Justification for no properties before measurement

[ggraham76]

Hi. I’m a PhD student at University of Toronto, in philosophy of physics.
I have a number of questions which are quite fundamental: just trying to get everything clear in my head in the most succinct way possible, so I’m sorry if the questions seem remedial.

First question: what is the reason/evidence, EXACTLY, for the position that particles have no properties before they’re measured? I know it’s part of Copenhagen, but what justifies that particular feature of Copenhagen? I’m looking for an answer that I could present to, say, a roomful of non-physics people.

 

[DrChinese]

Hi. I’m a PhD student at University of Toronto, in philosophy of physics.
I have a number of questions which are quite fundamental: just trying to get everything clear in my head in the most succinct way possible, so I’m sorry if the questions seem remedial.

First question: what is the reason/evidence, EXACTLY, for the position that particles have no properties before they’re measured? I know it’s part of Copenhagen, but what justifies that particular feature of Copenhagen? I’m looking for an answer that I could present to, say, a roomful of non-physics people.

Short answer: You could cite the Heisenberg Uncertainty Principle, literally applied. Or Bell's Theorem (1964) and other no-go theorems from that era which show that it is not possible to reconcile the statistical predictions of quantum mechanics with the assumption of simultaneously definite values for ALL particle properties. You can't even hand pick hypothetical values and accomplish that result.

On the other hand: there is nothing that says a particle doesn't have SOME specific properties absent a measurement. A particle observed to be in an eigenstate will generally remain in that state until some new interaction changes that.

 

[PeroK]

Hi. I’m a PhD student at University of Toronto, in philosophy of physics.
I have a number of questions which are quite fundamental: just trying to get everything clear in my head in the most succinct way possible, so I’m sorry if the questions seem remedial.

First question: what is the reason/evidence, EXACTLY, for the position that particles have no properties before they’re measured? I know it’s part of Copenhagen, but what justifies that particular feature of Copenhagen? I’m looking for an answer that I could present to, say, a roomful of non-physics people.

I'd say a particle has a well-defined mass and charge without being measured. It's the dynamic properties that require measurement: position, momentum, energy, angular momentum spin angular momentum. Although, that said, the total spin angular momentum is constant, it's just the spin in any given direction that takes different values.

Also, you can say a lot about the position of a particle without its actually having a definite position. If you are able to specify a probability distribution (pdf) for where you will find a particle if you measure its position, then that is different from it having no positional property at all. You could interpret the pdf as a property - although you have to be careful that that is the pdf of where you will find the particle if you measure it, not where the particle "actually is".

 

[atyy]

Mathematically, the state space of quantum mechanics is not a simplex, which does not mean no properties, rather, no unique properties (ie. more than one possibility about which property the particle has).

Mathematically, it can also be shown that if we choose, eg. position, as a property for the particle to have, then in many cases it cannot also have momentum, with position and momentum governed by laws having the same form as in classical physics.

However, those are not the only reasons for being agnostic about what properties a system has before measurement.

The main reason is that "measurement" is a fundamental concept in the Copenhagen interpretation. A measurement is said to occur when according to the subjective judgement of the observer, a measurement outcome is obtained, ie. something "really" happens.

This is in contrast to classical physics where measurement is not fundamental, and the theory describes things continually happening, even when no observations are being made.

Copenhagen does not so much claim that there are no properties before measurement. Rather it is agnostic to what really happens in between measurement, and it is agnostic about whether the quantum state or quantum wave function is a property of the system (as opposed to also encoding the subjective knowledge of the observer). Copenhagen says that while the dependence on "measurement" may be problematic, it does not prevent quantum mechanics from being a useful theory, since "There is no quantum world. There is only an abstract quantum physical description. It is wrong to think that the task of physics is to find out how Nature is. Physics concerns what we can say about Nature." https://arxiv.org/abs/quant-ph/0212084

That famous quote from Bohr is open to interpretation. However, the way I like to read it is that while there may be no quantum world, Bohr did not rule our the existence of the "classical" or "macroscopic" or "real" world. In fact, Copenhagen assumes that the "classical" or "macroscopic" or "real" world exists.

 

[ggraham76]

Short answer: You could cite the Heisenberg Uncertainty Principle, literally applied. Or Bell's Theorem (1964) and other no-go theorems from that era which show that it is not possible to reconcile the statistical predictions of quantum mechanics with the assumption of simultaneously definite values for ALL particle properties. You can't even hand pick hypothetical values and accomplish that result.

