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Macroscopic Realism

  1. YES

    12 vote(s)
  2. NO

    5 vote(s)
  3. Somewhere in between

    5 vote(s)
  4. I am not sure

    1 vote(s)
  1. Aug 27, 2009 #1
    In this thread I would like to start a debate on realism at the macroscopic level.
    To some of you this may be an issue that looks so trivial that it doesn't make any sense to try to talk about it. It is my hope that you still may be interested in making some comment, especially if someone else disputes what you have already classified as trivial.
    The kind of situation that I am referring to as macroscopic realism is in connection with large objects, such as those that we can see around us.
    It could be argued that this thread does not belong in QM but in classical mechanics. But in classical mechanics realism is a given. It is only by studying how the macroscopic world results from the interaction of microscopic systems that this issue sometimes arises.
    I see mainly two possibilities here with some people having positions in between.
    If you believe in realism at the macroscopic level, how do you define it?
    If you don't believe in it, could you still define it? not as a fact of nature but as a belief other people may have?. In this case, what is your picture of the world (only at the macroscopic level)?
  2. jcsd
  3. Aug 28, 2009 #2


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    FWIW here is my view.

    When I hear "realism" I associate to views of things that bypass the inference or measurement process. Ie. knowledge about something that is just assumed to be, independend of interaction/observation.

    But from the point of view of observation, perhaps a related concept would be to define realism as a kind of "objectivity", in the sense that all possible physical inferences would lead to the same final state.

    Then objectivity could be subjectively estimated as the extent to which one observer inferes the actions of it's environment to be consistent with something objective.

    In that sense, there is no clear line between micro and macro is a transition, I just see it as a varying degree of objectivity, where at one end you have complete subjectivit and an unpredictable symmetry between inferences/observations, and the other (macro) end of the scale you have an emergent symmetry whose variation is not distinguishable.

    So I do not "believe" in fundamental realism at any level, but my view can still make sense out of the effective de facto realism we do see at macro level.

    Clearly for two observers in the same environment to be in disagreement about the position of the moon would be highly unlikely simply because it concerns such a massive amount of information the disagreement about massive information would in my thinkinhg imply a very strong "force" that would quickly restore the objectivity (symmetry between observations) by deforming the physical observer.

    (Note that with observer here I talk about any interacting system, not humans)

  4. Aug 28, 2009 #3


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    To my mind the question is -- does an object exist, with determinate states and properties, independently of the web of physical interactions through which it makes a difference to other objects?

    Defined that way, I think it's clear that there can be no actual evidence for "realism". Still it seems to be a very useful belief at the macroscopic level. It allows us to describe the world as if all its information were contained "in the things themselves", which is vastly more efficient than describing the world as a web of discrete interactions communicating all this information. Macroscopically, where the interaction-web is extremely dense and finely-woven, the world certainly looks like an objective, even deterministic reality, to a very high level of precision.

    The problem is that we're very good at describing the world "realistically", and until QM came along, it seemed completely unnecessary to think about how things are measured, or how information is physically communicated. Since our entire intellectual framework was built on assumptions of "realism", based on taking the structure of the interaction-web for granted, here we are a century later, still struggling to fit the evidence of QM into this framework. We haven't really tried to describe the web in its own right, or asked about what kinds of interaction-structures are needed to communicate information between things.

    The many-worlds approach is a good example of the bizarre lengths to which we have to go to understand QM "realistically". Or the Bohm model, as another example. So I think we need to keep "realism" to the extent it works, but also learn from the actual evidence of QM about how all the information we have about "real things" actually becomes meaningful through the structure of interaction.
  5. Aug 28, 2009 #4


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    My initial reaction was that realism is the opposite of idealism (reality is only perception), but in the context of EPR-Bell's "local realism," I like ConradDJ's defining question:

    "To my mind the question is -- does an object exist, with determinate states and properties, independently of the web of physical interactions through which it makes a difference to other objects?"

