EPR paradox revisited, again. hehehe

[DrChinese]

A_d is a description of reality of A and B_d is a description of B. If when an action is performed on A so that A_d changes, B_d also changes, and if this has been observed to happen every time this action occurs, then I hold that the change in A_d causes the change in B_d. Blase Causality is the principle stating that the causing change should always occur before the resulting change, and always with enough time for the causing change to affect the resulting change. “Enough time” depends upon the mechanism via which the change or causality occurs.

Yes, sure. But a careful review will indicate that using your idea will indicate that causality does not exist - ever. Because only to the extent that you identify what you seek to label as "causal" can that actual setup be causal.

To be specific: A free falling object accelerates in a gravitational field. Except for chairs, cars, houses, etc. They do not appear to move at all. So our "cause" does applies only when there is nothing to make it not apply. Not very convincing, I'm afraid. The same is true of chance events. The world around us consists of a mixture of apparently chance and causal influences. Sometimes we tend to see one, sometimes the other.

The fact that a occurs before b is hardly sufficient to prove a causes b. By logic, I could use the same argument to prove that the future causes the past. I.e. can you prove that b didn't cause a? In the end, you define a as causing b only because you define a as preceding b - unless of course you give us the specific mechanism. That would be a bit difficult: no one actually knows the mechanisms... whether we are taking about relativity or quantum mechanics...

 

[vanesch]

A_d is a description of reality of A and B_d is a description of B. If when an action is performed on A so that A_d changes, B_d also changes, and if this has been observed to happen every time this action occurs, then I hold that the change in A_d causes the change in B_d. Blase Causality is the principle stating that the causing change should always occur before the resulting change, and always with enough time for the causing change to affect the resulting change. “Enough time” depends upon the mechanism via which the change or causality occurs.

In other words, Blase Causality = observed correlation + order in time, right ?

Order in time in *any* reference frame, or in just a specific one ?
(accepting special relativity...)

 

[vanesch]

I don't understand the MWI so I guess I can't really judge. Is it generally accepted that EPR experimental experiments are proof that MWQM is a wholy acceptable theory?

No, EPR experiments are not in general seen as a proof of MWI. I just see it that way, as do some others, and not even as "proof" but as "indication". As I said before, you're not going to be able to prove MWI. It is an *interpretation*.

Perhaps you could point me to an online introductory paper/article on MWI so that I may understand a bit more.

The Wiki article is not bad:
http://en.wikipedia.org/wiki/Many_worlds

There's IMO one serious error in it, namely that Hartle "proved" the Born rule. Nobody proved the Born rule without sneaking it in somehow through the back door (that's my heresy in MWI, but I'm pretty convinced of it: I found the "back door" in all derivations I've seen - this has been discussed here already).

I don't know... I direct you to my reply to Dr Chinese's post [post 102] MWQM presumably has an identified mechanism in EPR experiments which ensures Blase Causality is not violated, I have yet to hear about OQM's such mechanism. Also OQM is generally accepted, is MWQM generally accepted?

OQM has an "advantage", that is that it is used as starting point in about all textbooks (and I find that a good idea too: there's no need - or even possibility - to discuss seriously any interpretational issue in QM before one has learned the formalism). So people who somehow are not much interested in foundational issues (I'd say, the majority!), stick to the "default textbook" version. However, amongst people who ARE interested in foundational issues, I think that MWI has higher rates of popularity.

I direct you to my reply to Dr Chinese's post [post 102]. MWQM presumably has an identified mechanism in EPR experiments which ensures Blase Causality is not violated, I have yet to hear about OQM's such mechanism. Also OQM is generally accepted, is MWQM generally accepted?

