Undergrad The typical and the exceptional in physics

[Ken G]

Here is why I don't think the thermal interpretation should count as an ontology. As I understand it, if you have a hydrogen atom making a transition at the end of the era of recombination, then it produces a photon amplitude that starts spreading out throughout the universe, with a relatively low chance of interaction over most of the surface of a sphere that by now extends to tens of billions of light years in radius. When astronomers on Earth measure the arrival of that photon, the normal view is that its wavefunction "collapses" on the Earth. It sounds like Dr. Neumeier is arguing that what we do on Earth is a position measurement that is highly approximate, so although the photon wavefunction did indeed extend over much of the visible universe, our measurement localized it in our telescope out of a kind of measurement inaccuracy that could not detect the true spatial extent of that photon. Now, I admit that we are postulating the occurrence of a vastly unlikely individual event, that this particular photon should be detected in that tiny telescope has a truly miniscule probability, so somehow we are trading off the tiny chance of that particular photon (the indistinguishability of photons is of no significance here, the thermal interpretation can be applied similarly in a hypothetical universe where photons are distinguishable) being detected in that telescope against the vast number of possible photons that could have been detected, and this justifies an extremely unlikely hypothetical. But a quantum ontology that blames the uncertainty on the inaccuracy of the measurement must hold that any of those photons could have been detected anywhere in the universe. Now, that's a pretty darn inaccurate position measurement! Can we really say that is an ontology, can we claim we have an ontological description that says measurements are really that inaccurate, or must any ontology worthy of the name say that those photons really could have been detected anywhere on that huge sphere because they really could have been, in some sense, at that location on that sphere-- despite the impossibility of locally constrained unitary evolution giving such localization? I don't even see how MWI handles that case, it seems like no telescope could ever be involved in a unitary evolution that decoheres a wavefunction ten billion light years away.

On the other hand, if we treat the situation epistemologically only, we can just say that the wave function is a mathematical device for determining the probability that a given telescope will detect a given photon. The photon doesn't have a location until we say how we are going to define our meaning of its location, and that involves a position measurement that is correlated against all the other information we have in the problem, such as the information that was claimed in the scenario: a given atom emits a given photon. We don't have to say what the photon's position was prior to the measurement, because we don't have an ontology, and there is not any prescription in place for giving the photon a location except at the telescope. Epistemologically, any question never asked is also never answered, that's the difference between epistemology and ontology. We live in a universe where all is information, not because information is ontology, but because physics is epistemology.

 

[Simon Phoenix]

Inspired by this discussion I thought I'd read up a bit more on the ontology/epistemology issue. I haven't read through it all in detail yet (it is rather long and technical), but Matt Leifer's review on the issue seems to be an excellent place to start

http://mattleifer.info/wordpress/wp-content/uploads/2008/10/quanta-pbr.pdf

 

[maline]

I don't even see how MWI handles that case, it seems like no telescope could ever be involved in a unitary evolution that decoheres a wavefunction ten billion light years away.

I don't see any difficulty here. Coherence or decoherence is a property of the state as a whole, which exists in Hilbert space and is inherently nonlocal in physical space. The practical effects of decoherence- loss of quantum interference etc.- can only be felt once the various regions come into causal contact. So whether you describe the state as "decohered" or not can depend on which spacelike slice you look at; it has no physical significance until there is potential for interaction between the "branches".
In terms of the EPR setup: Alice's & Bob's measurements each "decohere" the entire state, but this is physically meaningless until the two results can be compared.

