I The typical and the exceptional in physics

[secur]

Eh? This is supposed to be the sharp pointy thing that cuts through the Gordian knot?

it automatically ends a common type of endless debate.

I fail to see how the rather obvious and trivial assertion that we can never prove anything to be real ...

Something so obvious is impossible to "prove".

... solves anything as far as the epi/onto problem in foundations physics is concerned.

This paradigm shift can never reveal any new scientific facts.

So given the assumption that there is an external reality independent of our senses and consciousness do we have to take an ontic or epistemic view of the quantum state, according to the structure of QM? That's the issue - ...

That is the question:

... and it isn't in any sense resolved by merely saying the initial assumption is unprovable or false.

This paradigm shift can never reveal any new scientific facts.

 

[Simon Phoenix]

@secur

Lol - I can't work out whether you're agreeing with me that Ken G's position is utterly irrelevant to the ontic/epistemic debate in the context of quantum foundations, and solves precisely nothing, or conversely whether you think it renders the whole foundations debate irrelevant, as Ken G appears to suggest.

I think on a physics forum we probably shouldn't spend too much time debating the fruitless philosophical frippery of whether there is an objective reality that exists independent of our senses (I wouldn't really describe this amusing diversion that we all went through sometime before high school as a 'paradigm' shift though).

I don't want this thread to get closed down because it's been pretty cool so far. So I'll shut up (and calculate)

 

[andrew s 1905]

How many ontic angels can dance on the head of a epistemic pin ?

Regards Andrew

 

[ddd123]

I think on a physics forum we probably shouldn't spend too much time debating the fruitless philosophical frippery of whether there is an objective reality that exists independent of our senses (I wouldn't really describe this amusing diversion that we all went through sometime before high school as a 'paradigm' shift though).

It's because it's not quite like this. I think few would take on the position that there is nothingness apart from our senses. The point is that being beyond the senses is inaccessible - it's different. The usual idea people have is this:

But it would be preposterous to say that the stuff outside the head is like that: the stuff inside is a representation extrapolated from the senses, that has evolved for millions of years to fulfill certain survival functions. I think
a) it's not hard to understand;
b) it's not trivial (not a high school existential moment, it's serious business);
c) it may be relevant at some point in the history of science.

 

[secur]

Lol - I can't work out whether you're agreeing with me that Ken G's position is utterly irrelevant to the ontic/epistemic debate in the context of quantum foundations, and solves precisely nothing, or conversely whether you think it renders the whole foundations debate irrelevant, as Ken G appears to suggest.

Well, I have said twice before in this thread that it does NOT solve the ontic/epi debate. Ken G does, indeed, seem to think otherwise. But it's not "utterly irrelevant" - it clears away some of the underbrush. This "trivial" point can extremely get in the way when discussing ontological issues more directly related to physics, because it's not accepted by everyone.

How many ontic angels can dance on the head of a epistemic pin ?

Case in point :-)

I think on a physics forum we probably shouldn't spend too much time debating [philosophy] ...

That's right. But half of this thread, and about half of PF and foundational physics in general, does exactly that. People keep getting sidetracked by this and a couple other equally obvious philosophical facts. BTW I didn't bring it up and have been avoiding it. As my kids used to say "It wasn't me! He did it."

... the fruitless philosophical frippery of whether there is an objective reality that exists independent of our senses (I wouldn't really describe this amusing diversion that we all went through sometime before high school as a 'paradigm' shift though).

You don't get it. You think you do - that's half the battle, I suppose. It's not a dormitory diversion but a fundamental fact of existence which can't - I mean, shouldn't - be forgotten in serious contexts such as foundational physics.

I don't want this thread to get closed down because it's been pretty cool so far.

Yes, let's drop it before Dale takes notice :-) And yet if ever there was a place that would benefit from understanding it, this is that place.

Re. ontic/epi issue, I'm ontic. Still working out how to defend my stance. Believe it or not there are some new things to say about it.