    My answer to ConradDJ's question is "no," because I subscribe to a variation of ontological structural realism, i.e., relata are made from relations. So, in a sense, an object is just its "web of physical interactions."
  6. Aug 28, 2009 #5
    OK. So, how would you answer these (old) questions:

    (1) If a tree falls in the forest and nobody is watching. Does it make a noise?
    (The tree definitely qualifies as macroscopic)

    (2) You have conducted Schodringer's cat experiment 2 hour ago. You have a "real" cat (not a rabit or a mesoscopic system). The poison should work in about 5 minutes. Would you assume that the cat in a superposition of macroscopic states? (dead and alive) or it is in a definite state (dead or alive) before you open the door?.

    (3) Consider a block of iron of about 1kg moving at about 10 cm/s on a track in a school physics lab. Would you consider it possible to say that the block definitely exists? would you say that it is possible to determine it's position and momentum to a very high degree of precision? Do you think that most of the time someone standing next to you woud agree that the block is there and moving? (even if you don't agree in your measurements).

    (4) Is your body real or is it an illusion?
  7. Aug 29, 2009 #6


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    I fully agree with this -
    - which is why my answer to this questions of yours is a rejection of the question as nonphysical. [Meaning I don't see how this question(seen as an action) from a physical system(observer) would evolve; it corresponds to a non-physical action]

    So I don't think the answer is yes or no, the answer is that the question itself is unphysical, and never appears in an intrinsic scenario, because if I am indifferent to the answer, than what is the point/motivation asking it?
    Yes and from my personal point of view this translates to ask, which questions are preferred? Ie. what are the physical questions/actions of a given observer?

    I think the idea is hard to convey, but this is what I meant in my first post. An observer would evolutionary, ask questions(ie. performa actins) that are beneficial. A question to which the quesioner is indifferent since there exists no evidence-feedback is thus rejected as a non-physical or "improbable" question.

  8. Aug 29, 2009 #7


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    If you understand "noise" as something a human being hears, and there's no human being, there's no noise. If you mean by "noise" vibrations in the air, it makes a noise. This is not hard.

    Bu this illustrates a problem I was trying to get around by posing the question the way I did. Lots of people seem to assume that EITHER things exist "in themselves" in an absolute sense, OR they only exist "in our minds" -- either objective or subjective. This doesn't make sense to me, because it leaves out the web of physical interaction, which is all anyone can possibly experience of the world -- and there's no reason to imagine this only exists "in our heads"!

    I think your questions (3) and (4) reflect the same false dichotomy. Of course the iron block and my body exist. The question is, do they exist in some absolute sense? I think Ockham posed the question this way: would they still exist in the same way if everything else in the universe were eliminated? Ockham said, yes they would, if they "really exist".

    My own feeling is that "exist" has meaning because of the web of interactions things participate in. Things exist insofar as they make a difference to each other's existence. Because we take the web for granted, we say "exist" as though it had some absolute, logical significance.

    As to (2), this is a bit different. If I'm outside the unopened box, QM clearly tells me how to describe what's in the box, i.e. as a superposition. The assumption here is that the cat and I do NOT participate in the same interaction-web -- otherwise I could get information about the state of the cat, and there would be no superposition.

    If I'm in the box with the cat, there's no superposition, for me. But if you're outside the box, then for you, the cat and I are both in a superposition.

    If you take these as statements about "what really exists" in an absolute sense, you feel there's a paradox. But then the paradox is in the very idea of a superposition, that something can be in two states or two places at once.

    My understanding of this is that QM is not describing an absolute reality "in itself" -- and certainly not something "in our heads" -- but the structure of the interaction-web in which we participate. And "superposition" tells us something we haven't yet learned to see clearly, about how the web of real-time interaction is structured at a deep level -- where different parts of the web are evolving separately, they don't yet share a common definition of what's real.
  9. Aug 29, 2009 #8


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    I believe that to REALLY get the cat into a superposition state it would have to be screened off, which means, in part, no interactions with the box. People don't point that out, but such interactions constitute an ongoing measurement process, so any QM state for the cat is always collapsed in this case. But, suppose you were able to actually screen off something as large as a cat (it's been done with molecules containing 60 atoms), then I would say the cat doesn't exist before you open the door. Again, I say this because I believe relations are fundamental to relata (no relations --> no relata) and a screened off entity, by definition, does not interact with its universal complement, so there are no relations to give it existence (make the relata).
  10. Aug 29, 2009 #9
    OK. Let's leave this out. I just mentioned because I have seen it a lot in many books.