 

[Ken G]

I see what you're saying, that MWI would allow a decohered subspace that regards the density matrix as locally diagonal, and calling that a "world" by allowing that subspace to create a new normalization for that subspace in which the probability is "1", even though in the "master wavefunction" most of the density matrix is still not diagonal and attributes a minute probability to that decohered subspace. It is as though in our "world", the wavefunction is "collapsed" (in both senses of decohered and given a new probability of 1), but in most other "worlds", the wavefunction remains uncollapsed (in the broader sense of not even decohered). So MWI can handle that, because it is indeed an ontology-- albeit a bizarre one. But it seems even more bizarre to call it an ontology that holds that purely unitary evolution occurred, and it was just a very inaccurate measurement that attributes a location to a particle that is, in fact in the thermal ontology, still spread all over the universe. Maybe that's no less bizarre than MWI, since both say that we are in some sense vastly misinformed about the true state of things, but MWI seems more like an ontology in the sense that it does not sweep "off the page" the "other worlds", it holds that they still exist in the mathematical description. The thermal interpretation seems to focus so much on what we are making of our measurements that I don't really see what the claim is about the "true ontology" of that photon. (Of course I don't ascribe to the existence of a true ontology of anything, as I think that the intersection of ontology and science equals epistemology, so I regard ontology as akin to a religious belief. I do, like everyone, create a kind of mental ontology to help me picture what is going on, I just don't take it seriously.)

 

[maline]

an ontology that holds that purely unitary evolution occurred, and it was just a very inaccurate measurement that attributes a location to a particle that is, in fact in the thermal ontology, still spread all over the universe.

I don't have any understanding of the ""thermal interpretation", but this description certainly sounds wrong. A photon that was absorbed here will definitely not be detected anywhere else (except possibly in a different "world").

 

[David Lewis]

You're dealt a hand in cards, and it could be anything, but you pick the hand up and look at it, and now you have new information.

But the new information existed before you looked at it. The new information is discovered, not brought into existence by the act of looking.

 

[Ken G]

But the new information existed before you looked at it. The new information is discovered, not brought into existence by the act of looking.

Any time the density matrix is decohered, so diagonalized, everything that happens will be consistent with saying the information pre-existed, and everything that happens will be consistent with saying it was discovered. It is purely our preference of philosophy that chooses the former. Nevertheless, I agree that is a natural choice to make, my point is only that the physicist never uses that choice over the other, it is always irrelevant to the tests that are done. All we need is that there is completely successful correlation between all the measurements that follow with saying that the density matrix is diagonal. So what I'm saying is that the MWI already existed for classical mechanics, and suffered no contradictions with observations-- it was just not a popular choice of interpretation in that context.

But more to the point of the thermal interpretation, it seems to me that interpretation is saying that when you look at the cards you are dealt, and do a "measure the card's identity" operation on it, your measurement is subject to significant uncertainty. Hence you might think the card identity is quite a bit different from what it actually is, but this error propagates through all other measurements of that and all other cards, so everyone is similarly mistaken. Only when you repeat the measurement many times do you see the full range of cards that your measurement could have shown, and that is the actual uncertainty in your measurement, not a reflection of the various different states of the reality. For if it is different states of the reality, then the measurement does not have any error, and the distribution of outcomes must be inherent in the ensemble, making the result a type of "collapse" on the actual state of each card. That is if I understand the interpretation correctly, mapped from position measurements of a broad wavefunction onto card identities after a shuffle produces a diagonal density matrix.

 

[David Lewis]

Any time the density matrix is decohered... everything that happens will be consistent with saying the information pre-existed, and everything that happens will be consistent with saying it was discovered.

When a photon has two paths to get to a detector screen, whether it travels as a wave or as a particle is not information that pre-exists waiting to be discovered before observations are made.

 

[Ken G]

When a photon has two paths to get to a detector screen, whether it travels as a wave or as a particle is not information that pre-exists waiting to be discovered before observations are made.

The situation you describe is not treated with a diagonal density matrix, so is not relevant to my statement. But the spots made on the screen are, so they would be relevant.

 

[stevendaryl]

Any time the density matrix is decohered, so diagonalized, everything that happens will be consistent with saying the information pre-existed, and everything that happens will be consistent with saying it was discovered.

Right. That's the frustrating (for me) thing about quantum measurements. On the one hand, whatever it is that we measure, it's as if it always had that value, and we're just discovering it. On the other hand, Bell's theorem shows that it can't be the case that every quantity that we might measure has a pre-existing value. (Or at least, it's impossible to make sense of such a thing using standard reasoning about probabilities).