 

[A. Neumaier]

How many ontic angels can dance on the head of a epistemic pin ?

http://arnold-neumaier.at/physfaq/topics/needle

 

[Simon Phoenix]

You don't get it. You think you do

Well both you and ddd123 have now suggested that the fact that our brain constructs a representation of 'reality' which may, or may not, have some substantial overlap with 'reality' (assuming we have taken the blue pill, or was it the red pill?) - is some tremendously important fact. I think it's kind of obvious and of complete irrelevance to physics (or pretty much anything else, for that matter).

So what am I not getting?

 

[ddd123]

Well both you and ddd123 have now suggested that the fact that our brain constructs a representation of 'reality' which may, or may not, have some substantial overlap with 'reality' (assuming we have taken the blue pill, or was it the red pill?) - is some tremendously important fact. I think it's kind of obvious and of complete irrelevance to physics (or pretty much anything else, for that matter).

Tell that to the foundations guys

 

[ddd123]

To think of it, you are among them! You have said the exact opposite just a little earlier:

So given the assumption that there is an external reality independent of our senses and consciousness do we have to take an ontic or epistemic view of the quantum state, according to the structure of QM? That's the issue

So is it an issue, or is it completely irrelevant?

 

[Simon Phoenix]

So is it an issue, or is it completely irrelevant?

The issue, as I see it anyway, isn't anything at all about how our conscious brains perceive reality, or construct that reality from our sensory inputs, or whether there is such a thing as an external reality (you could just be some part of the computer simulation that is responsible for what I perceive as consciousness, and so on, and so forth). These are all red herrings.

Let us assume there is such a thing as an external reality and that our consciousness constructs a reasonably faithful and consistent representation of that for us. Since we can't really say one way or the other we may as well assume the simplest explanation. If we wanted to be more sophisticated we could couch everything in terms of perceptions of reality - but then all we'd be doing is to have to add a zillion qualifiers to everything when we talked about reality and our models of it - and we wouldn't have changed the basic problem one iota - just nested it within a quagmire of qualification.

We can all agree, I think, that the mathematical models we write down, the squiggles on the paper are not 'reality' themselves, but are (at best) merely representations of some underlying reality. The underlying reality doesn't change if we use a different notation, for example. This is the ontic view. When we describe a particle with a p and a q we are saying that the particle has some position and some momentum which we label with the symbols p and q. The concepts of position and momentum have some objective meaning and are not just mental constructs. Certainly the transfer of momentum of a golf ball as it hits our heads is far from being simply a mental construct - the particle has objective properties (we use mental constructs to reason about those properties but the properties themselves are more than merely mental constructs). This is the ontic view.

Yes in some sense momentum is a 'mental construct' but it maps onto a real property in the real world in the ontic view - and I would describe classical physics as ontic in this sense.

The question is whether the quantum state also maps onto some real property of the real world - is it describing some objective reality, or is it just a convenient mathematical tool to describe what we can know about something?

This issue is not resolved or sidestepped or brushed away if we adopt some notion of perceived reality constructed by our consciousness. We'd just have to qualify everything in terms of a model of the perceived reality (perceived ontic) or what we can perceive we can know about this perceived reality (perceived epistemic). So in my view trying to emphasize that we can never know whether there is an external reality and, if there is, whether our evolved consciousness constructs a faithful representation of that for us, does not help us in any way whatsoever with the ontic/epistemic debate.

 

[ddd123]

how our conscious brains perceive reality, or construct that reality from our sensory inputs

The concepts of position and momentum have some objective meaning and are not just mental constructs. [...] Yes in some sense momentum is a 'mental construct' but it maps onto a real property in the real world in the ontic view - and I would describe classical physics as ontic in this sense. [...] The question is whether the quantum state also maps onto some real property of the real world - is it describing some objective reality, or is it just a convenient mathematical tool to describe what we can know about something?

I think these two quotes address essentially the same issue. Don't we perceive position, in the sense that that's what the brain does?