None are "generally accepted". But if your question is: is MWI a fringe view, helt up by a few lunatics, then I'd say, no, it is taken seriously. It is in fact easier to take seriously than OQM, which is *inconsistent*.
What is inconsistent in OQM is that there are two different rules for describing a physical process, which lead to different descriptions (though, for all practical purposes, lead to identical empirical predictions... except in special cases!). These rules are 1) the Schroedinger equation and 2) the collapse postulate. According to whether something happening, is called a physical interaction, or a measurement, we should use rule 1 or rule 2, and it is clear that rule 1 is going to give a DIFFERENT result than rule 2. And then, the question arises: is a voltmeter acting on the system, going to be a physical interaction between "voltmeter" and "system", or is it NOT a physical interaction, but a "measurement" ? You'll say, both. Well, then now we have two non-equal state descriptions of the voltmeter, and that's an internal inconsistency.

What are the ways out ? The simplest one is to make a list of what happenings are NOT physical interactions, but measurements. Although this can be done, there seems to be no general principle which makes up this list. Not the number of particles involved (like in superconductivity or other condensed matter phenomena), not the spatial extention of the quantum system (EPR experiments over 50 km)...

The way out of an inconsistent theory is to use your gut feeling of when to use one path of reasoning, and when the other. That's actually what most practitioners of quantum theory do, and the "gut feeling" is usually induced by "it becomes too complicated to calculate explicitly".

The other idea is to say that the wavefunction is some kind of statistical mechanics and that there will be an underlying classical theory explaining all this. As such, we should not be surprised that a measurement is a collapse, because it simply updates our knowledge. Then I'd like to see WHAT underlying theory we're talking about. Claiming vague properties of a non-defined theory is too easy to weasel out. And once you DO specify a specific theory (such as Bohmian mechanics), then you do not have the OQM view either anymore.

 

[jackle]

Bury the hatchet

Leandros,

if anyone else can make sense of your objections and put them forward in a clear manner I shall try and answer them. I'm sorry but I do not have a clue what you are talking about.

I think Leandros is saying that if you tried to set up an EPR type experiment, you would encounter relativistic contraints. You would also have to be extremely careful that any information you thought you were transmitting faster than light was actually something you didn't already have to know in advance and could be controlled in a way that it could be used to send actual messages.

His objections made sense to me on this level and I think you were unfair to him.

jackle no one is talking about determinism here... you are confused. I recommend the philosophy forum.

When I skim-read the last few pages, I noticed that the bottom line here (on many recent threads) is about how to interpret quantum mechanics and I see causality coming up again and again. Something you said earlier made me think that you were simply assuming determinism and this is why I tried to challenge this early on. I still think you need to check your assumptions.

I studied QM at university (rather than primary school!) it is a lot of heavy maths that describe nature. I think you will find that a lot of the issues you are raising are in fact philosophy rather than quantum physics. I would be very happy to discuss this with you in the Philosphy forum. I admit I have forgotten the maths.

We can abandon the insults and have a grown up conversation if you like. I'd also be interested to find out your academic background because I am having trouble "placing" you.

 

[alfredblase]

unless of course you give us the specific mechanism. That would be a bit difficult: no one actually knows the mechanisms... whether we are taking about relativity or quantum mechanics...

In other words, Blase Causality = observed correlation + order in time, right ?

Order in time in *any* reference frame, or in just a specific one ?
(accepting special relativity...)

well I always assumed that such mechanisms were identified in all theories,,, well anyway I’ll try to answer both your posts [drchinese and vanesch] with an example of Blase Causality,,

the discussions taking place in this thread and in the “yet another cat question” thread are in my view important and my ideas on the issues raised in these threads are begging to coalesce; I think they answer quite a few unanswered questions,,, I have never claimed that QM is not a completely truthful theory, I have been merely struggling to understand how it is generally accepted when there are so many unanswered, important questions,,