On the other hand: there is nothing that says a particle doesn't have SOME specific properties absent a measurement. A particle observed to be in an eigenstate will generally remain in that state until some new interaction changes that.

Thx Dr. Chinese. I think the position I’m referring to, apart from being part of Copenhagen, is also inherent in the ‘no hidden variables’ view, which is asserted by Bell’s theorem. What I’m looking for is something like a knock-down reply to the objection: ‘It’s just a matter of ignorance. Of course there are properties, we’re just unable to access or observe them.”

 

[PeroK]

What I’m looking for is something like a knock-down reply to the objection: ‘It’s just a matter of ignorance. Of course there are properties, we’re just unable to access or observe them.”

To be honest, that's not really a question I can easily associate with PhD research!

 

[ggraham76]

Mathematically, the state space of quantum mechanics is not a simplex, which does not mean no properties, rather, no unique properties (ie. more than one possibility about which property the particle has).

Mathematically, it can also be shown that if we choose, eg. position, as a property for the particle to have, then in many cases it cannot also have momentum, with position and momentum governed by laws having the same form as in classical physics.

However, those are not the only reasons for being agnostic about what properties a system has before measurement.

The main reason is that "measurement" is a fundamental concept in the Copenhagen interpretation. A measurement is said to occur when according to the subjective judgement of the observer, a measurement outcome is obtained, ie. something "really" happens.

This is in contrast to classical physics where measurement is not fundamental, and the theory describes things continually happening, even when no observations are being made.

Copenhagen does not so much claim that there are no properties before measurement. Rather it is agnostic to what really happens in between measurement, and it is agnostic about whether the quantum state or quantum wave function is a property of the system (as opposed to also encoding the subjective knowledge of the observer). Copenhagen says that while the dependence on "measurement" may be problematic, it does not prevent quantum mechanics from being a useful theory, since "There is no quantum world. There is only an abstract quantum physical description. It is wrong to think that the task of physics is to find out how Nature is. Physics concerns what we can say about Nature." https://arxiv.org/abs/quant-ph/0212084

That famous quote from Bohr is open to interpretation. However, the way I like to read it is that while there may be no quantum world, Bohr did not rule our the existence of the "classical" or "macroscopic" or "real" world. In fact, Copenhagen assumes that the "classical" or "macroscopic" or "real" world exists.

Mathematically, the state space of quantum mechanics is not a simplex, which does not mean no properties, rather, no unique properties (ie. more than one possibility about which property the particle has).

Mathematically, it can also be shown that if we choose, eg. position, as a property for the particle to have, then in many cases it cannot also have momentum, with position and momentum governed by laws having the same form as in classical physics.

However, those are not the only reasons for being agnostic about what properties a system has before measurement.

The main reason is that "measurement" is a fundamental concept in the Copenhagen interpretation. A measurement is said to occur when according to the subjective judgement of the observer, a measurement outcome is obtained, ie. something "really" happens.

This is in contrast to classical physics where measurement is not fundamental, and the theory describes things continually happening, even when no observations are being made.

Copenhagen does not so much claim that there are no properties before measurement. Rather it is agnostic to what really happens in between measurement, and it is agnostic about whether the quantum state or quantum wave function is a property of the system (as opposed to also encoding the subjective knowledge of the observer). Copenhagen says that while the dependence on "measurement" may be problematic, it does not prevent quantum mechanics from being a useful theory, since "There is no quantum world. There is only an abstract quantum physical description. It is wrong to think that the task of physics is to find out how Nature is. Physics concerns what we can say about Nature." https://arxiv.org/abs/quant-ph/0212084

That famous quote from Bohr is open to interpretation. However, the way I like to read it is that while there may be no quantum world, Bohr did not rule our the existence of the "classical" or "macroscopic" or "real" world. In fact, Copenhagen assumes that the "classical" or "macroscopic" or "real" world exists.

Thx atyy, especially for the elucidation about Copenhagen. I’m keenly interested in a response to the position, as you put it, that the anti- realism about (some) properties is simply an expression of “ the subjective knowledge of the observer.” I think the evidence for no hidden variable theorems, like the EPR/Aspect experiments, support the anti-realist position. Do you concur?

 

[ggraham76]

To be honest, that's not really a question I can easily associate with PhD research!

Lol really, why do you say that?