We can all agree, I think, that the mathematical models we write down, the squiggles on the paper are not 'reality' themselves, but are (at best) merely representations of some underlying reality. The underlying reality doesn't change if we use a different notation, for example. This is the ontic view.

I think I'm lost here. The epistemic view holds that the underlying reality changes instead?

This issue is not resolved or sidestepped or brushed away if we adopt some notion of perceived reality constructed by our consciousness. We'd just have to qualify everything in terms of a model of the perceived reality (perceived ontic) or what we can perceive we can know about this perceived reality (perceived epistemic). So in my view trying to emphasize that we can never know whether there is an external reality and, if there is, whether our evolved consciousness constructs a faithful representation of that for us, does not help us in any way whatsoever with the ontic/epistemic debate.

I'm lost again. Haven't you simply redefined the problem, and now you are asserting that THAT is what we care about, eventually perceived ontic vs perceived epistemic and not just simply ontic vs epistemic? So of course if that's the nature of the problem, and I wasn't informed, then it is a red herring, but it seems to me even more "irrelevant" than the actual ontic vs epistemic debate: why would we care about such a thing?

 

[Simon Phoenix]

why would we care about such a thing?

That's a very good question :-)

Since we get the same results in QM if we adopt an ontic picture or an epistemic picture then many people would suggest that it doesn't really matter.

I tend to think of the quantum state as a real thing and think of measurements projecting into a new state - so broadly speaking a collapse picture. That's just the view that helps me get the right answers in calculations. Is it what's actually happening? Buggered if I know.

I would like to have a clearer 'physical' picture of what's 'really' happening - but that's more of an emotional response on my part than any rational requirement. I just see part of the job of physics as explaining the 'why' as much as it can - as well as the 'how'. We need to be able to predict stuff because we want to test our ideas of the why and how as best we can.

 

[ddd123]

In any case, when physicists say, to use exactly your own words:

"momentum is a 'mental construct' but it maps onto a real property in the real world"

or

"no, momentum is only a 'mental construct' and it doesn't map onto a real property in the real world",

they are talking exactly about the issue I was talking about above, with the colorful cartoon I posted. Or if they aren't, how can I tell?

 

[ddd123]

IMHO it seems that exactly the same problem, for you, is once irrelevant for physics (and for everything else!) and another an interesting issue, depending on whether you put on it the philosophy badge or the foundations badge respectively.

 

[Simon Phoenix]

"no, momentum is only a 'mental construct' and it doesn't map onto a real property in the real world",

I think this is all part of the dissociative personality disorder induced by QM. I don't think this kind of thinking would have made any sense before the advent of QM - and as has been mentioned I believe there has been some attempt to redefine what 'science' is about because of the difficulties introduced by QM. Some in this thread have posited that this is a more correct view of science, but I am not convinced.

I am not aware of any textbook dealing with classical physics that would not implicitly (or explicitly) adopt an ontic view. There may be some I suppose, but in my experience classical physics is almost never introduced, taught or thought about as an epistemic theory (that is a theory that describes what we know, rather than a theory that describes some reality). Maybe I've just read the wrong stuff :-)

I have never read any description of a (purely) classical process that does not talk about position, momentum, energy, electric field, etc as if they were anything but descriptive of something objectively real. Again - this doesn't mean that epistemic descriptions don't exist - but I think it would be fair to say that from the perspective of classical physics it would be a bit weird to say that momentum doesn't map onto some real world property.

But clearly, when QM came along, something went disastrously wrong with this way of looking at things - and this I think is why there were so many struggles to get to grips with it in the early days. This disconnect with previous classical ways of thinking is beautifully illustrated by Dirac in the introduction to his textbook (which I think should be required reading for anyone studying QM). He describes the problem of figuring out specific heat capacities for atoms. Assuming there were classical variables which contributed, as any classical degree of freedom would, to the specific heat capacity gave the wrong answer. So it was clear from the early days that something different than 'classical variables' was required. In one way we can see this as a very persuasive early argument against the existence of classical-like hidden variables.