 

[ddd123]

Maybe the whole lesson is that spacetime isn't as fundamental and consistent as we assumed. At least it sounds better to me than recurring to a preferred frame or changing the probability rules etc.

 

[Ken G]

Right. That's the frustrating (for me) thing about quantum measurements. On the one hand, whatever it is that we measure, it's as if it always had that value, and we're just discovering it. On the other hand, Bell's theorem shows that it can't be the case that every quantity that we might measure has a pre-existing value. (Or at least, it's impossible to make sense of such a thing using standard reasoning about probabilities).

I interpret that as saying that we are not terribly used to correlating observations with strange relationships, like a spin in one direction of one particle to a spin in some 45 degree direction of the other particle. If we were used to doing that, and had lots of entangled systems in our environment, then we would be used to the problems with imagining that observational values pre-exist. When all we deal with is simple diagonal density matrices, we of course build an intuition that we can imagine the outcomes pre-existed, but we forget that just because a given picture usually works well for us, it doesn't require this is what is really happening. I might go so far as to say the entire history of science is trying to tell us, loud and clear, that all uses of ontology, all attempts to say what is really happening, simply falls into this "fallacy of oversimplification." It's fine to simplify, we always imagine cannon balls fly in an absence of air resistance, but we don't have to tell ourselves the simplification is the truth. Idealization is epistemology; believing the idealization is ontology.

 

[zonde]

I interpret that as saying that we are not terribly used to correlating observations with strange relationships, like a spin in one direction of one particle to a spin in some 45 degree direction of the other particle. If we were used to doing that, and had lots of entangled systems in our environment, then we would be used to the problems with imagining that observational values pre-exist. When all we deal with is simple diagonal density matrices, we of course build an intuition that we can imagine the outcomes pre-existed, but we forget that just because a given picture usually works well for us, it doesn't require this is what is really happening.

You don't have to imagine that observational values pre-exist to run into the problems with entanglement. It's enough to imagine observation results as factual and that observations are independent from non local affairs.

I might go so far as to say the entire history of science is trying to tell us, loud and clear, that all uses of ontology, all attempts to say what is really happening, simply falls into this "fallacy of oversimplification." It's fine to simplify, we always imagine cannon balls fly in an absence of air resistance, but we don't have to tell ourselves the simplification is the truth. Idealization is epistemology; believing the idealization is ontology.

Yes, we know. The map is not the territory.

 

[Ken G]

You don't have to imagine that observational values pre-exist to run into the problems with entanglement. It's enough to imagine observation results as factual and that observations are independent from non local affairs.

Yet all the problems stem from insisting that observations are not just information, they are information about something. Get rid of the "something" and all the problems go away, yet you still have what you actually use: the information.

Yes, we know. The map is not the territory.

Actually, what I'm saying is that the territory is a map too. It's all maps, at least in science.

 

[ddd123]

Yet all the problems stem from insisting that observations are not just information, they are information about something. Get rid of the "something" and all the problems go away, yet you still have what you actually use: the information.

No, this can't be right. Information without an object is like saying: I have the following information "10001011101101". And you ask "what is it about?" And I say "nothing". You have no problem but no information either. Maybe what you mean is: you don't have information about physical entities, but about experimental procedures (i.e. outcomes of measurements). In that case though, if you frame outcomes in space and time (which you can do, of course) then, as zonde says, you still get nonlocal behavior: so you have to consider also information on space and time as information about procedures, then the whole story becomes abstract enough that even confronting results of outcomes in different places and times is information on an outcome of an experimental procedure and not about a physical event. It does still feel forced though because it impacts real life, even our subjective experience of time when we do all this is put to question. I don't know if I'm making sense though :)

 

[stevendaryl]

Yet all the problems stem from insisting that observations are not just information, they are information about something. Get rid of the "something" and all the problems go away, yet you still have what you actually use: the information.

Well, information is pretty useless if it's not about something (I would say that it's not information if it's not about something). For quantum mechanics, at the minimum, the information is about future measurement results. And that's the problem I have with it; it gives a reality to macroscopic objects and processes that it denies (or seems to) for microscopic objects and processes. Since macroscopic objects are (presumably) made up out of microscopic objects, it's hard to see how the former can have more reality than the latter.