I am going to attempt to write a paper that I hope will express my ideas,,,

I will not continue big discussions on these subjects via this forum until I at least make an attempt to write the paper as it is not a very efficient way,, this forum is titled “Physics Help and Math Help” not “express your new ideas” after all :P ,,, if you are still interested in the discussions taking place and what I may have to say on the subjects, I ask you to wait until my “paper” is written,,, I’ll let you know, xD but you probably have more important things to worry about, hehe; thanks for your attention and patience so far, you have all helped to clarify my ideas about QM, and I have enjoyed the discussions very much, and look forward to continuing them =)

 

[alfredblase]

I think Leandros is saying that if you tried to set up an EPR type experiment, you would encounter relativistic contraints. You would also have to be extremely careful that any information you thought you were transmitting faster than light was actually something you didn't already have to know in advance and could be controlled in a way that it could be used to send actual messages.

His objections made sense to me on this level and I think you were unfair to him.



When I skim-read the last few pages, I noticed that the bottom line here (on many recent threads) is about how to interpret quantum mechanics and I see causality coming up again and again. Something you said earlier made me think that you were simply assuming determinism and this is why I tried to challenge this early on. I still think you need to check your assumptions.

My arguments where no different to EPR arguments, that have been made before by infinately better scientists than myself, I just modified them very slightly in order to take the issue to causality,, I am sorry about being rude, I know I have been, but sometimes I suffer from complete lack of patience :P I'm sure we all agree that what matters in PF is really the physics, not whether I'm nice no? :P heh

I studied QM at university (rather than primary school!) it is a lot of heavy maths that describe nature.

I'd also be interested to find out your academic background because I am having trouble "placing" you.

Our backgrounds are similar then: I am a recent MPhys graduate [2:1] from good uni =) but I honestly believe my background shouldn't matter, one of the great things about PF is that if your posts are sound enough then you can discuss things with some great physicists [not counting myself as such a physicist of course] even if you are a layman,,

If you want to hear my arguments properly watch this space [see my previous post] =)

 

[DrChinese]

if you are still interested in the discussions taking place and what I may have to say on the subjects, I ask you to wait until my “paper” is written,,, I’ll let you know

Looking forward to seeing more...

 

[jackle]

I'm sure we all agree that what matters in PF is really the physics, not whether I'm nice no?

Yeah it is.

If you want to hear my arguments properly watch this space...

OK

 

[Sherlock]

To "go epistemic" as a way of eluding the charge of nonlocality is *not* to defend the locality of one's theory. It's to remove one's theory from the class of things to which concepts like locality/nonlocality are applicable.

I agree. I've thought of this differently in the past, but now I think that concepts like locality-nonlocality don't apply to quantum theory.

The orthodox interpretation is about what the theory is, not what it might be. The theory is a mathematical scheme that assigns probabilities to qualitative instrumental behavior. Attributing some speculative significance (in terms of a correspondence to an underlying quantum world) to the qm algorithm or any part thereof is beyond the scope of the theory itself (and apparently beyond the scope of physics, at least for the foreseeable future).

What about Bohmian Mechanics? You can't just arbitrarily say it's "beyond the scope of physics for the foreseeable future" when there already exists an empirically viable theory that does precisely this.

It's beyond the scope of physics to determine that theory A more closely approximates an underlying quantum world than theory B. Underlying quantum world is undefined --- its meaning is as ambiguous as there are metaphysical visions of it.

If a theory is something that provides an account of the state of the quantum system, then no Bell Local theory can agree with experiment. Of course someone can refuse to put forth a theory, or can put forth a calculation recipe that they *call* a theory but which is not a theory in the sense I've just defined it. That doesn't magically count as a "local theory" though. It's just a calculation recipe, and doesn't effect one whit the two part argument. No theory can be Bell Local and still agree with experiment.

What we're talking about is not all the possible ways of avoiding making a certain kind of false statement. What we're talking about is whether one can have a *theory* that respects Bell Locality and still agrees with experiment. I claim Bell proved (with the 2 part argument) that we can't. And you don't refute this proof by pointing out that there are other things one could utter (things which aren't theories) which "don't imply nonlocal causality." All sorts of things don't imply it -- by virtue of their not making any causal claims in the first place.