 

[ddd123]

So you simply disagree with this foundations debate, the epistemic view within foundations is disastrous, and the interesting aspect is how to get back to the ontic?

That's different, though, than calling my explanation completely irrelevant to physics or to anything else.

 

[Simon Phoenix]

So you simply disagree with this foundations debate, the epistemic view within foundations is disastrous, and the interesting aspect is how to get back to the ontic?

Well if I've given you these impressions from what I've written - then I apologise.

I think the foundations debate is interesting - is the wavefunction describing something real, or is it just a device to describe our knowledge? I think if this question could be answered definitively then it would be a wonderful thing (either way). I would certainly prefer it if the answer came out on the ontic side - but either way it would represent a very significant advance.

At the moment we can't tell - the predictions from all of the various interpretations/formulations of QM are the same. It would be nice to have some experimental way to rule out some of these.

I just don't see how delving into an appreciation that our consciousness constructs a perception of reality for us helps us to progress towards an answer to these questions. If I've missed the point of your argument I apologize and would appreciate being corrected.

 

[Nugatory]

I don't want this thread to get closed down because it's been pretty cool so far.

Speaking for myself here... I don't worry overmuch about what consenting adults are doing in the privacy of the 20th page of an interpretations thread. Just be sure that the children aren't watching.

 

[ddd123]

I just don't see how delving into an appreciation that our consciousness constructs a perception of reality for us helps us to progress towards an answer to these questions. If I've missed the point of your argument I apologize and would appreciate being corrected.

No problem, I don't say it helps us progress, I say it's just a rephrasing of the same question. From our perception of reality (i.e. mental construct) comes the concept of point in space for example, and thus classical mechanics is just taking that on. Along these lines, even if we construct a completely counterintuitive "ontic" model (say, holographic principle) and say THAT is what is outside our heads, it simply means that our mental constructs have refined themselves enough to somewhat depart from the old intuition, but we are still debating on: our mental constructs have a correspondence outside or not. Which is exactly the foundations debate we're talking about.

 

[Simon Phoenix]

Just be sure that the children aren't watching.

lol

whenever my kids were playing up I'd threaten them with a lecture on interpretations of QM - worked a treat :-)

 

[Simon Phoenix]

our mental constructs have a correspondence outside or not. Which is exactly the foundations debate we're talking about.

OK - that's a nice way of putting it. My issue with going into detail about perceptions of reality and so forth is that we end up talking about mental constructs of mental constructs (our mentally constructed models of a perceived reality which is itself a mental construct) and so we end up with this tortuous nesting of mental constructs.

So I like to keep things simpler and just assume that there is an external reality, that our mental construct of that reality, derived from sensory inputs, is pretty faithful. So the question then becomes one of whether the models we adopt to explain things have a mapping to some external reality or whether those models merely describe what we can know about that reality.

Although I would prefer an ontic answer and despite the fact that when doing QM I think in ontic and collapse terms I actually can't help but think that the epistemic approach is actually more logically cohesive. Problem is I find it more difficult to actually do calculations if I try to think in terms of epistemology

 

[atyy]

Although I would prefer an ontic answer and despite the fact that when doing QM I think in ontic and collapse terms I actually can't help but think that the epistemic approach is actually more logically cohesive. Problem is I find it more difficult to actually do calculations if I try to think in terms of epistemology

That is the beauty of Copenhagen. Ontology is a means to epistemology.

 

[atyy]

In this sense, of course the Heisenberg cut is there. The only thing I see no justification for is to claim that on a fundamental level there is a "quantum world" and a "classical world" governed by different dynamical laws. I think the classical behavior of macrocopic objects under usual conditions is a phenomenon that can be understood from QT, using the standard ("coarse-graining") techniques of many-body theory.

There is no absolute cut. The cut is subjective and observer dependent. In the standard interpretation, anything can be quantum, but not everything can be quantum.