 

[ggraham76]

I'd say a particle has a well-defined mass and charge without being measured. It's the dynamic properties that require measurement: position, momentum, energy, angular momentum spin angular momentum. Although, that said, the total spin angular momentum is constant, it's just the spin in any given direction that takes different values.

Also, you can say a lot about the position of a particle without its actually having a definite position. If you are able to specify a probability distribution (pdf) for where you will find a particle if you measure its position, then that is different from it having no positional property at all. You could interpret the pdf as a property - although you have to be careful that that is the pdf of where you will find the particle if you measure it, not where the particle "actually is".

Thx PeroK, for clarifying that point. You’re right, I should have spoken more precisely.

 

[Jilang]

Thx Dr. Chinese. I think the position I’m referring to, apart from being part of Copenhagen, is also inherent in the ‘no hidden variables’ view, which is asserted by Bell’s theorem. What I’m looking for is something like a knock-down reply to the objection: ‘It’s just a matter of ignorance. Of course there are properties, we’re just unable to access or observe them.”

You need to be clear what is meant by “properties” Bells inequality would rule out classical “properties”. Not quantum ones.

 

[PeroK]

Lol really, why do you say that?

I think there is plenty of material out there that will quickly take your knowledge beyond a question like that. In principle, Bell's theorem and the experiments in support of it "proved" that there are no hidden variables.

But, the debate has moved on - and this is beyond my knowledge of the subject - into arguments about "counter-factual definiteness". I don't doubt the validity of these debates, but they don't interest me as much as learning about the core physics.

It would be nice if Bell's Theorem were the last word on hidden variables, but it's not. So,there is no knock-down argument.

I think you need to get yourself up the state of knowldedge where you can follow and assess these debates, post Bell's Theorem, on the nature of QM.

 

[mikeyork]

It is a fundamental principle of science that the information we gather about a physical system is logically self-consistent. If some information had been previously recorded (or prepared) about an object and there were no reason to assume it had changed then any future measurement cannot contradict it. But if there is no previously recorded information then there is nothing that a subsequent measurement must be consistent with. Quantum uncertainty is all about the presence of previously unrecorded information in the observer's knowable world. It says nothing about objective reality.

The probabilistic projection of a superposition onto an eigenstate -- that Copenhagen people call "collapse" -- describes the limited view of reality available in the observer's chosen context of observable and frame of reference..

 

[mikeyork]

I’m keenly interested in a response to the position, as you put it, that the anti- realism about (some) properties is simply an expression of “ the subjective knowledge of the observer.”

No but it is an expression of the limits of the objective knowledge available to human observers. That is not the same thing.

 

[atyy]

Thx atyy, especially for the elucidation about Copenhagen. I’m keenly interested in a response to the position, as you put it, that the anti- realism about (some) properties is simply an expression of “ the subjective knowledge of the observer.” I think the evidence for no hidden variable theorems, like the EPR/Aspect experiments, support the anti-realist position. Do you concur?

Bell's theorem and the violation of the Bell inequalities by quantum mechanics do not mean that hidden variables are impossible. They only mean that certain types of local hidden variable theories are impossible.

Nonlocal hidden variable theories remain possible. For non-relativistic quantum mechanics, non-local hidden variable theories have been constructed.

Other important approaches for removing "measurements" as fundamental in quantum mechanics include the many-worlds interpretation.

Some varieties of Copenhagen are compatible with these realist research programmes. In these varieties of Copenhagen, Copenhagen is understood to be a practical interpretation, not an anti-realist one.

 

[Mentz114]

Hi. I’m a PhD student at University of Toronto, in philosophy of physics.
I have a number of questions which are quite fundamental: just trying to get everything clear in my head in the most succinct way possible, so I’m sorry if the questions seem remedial.

First question: what is the reason/evidence, EXACTLY, for the position that particles have no properties before they’re measured? I know it’s part of Copenhagen, but what justifies that particular feature of Copenhagen? I’m looking for an answer that I could present to, say, a roomful of non-physics people.

This is probably the most misunderstood area of standard QM because of the confusion about what a 'property' is.
An electron has intrinsic angular momentum $1/2$. That is a property. The direction of the axial vector is not a property it is part of a configuration that belongs to actual particles. Is the direction that your car is pointing when it is parked a property of the car ? I don't think it is.

If an apparatus is used to rotate the spin alignment ( projective measurement) into $|Z_+\rangle$ then something is known about the configuration of the spin, but the spin has not changed. If we make another projection into $|X_+\rangle$ then asking 'what happened to $|Z_+\rangle$' is not hard to answer - it got rotated away and no longer exists.