I agree that Bell proved that we can't have an empirically viable, locally causal description of underlying reality. But, this has to do with the issue of what can be experimentally determined, rather than locality-nonlocality.

But citing a bunch of such things isn't a good strategy for refuting Bell's argument ...

I think that's true. I also think it's true that Bell's argument doesn't require the conclusion that nonlocality is a fact of nature. The only facts of nature that exist are our qualitative sensory perceptions of it (the material-instrumental output).

OQM quantitatively organizes these facts in a certain way. BM does it in a somewhat different (maybe a clearer and more succinct) way.

There's presently no physical (factual) basis wrt which one can argue that one approach more closely corresponds to the composition and behavior of an underlying reality than the other.

 

[ttn]

It's beyond the scope of physics to determine that theory A more closely approximates an underlying quantum world than theory B. Underlying quantum world is undefined --- its meaning is as ambiguous as there are metaphysical visions of it.

Can you give any reasons why this should be the case for quantum theory, but not for (say) every other apparently-similar situation in the history of science? How come it wasn't "beyond the scope of science" to discover an underlying causal mechanism for Kepler's laws? Or for the macroscopic laws of classical thermodynamics? Or for Mendel's laws of genetic inheritance? and so on and so on...

There's presently no physical (factual) basis wrt which one can argue that one approach more closely corresponds to the composition and behavior of an underlying reality than the other.

I agree (at least partially), but only if you mean this as an assessment of the current state of evidence. I'd be the first to admit that we don't know today that Bohmian Mechanics is true. There are too many unanswered questions, and too many (meaning, at least one) alternative that seems also to be logically coherent and empirically viable. (The one I'm thinking of is the GRW type theories.)

But if (as you seemed to be hinting before) you think this is a deep metaphysical barrier that cannot be crossed, I disagree entirely. Yes, there is not yet sufficient evidence to know which picture of quantum reality is the correct one; but there is nothing whatever to suggest that quantum reality doesn't exist or can never be understood or anything like that. Any such suggestion is pure anti-scientific philosophical nonsense.

Note, by the way, that I see the issue as whether (a) Bohmian mechanics is right, (b) GRW is right, or (c) something not yet known turns out to be right. Orthodox QM *cannot be right*. One doesn't have to know the right theory to know that some proposals are fatally flawed.

 

[Sherlock]

It's beyond the scope of physics to determine that theory A more closely approximates an underlying quantum world than theory B. Underlying quantum world is undefined --- its meaning is as ambiguous as there are metaphysical visions of it.

Can you give any reasons why this should be the case for quantum theory, but not for (say) every other apparently-similar situation in the history of science? How come it wasn't "beyond the scope of science" to discover an underlying causal mechanism for Kepler's laws? Or for the macroscopic laws of classical thermodynamics? Or for Mendel's laws of genetic inheritance? and so on and so on...

These are higher order, more complex (than quantum) phenomena that have been explained in terms of micro, meso, and macroscopic phenomena and organizing principles.

The underlying quantum world remains qualitatively undefined --- (at least for the foreseeable future).

There's presently no physical (factual) basis wrt which one can argue that one approach more closely corresponds to the composition and behavior of an underlying reality than the other.

I agree (at least partially), but only if you mean this as an assessment of the current state of evidence. I'd be the first to admit that we don't know today that Bohmian Mechanics is true. There are too many unanswered questions, and too many (meaning, at least one) alternative that seems also to be logically coherent and empirically viable. (The one I'm thinking of is the GRW type theories.)

Yes, that's what I mean --- an assessment of the current state of evidence.

But if (as you seemed to be hinting before) you think this is a deep metaphysical barrier that cannot be crossed, I disagree entirely. Yes, there is not yet sufficient evidence to know which picture of quantum reality is the correct one; but there is nothing whatever to suggest that quantum reality doesn't exist or can never be understood or anything like that. Any such suggestion is pure anti-scientific philosophical nonsense.