However, I sense that you believe that in principle there is a quantum state of the universe, including the observer, and the measuring apparatus can be obtained by coarse graining. This is not the standard or minimal interpretation. It is true that classical mechanics is a limit of quantum mechanics. However, that does not mean we can do away with postulating a a "classical world" on one side of the cut that is in fact more fundamental than the "quantum world" on the other side of the cut.

 

[RockyMarciano]

The way I usually try to frame this ontic/epistemic duality so it doesn't lead me to a stalemate is by trying to concentrate on the physics, and to me this means observations, specifically observations that can be translated into physical measurements.
Now if you look at it from the classical mindframe(wich we all tend to do by habit even after the quantum revolution since the formalism has been basically inherited from classical physics-Hamitonians, linearity, Hilbert spaces from symplectic,...etc- and the observables are classical), and since classically there was a clear cut between measurement and measured system, we tend to hang on intuitively to the separation between the ontic and the epistemic that reflects that cut.

But if you concentrate on the quantum phenomenology and let go of the inertia that holds you back to the classical thinking, you can dissolve the antinomy because it really is not relevant if all you care about is measurements. You are led to a natural fusion. Noone will deny their existence("onticity") since they are all we have in physics in the end and they can also be regarded as purely "informational" or epistemic.

Of course the problem here is we don't yet have a quantum measurements-only theory in this sense, we have an operationally efficient theory for predicting quantum measurements probabisitically that ironically leaves measurements out of the mathematical formalism.
This already happened in classical physics, where the phenomenology didn't require to fuse measurement and system measured, they were independent, but it is a problem with the quantum phenomenology where it is very difficult to maintain that measurement and measured system are independent. If the formalism doesn't reflect this dependency that actually contradicts its mathematical premises there are going to be interpretational issues for sure.

From this point of view it is easy for me to be understanding both with those that don't see any problem in quantum foundations or any measurement problem whatsoever(i.e. vanhees71 or Neumaier) and those that really feel there are deep problems(i.e. stevendaryl and others). It all depends on how attached(most likely unaware of it) you are to the classical perspective.

 

[secur]

1. Subjective Reality Principle

... our senses are turned by our brain into intelligible experience as a representation, which not necessarily (and most likely not) corresponds to "what is out there".

Since all our information comes very indirectly through the senses, we don't know what's out there (if anything) and never will.

I take it we all agree on this fundamental fact: "Reality" is (when you get right down to it) subjective. This principle needs a name, how about "Subjective Reality Principle" (SRP).

The way I usually try to frame this ontic/epistemic duality so it doesn't lead me to a stalemate is by trying to concentrate on the physics, and to me this means observations, specifically observations that can be translated into physical measurements.

Yes, the scientific version of SRP is something like "we know only physical observations or measurements, not any 'reality' we might imagine was responsible for them".

2. What good is it?

It's not a dormitory diversion but a fundamental fact of existence which can't - I mean, shouldn't - be forgotten in serious contexts such as foundational physics.

I just don't see how delving into an appreciation that our consciousness constructs a perception of reality for us helps us to progress towards an answer to these questions.

I'll try to explain what good it is in a later post. If I get to it.

There are a lot of things we think of as ontically real, like people, rocks and wavefunctions. Continue calling them "real" but translate, in your thoughts, to "appears real to me, based on the persistent and convincing data of my senses, but of course there's no way to tell if it's really real."

We have to keep the SRP in mind and deal with it, for foundational issues.

Let us assume there is such a thing as an external reality and that our consciousness constructs a reasonably faithful and consistent representation of that for us. Since we can't really say one way or the other we may as well assume the simplest explanation.

With this over-simplifying assumption, SRP is "resolved or sidestepped or brushed away" entirely. Given this approach, of course SRP has no impact on foundational issues. That's why you don't "get" its importance IMHO.

I fail to see how the rather obvious and trivial assertion that we can never prove anything to be real solves anything as far as the epi/onto problem in foundations physics is concerned.

So given the assumption that there is an external reality independent of our senses and consciousness do we have to take an ontic or epistemic view of the quantum state, according to the structure of QM?

