|May16-09, 09:06 AM||#1|
Where can we go with Relational QM?
I’m looking for help on a project connected with Rovelli’s Relational Quantum Mechanics, which has been talked over a few times in this forum (see links below). The original papers on this are –
http://arxiv.org/abs/quant-ph/9609002v2 (“Relational Quantum Mechanics”)
http://arxiv.org/abs/quant-ph/0604064v3 (“Relational EPR”)There’s a lot going on in these papers, especially the first, and they open up more issues than they resolve. But they aim at a basic shift in perspective that I want to explore. To summarize –
(A) Usually physics operates from the common-sense assumption that things like particles, fields and spacetime structure are just “given” in reality, and have definite properties “in themselves”. Obviously this notion works very well for most practical and theoretical purposes, but not so well when it comes to QM, where we end up with descriptions of reality in terms of superpositions.The (A) perspective can be thought of as a “hardware” view, where the world consists of things with “built in” properties and intrinsic modes of behavior. We as observers have no direct perception of this underlying hardware, but we can make reasonable inferences about it based on observation, and verify hypotheses by experiment. This is standard physics.
The (B) perspective of RQM is more like a “software” view. Here the physical world is made of communicative interaction-events – “observations”. That means ultimately physics is describing the same world that we all actually experience – the “informational environment” from the standpoint of each individual system in communication with others. Yet we’re not talking about something subjective, in the sense of mind-dependent, “in our heads”. We’re talking about the physical world “out there” as the information-defining interactional environment we all live in, moment to moment.
Now clearly, to the extent that the (A) hardware description works, we should use it. It’s vastly more efficient to say what a thing really is, than to say how it looks from the changing perspectives of each thing it interacts with. To say “the electron has mass Me” (a hardware description) summarizes in a single datum many different regularities occurring in countless individual interaction-events. So the goal of RQM can not be to do away with (A) descriptions of reality. However, it raises the question of why the (A) description generally works so well.
If Rovelli is right, and the software view is ultimately more fundamental, then QM must be showing us how the real-time network of communications-events defines, maintains and constantly updates this consistent, verifiable picture of reality we all share. In other words, it’s possible that without discovering new data or inventing new theories, we could learn to interpret the known physics as describing not only the “content” of this messaging system – the picture of reality it sustains – but also the functionality of the system itself.
That could open up a much more powerful way of explaining the complex structure of known physics. So long as we’re just describing reality as a body of given fact, ultimately the best our theories can do is develop a compact, efficient way of summarizing all the facts in a minimal set of equations. On the other hand, if what we’re dealing with is not just given facts but a functional system, this gives us (at the least) an approach to understanding the multi-faceted “fine-tuning” of the universe. We can guess that the universe has to be more or less like this, in order to define itself as a coherent body of communicated information, and see if that leads anywhere. We don’t need to suppose the universe exists so we humans can observe it – in RQM “observing” is how every entity participates in defining a collective reality.
Beyond that, there’s the possibility of developing an evolutionary explanation for base-level physics. If we can describe how this self-defining, self-communicating information system works, we might be able to see how something like this could evolve by “natural selection” out of the chaotic background of a quantum vacuum – i.e. out of a random superposition of all possible interaction-webs (most of which wouldn’t succeed in defining any information).
So I think what RQM offers us is the possibility that a much more profound understanding of existing theory might be reachable, without introducing any new assumptions.
Essentially we know two things about the world. One is that it looks very much like a given, objective (and nearly Euclidean / Newtonian) reality. The other is that it functions as a real-time informational environment that consistently supports that picture of reality. Because regardless of how we choose to interpret QM, it’s clear that the physical world does operate as a communications system. Not only can we humans send messages to each other, but everything anyone can know about the world gets physically defined / communicated through interaction. The question is just whether this is just a trivial fact about the world, to be taken for granted, or something important for physics to analyze and understand.
Rovelli’s RQM papers don’t analyze this functionality, but they do certainly open up the question – how does the interaction-web described by the QM formalism coordinate the data exchanged between different observers, to produce a shared picture of reality? And despite their limited focus on interpreting QM, I think there are important indications in these papers about the structure of a quantum communications system. I’ll try to summarize some of those in separate posts.