I agree. It isn't known if the current evidentiary barrier can or cannot ever be crossed. I think the evidence suggests that there is an underlying quantum world. There's just not enough of it to form a complete picture.

Note, by the way, that I see the issue as whether (a) Bohmian mechanics is right, (b) GRW is right, or (c) something not yet known turns out to be right. Orthodox QM *cannot be right*. One doesn't have to know the right theory to know that some proposals are fatally flawed.

I don't think enough is known yet to say that the issue is whether (a), (b) or (c) is "right". Hopefully, that will become the issue. For now, OQM is the conventional "workaround".

It's like the situation with light. The evidence suggests a medium, but it isn't known how to detect it or even if it is in any sense detectable. But there's SR.

Some aspects of quantum theory might be "right", but, taken as a whole, and wrt the orthodox interpretation (and the current state of the evidence) it doesn't make much sense to say that it is about an underlying reality.

Quantum theory is, if nothing else, about what can be experimentally determined, and if its principles wrt what can be experimentally determined are correct, then the "rightness" of (a), (b) or (c) isn't the issue.

But, for the foreseeable future, this is all a matter of speculation.

 

[vanesch]

Some aspects of quantum theory might be "right", but, taken as a whole, and wrt the orthodox interpretation (and the current state of the evidence) it doesn't make much sense to say that it is about an underlying reality.

Quantum theory is, if nothing else, about what can be experimentally determined, and if its principles wrt what can be experimentally determined are correct, then the "rightness" of (a), (b) or (c) isn't the issue.

But, for the foreseeable future, this is all a matter of speculation.

Take the spirit of what you say about theory (T) - here, quantum theory - as S(T), that is: "Some aspects of theory T might be right, but taken as a whole ..."

Now, take the time t on the history and future of humanity, and call T(t) the "most fundamental" theory known at moment t. Don't you think, that for ALL future and past t, we can make the statement S(T(t)) ?

 

[Sherlock]

Take the spirit of what you say about theory (T) - here, quantum theory - as S(T), that is: "Some aspects of theory T might be right, but taken as a whole ..."

Now, take the time t on the history and future of humanity, and call T(t) the "most fundamental" theory known at moment t. Don't you think, that for ALL future and past t, we can make the statement S(T(t)) ?

I don't know what you're trying to say. Would you elaborate please ?

 

[vanesch]

I don't know what you're trying to say. Would you elaborate please ?

I meant, doesn't what I quoted in post number 117 apply at all times to the "at that time most fundamental, known, theory" ?
Meaning, you could say that about Newtonian mechanics in the 18th century, you could say it about Maxwellian electrodynamics at the end of the 19th century, you say it now about 20th century quantum theory... and probably you can say it too about just any next theory that will come along.

That said theory will have some aspects "right", but that it doesn't make much sense to say that it is about an underlying reality. And that said theory is, if nothing else, about what can be experimentally determined, and if its principles wrt what can be experimentally determined are correct, then the rightness of any interpretation or view on it isn't the issue.

 

[alfredblase]

but important questions remain to be answered,,,

previously I gave a definition of causality but then I realized that to be able to apply that definition, i had to identify what mechanism collapses a superimposed description,, decoherence seems to be a good candidate for this but then I came up with a scenario which I think refutes the idea,,, regardless of this point I'm having a great deal of trouble identifying the collapse condition in a double slit quantum eraser experiment,,, [walborn, cunha, padua, monken 2002]

 

[Sherlock]

I meant, doesn't what I quoted in post number 117 apply at all times to the "at that time most fundamental, known, theory" ?
Meaning, you could say that about Newtonian mechanics in the 18th century, you could say it about Maxwellian electrodynamics at the end of the 19th century, you say it now about 20th century quantum theory... and probably you can say it too about just any next theory that will come along.

