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.

 

[lightarrow]

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.”

Sorry, I haven't read all the post after this so don't know if it's already been written.
I think your request is, in general, impossible, but you could answer something like what writes Griffiths at the beginning of his QM book: consider a particle in a state described by a wavefunction which is, essentially, a train of waves along some spatial directions, with a lot of cycles: which is the "position" of this train of waves? You could take the beginning of the train as well as the end or any other value in between.

--
lightarrow

 

[mikeyork]

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.

Yes, there is a huge distinction between ignorance and unknowability. A quantum superposition represents the latter. Suppose someone else had already projected a superposition onto their chosen basis and thereby measured an observable. A subsequent observer may still be ignorant of this result, but the result is now knowable by communication with the other observer and any subsequent measurement must be consistent with the original.

 

[Mentz114]

Ok thank for the answer. For my understanding.

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

View attachment 217960
Best regards
Patrick

Have a look at this article about intrinsic spin.

https://en.wikipedia.org/wiki/Spin_(physics)

and

http://hyperphysics.phy-astr.gsu.edu/hbase/spin.html

 

[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 see, that sounds good that you have a physics background.

I have spent enough time on the philosophical foundations myself, so i have my own views quite clear in my head.. And I agree its important. But to convey it so someone else is a different challenge.

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

Good question :) The truth is that its a bad habit of mine, i find myself often put lots of things in quotation even when it doesn't always make sense. Sorry about this.

But i think what i meant in this case is that philosophical training could mean two things to me, either its simply that spend time on the thinking about foundations in a philosophical manner and train yourself to analyze things (I think physicists does this too to some extent), and then there is the philosophical training that means studying traditional philosophy as whatever they do at philosophy departments, where the emphasis isn't the physics but the process of analysis.

I have also a physics background but i never ever formally studied any philosophy whatsoever. Except I read some random books, like poppers terrible book, some books on history of probability theory. My own philosophical contemplation has arised and been driven strictly from open problems and interpretational issues in physics. This is why my experience that the "philosophy of physics" as per physicists which are then formally "amateur philosophers" are quite different from that of professional philosophers tha. But I think you can get a good combo if you combine background in both.

I actually remember one old physics teacher i had, I had him in both analytical mechanics and then in QM1 or QM2 (dont remember), and while obviously beeing a physicist he also had a formal background in philosophy (ie. having studied philosophy at philosophy department) and he had a distinguished ability to understand fuzzy questions in a way that some other teachers that were more narrow minded couldn't. He couldn't necessarily ANSWER the questions as they were admittedly open questions, but he did understand and acknowledge the questions, that others either didn't understand or pretended to not understand. The latter thing is a very bad thing todo to students. Some researchers may be clever but may be less suitable to teach. I have even experience cases where teachers almost get offended when getting deep questions from undergraduates that they could not handle as some even said straight out "an undergraduate are not supposed to ask these things" etc.

/Fredrik

 

[romsofia]

I'm in the same place you are currently, doing a PhD program and am questioning a lot of things fundamentally. I'm a relativist at heart, and I know that "hidden variable" theories are pretty much dead, and that's just something I'll have to accept currently, but I digress.

The best answer that I've found is that "quantum information" is not coupled to the environment. As I understand it, it's once you MAKE a measurement does this "pure" state become "mixed" with the environment. The only way for me to believe this, is to believe that the states DO have some inherent information, but until we actually act on it, we can't say what it is. I don't like it, but that's life. Physics doesn't care if I like something or not, it is what it is.

Now, the last part is the only way I can, personally, come to terms with it. If there is no inherent information, and we act on it, the state has to be ready to show me what it knows, right? Otherwise, it'd be saying the information comes from no where.

I think to explain it to a non-physics audience would be to relate it to cards. You know that if you deal someone cards, it will be of 4 suits, this or that, etc. Similar to particles, we know when dealing with certain particles they will follow these properties. Just like in cards, however, whether the person has a jack, a queen, etc won't be known until we ask. This can be related to quantum mechanics, we won't know the properties of the state of the particle until we make a measurement. I think the key idea for them to understand is that until you act on a state, you won't know. But to say there is no inherent information is just silly to me the more I think about it.