I’m very interested in your thoughts on this, and also in other approaches to informational functionality in physics. What I’m after here is to clarify a certain perspective for thinking about physics, rather than advocate for a particular theory. Just as one other example of work that might point in this direction, I’ll mention a recent paper of Zurek’s on “quantum Darwinism” – an outgrowth of the decoherence approach:
FYI, here are links to some previous PF threads on RQM. These are mainly about Rovelli’s second paper interpreting the EPR “paradox”, and about the relationship of RQM to the many-worlds interpretation.
|May17-09, 12:14 PM||#2|
In Relational Blockworld (RBW) relations are also fundamental to relata, but RBW goes one step further in acknowledging the blockworld implications of the relativity of simultaneity. In this view, dynamical language (cause and effect) is to be replaced at the fundamental level with a self-consistent co-construction of space, time and "things." Discussion of "reality emerging from chaotic quantum fluctations," for example, has no place in a spatiotemporally holistic view unless one introduces an extra temporal dimension (same is true in ordinary general relativity). In this approach, computations are via a spatiotemporally complete picture (initial and final boundary conditions included) as in the path integral formalism. Then one doesn't ask, "What will happen?" but "How likely is THIS to happen?" If you're interested in checking out RBW, see quant-ph/0903.2642. If we're not allowed to reference non-refereed papers here, see:
“Reconciling Spacetime and the Quantum: Relational Blockworld and the Quantum Liar Paradox,” W.M. Stuckey, Michael Silberstein & Michael Cifone, Foundations of Physics 38, No. 4, 348 – 383 (2008), quant-ph/0510090.
The problem with refereed pubs is that the material is dated. This Foundations paper was originally arXived in 2005 but did not appear until 2008. That's why I gave you the latest arXiv reference (currently under review).
|May18-09, 08:45 AM||#3|
Thanks for the response!
But, to me the Relational Blockworld approach is almost opposite to Rovelli’s, in that – if I understand it correctly — it envisions the world as a certain kind of object (a self-consistent set of interaction-events), whose structure can be described objectively, from a point of view outside the system.
I’ve posted some thoughts on the “blockworld” idea elsewhere – basically, I see it as a misreading of Minkowski spacetime as static rather than dynamic. I realize that’s a minority view!
Anyhow in Rovelli’s RQM, the world exists only from some point of view inside it – i.e. for some actual “observer”. So it’s not “objectively real” in the traditional sense, as an entity existing “in itself”, whose characteristics can be described from no point of view in particular.
But, the world is also much more than the subjective world of any one observer, because it involves information being communicated among all systems as “observers” of each other. This network of interaction-events somehow creates and maintains an evolving body of facts that different observers agree on – i.e. it approximates a reality by communicating about it. So here the self-consistency of facts is not just “given”, but gets defined dynamically in the web of real-time interaction.
What’s most interesting about this approach is not that it resolves the problems with QM in a superior way – which is the claim also made for the Relational Blockworld, or Many Worlds, etc. It’s that it creates a whole new problem with QM, that I think might be fruitful to pursue, because it’s an issue of functionality – i.e. how is the consistency of "the real world" being reproduced and updated through physical interaction? For example, how does this network have to be structured so that measurements can define information, contributing to the environment that enables further measurements?
Now the structure of the quantum interaction-web is not unknown – it’s probably more completely known and more thoroughly analyzed than any other part of the natural world. If Rovelli’s point of view is right, this body of knowledge is not just about what the world is (as a self-consistent mathematical structure) but also about what it does and how it works (to define that structure interactively).
|May18-09, 09:38 AM||#4|
Where can we go with Relational QM?
Hello Conrad, I haven't read the entire thread. I'm lagging at the moment as I have some papers to skim as well, but I think I connected a little bit to your reasoning, so this is a quick reflection without me having analyzed in detail everything you've wirtten.
It's easy to take a simple relational view and consider the relations more fundamental, than the relata. But that is almost as bad. I thikn rovelli seems to gold almost a structural realist view. In that the relations at some level are immune to scientific questioning, it's a realism layer present at some point. I don't like this.
In one part (don't have the paper herE) he says that there are only relations, moreover there ar no absolute relations, only relational relations. In particualr does he imagine that relations are only compare by means of physical interactions. so far that's fine. but then he throws in the old QM and assumes the interactions are described by QM.