Thinking this way makes it clear what can be said to be the case and what cannot. Things that are parameters of a particular configuration are not properties.

I don't think there is much material in this question because the standard formalism is clear, but misunderstood by some.

 

[bhobba]

First question: what is the reason/evidence, EXACTLY, for the position that particles have no properties before they’re measured? I know it’s part of Copenhagen, but what justifies that particular feature of Copenhagen? I’m looking for an answer that I could present to, say, a roomful of non-physics people.

The theory does not say that. Some early versions of the Copenhagen Interpretation may - but that is just an interpretation ie the view of some of the founders of QM - other founders like Einstein did not agree with it. The theory, as opposed to an interpretation of it, is silent on if it has any properties before being measured. It may have, may not, it may be on Mars for all we know.

Now why is the theory silent on that? Because its a theory about the probability of observations. That's the theory, it works and basically that's the only reason needed in science.

Of course that does not stop people conjecturing about what's happening when not observed - and we have quite a few of those.

As a philosophy student you would probably like what's called Bohmian Mechanics:
http://philsci-archive.pitt.edu/3026/1/bohm.pdf

In that its more in line with everyday intuition. Trouble is there is no way of proving it true. That's the real crux of the thing - we have interpretations where it has properties even when not measured - but we can't prove it.

Personally, for what it's worth I hold to the ensemble interpretation advocated by Einstein (yes its a misconception that Einstein didn't think QM true - he just thought it incomplete)::
https://en.wikipedia.org/wiki/Ensemble_interpretation

So the answer to your question is - it may indeed have properties when not observed - or not - its just nobody can figure out an experiment to decide one way or the other.

Thanks
Bill

 

[bhobba]

To be honest, that's not really a question I can easily associate with PhD research!

Ahhhhhh. Schlosshauer carefully examines the issues with QM in Decoherence and the Quantum-to-Classical Transition:
https://www.amazon.com/dp/3540357734/?tag=pfamazon01-20

The conclusion, using our modern current knowledge, is there is an unanswered question - why do we get any outcomes at all - which is deeply intertwined with what the OP is asking. In technical parlance its how does an improper mixed state become a proper one. My view is - who cares - you can't tell the diffidence observationally so its of no concern. But that is just my view. Others, especially philosophers may have a different take.

The answer leads directly to the why of various interpretations - exactly what issue they are trying to grapple with and how they do the 'grappling'. It would be a perfectly valid PhD research paper to philosophically look at these and see how each comes to grips with the issue.

We don't worry about such on this forum - ie the philosophy of it all - but I do like that this student comes here to get the facts. I have read far too many papers by philosophers on QM that to be blunt - makes you want to chunder - they don't understand QM at all.

To the OP I have to go to dinner now, but when I return will give you a reading list that will explain what I think you need to know to do your research, and the order you should read it.

Thanks
Bill

 

[ggraham76]

The theory does not say that. Some early versions of the Copenhagen Interpretation may - but that is just an interpretation ie the view of some of the founders of QM - other founders like Einstein did not agree with it. The theory, as opposed to an interpretation of it, is silent on if it has any properties before being measured. It may have, may not, it may be on Mars for all we know.

Now why is the theory silent on that? Because its a theory about the probability of observations. That's the theory, it works and basically that's the only reason needed in science.

Of course that does not stop people conjecturing about what's happening when not observed - and we have quite a few of those.

As a philosophy student you would probably like what's called Bohmian Mechanics:
http://philsci-archive.pitt.edu/3026/1/bohm.pdf

In that its more in line with everyday intuition. Trouble is there is no way of proving it true. That's the real crux of the thing - we have interpretations where it has properties even when not measured - but we can't prove it.

Personally, for what it's worth I hold to the ensemble interpretation advocated by Einstein (yes its a misconception that Einstein didn't think QM true - he just thought it incomplete)::
https://en.wikipedia.org/wiki/Ensemble_interpretation

So the answer to your question is - it may indeed have properties when not observed - or not - its just nobody can figure out an experiment to decide one way or the other.

Thanks
Bill

Bill, thank you very much. I greatly appreciate your support.

 

[microsansfil]

hi all,

An electron has intrinsic angular momentum 1/2. That is a property. The direction of the axial vector is not a property it is part of a configuration

And also the states variables position r and momentum p , to which it correspond linear Hermitian operator, can't be consider as intrinsic properties of microsystems/particle. isn't it ?