 

[Quandry]

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.

"Absence of evidence is not evidence of absence" - Carl Sagan
We should not confuse no evidence of effect and evidence of no effect.
If there are particles in the universe, then they have properties be they known, unknown, or even unknowable.
If there are no particles until some interaction creates them, then they have no prior properties and so cannot be knowable until after the interaction.
We can only state that particles have properties if they have been experimentally observed.
We can only state that they do not have properties if it has been experimentally observed that they do not.
Given that the latter would appear to be impossible to achieve (given the definition of the issue) we cannot state whether particles have properties or not prior to 'measurement'.

 

[PeterDonis]

If there are particles in the universe, then they have properties be they known, unknown, or even unknowable.

We can only state that particles have properties if they have been experimentally observed.

These two statements are not consistent.

We can only state that they do not have properties if it has been experimentally observed that they do not.

How would you experimentally observe that a particle does not have a property? Or, as you say:

Given that the latter would appear to be impossible to achieve

...then it is meaningless to talk about what we could or could not state if it happened.

 

[Quandry]

These two statements are not consistent.

They are not meant to be consistent. They are two separate statements. One says if there are then they exist. The other says that we can only state what we know.

How would you experimentally observe that a particle does not have a property? .

I was not offering a method doing that, and it seems that no-one has worked it out yet,

Or, as you say
...then it is meaningless to talk about what we could or could not state if it happened.

It is never meaningless to talk about what you do or don't know. In this case we know that we cannot know what the state was before we performed the measurement.

Measurements are the evidence, so the position that particles don't have properties before they're measured is simply sticking to the evidence

Evidence resulting from measurements is not valid evidence of the state before measurement.

 

[bhobba]

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

Yes it is - but also remember the counter example - Bohmian Mechanics. It has exactly those strange properties Dr Chinese talks about. So really you can't win this one - you can simply put up reasonableness arguments.

Thanks
Bill

 

[PeterDonis]

They are not meant to be consistent.

Then which one do you think is right? They can't both be. Or do you think they're both wrong?

 

[PeterDonis]

Evidence resulting from measurements is not valid evidence of the state before measurement.

I didn't say it was.

 

[microsansfil]

Have a look at this article about intrinsic spin.
http://hyperphysics.phy-astr.gsu.edu/hbase/spin.html

Thank. Here an answer of my interrogation : https://physics.stackexchange.com/questions/216216/spin-and-angular-momentum

the Stern-Gerlach experiment shows that spin, like angular momentum, carries a magnetic moment. The conclusion is that the electron's spin is a quantum degree of freedom of the nature of angular momentum that carries a magnetic moment. It characterizes the electron's state independent of its position(or momentum)-dependent wave function, or as you observed, it is intrinsic. The orbital angular momentum, on the other hand, concerns the spatial wave function and is the analog of the classical angular momentum.

Best regards
Patrick

 

[Quandry]

Then which one do you think is right? They can't both be. Or do you think they're both wrong?

The first one cannot be wrong, because it is a statement of if. However, if you mean that in the case that there are protons they could have no properties then that, of course, cannot be true. The very existence defines a need for properties (based on experimental evidence that everything we know that exists has properties).
However, defining a need for properties does not define what these are (although we could make some basic assumptions). To be able to state (empirically) what these properties are, requires that they be experimentally observed. To state empirically that they do not have properties requires that the lack of properties must be experimentally observed.

Both of my statements are true.

The purpose was to encourage the OP to consider that the postulate that "the position that particles have no properties before they’re measured?" assumes evidence that has not been discovered.

 

[Quandry]

I didn't say it was.

Then I misinterpreted your comment

 

[DrChinese]

How would you experimentally observe that a particle does not have a property?

This is a great point in the context of this thread. Clearly, there is no experimental proof that non-commuting properties cannot have simultaneously well-defined values. (You also cannot disprove the existence of God by experiment.)

The "backup plan" for that is the no-go theorems, of which Bell is the best known. Again, we no there are no value sets that fit the predictions of QM. That's as good as it gets.