    If you are a solipsist (I know you are not) then things "exist" only in you mind. But if you are a "realist" that does not mean you are rejecting your subjective experience. In this case you would have to accept the fact that all you may know about the physical world you acquire by interacting with it (directly or indirectly). I don't think that people who label themselves as realists would reject this.
    I like your concept of a "web of physical interactions"

    I understand we could go a little deeper into what we mean when we say something "exists". I wasn't trying to get too philosofical, but I'll try to tell you the way I see this in a different post. With respect to the block, this is something that is part of your web of interactions. So you would say it exists. If you never saw it but you talked to a lot of people who claim to have seen it and even show you a piture of it, I think you would agree that there is a high probability that it exists and that this is not some kind of conspiracy right?. Even if you haven't had direct contact with the block, you have come in contact with a web of interactions (the evidence presented to you) that include the block.

    I agree with your line of reasoning. Now as Ruta pointed out, it is very hard to keep a microscopic object isolated. Even if the cat is in a closed box, and you are not seing it, one way or another information leaks out of the box. So, you do inadvertently become part of the web of interactions. This information is not registered in your brain, but perhaps you could do some kind of experiment outside the box by (as in a criminal investigation) you can find six hours later at what time the cat died or if it is still alive.
    Schrodinger didn't think it made any sense to assume that the cat ould be dead and alive at the same time as we never see this happening. I think most people would agree with this.
    I also think that we could lablel this as "macroscopic realism". In this context it would mean that we can make a definite statement about the cat being dead or alive and be able to prove it false or true.

    I think in the macroscopic world, we can look at things many times and confirm their existence. We can talk to others who can confirm for us that they have seen it, etc.
    When this is the case we can assume that this object exists independently from us as we share the same perception with a lot of people. So in other terms, there is a "world" out there that we all perceive or can perceive. I think this is the way most people think.
    Now, we can run into some paradoxes, but most of the time our perceptions of the macroscopic world are quite consistent. That's why there aren't that many paradoxes in classical mechanics. Now, for quantum systems that's a different story. But when I initiated this thread I was talking about macroscopic systems, which in acordance to the correspondence principle show classical behavior.

    I was trying not to discuss here superpositions of microscopic (quantum) systems. This is definitely something that for the moment depends on interpretation.