I think he does this to be conservative, as his aim doesn't seem to reformulate QM, jsut to cast it into a relational form suitable for his later scheme.
But it's not a fully relational picture IMO, since he leaves some baggage unresolved. I think that if this is analyzed differently, one does not need to add assumptions such as "communication between observers" obeys QM. Becuse it may be that the analysis itself - regressive as it admittedly is - evolves a communication structure that proves to be exactly QM in some limit.
So I think rovelli's RQM is not an attempt to understand QM, it's just a sort of reinterpretation in relational form, that IMO still fails to be relational on the most important point (probability determinism).
|May19-09, 07:12 AM||#5|
When I first started reading rovellis RQM paper, I thouhgt (while not having finished it) that the author here has a great vision, to take the relational ideal, to it's extreme (this is how I interpret his statement that not only are their only relations, there are no absolute relations) and show how that IMPLIES quantum logic or quantum mechanics.
Unfortunately this bubble bursted when he throwed up QM as "communication is described by QM".
This is why I personally think that RQM as rovelli ultimately ends up with it, does not solve that many problems. But I think part of the spirit I at least thought he had could prove to be worked out in a different way so that a difference could be made.
My personal guess is that maybe Rovelli accepts this this wet sock because his main focus seems to be on spacetime and geometry specifically, and not structure of theory and law in general. This impression is I think strengtened by the fact that at least from a fundamental poitn of view, LQG seems to be more of a more pure gravity theory. The idea that there can be a fundamental pure gravity theory and then add matter without distorting the theoretical structure seems to me to be analogous to considering spacetime structure in a relational view, separation from the other structure in the theory (hilbert spaces, hamiltonians etc etc).
|May19-09, 08:31 AM||#6|
Fra – I appreciate your taking a look at this.
If I understand what you mean about “structural realism” – I would say the term “relational” only takes us one step. We can go from envisioning a given reality made of things to a given reality made of interactions – but the still describe the interaction-structure “from the outside”, rather than from the point of view of a participant doing the interacting. So in effect the “relations” just become part of the hardware. I think that might describe the Relational Blockworld approach.
But the RQM papers go much further than this. Because this is very unfamiliar territory, there’s a lot that’s not yet clarified – for example, the central issue of what it means for a system to “have information about” another system, or for two systems to “exchange information” when they interact. We know these concepts are meaningful, because we certainly are physical entities and we certainly have information about things that we get by interacting with them. But it’s not clear what these concepts should mean when we’re talking about atoms or “physical systems” in general. Rovelli’s remarks about this don’t always seem very helpful or even consistent.
But the thing is, he’s not starting from any a priori definition of physical information – he’s trying to understand what QM is telling us about this... and I think he partly succeeds, even though the picture is still far from clear. What you call his “conservatism” in sticking to “the old QM” is important – he doesn’t want to start inventing theories about the informational structure of the world before we’ve gotten clear as to what we already know about this from established theory.
The temptation is to bypass all the murkiness surrounding quantum measurement, and just assume some simple notion about what “information” is, physically – so we can use that to build a mathematical model. (And maybe that’s what Rovelli is doing with Loop Quantum Gravity? – though I don’t understand that program well enough to comment.) But anyway, his approach with RQM is to try to see what basic ideas about information are built into the QM formalism itself. Specifically, by stating certain postulates and trying (with partial success) to derive the formalism from them.
I’ve been struggling to come up with a brief description of his postulates and what I think they mean, but it’ll have to wait... no more time this morning!
|May19-09, 09:52 AM||#7|
I think what you'd call "inventing theories about information structures" is what I have in mind.
My aim is to explain the QM formalism, not ask what QM formalism can explain, because I think for various reasons I see QM formalism as effective, and it's validated only in a particular actual experimental domain. Rovelli tries to extrapolate this formalism into yet unverified domains of the theory. This does not appeal to me, but that's a personal view I guess, that has more to do with my preference for scientific strategy, than it has to do with what I know about nature.
I don't have enough faith in the QM formalism to extrapolate it beyond the tested domains. This extarpolation is ambigous anyway, so the degree of "conservatism" is I think relative to your strategy.
I sort of think that my I picture is more conservative. Not in the sense of the history of science, and relative current theory, but conservative relative to what I actually know. I think Rovelli's extrapolation of QM into untested domains is "more speculative" than is the idea of trying to understand the QM logic from an inside perspective.