And thus all http://www.physics.rutgers.edu/~steves/501/Lectures_Final/Lec11_The_Postulates_of_Quantum_Mechanics.pdf (angular momentum L = r×p, ...) can't be consider as intrinsic properties of microsystems/particle. isn't it ?

Best regards
Patrick

 

[PeterDonis]

what is the reason/evidence, EXACTLY, for the position that particles have no properties before they’re measured?

Measurements are the evidence, so the position that particles don't have properties before they're measured is simply sticking to the evidence and not going beyond it at all. So asking for evidence of a position that just amounts to believing nothing beyond the evidence doesn't seem right.

 

[Fra]

Hi. I’m a PhD student at University of Toronto, in philosophy of physics. I know it’s part of Copenhagen, but what justifies that particular feature of Copenhagen? I’m looking for an answer that I could present to, say, a roomful of non-physics people.

This can be a challenge. In order to adapt arguments to your audience you need to know of their background. If non-physicists means they are also har poor mathematical knowledge then they might not be able to follow any detailed theorems or so. Then you need to explain things at a different level. The question is what kind of explanations do you seek?

Regarding yourself, i would recommend from a philosopher of physics to at least have some training in physics, so you can follow the basics of say hidden variables also mathematically from your own understanding. I agree with bhobba that quite often I've found the writings on philosopy of physics, from so called "pure philosophers" to indicate they are analyzing things where you wonder if they have the basic issues right from the beginning or not. IMO from the perspecitve of foundational physicsts, the best "philosophical writings on physics" are from physicists. That are experts in the field, but somtimes without formal "philosophy training".

So the question is, if with your work you want to "impress physicists" or "impress fellow philosophers"?

/Fredrik

 

[Fra]

What I’m looking for is something like a knock-down reply to the objection: ‘It’s just a matter of ignorance. Of course there are properties, we’re just unable to access or observe them.”

The insight from QM is that in quantum phenomena the ignorance is not a matter of practical limitations, like it is in classical mechanics.

Another thing that is very often confused in philosophy of QM, is that the epistemological limitattions on what can be said about the system, are often wrongly associated to a "human observer". It is quite clear that this is not what QM says. And the published things that drag this into physics cause only confusion.

The epistemological limitations is about the physical relations between the observed system and the measurement device. The measurement device is the observer. So whenever one speaks of an "observer" in quantum mechanics what that means is a measurement device, that can interact with the system, register and store outcomes. Moreover this measurement device must be a classical device, otherwise we do not know how to describe it.

Then "relation" between the atual human experimenter and the measurement device is then ideally described by classical mechanics, special realtivith etc, and thus beeing "trivial" in this context.

Note that in a physics lab, where you have accelerators and colliders these "classical/quantum" divide is well satisfied. The crazy quantum phenomena going on on subatomic level can be so well described, simply THANKS to the highly rigid and classical "background" that constitudes the "measurement device" or "observer".

/Fredrik

 

[Mentz114]

hi all,And also the states variables position r and momentum p , to which it correspond linear Hermitian operator, can't be consider as intrinsic properties of microsystems/particle. isn't it ?

And thus all http://www.physics.rutgers.edu/~steves/501/Lectures_Final/Lec11_The_Postulates_of_Quantum_Mechanics.pdf (angular momentum L = r×p, ...) can't be consider as intrinsic properties of microsystems/particle. isn't it ?

Best regards
Patrick

Hi,
intrinsic quantum spin cannot arise from $L=\vec{r}\otimes \vec{p}$ because there is no tangential momentum operator. The operators for spin are the Pauli matrices. Momentum and position are not governed by that symmetry. Momentum is frame dependent in any case.

The original question is about when we can meaningfully assume values before measurement - for spin we cannot. But if an atom is confined in a trap then it must have a position even before measurement because it was thus prepared. The only thing in question is how accurately the position can be measured.

 

[bhobba]

Hi Graham

As promised here is my suggested reading list for your purposes.