    In summary Conrad, I don't think we dissagree as it first appeared.
    Last edited: Aug 29, 2009
  11. Aug 29, 2009 #10
    Ruta, you said "people don't point that out" but I think they do. You can show a little of superposition with a mesoscopic system such as fullerene molecule, but due to decoherence that superposition does not last long. In the case of Schrodinger's cat, I think most physicists today would consider that decoherence (or some other process) causes a macroscopic system to loose it's superpositions and appear classical. In the many-worlds interpretation, you would conserve the superposition but in each version of yourself you would perceive only one outcome, so your experience consists of only a dead or live cat and you never experience interference between the different "branches". But most people who know something about physics don't like the MWI. And I would say that people who never heard of it, think of the world as only one "thread". There are some very highly regarded physicists which do like the MWI, but I still think they are a minority with respect to the rest of the physics community.
  12. Aug 29, 2009 #11
    I think classical physics is concerned with the study of the world by observing, taking measurements, etc. This represents interactions. Some times you see the object directly and some other times you infer their existence by their effect on other objects.
    We also assume that objects that we can observe repeatedly, not only exist while we are observing them but also in between observations. Believing that this is the case does not necessarily mean that you are a "realist", because you could still deny an underlying reality in connection to quantum phenomena. But I would say that you are a "macroscopic realist"
  13. Aug 29, 2009 #12
    Realism is a word that describes a point of view that probably has been around way before there were philosofers or physicists. Of course the word itself appeared much later, only when people started thinking about these things.
    But what do we mean by "reality", "realism", "existence", etc. ?
    When we have a dream, usually the scene changes form moment to moment, we come back to a place and every thing has changed. But when we are awake, there is an order to our experiences. The things we see don't dissapear they just move or get transformed. When we see an object, most of the time when we look a few seconds later we still see it. So we come to have confidence in this. When we communicate with other people (once we learn to talk) we notice that they see the same things we see. So, it makes sense to assume that there is something that we consider to be "outside of us" and that this thing is there even if we are not looking. So we call this thing "reality". If someone sees something that most people can't see, we assume that this person is hallucinating and consider that perception as not being in correspondence with "reality".
    With respect to "existence", althought this word can be used in an abstract way in logic and math, in physics and in day-to-day living we say that something "exists" if it is part of "reality".
    Now, at some point philosofers started examining our implicit belief in "reality" and questioning it. So we had positivism and solipsism (and other ideas). But in day-to-day living and thinking, I think probably even these philosofers relied on the usefull assumption of realism.
    Later in history, quantum mechanics started to show a number of paradoxes that made us question the meaning of reality at the microscopic level. But even then, we have assumed that the macroscopic apparatus that we use to measure the quantum system is "real" and even the quantum object being measured is "real". What is disputed is the reality of definite values of certain variables before measurement or when they are not being observed.
    It doesn't seem to me that "reality" at the macroscopic level is disputed even by physicists who adhere to the Copenhagen interpretation.
    Einstein, Schrodinger, Bohm and others on the other hand argued that all parameters being used in the theory needed to correspond to an element of reality. So for instance the wave function needed to correspond to something in this reality (in the physical world).
    Bohr and others terminantly rejected this type of realism in conection to microscopic systems. But I don't think they rejected the idea that macroscopic systems are "real" or that in the macroscopic world we can obtain quite definite values for their properties. The correspondence principle is a good example of their reliance on the "reality" of macroscopic systems and their macroscopic properties.
  14. Aug 29, 2009 #13
    It seems to me that even though at a deep or philosofical level you have some reservations about macroscopic realism, you consider it as a pretty good assumption. Now with respect to which questions we ask, I think even before language developed, we would be interested in knowing if there is a lion outside the cave or how fast the animal we are trying to catch is moving. So we believe that we can say with certainty if a big object is there or not. Even if we don't know if the object is there, we assume that this is a question that must have a definite answer. If one person tells us that it is there and another tells us it is not, we assume one of them is lying. A policeman investigating a crime would certainly think this way. I would label this kind of thinking as "macroscopic realism". Now, you may say: "I have not seen this type of realism in my philosofy book" but the thing is that most philosofies we know today were originated before quantum mechanics, and at that time there was not need to make a distinction.
  15. Aug 29, 2009 #14
    Wheeler once came up with a delayed-choice thought experiment in which depending on the choice of measurement he performed here on Earth he could cause a photon coming from a far away star to have passed on one side of a galaxy that forms a gravitational lens or to have taken both paths in superposition. Now, in our day-to-day experience events in the past can't be changed. Once they happened they happened. So this seems to be a paradox. Does the past contain different alternative events from which we can choose?. Now the thing here is that our expectation of events in the past being "there" and not being able to chage things comes from our experience with macroscopic systems. In this case we are talking about photons, and while a photon is in flight it is described as a wave. So we have come to accept that even if Wheeler's expected result of this experiment is surprising, most likely if someone was able to do the experiment, they would indeed find that the results agree with Wheeler's prediction. Now, this experiment runs against the idea of realism. In this case it refers to the "reality" of the past and even if the experiment relates to events happening at a very large scale in space, the particle it deals with is a photon, which is microscopic and exhibits quantum behavior. So we could say that this experiment does not contradict macroscopic reality.
    If on the other hand, we could prove that some photon coming from a distant planet which carries information about a macroscopic event there, and depending on which type of experiment we run on the photon, we can make a choice as to what macroscopic event happened, this would definitely violate macroscopic realism as most of us understand it. Wouldn't it?
    I don't think anybody has come up with such experiment. So I think we are safe here.
  16. Aug 30, 2009 #15


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    I really don't come from the philosophy background. The only philosophy book I ever read are a few on the philosophy of science and the history of probability theory and induction. Other than that I studied physics and I know QM well. The fact that I'm philosophically inclined on these questions is a coincidence :)

    There are others on here that I think are more from a formal philosophy background, but it's not me.