So degree of speculation is also relative.
|May19-09, 02:46 PM||#8|
It sounds like RQM is trying to "save the appearances" which today is that of dynamism. If so, then you're right, RBW is in stark contrast to RQM because RBW is fundamentally adynamical.
|May20-09, 08:17 AM||#9|
Fredrik – I think you outline quite clearly the difference between our points of view on QM. I tend to see it as giving us the basic information-structure of the world, and you see it as a (maybe misleading) approximation to an underlying structure that would make more sense. I just want to say, I’m much in sympathy with your quest to understand where information comes from, in the relationships between observers’ points of view.
As you’ve said before, it’s hard to identify a reliable starting-point for this research. If we don’t assume that data is just given in the reality of “what things are in themselves”, then what aspect of “information” do we take as fundamental?
Let’s focus on the stumbling-block you ran into with RQM – where Rovelli insists that “communication between observers” is also a physical interaction to be described via QM. You call this an “additional” assumption, but it’s basic to his whole approach, that the QM description applies to everything.
His story goes: Observer O makes a measurement on a quantum system S, and gets a definite result for parameter q. Before the measurement, S is described as being in a superposition with respect to q, and this “collapses” into the observed state with the measurement. Another observer P knows that O has made the measurement, but doesn’t yet know which result O got for q. Rovelli says – For P, both S and O remain in an (entangled) superposition of states with respect to q. That superposition “collapses” when O tells P what result was obtained. So the measurement of a quantum system is seen as essentially the same kind of physical event as O speaking to P (or writing to P!) with information about q.
I can easily understand why this seems crazy. When we think about the world, we’re used to taking the “objective” standpoint – imagining “what really happens” (from no particular point of view) rather than what things look like from some particular viewpoint. QM has more or less forced us to accept that sub-microscopic systems can be in many places at once, etc. – so “reality is weird” at this level. But there’s no reason to believe a person can be in a superposition of states! So we tend to imagine that at some point in time a “collapse” actually happens and the superposition changes into a definite fact.
But RQM says – this “view of the world from outside” is the problem. There is no point at which the “collapse” actually happens, in reality – it happens only for each observer insofar as they gain information. The communication of information through the interaction-web is the “collapse”. So RQM tells us – if we want, we can envision the universe as if we could stand “outside” and look at it without participating in the interaction. But then we should envision it as a superposition of all possible states of all possible universes, defining no specific information whatever. The “determination” of all the information that constitutes our “real world” happens in the network that physically communicates that information.
This is hard to adjust to, because we think of data as “given” and of communication as merely transmitting data from one locus to another. Again, we can maybe believe that at the quantum level information-transfer involves a mysterious “collapse” – but surely not at the human level! Say I didn’t get to see last night’s basketball game – does the outcome remain in a superposition of states, in which both teams win, until I open the morning paper and read the final score?
Yes it does – for me. I know there are thousands of people who saw the game last night, and I don’t have to believe that “in reality” they were all hovering in a superposition of states until I opened my paper. In RQM “superposition” does not describe a “reality” (the state of system S prior to the measurement), but a part of the communications network, namely the part that is in a given observer’s future.
The difference between the submicroscopic quantum realm and the world we experience is that at our scale, there is a huge redundancy in available information. Knowledge of the game’s final score is “out there” in the interaction-web in countless versions. When O makes a measurement on S, on the other hand, there are no other interaction-channels in which q has a definite value.
Because there are so many different ways in which I could get information about the game, it makes very good sense for me to think of its outcome as a given objective fact that happens to be unknown to me, until I open the paper. At our level, that simplification works – there is no detectable “interference” between different outcomes of the game, in my informational environment. It works for several reasons, discussed at length in Zurek’s paper (cited at the end of the OP). So I may as well think of my world as “real in itself” and ignore the fact that this world only exists for me as information in the interaction-web.
At the quantum level, where information redundancy is minimal, this simplification no longer works. The statistical structure of the information-network is quite different here – we have complex states and Born’s rule, interference effects, etc. But QM is describing the same world at all levels – a “software” world made of communicated information, not a “hardware” world made of things-in-themselves.