 

[Ken G]

No, this can't be right. Information without an object is like saying: I have the following information "10001011101101". And you ask "what is it about?" And I say "nothing". You have no problem but no information either.

I just don't think information works like that-- it makes no difference "what it is about", it's information, that's all that matters. Let's take an example. A deer sees a car coming, and has the sense to leap out of the way. It makes no difference at all if the deer knows "what a car is", say from the perspective of what a human thinks a car is, all that matters to the deer is the information that matters to the deer: jump out of the way. The act that saves the deer has nothing to do with what that information is "about", the information is that there is danger and the deer must leap or die. The deer doesn't need to know anything else, and anything else that it might imagine it knows is most likely a form of self delusion.

Maybe what you mean is: you don't have information about physical entities, but about experimental procedures (i.e. outcomes of measurements).

I would say that it doesn't matter if I regard it as information about physical systems, or information about experimental outcomes, what matters is that it is information. Yes, the concept of an experiment is crucial to science, so that's just more information-- the information that what we regard as an experiment is happening, I'm going to process that information too. But it doesn't matter what an experiment is "about", it only matters how I relate to the information that an experiment is happening. All we ever use is what we make of that information, it never needs to be "about" anything other than that, like the deer.

Now, I will not deny that the way we do process information is that we create ontologies. This clearly helps us use information. So how we use information is helped by imagining that the information is "about" something, I've no problem with that. But it only matters to how we process and use that information, what we think the information is "about" is not, itself, information, it's more like a kind of crutch that supports our information processing without adding anything to that information. It's how we think, so it's just more epistemology, disguised as what the epistemology is "about." That's the irony of ontology, and I believe it is the source of the unease we feel about quantum interpretations. We are asking our ontology to be something it isn't able to be, it is simply overmatched by the topic.

I don't know if I'm making sense though :)

I think I understand, you are trying to generate a working ontology. That's a perfectly normal thing to do, the deer probably does it too-- but what the deer thinks a car is is probably nothing at all like what we think it is, and neither are at all like what a car is in some absolute sense because the "territory" that is a car has meaning only as another map.

 

[Ken G]

Well, information is pretty useless if it's not about something (I would say that it's not information if it's not about something).

We probably need to refine what we mean by "information." I think if we do that, we will find that it is not important to be "about" something. Information is something like a culling process, where you get answers to yes/no questions that allow you to reduce the possibilities of what you expect to happen. Let's say you are playing bridge, and your partner bids something-- that's information to you, it narrows down the possibilities for their hand. So it would be natural to say that means it is information "about" their hand, but what if the bridge game is being played online, and there is no hand-- it's just digital 1s and 0s. You can play as if the information is "about" a hand, but it doesn't matter that there is no hand, it's just information. You don't play any differently, none of the strategies change-- that should tell us the "about" part is very optional.

For quantum mechanics, at the minimum, the information is about future measurement results.

I'm fine with that, but that's not an "about" in the sense of an ontology., because there is no need to think the measurements have an ontology either-- they are information. Information does have to relate to something, it must allow you to cull your expectations somehow, so it is "about" culling expectations. But what I mean is that it doesn't need to be "about" an observation in the sense that the observation is anything more than just additional information. We never use anything except information, so it needs no other "object" than itself.

And that's the problem I have with it; it gives a reality to macroscopic objects and processes that it denies (or seems to) for microscopic objects and processes. Since macroscopic objects are (presumably) made up out of microscopic objects, it's hard to see how the former can have more reality than the latter.

Yet that makes perfect sense if the "aboutness" is just how we think about things. It's not surprising we deal more effortlessly with "aboutnessess" that we are already used to from a lifetime of ontological idealizations that we have come to expect more from than we really have any right to.

 

[stevendaryl]

Yet that makes perfect sense if the "aboutness" is just how we think about things. It's not surprising we deal more effortlessly with "aboutnessess" that we are already used to from a lifetime of ontological idealizations that we have come to expect more from than we really have any right to.