    Yes, definitely. This is why I pointed out that that by observer I don't mean human, I mean any physical system. And the language is not verbal, it's any physical state, and communication is physical interaction rather than talk.

    But to stick to your example, there are certainly alot of ways of communicating and formulating that question without words. All sensory inputs and outputs is communication. No need for speech. Fear can be measures as a state of the biological system, and indeed if the environment signals threat, the human must decide what to do.

    But the real question for this guy in the cave, is what action to take. He does need to KNOW what is really outside the case to take an action. Almost all actions our brain take on a daily basis are based on incomplete information. That usually means that as a precaution our actions are based on all possibilities at once. ie. we are prepared for several possibilities.

    Suppose I can carry only ten weapons, and the evidence I have says there is 60% chance that outside is lion, and 40% suggests it's a snake. Then a rational action might be to bring 6 lion guns and 4 snake guns :)

    If you stall the decision until you have complete information, you're toast.

    A fundamental conjecture in my personal view is that physical actions are in strong analogy with decisions based upon incomplete information. And it's exactly the fact that different actions follow from different information, that we have interactions.

    Do you assume that you need perfect information in order to make a decision?

    I think this is an incorrect conclusion if you by lying mean "by purpose stating something you know is wrong". An equally possible conclusion is that both persons are telling the truth, as far as they can infer it, and the apparent contradiction could be because they simply have different information, this is not uncommon.

    Two people might have witnessed two different perspectives or selections of some happening, and they both make RATIONAL inferences of what most probably happened, based on what they have seen. They can come to different probable conclusion, and this does not mean one of them is lying. It's just a fact of how relative inference, and probably truth is.

    If these two people meet, and are exposed to each others inconsistent statements, then clearly we have an "interaction" (analogy) that probably results in some new negotiated position of both.

    My point is that the "relgious belief" in an objective reality might inhibit negotiation processes and progress. No matter how certain I am about something, it never gets more than my opinon.

  17. Aug 30, 2009 #16


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    There is I think no certain way to tell someone is lying, but in this example for someone to at least claim that one is PROBABLY lying, then one need to with high probability exclude the clear possibility of alternative rational inference histories that lead to different probable conclusions.

    Sometimes it might be possible to do this if it seems "unreasonable" that anyone could possible interpret something in a particular direction.

    If serveral people, make the some judgement of "reasonable doubt" that somehow prooves nothing with certainty, but it's OTOH the best we can do - it's at least a rational action.

    It happens that innocent people are judged to be guilty, does this mean that the judge or the jury has been lying or been dishonest? Not necessarily. They might simply have made a probably inference, which is not perfect, but it's at least rational.

  18. Aug 30, 2009 #17


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    Note that I here distinguish between lying and claim something someone else evaluates as false - lying is claiming something false on purpose - ie. beeing dishonest.

    Last edited: Aug 30, 2009
  19. Aug 30, 2009 #18


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    I certainly believe there's something "out there" we all experience together, each in our own way. And very clearly treating it as "objective reality" works very well -- this means we ignore what's specific to each of our viewpoints, and focus on the facts we all share in common. In many situations it's very important to establish the objective facts, and the process of getting to the facts is also very important to develop.

    But -- even at the macroscopic level -- there's more to the world than "reality". This "more" is not just "what's in our heads" -- there's more out there in the world between us, than what's "objectively real in itself" -- there's also the structure of one-on-one interaction that lets us see things and communicate with each other.

    We can describe aspects of the interaction-web as real objects "in themselves" -- for example, the electromagnetic field. That kind of description isn't wrong -- but my own belief is that it misses something important about the structure of the physical world. I think this is what comes out in Quantum Mechanics.

    You say, we observe some things "directly" -- but all we observe "directly" are moments of interaction with the world, that convey information in the context of other interactions we're having, in real time. There's no good framework yet for describing this aspect of physical structure, where each type of interaction depends on other kinds of interactions to communicate information between different points of view.

    So I would say, there are "real things" out there (and objective facts about them), and besides that each of us has our own world "in our heads" (to some extent correlated with reality). But dividing the world into these two categories misses something that may be more fundamental than either.