Without getting into a protracted discussion about it ... yes. The point being that physics, the enterprise of quantitatively accounting for the phenomena that nature presents to our senses, isn't a 1-1 mapping of physical reality --- and this is especially true of whatever reality underlies quantum experimental phenomena.

There's just no evidentiary basis wrt which we should expect that (a), (b) or (c) can ever be evaluated as being 'right' (provided they're all generating the same experimental predictions).

On other grounds, not necessarily purely esthetic, there are of course cases to be made for preferring, say, Bohmian Mechanics or MWI or whatever.

In the words of Werner Heisenberg:
"Every description of phenomena, of experiments and their results, rests upon language as the only means of communication. The words of this language represent the concepts of daily life, which in the scientific language of physics may be refined to the concepts of classical physics. These concepts are the only tools for an unambiguous communication about events, about the setting up of experiments and about their results. If therefore the atomic physicist is asked to give a description of what really happens in his experiments, the words 'description' and 'really' and 'happens' can only refer to the concepts of daily life or of classical physics. As soon as the physicist gave up this basis he would lose the means of unambiguous communication and could not continue in his science. Therefore, any statement about what has 'actually happened' is a statement in terms of the classical concepts and -- because of thermodynamics and of the uncertainty relations -- by its very nature incomplete with respect to the details of the atomic events involved. The demand to 'describe what happens' in the quantum-theoretical process between two successive observations is a contradiction in adjecto, since the word 'describe' refers to the use of the classical concepts, while these concepts cannot be applied in the space between the observations; they can only be applied at the points of observation."

 

[ttn]

There's just no evidentiary basis wrt which we should expect that (a), (b) or (c) can ever be evaluated as being 'right' (provided they're all generating the same experimental predictions).

I think the whole point here is to distinguish the present tense from the future. It may be that, for all we can tell today, the different theories make the same empirical predictions. That's (as you say) exactly what makes it hard to decide which one is right. But if history is any indicator, the theories won't continue to be empirically equivalent forever. This is just the natural order of things in science. The geocentric and heliocentric theories of the solar system made identical empirical predictions for what could be observed up to the time of Copernicus, which is precisely why (well, part of why) the debate between the two camps was so vigorous. But then eventually along came Galileo and his telescope (and then even more stuff later) which settled the matter empirically. Same story over and over again in science... to the extent that I think one would have to be crazy to claim that the current debates about QM are somehow fundamentally different.

On other grounds, not necessarily purely esthetic, there are of course cases to be made for preferring, say, Bohmian Mechanics or MWI or whatever.

There are some other grounds, yes, but I don't think it's quite right to call them "purely esthetic". For example, that OQM suffers from the measurement problem is not a mere esthetic issue. As Bell said, this means that OQM is "unprofessionally vague and ambiguous" -- which is surely worse than its merely having equations that "don't look pretty" or whatever.

In the words of Werner Heisenberg:
"Every description of phenomena, of experiments and their results, rests upon language as the only means of communication. The words of this language represent the concepts of daily life, which in the scientific language of physics may be refined to the concepts of classical physics. These concepts are the only tools for an unambiguous communication about events, about the setting up of experiments and about their results. If therefore the atomic physicist is asked to give a description of what really happens in his experiments, the words 'description' and 'really' and 'happens' can only refer to the concepts of daily life or of classical physics. As soon as the physicist gave up this basis he would lose the means of unambiguous communication and could not continue in his science. Therefore, any statement about what has 'actually happened' is a statement in terms of the classical concepts and -- because of thermodynamics and of the uncertainty relations -- by its very nature incomplete with respect to the details of the atomic events involved. The demand to 'describe what happens' in the quantum-theoretical process between two successive observations is a contradiction in adjecto, since the word 'describe' refers to the use of the classical concepts, while these concepts cannot be applied in the space between the observations; they can only be applied at the points of observation."