1. Quick Calculus:
https://www.amazon.com/dp/0471827223/?tag=pfamazon01-20
2. Theoretical Minimum
https://www.amazon.com/dp/0465075681/?tag=pfamazon01-20
3. Susskind - Quantum Mechanics::
https://www.amazon.com/dp/0465036678/?tag=pfamazon01-20
4. Structure And Interpretation Of QM:
https://www.amazon.com/dp/0674843924/?tag=pfamazon01-20
5. Quantum Probabilities:
https://www.scottaaronson.com/democritus/lec9.html
http://math.ucr.edu/home/baez/bayes.html
6. Consistent Histories - It'a a modern interpretation favored by Gell-Mann and towards the end by Feynman - sometimes categorized as Copenhagen done right:
http://quantum.phys.cmu.edu/CQT/index.html

I think that's enough to start with - get back with any questions.

Thanks
Bill

 

[microsansfil]

Hi,
intrinsic quantum spin cannot arise from $L=\vec{r}\otimes \vec{p}$ because there is no tangential momentum operator.

Ok thank for the answer. For my understanding.

The corresponding quantum angular momentum L is also called spin angular momentum !?

Best regards
Patrick

 

[DrChinese]

What I’m looking for is something like a knock-down reply to the objection: ‘It’s just a matter of ignorance. Of course there are properties, we’re just unable to access or observe them.”

In my view, Bell is the easiest argument to explain. The problem is that in the scheme of things, it is far beyond what you can explain to someone at a cocktail party. I have found that even many interested parties on this forum won't take the time to follow the full Bell argument. There are some webpages that have simplified Bell proofs. I humbly have one too: http://drchinese.com/David/Bell_Theorem_Easy_Math.htm - but in the mean time, try this one:

Premise: If quantum properties (such as spin or polarization) exist at all times - but we just don't know their values... WHAT COULD THOSE VALUES BE?

Place 3 quarters flat on a table in a triangle, any way you like as to H/heads or T/tails up. You will see that no matter how you select H or T, at least 2 will match. Right? And if you randomly select any pair out of the 3 quarters, they will match no less than 1/3 of the time. If you had 100 sets of 3 quarters and did the same thing (randomly selecting 2 of the 3), you'd get about 33 of the 100 at a minimum matching. Your rule is: you get to hand pick how you want the 3 quarters to be presented (H or T), but pick the 2 to compare randomly and without consideration of whether it is H or T. Your goal is to mimic a quantum experiment by picking a H or T value for the other quarter, the one not compared. After all, that's our premise!

Now think of the quarters as representing 3 specific polarization angles of a photon, and whether its polarization would be H or V if measured (H or V is actually relative to the selected angle, it is not absolute). The analogy is that H or V values map to H or T on the quarter. According to the idea that these simultaneously have values, you just don't know what they are: then you already know from the above paragraph that the quantum version of the test should not provide matches of less than 1/3. It's the same constraint exactly. But for certain settings in a real quantum test (details not provided), the actual percentage is 1/4 (25%). That's impossible if all 3 values pre-existed!

Conclusion: there are NO SUCH POSSIBLE VALUES. Ignorance has nothing to do with it, you can't even make up values by hand that make it work. Note that the only way to "cheat" is for you to select H or T for the 3 quarters knowing in advance (seeing into the future) which 2 you plan to compare. In the quantum world, there may be ways that happens. It's sometimes referred to as quantum nonlocality.

 

[bhobba]

What I’m looking for is something like a knock-down reply to the objection: ‘It’s just a matter of ignorance. Of course there are properties, we’re just unable to access or observe them.”

That's easy - nobody has ever been able to find such if they exist - and if they do they would have to have rather strange properties as Dr Chinese explains. Of course who you tell it to may not agree its a knock down argument - unfortunately science has nothing to say beyond what experiment says - if they want more - its not science. They then may retort we want to go beyond that - of course that's their right - but in that we can't help.

Thanks
Bill

 

[mikeyork]

The insight from QM is that in quantum phenomena the ignorance is not a matter of practical limitations, like it is in classical mechanics.

Another thing that is very often confused in philosophy of QM, is that the epistemological limitattions on what can be said about the system, are often wrongly associated to a "human observer". It is quite clear that this is not what QM says. And the published things that drag this into physics cause only confusion.

The epistemological limitations is about the physical relations between the observed system and the measurement device. The measurement device is the observer. So whenever one speaks of an "observer" in quantum mechanics what that means is a measurement device, that can interact with the system, register and store outcomes. Moreover this measurement device must be a classical device, otherwise we do not know how to describe it.

All human knowledge requires human observers whatever apparatus they use or rely on others having used. The error that people make concerning human observers is to assume that a human can only know what they have themselves directly observed, not that the acquisition of human knowledge requires human observers.