SRP is relevant to all foundations physics, particularly QM wavefunction ontology question. It doesn't "solve" anything by itself but is a vital principle for analysis. IMHO.

3. QM Interpretations

I tend to think of the quantum state as a real thing and think of measurements projecting into a new state - so broadly speaking a collapse picture.

Me too. It's simple and intuitive, and accounts for all experimental data. The collapse interpretation has always been the most popular way of looking at it.

Since we get the same results in QM if we adopt an ontic picture or an epistemic picture ...

I actually can't help but think that the epistemic approach is actually more logically cohesive.

I haven't seen an epistemic approach that actually works to reproduce QM phenomena although some people claim such exists. At the moment, I don't think it's more logical, either. Admittedly there's a lot I don't know about the subject.

4. Miscellaneous

We need to be able to predict stuff because we want to test our ideas of the why and how as best we can.

Practical people would instead say that we want to predict how things will behave under given conditions so we can build cars, cell phones and bombs instead of relying on horses, letters and clubs. That it's not about testing ideas but making a product.

I have never read any description of a (purely) classical process that does not talk about position, momentum, energy, electric field, etc as if they were anything but descriptive of something objectively real.

This sort of debate has occurred often in history of science. GR says that absolute position doesn't exist. Zeno argued that velocity (momentum, motion) wasn't real. Ontological status of energy was questioned by many. Read "On Action At A Distance" by Maxwell http://www.informationphilosopher.com/solutions/scientists/maxwell/action_at_a_distance.html where he argues that the electric field is objectively real, against the majority opinion of the day (led by Ampere). It sounds a lot like this thread, in places.

 

[ddd123]

Maybe a less controversial term would be Intersubjective Reality Principle: since we share the same basic sensory-cognitive apparatus, we can agree on / synchronize our subjective information through language.

Digression: it's often said that intelligent aliens would understand the same physics / maths we do, like in the movie Contact when they find ET radio signals with prime numbers. But an alien with a completely different sensory-cognitive apparatus may not translate reality in that way (e.g. using numbers), but in a way so radically different as to be mute for us (we may even not be able to recognize such a species as intelligent). That's because we are "trapped" in that set of basic thinking categories (try to do away with quantities...).

 

[A. Neumaier]

This sort of debate has occurred often in history of science. GR says that absolute position doesn't exist. Zeno argued that velocity (momentum, motion) wasn't real. Ontological status of energy was questioned by many. Read "On Action At A Distance" by Maxwell http://www.informationphilosopher.com/solutions/scientists/maxwell/action_at_a_distance.html where he argues that the electric field is objectively real, against the majority opinion of the day (led by Ampere). It sounds a lot like this thread, in places.

Nice paper!

 

[secur]

...Intersubjective Reality Principle [IRP]...

Good point. SRP threatens to get into hard-core philo: solipsism, "Cogito Ergo Sum", Leibniz's Monads, that sort of thing. As soon as we engage in discussion, much less science, we move to IRP. Discussion makes no sense without inter-subjective reality principle.

However SRP is still relevant to science here and there. We can use both acronyms. BTW these days everybody seems to want their own acronym, principle, or theory - but that's not my motivation. We just need a brief way to refer to the principle(s).

SRP, as opposed to IRP, is relevant to the practice of science: how do we know we can trust other's results? Brings up topics like peer review process (PRP :-), replication of experiments, and fraud/fudging. But far more relevant is the following.

In typical Bell gedanken, the "weird" part is that Alice and Bob's results seem to depend on each other at the moment of observation, even though they're spacelike separated. But there's no problem locally: each gets random sequence of 1's and 0's as expected. We don't know they're correlated until the results are brought together. Now, according to SRP, no possible observer can know both sets of results until they're brought together. The illegal observer who can somehow do that has been derisively referred to as the "God's-Eye view". Thus SRP provides a way to dismiss the whole puzzle. It never happens that a single observer, at one point of spacetime - the only allowed type of observer -, knows something that requires FTL influence or similar.