Well, we certainly don't have a right to understand anything at all about the universe. I would prefer to, though.

I really don't understand what you can mean by "information" that lacks "aboutness". As ddd123 says, a string of numbers is not information (or at least, is not meaningful information) unless the numbers are about something: the number of fish caught in a certain pond over the last few days, for example.

Measurement results are part of an ontology that is necessary to make sense of quantum mechanics. You can't actually do quantum mechanics, at least in the usual interpretation, without talking about measurement results. My complaint, as I said, is that measurement results are about us. Having a physics whose only ontology is observations by physicists is way too narrow and solipsistic for my tastes.

 

[secur]

Science provides only quantitative information from experiments and observations. It tells you nothing about underlying reality. It's epistemological, not ontological. All these "endless debates" concern ontology and can't be decided scientifically - which, of course, is why they're endless.

Last century quantum "interpretations" were called quantum "ontologies". I don't know when the name changed, but that's really what they are. Copenhagen and MWI, for instance, use the same math and predict all the same experimental results, the same information. Their difference is purely ontological. That's why we have to invoke vague philosophy like Occam's Razor, falsifiability, and "elegance" or "beauty", when arguing about them.

Ken G is right to point this out. It's a very valuable observation, especially since many people don't know it. But it won't stop scientists from developing ontological models: speculating about what's "really" there. To make sense of the data you have to develop models - like atoms, billiard balls, and galaxies - even though we can't prove they exist. Intuition says they exist, and it's probably right.

Where you go too far, Ken G, is asserting that there really is no reality. Instead, we must be agnostic about ontology, scientifically. You try to reject all ontology by claiming that science constitutes all of our knowledge; but that's not so. There's also what we can loosely call intuition, or whatever. Intuitively we all know there's something real which the measurements are measuring, and the observations are observing.

Anyway all this is mere philosophy. Epistemology vs. Ontology just as much as Copenhagen vs. MWI. A certain amount of such discussion is valuable and necessary. But remember it's not science and will never furnish any new scientific information.

 

[stevendaryl]

Science provides only quantitative information from experiments and observations. It tells you nothing about underlying reality. It's epistemological, not ontological. All these "endless debates" concern ontology and can't be decided scientifically - which, of course, is why they're endless.

I don't agree that that is what science is about. I think that's revisionism. The scientific theories prior to quantum mechanics could be described in terms of an ontology. Newton's theory claimed that there was 3-dimensional space and universal time. There are physical objects that take up space and that have mass. There are forces that act between physical objects.

Special Relativity has a unified spacetime and has a universal velocity.

Etc.

I would say that up until quantum mechanics, proposing a scientific theory meant proposing something like an ontology.

Anyway all this is mere philosophy. Epistemology vs. Ontology just as much as Copenhagen vs. MWI. A certain amount of such discussion is valuable and necessary. But remember it's not science and will never furnish any new scientific information.

I think you're defining "scientific information" in a way that makes that a tautology. And I would say to me it's a matter of making necessity into a virtue; because nobody can come up with a sensible ontology for quantum mechanics, people like to say that that was not a worthwhile goal. And people like to engage in revisionism and say that it was never a goal for science.

Anyway, standard quantum mechanics does have an ontology, and I don't think that quantum mechanics would be worth anything without it. It posits that there are things called "measurements", and that a measurement always results in an eigenvalue of the corresponding operator. So with the purely minimalist ontology for quantum mechanics, it's a theory about predicting the results of future measurements from past measurements.

 

[stevendaryl]

To me, there is a very different character to a theory that posits "there is a tensor-valued field F_{\mu \nu} that obeys such-and-such equation of motion..." and a theory that posits "if you do such-and-such, you will get such-and-such result with probability such-and-such". The first seems to be about reality, while the second seems to be about ME (or about physicists). I think it's weird for people to say that physics is always about the latter (what happens when you do certain things), and is never about the former (what exists, and how does it behave). Every theory before quantum mechanics was the former type, so it seems like revisionism to say that only the latter counts as "science".

