knowing.

And how would you describe "that what remains" using physical terms? The latter because we talk about a describable physical reality.

 

[Fra]

‘Physical reality’ is that what remains if the observer as a component can be eliminated from the process of knowing.

I think if you remove the observer from the inference process, there is no context for inference and no context of knowing.

Can we by analogy separate out and characterize "ultimate truth", from the scientific process?

I think not. Neither do I find it necessary.

/Fredrik

 

[Lord Jestocost]

And how would you describe "that what remains" using physical terms? The latter because we talk about a describable physical reality.

In principle, that boils down to one simple question. To quote Harald Atmanspacher: Can nature be observed and described as it is in itself independent of those who observe and describe – that is to say, nature as it is “when nobody looks"?

 

[timmdeeg]

Hm, supposed the wave function describes the physical reality of say an Elektron prior to measurement. Isn't this description meaningless if it doesn't refer to certain physical properties of that state of the Elektron?

 

[PeterDonis]

So its not decidable whether the wave function describes a quantum state or just probability distributions.

In the sense that there are QM interpretations that say each of those things, yes.

If it describes a quantum state what does this mean if things like position, a definite path ... aren't describable prior to measurement?

They are in some interpretations--for example, Bohmian.

In the MWI there are no "positions" of anything; the wave function is the entire physical reality.

 

[PeterDonis]

Hm, supposed the wave function describes the physical reality of say an Elektron prior to measurement. Isn't this description meaningless if it doesn't refer to certain physical properties of that state of the Elektron?

In an interpretation like the MWI, the wave function is the physical property. It's all there is.

 

[timmdeeg]

In an interpretation like the MWI, the wave function is the physical property. It's all there is.

To me its weird to identify a mathematical expression with physical properties. Physical properties are due to measurements. Which equipment do we need to measure a mathematical expression?

 

[PeterDonis]

To me its weird to identify a mathematical expression with physical properties.

This is done in every area of physics. You can't avoid it. Physics has to describe what we observe and measure using math.

What would be weird would be to insist that every mathematical expression in a physical theory must be identified with a physical property. But no theory of physics does that--not even QM with the MWI.

 

[physika]

experiments thinkable which clarify if the wave function collapses

Gravity induced collapse

“Being able to measure the time difference on such a minute scale could enable us to discover, for example, that gravity disrupts quantum coherence, which could be at the bottom of why our macroscale world is classical.”



---------
https://www.nature.com/articles/s41586-021-04349-7

 

[timmdeeg]

Thanks to everybody for helpful contributions.