    It's true that the structure of the physical "messaging system" in which we live becomes unavoidably obvious only at the atomic level, and below. There the interaction-web is less dense and less finely-woven, so interpreting what it communicates as "objective reality" becomes problematic. We run into paradox if we assume the reality of things-in-themselves at that level.

    But I think QM points toward an aspect of the world that's also very obvious at the macroscopic level -- since in fact, this "messaging system" of real-time physical interaction all anyone ever experiences. It's just that we take it for granted.

    Just my own way of imagining things... but that's why I voted "Somewhere in between".
  20. Aug 30, 2009 #19
    I don't see anything wrong in what you are saying. But the situation I had in mind was different. I understand two people may see the same situation from diferent angles. I also understand that they may arrive to the wrong conclusion without intent and therefore this can't be construed as lying. In a legal situation you could make these arguments and it would be very reasonable. When we have incomplete information we tend to make an inference based on probability. Sometimes we may even say: "I am sure" about this or that. But we actually mean "I am almost sure". As you say, if there is "reasonable doubt" then nothing is proven. But I would go a step further and say that if there is "any doubt" nothing is proven. I don't know in what country you live, but in the US someone can be still convited if the doubt is not "reasonable".
    But I was thinking about a situation that is clear-cut. In classical physics we mostly study situations that don't lend themselves to much interpretation. Even if we can look at the situation from different angles or reference frames, there is always some mathematical transformation that allows you to move from one perspective to the other, be it a rotation in space or a Lorentz transformation. So we assume that there is an underlying reality.
    In day-to-day life we encouter clear-cut situations all the time. So I was trying to concentrate on those types of situations, not on the murky ones where we have incomplete information or perhaps the question we ask does not allow for a clear and definite answer.
    Even in those cases where we have incomplete information, such as in a crime investigation, we usually assume that there is an underlying reality. It is just that we don't have enough information to know for certain what actually happened. But we would usually accept the fact that if we had much better technology, more money and time then eventually we might be able to clear most of our doubts. Now, although we can increase our certainty about an event that we have not witnesed, there may always be a very, very small doubt. But the doubt itself means that we are not sure we know the truth, this implying that there is a truth and therefore some reality of which we don't have 100% complete infromation.
    But the kind of events that I wanted to foucus on are the type where we have direct and clear evidence. In these cases, our assumption of some "reality" is even more compelling.
    Let's say you have a box in front of you. You open the lid and see a ball inside the box. You grab the ball and notice that this is a solid, heavy billiard ball. Then you put the ball back in the box and put the lid back. You ask somenone else to open the box and tell you what he/she sees. What would you conclude if this person tells you there is nothing inside the box?. Maybe you want to give this person the benefit of the doubt. You may not want to jump to conclusions and say he/she is lying. You may think that he/she may have not looked carefully, or may not have good eye-sight or may have some mental problem. Or you may even question your own mental stability. I think you would consider one or all of these possibilities (or other which I may not have thought about) before you consider the posibility that there is no underlying reality. That one person may see the ball and another not because the existence of the ball is relative to the observer or undefined in some other way. (we are not talking quantum mechanics after all, we are talking about a large, classical ball)
    Last edited: Aug 30, 2009
  21. Aug 30, 2009 #20
    I agree with this. But I would go even further. Some of the viewpoints may offer information not contained in the other viewpoints. In order to get a more "complete picture" we could put together the information from different viewpoints. In the case where the differences are due to each of us being in a different reference frame but each view has complete information, we can devise transformations that take you from one reference frame to another.

    I think I understand your position. You consider macroscopic realism a reasonable hypothesis. But you think that quantum mechanics has shown us that there are these interactions at a more fundamental level which defy realism. Being the fact that the macroscopic world consists in a collection of all these miroscopic systems and interactions, then It would be reasonable to think that this "weird" (as some people call it) behavior at the microscopic level might percolate somehow to the macroscopic level.
    Now, here I am not too sure if you think that this might imply the possibility of detecting "weird behavior" at the macroscopic level, or that a better understanding of the processes at the microscopic level could give us a better framework for explaining the macroscopic even if behavior at the macroscopic level is just as we have assumet it to be so far (without any "weird" behavior).
    Maybe you can clarify this for us.
    Oh! by the way. Is this "interaction web" thing your own idea or are you referring to some interpretation you have read about?
  22. Aug 30, 2009 #21
    Do you think that we also run into paradoxes if we assume the reality of things at the macroscopic level?
  23. Aug 30, 2009 #22


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    I agree completely with this. It doesn't contradict any of my views.