 

[Ken G]

I really don't understand what you can mean by "information" that lacks "aboutness". As ddd123 says, a string of numbers is not information (or at least, is not meaningful information) unless the numbers are about something: the number of fish caught in a certain pond over the last few days, for example.

It's not the general concept of "aboutness" that I mean-- I agree that information has to change our expectations in some way, so it's "about" that-- changing expectations. But what I'm talking about is the need to think that the "aboutness" of the information has some existence beyond the way we are using the information to alter our expectations. When that is demonstrably all we ever use information for, why do we have to believe it is something more than that, in order to use that information? Why does a deer need to think it knows what a car is in order to have the sense to jump out of the way of it?

Measurement results are part of an ontology that is necessary to make sense of quantum mechanics.

But look at what you just said there-- ontology is necessary to make sense. But isn't all making sense epistemology? The core idea of ontology is that it needs to be true or else it wouldn't work to help us make sense, but if all we are using the ontology for is to make sense, then why does it need to be true? If all we are doing is making sense, then it's all epistemology, the ontology part is a pretense-- what we are telling ourselves in the process of making sense. It's a voice inside our own heads. Have you referred to anything else? That's what I mean that is you look at what we use ontology for, you can see that it is epistemology in a convincing disguise. It makes the epistemology easier to swallow, somehow.

You can't actually do quantum mechanics, at least in the usual interpretation, without talking about measurement results. My complaint, as I said, is that measurement results are about us. Having a physics whose only ontology is observations by physicists is way too narrow and solipsistic for my tastes.

It's not solipsism, let's be clear on that. Solipsism is another form of ontology, because it claims that what exists is what is inside our heads. I'm saying something different-- I'm saying that all we are doing is processing information, so there isn't even solipsistic ontology. It's all epistemology, because that's all we ever use. It's all you are using too-- you are saying that it is not to your taste. What is to your taste is epistemology-- why should ontology be to someone's taste?

 

[secur]

... because nobody can come up with a sensible ontology for quantum mechanics, people like to say that that was not a worthwhile goal. And people like to engage in revisionism and say that it was never a goal for science.

Yes, people do say such things, but they're wrong. BTW, perhaps I should mention: my post says neither of those, so none of your comments apply to it.

 

[Mentz114]

To me, there is a very different character to a theory that posits "there is a tensor-valued field F_{\mu \nu} that obeys such-and-such equation of motion..." and a theory that posits "if you do such-and-such, you will get such-and-such result with probability such-and-such". The first seems to be about reality, while the second seems to be about ME (or about physicists). I think it's weird for people to say that physics is always about the latter (what happens when you do certain things), and is never about the former (what exists, and how does it behave). Every theory before quantum mechanics was the former type, so it seems like revisionism to say that only the latter counts as "science".

I mostly agree with the previous post, and this somewhat less. There is no need to mention physicists, only how they prepare states and what the probable states of the measuring apparata will be.

If the same scenario happened accidentally we would expect the same distribution of outcomes, surely ?

(Or replace the Physicist by an equivalence class ...)

 

[Ken G]

Where you go too far, Ken G, is asserting that there really is no reality. Instead, we must be agnostic about ontology, scientifically.

Actually I agree with that-- the scientist must be agnostic about reality. But that's all I'm saying-- if we don't use something, then it's not in our science. So I'm saying reality (as in, ontology) is not in science, because we never use it, what we do is picture it, while we are actually using information. People will create ontologies, I do it too, the difference is I don't take them seriously. I am even more than agnostic about ontology, I'm skeptical of it. I don't think any human ontology will really matter much, except as a kind of epistemological crutch for us. Which is fine-- that's what ontology is, an epistemological crutch. So that's all I'm saying-- our ontologies are epistemologies in disguise. I have no idea if there is actually a territory-- but if there is, it isn't what we mean by the word, because we are using territory to mean just another kind of map for us to picture.

You try to reject all ontology by claiming that science constitutes all of our knowledge; but that's not so. There's also what we can loosely call intuition, or whatever. Intuitively we all know there's something real which the measurements are measuring, and the observations are observing.