 

[vanhees71]

Non-commuting observables can have simultaneously determined values, if there is a common eigenvector. An example are the three components of angular momentum $\hat{\vec{J}}$ for $J=0$ (i.e., $\hat{\vec{J}}^2 |J=0,M=0 \rangle =0$). Obviously for this state $\hat{\vec{J}} |J=0,M=0 \rangle=0$.

However, only if the self-adjoint operators representing two variables are commuting, you have a complete orthonormal set of simultaneous eigenvectors and only then we consider these observables as compatible, i.e., for any possible value $a$ of the observable $A$ you can simultaneously also determine the value of observable $B$ to be any possible value $b$. This is, e.g., the case if the system is prepared in the state described by the common eigenvector $|a,b \rangle$ of the operators $\hat{A}$ and $\hat{B}$.

 

[PeroK]

We can only state that they do not have properties if it has been experimentally observed that they do not.
Given that the latter would appear to be impossible to achieve (given the definition of the issue) we cannot state whether particles have properties or not prior to 'measurement'.

In a sense Bell's theorem does provide an experiment to show that certain properties no not exist until they have been measured - by an ingenious use of statistical analysis.

You did say "appear" to be impossible!

 

[itfitmewelltoo]

All too often, people want to jump to the philosophy before having learned the details of experiment, mathematics, and theory. This leads to every manner of absurd statements.

There is an old joke asking the difference between a mathematician and a philosopher. A mathematician needs a pencil, paper, and a waste basket. A philosopher needs only a paper and pencil. (Present company not included, of course).

Feynman has a lecture series in which he does a fairly good job of explaining QM to a general audience.

My sense is that, at a most fundamental level, space and time must be "fuzzy". If it did not, nothing would move, space and time would not exist. If a particle we located at some infinitely specific location in space, it could never then exist in the location at an infitesimal distance away. Somehow, the span of space must connect.

Time, as we understand it, does not exist. It isn't some thing separate from the dimensions of space. In all measures that I am aware of, time is a measurement of distance. We measure time by the occurrence of a periodic event that occurs in some location. The hands of a clock rotate about it's center. A minute is the movement of the second hand about the distance of the circumference. A day is the movement of the Sun from a point in the sky to the same place again as the Earth makes one revolution on it's axis. In every measure of time is a measure of some periodic motion. Time is simply that things have changed. The existence of time as some thing, distinct in it's existence from the objects and the extent that they inhabit is an illusion of how we conceptualize our environment.

Those that presume to philosophize about physics, before having learned the measures and mathematics of physics, also tend towards a certain "absoluteness" in their perception. They see there to be some underlying absolute amd fundamental properties that explain the why of the universe. In the larger body of physics, there are no fundamental properties. Everything in physics is a comparison of one thing to another thing. Physics is a collection of correlations of one thing to something else. At some point, reality is simply irreducible. There are no further underlying properties of hidden variables to further explain "why?".

So, as I see it, at the finest level to which space is, point A is simply not distinguishable from point B. A point A, at x meters cannot exist as distinct and separate from a point B at x+0.000000000001 meters. Heisenberg uncertainty are actually coupled pairs so this is may be oversimplification. Never the less, points A and B cannot be distinct because if this were so, an object at point A could not move to point B. Points in space, A and B, are connected. We know they are connected. We can move from one place to the next. Quantum uncertainty is simply the natural outcome of space having an extent.

That being said, I have a trash can around here somewhere.

 

[Quandry]

Clearly, there is no experimental proof that non-commuting properties cannot have simultaneously well-defined values. (You also cannot disprove the existence of God by experiment.)

I am not sure that this statement is true in all cases (the first part, that is). I am sure that an experiment can be designed to show that any property that requires time as part of its definition cannot be measured without taking time to do so, hence ensuring that simultaneity (dictionary definition) cannot be achieved.

Not disagreeing, just commenting.

 

[PeterDonis]

My sense is that, at a most fundamental level, space and time must be "fuzzy".

This is a common speculation, but we have no evidence one way or the other, and there is no current theory with this property--trying to find such a theory and make it testable is a primary motivation for current research in quantum gravity. In the absence of evidence or theory, there's not much we can discuss about it.