Another nice feature of Bohmian Mechanics is that it provides one with a beautifully clear counterexample to this kind of philosophical crap.

 

[alfredblase]

hahaha lol

I liked the style of your last sentence, very funny ttn =)

 

[Sherlock]

I think the whole point here is to distinguish the present tense from the future.

Point taken.

... if history is any indicator, the theories won't continue to be empirically equivalent forever.

Changes will be precipitated by experimental phenomena that current theories can't handle. It would be surprising if, say, BM could quantitatively deal with something that OQM couldn't ... wouldn't it? BM isn't predicting anything new, is it ?

There are some other grounds, yes, but I don't think it's quite right to call them "purely esthetic".

I agree, and I didn't. That's what the "not necessarily" in front of "purely esthetic" means.

Another nice feature of Bohmian Mechanics is that it provides one with a beautifully clear counterexample to this kind of philosophical crap.

I wouldn't call observations on, and requirements for, unambiguous statements about the world "philosophical crap". BM might, for all we know, be philosophical crap, but Heisenberg's statement about unambiguous communication of experimental preparations and results isn't. There has to be some qualitative, sensory apprehension of what is being counted --- and it's precisely a qualitative, sensory apprehension of an underlying reality that we don't have.

It is interesting that any empirically viable realistic theory has to be nonlocal. Whether this is a constraint due to intrusive measurements or whether it's due to nonlocality being a fact of nature is still a question in my mind.

Are there any situations in which non-intrusive quantum measurements are made? And, if so, do they shed any light, so to speak, on the problem ?

I'm still in the process of rereading both of your papers to make sure I'm understanding any assumptions involved in your (Bell's) two-part argument.

Every once in a while I get this gut-wrenching feeling that I'm never going to fully understand it. But, it's probably that I just don't know enough yet. So, there's hope.

 

[ttn]

Changes will be precipitated by experimental phenomena that current theories can't handle. It would be surprising if, say, BM could quantitatively deal with something that OQM couldn't ... wouldn't it? BM isn't predicting anything new, is it ?

I doesn't really predict anything new, but that doesn't mean that different theories won't be more or less able to handle new phenomena in the future. Just to give one trivial example, suppose we find some kind of exotic situation in which measurement outcomes don't obey the Born rule. This is easily accounted for in Bohm's theory by saying that we are out of quantum equilibrium. Maybe not so easy to deal with in OQM, since there measurement (and the associated Born rule) is a fundamental law of nature.

I wouldn't call observations on, and requirements for, unambiguous statements about the world "philosophical crap". BM might, for all we know, be philosophical crap, but Heisenberg's statement about unambiguous communication of experimental preparations and results isn't. There has to be some qualitative, sensory apprehension of what is being counted --- and it's precisely a qualitative, sensory apprehension of an underlying reality that we don't have.

But I think Heisenberg's point was that, because of these limitations that are built into the structure of language, it's *impossible*, in principle, to ever give a detailed story for what happens between observations. He argues that it would be a contradiction in terms to imagine such a story. And yet Bohm's theory provides one. (Not that this proves Bohm's theory is true; it just proves that fuzzy philosophical arguments about language and such are often far less rigorous than they appear.)

Are there any situations in which non-intrusive quantum measurements are made? And, if so, do they shed any light, so to speak, on the problem ?

Einstein once said (and Bell repeated, stressed, and clarified) that it's the theory which decides what can and can't be measured. For example, according to Bohm's theory, any time you measure a particle's position, the measurement is non-intrusive in the sense that your measurement outcome simply reveals the pre-existing position. Not so, in Bohm's theory, for some other things such as momentum or (in a different way) spin. But a different theory like OQM says that *none* of these are non-intrusive measurements. So... take a particle prepared in a certain way (and hence with a known wf). Now measure its position. Was that measurement "non-intrusive"? Depends on which theory you believe.