Ambiguity in language is a major problem with quantum mechanics and leads people into making false claims that seem perfectly reasonable on first sight without critical examination.

 

[ggraham76]

This can be a challenge. In order to adapt arguments to your audience you need to know of their background. If non-physicists means they are also har poor mathematical knowledge then they might not be able to follow any detailed theorems or so. Then you need to explain things at a different level. The question is what kind of explanations do you seek?

Regarding yourself, i would recommend from a philosopher of physics to at least have some training in physics, so you can follow the basics of say hidden variables also mathematically from your own understanding. I agree with bhobba that quite often I've found the writings on philosopy of physics, from so called "pure philosophers" to indicate they are analyzing things where you wonder if they have the basic issues right from the beginning or not. IMO from the perspecitve of foundational physicsts, the best "philosophical writings on physics" are from physicists. That are experts in the field, but somtimes without formal "philosophy training".

So the question is, if with your work you want to "impress physicists" or "impress fellow philosophers"?

/Fredrik

This can be a challenge. In order to adapt arguments to your audience you need to know of their background. If non-physicists means they are also har poor mathematical knowledge then they might not be able to follow any detailed theorems or so. Then you need to explain things at a different level. The question is what kind of explanations do you seek?

Regarding yourself, i would recommend from a philosopher of physics to at least have some training in physics, so you can follow the basics of say hidden variables also mathematically from your own understanding. I agree with bhobba that quite often I've found the writings on philosopy of physics, from so called "pure philosophers" to indicate they are analyzing things where you wonder if they have the basic issues right from the beginning or not. IMO from the perspecitve of foundational physicsts, the best "philosophical writings on physics" are from physicists. That are experts in the field, but somtimes without formal "philosophy training".

So the question is, if with your work you want to "impress physicists" or "impress fellow philosophers"?

/Fredrik

Hi Fra,

I do have a physics background, but would like to be as non-technical as possible, hence my asking the question very simply and plainly (also, that’s a good exercise, I think, for anyone: to have the philosophical foundations clear in your head). The reason is, as you have guessed, my audience. I’m not sure about what amount of the math formalism of QM they’re familiar with.

I appreciate your help very much. If you don’t mind my asking, though, why do you put “philosophy training” in quotes?

 

[ggraham76]

In my view, Bell is the easiest argument to explain. The problem is that in the scheme of things, it is far beyond what you can explain to someone at a cocktail party. I have found that even many interested parties on this forum won't take the time to follow the full Bell argument. There are some webpages that have simplified Bell proofs. I humbly have one too: http://drchinese.com/David/Bell_Theorem_Easy_Math.htm - but in the mean time, try this one:

Premise: If quantum properties (such as spin or polarization) exist at all times - but we just don't know their values... WHAT COULD THOSE VALUES BE?

Place 3 quarters flat on a table in a triangle, any way you like as to H/heads or T/tails up. You will see that no matter how you select H or T, at least 2 will match. Right? And if you randomly select any pair out of the 3 quarters, they will match no less than 1/3 of the time. If you had 100 sets of 3 quarters and did the same thing (randomly selecting 2 of the 3), you'd get about 33 of the 100 at a minimum matching. Your rule is: you get to hand pick how you want the 3 quarters to be presented (H or T), but pick the 2 to compare randomly and without consideration of whether it is H or T. Your goal is to mimic a quantum experiment by picking a H or T value for the other quarter, the one not compared. After all, that's our premise!

Now think of the quarters as representing 3 specific polarization angles of a photon, and whether its polarization would be H or V if measured (H or V is actually relative to the selected angle, it is not absolute). The analogy is that H or V values map to H or T on the quarter. According to the idea that these simultaneously have values, you just don't know what they are: then you already know from the above paragraph that the quantum version of the test should not provide matches of less than 1/3. It's the same constraint exactly. But for certain settings in a real quantum test (details not provided), the actual percentage is 1/4 (25%). That's impossible if all 3 values pre-existed!

Conclusion: there are NO SUCH POSSIBLE VALUES. Ignorance has nothing to do with it, you can't even make up values by hand that make it work. Note that the only way to "cheat" is for you to select H or T for the 3 quarters knowing in advance (seeing into the future) which 2 you plan to compare. In the quantum world, there may be ways that happens. It's sometimes referred to as quantum nonlocality.

That is an excellent answer, and a very accessible explication of Bell’s inequality. Thanks.