There might be some value in parsing the difference between knowledge and intuition, but they both sound like ways that we think-- neither sounds like what is, independent of those ways we think.

Anyway all this is mere philosophy. Epistemology vs. Ontology just as much as Copenhagen vs. MWI. A certain amount of such discussion is valuable and necessary. But remember it's not science and will never furnish any new scientific information.

Yes, that is very much the purpose of recognizing that our ontology is just epistemology. We don't actually get any new scientific information by taking the information we do have, and picturing some kind of "real" scaffolding that supports it. That is the self delusion-- that we can bootstrap our way to reality by imagining there is a "territory" there, when what we have actually found to be the case is that the only way to learn about reality is by looking for new information. That's epistemology, not ontology. Is this not the lesson of history?

 

[Simon Phoenix]

I think it's weird for people to say that physics is always about the latter (what happens when you do certain things), and is never about the former (what exists, and how does it behave).

Yes, I confess I can't quite get my head around the viewpoint that the wavefunction is merely descriptive of our 'state of knowledge' - whatever that rather vague phrase actually means - and that measurement simply represents an 'update' to that knowledge. I don't think there's anything actually wrong with that (notwithstanding the PBR theorem) and this perspective certainly does neatly cut through all the troublesome locality issues in assuming the quantum state represents something 'real'. But I'm kind of a bit old-fashioned I guess in that I'd like my physics to be (at least partially) descriptive of something 'real'.

But epistemic approaches occur in classical physics too. In a complex system we might posit that our object of interest has some definite state described by a point in a phase space (thinking classically) - but, because of our ignorance (that is, our 'state of knowledge') we have to describe things using a distribution in phase space. Is this distribution 'real'? I don't think so - so it has this epistemic character even though we suppose there actually is some underlying 'real' state in a classical view. There will be many probability distributions consistent with our (assumed) actual 'real' phase space point.

But the best we can do in QM, in terms of pinning things down given our knowledge, is to assign a pure state to something - and a pure state is quite different to a point in a classical phase space. A pure state isn't even a probability distribution but something like a 'complex square root' of one - and some authors describe it as a 'pre-probability' which is a term I don't fully get.

It's all further muddied when we throw mixed states in there. If I prepare a 'proper' mixture of up and down spin-1/2 states in a given basis (up and down chosen uniformly at random) then this is, mathematically at least, precisely equivalent to preparing the same kind of proper mixture in any spin basis - yet I think we would be entitled to say that there is a definite physical difference (albeit one with no experimental consequences) between a proper mixture (as described) of spin-z states and a proper mixture of spin-x states.

For me the key feature is the different way classical and quantum approaches handle distinguishability - it's all in the overlap :-)

 

[zonde]

I have no idea if there is actually a territory-- but if there is, it isn't what we mean by the word, because we are using territory to mean just another kind of map for us to picture.

Of course we don't know if there actually is a territory but ... some maps simply don't work and if we ask why they don't work assuming that there actually is a territory gives explanation why they don't work. In that sense there is simply no point in assuming that there is no territory.

 

[zonde]

To me, there is a very different character to a theory that posits "there is a tensor-valued field F_{\mu \nu} that obeys such-and-such equation of motion..." and a theory that posits "if you do such-and-such, you will get such-and-such result with probability such-and-such". The first seems to be about reality, while the second seems to be about ME (or about physicists). I think it's weird for people to say that physics is always about the latter (what happens when you do certain things), and is never about the former (what exists, and how does it behave). Every theory before quantum mechanics was the former type, so it seems like revisionism to say that only the latter counts as "science".

I suppose that the second type of theory is called "phenomenological". And I think that this type of theory has utility but it does not directly advance our understanding of reality. However it can advance our understanding indirectly as it shows what fundamental models are not going to work.

 

[Ken G]

Of course we don't know if there actually is a territory but ... some maps simply don't work and if we ask why they don't work assuming that there actually is a territory gives explanation why they don't work. In that sense there is simply no point in assuming that there is no territory.

Surely the burden in science is on the claim that there is a territory. You say we need it to explain why some maps don't work, but it seems to me that is something that needs no explanation.