    If you believe in the reality of timeless law yes, otherwise not.

    For some thoughts on this perhaps you've seen this talk of Lee Smolin
    "On the reality of time and the evolution of laws"

    To make sense out of physics it's not necessariy to assume an underlying reality even the structural realism that is indeed common for many physicists which is pretty much the idea that physical laws does constitute a timeless reality independent of observation, in the sense of realism.

    But I reject also this form of "modern realism" that is compatible with QM etc.

    I would almost say that ALL situations we encounter are incomplete information cases.
    Can you give me an example of a clear cut situation? :)

    If you're suggesting that for example physical law is clear cut, then I think you completely missed one of my personal pet points. Physical law as we humans know it is a result of a scientific inference process, and one of my personal conjectures is that the situation is no different for a physical system. Laws are inferred from a history of interactions and encoded in the microstructure of matter as symmetries, but these are evolving.

  24. Aug 31, 2009 #23


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    No. To me the point of challenging traditional "realism" is to point out that there is another kind of structure to what we see in the physical world, in addition to the one traditionally described by physics. At the macroscopic level, the world appears "real" to a remarkable degree of precision... i.e. we can establish context-independent facts about things that are valid for all observers. But the point is -- whether or not we believe in a reality-in-itself at the basis of what we observe -- all the information anyone can have about reality has to be communicated through interaction. To convey information, an interaction has to make a difference to some other context of interaction. To observe the electric charge of a particle we need to be able to observe its mass. If there were only one kind of interaction in the world, I imagine no information would be observable or communicable.

    So the idea is not about discovering anything new or strange about macroscopic physics, but about learning how not to take for granted the aspect of physical structure that supports information-exchange. "Realism" is an issue, only because it seems to be taken for granted that if something is "real in itself" then of course there will be some way to observe it. In that case the issue is only establishing the facts about "what really is." My point is just that there is necessarily another issue -- what does it take for anything to observe anything, through physical interaction? The importance of QM, as I said, is just that this issue becomes obvious, where in the macroscopic world it's easy to ignore.

    By "interaction-web" I don't mean anything other than the kinds of interaction already very well-described in physics... so it's not at all a new idea. But maybe it's a new question I'm trying to pose, about what it takes to communicate information through such a web. If others have worked on this problem, I'd love to know about it!
  25. Aug 31, 2009 #24


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    Interesting posts in this thread. So I would like to add my comment.
    To me it seems that "real" is about discovering common things in different cases and inevitably discarding unique aspects of each case.
    So that property of "real" is something belonging to interaction network ConradDJ mentioned. But property of "unreal" belonging to things beyond influence of this interaction network in two different scales: big scale - anything that we can not influence differently by sharing information, small scale - everything that's inside each separate unity at the end of interaction thread.
    So my answer is - yes, I believe in macroscopic realism as I feel that this scale is right at the middle of our domain of "real".
  26. Aug 31, 2009 #25
    Sorry, I missed the discussion.
    So, what is your conclusion, are virtual particles real?
    Here is another post I posted few minutes ago, but it best fits here:

    Are virtual particles "real"?
    A) Yes, like the regular ones. They just don’t have 'free' energy. But on the short time intervals there is almost no difference.
    B) No, only real particles are real. Virtual particles are just a mathematical method of calculation of interaction between real particles.
    C) Both real and virtual particles are just mathematical methods of calculation of the correlation between macroscopic events. Only macroscopic events are real.
    Note: Hawking radiation and Unruh effect (so different accelerated observers do not agree on the number of ‘real’ particles) strongly suggest A) or C), not B)
    Note 2: In MWI there are no particles at all, but question is still valid if we talk about wavefunction.
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