I don't agree that solves the conundrum, although it does ameliorate it. But that's not the point anyway. The point is, this is one example showing SRP's relevance to the quantum ontic/epi question.

IRP is like co-moving observers in GR. SRP says you've got to be careful with that concept. In truth there's only one observer at one spacetime point. Co-moving observers are very useful, but when you get right down to it, remember they are an abstraction. Their results must be brought together in one place, for one observer (scientist) to make sense of. That explains some GR oddities, which I won't get into.

I'll show SRP/IRP's relevance to science, QM, and ontic/epi wavefunction question in many other ways, if I get around to it. The only problem is organizing the plethora of examples! Anyone is welcome to come up with a few. The best attempt wins a surprise prize.

...an alien with a completely different sensory-cognitive apparatus may not translate reality in that way (e.g. using numbers), but in a way so radically different as to be mute for us...

That's certainly conceivable. But, as you know, the hypothetical alien's alien mode of thought is NOT conceivable to us.

Nice paper!

It really sounds a lot like today's controversies in places. Here's a quote from Maxwell, where he paraphrases his opponents:

"If we are ever to discover the laws of nature, we must do so by obtaining the most accurate acquaintance with the facts of nature, and not by dressing up in philosophical language the loose [my bold] opinions of men who had no knowledge of the facts which throw most light on these laws. And as for those who introduce aetherial, or other media, to account for these actions, without any direct evidence of the existence of such media, or any clear understanding of how the media do their work, and who fill all space three and four times over with aethers of different sorts, why the less these men talk about their philosophical scruples about admitting action at a distance the better."

It's amusing that the word "loose" is also used by Murray Gell-Mann, with the same pejorative intent, in characterizing his opponents: "People say loosely, crudely, wrongly ...". There's nothing new under the sun; furthermore, plus ça change, plus c'est la même chose.

 

[ddd123]

I'll show SRP/IRP's relevance to science, QM, and ontic/epi wavefunction question in many other ways, if I get around to it. The only problem is organizing the plethora of examples! Anyone is welcome to come up with a few. The best attempt wins a surprise prize.

I would really like to hear them. I can think of examples in physics where SRP/IRP can be brought in for comment, but none in which they have a clear impact.

 

[stevendaryl]

If you're going to suggest that the phase space point (p,q) of classical physics is an example of an epistemic state, then we'll just have to agree to disagree :-)

I'm not sure if this is missing the point (no pun intended), but while I agree that a point in phase space is ontic in classical physics, in practice, we don't have a point, but a region of phase space or a probability distribution on phase space. We don't know precisely where the actual system is, we only know it probabilistically. The distribution is epistemic, rather than ontic (it reflects our subjective knowledge).

The corresponding thing in quantum mechanics to a "point in phase space" is a pure state, and the corresponding thing to a distribution in phase space is a density matrix. The weird thing about quantum density matrices is how they mix up ontic and epistemic. There is no unique way to determine which parts of the density matrix are due to our lack of information, and which parts are due to objective facts about the system. What makes quantum measurements strange, and what it makes it impossible to prove that they violate causality, is that in a measurement, the same uncertainty seems to shift from ontic to epistemic. After the measurement is done, you can pretend, after the fact, that it was epistemic all along.

Concretely, if you have a pair of particles in an entangled state (measured by Alice and Bob at a spacelike separation), then the density matrix that you obtain from one of the particles by tracing out the degrees of freedom of the other particle is a density matrix. This density matrix is not epistemic, but ontic---you started with a pure state, where presumably you know everything there is to know about the composite two-particle system. After Alice measures her particle's spin and "collapses the wave function", you can think of Bob's particle as being in a definite spin state. But Bob doesn't know WHICH definite spin state, so he would describe his particle using a density matrix---the same density matrix that he originally constructed from a pure two-particle state by tracing out Alice's particle's degrees of freedom. So the wave function collapse has no effect on the density matrix used by Bob, it just changes it from being ontic to being epistemic.