To summarize – for standard QM, “superposition” is a weird form of objective reality, and the “collapse” is some sort of real event that (due to the linearity of QM) is weirdly difficult to pin down. For RQM, “superposition” describes the structure of possible information that is yet to be gained, and “collapse” is just another word for “communication”.
I don’t know whether this makes sense to you. In any case it’s not a “solution” to the problems with QM, because it leaves open very basic questions about the structure of the communications-web and how it works to define information interactively. But my main point above is that these are potentially very powerful questions, because they address the functionality of base-level physics – not just what the world really is, but what it’s doing that supports all these phenomena we observe.
By the way, what you say about Rovelli’s approach in LQG seems right to me, though quantum gravity research is quite a bit over my head.
Thanks again -- Conrad
|May21-09, 01:02 AM||#10|
If we don't care about gravity, and thus already here, make idealisations, then of course, then there is no major complaints on QM. If we are allowed to make certain idealisations, then I have no problems whatsoever with QM. But if you relax those idealisations, then QM as it stands, can not qualify as the fundamental information theory we need. This is my opinion.
I don't I was able to convey my objection clear enough.
Let me try to describe it again...
The problem is the next step: so what ARE the physical interactions. Rovelli assumes that he already have the perfect answer, and it's QM. Here I object, he is too fast! He ignores here several things. Here is where my objection is.
Anyone who accepts QM, since it's "proven correct" and then tries to see what it implies are bound to completely ignore exactly the point where my objection is rooted.
I suggest that Rovellis does not take his own initiated reasoning far enough. In as much as you requierd observer communications to be physical interactions, there should also be a "physical basis" for the information structures, and the observers memory. This includes the physical reconstruction of the QM baggaged, ensenmbles, and probability spaces. I wouldn't object to this unless I thought there was a way to do this.
Rovellis ignores these problems. In one of his papers or books (don't remember) he has a footnote noting that he "does not want to discusse the meaning of probability", now that is pretty much an explicity declaration of ignoring the problem I object to.
The part you describe, IS the GOOD part of rovelli. I however think that it could be better.
My point is that QM should rather follow from, rather than constitute the more fundamental framework we are looking for. And if that's found, there are implications for many things, unification of ALL forces, as well as fine tuning and initial value problems.
From my perspective, which focuses alot on the logical structure of methodolgy Rovelli is not quite consistent when he first insists that observer-obseserver communication is constrained by physical interactions, and at the same time doesn't want to demand that _information_ is also constrained by physical strucutres. Becase to me it's obvious that the information content in QMs baggage, hilbert spaces, continuums etc are more than is allowed by a finite observer.
I picture the context of information, beeing the physical makeup of the observer, which of course (in the extended rovelli spirit I MISS here) simply the microstructure of matter. Now that's why again my objection to Rovelli here goes hand in hand with my objection to LQG as first of all beeing a theory of pure gravity. The coupling to matter microstructure (read here observers memory structure) is requierd for consistency. So I don't quite understand the quest for a "pure gravity" theory. I'm not sure there is such a thing as pure gravity.
|May23-09, 08:59 AM||#11|
Hi Fredrik —
Yes, I certainly agree that “there should be a physical basis for the information structures and the observer’s memory,” and that this is implicit in the RQM approach. Though I don’t fault Rovelli for failing to go that far in his papers, which really just aim at establishing a starting-point.
I also agree that a “pure gravity theory” won’t give the answer. If what we’re looking at is a system where all the information has to be defined in physical communications, then we need to take seriously that to define or communicate any kind of physical information requires a context of other kinds of information. Apart from virtual interactions, which don't carry definite information, even the simplest interactions communicate more than one kind of data. Even a photon, for example, carries both analog momentum-data and digital spin-orientation data. And photons aren’t “observed” by other photons, but by charged particles. Every interaction involves a “translation” from one information-channel into a different one. Any observation registers data about one kind of system in terms of its effect on a different kind of system. Multiple interaction-contexts are always involved, as well as multiple reference-frames.
So I think RQM ultimately implies an inherent complexity in base-level physics, that’s at odds with the goal of an ultimate “unification”. In a world structured only by gravitation, or only by an electromagnetic field, or whatever, I’m pretty sure that no sort of measurement or communication would be possible in principle.
Anyhow, you’re right that physics needs to account for something like memory, among other things, if the goal is to account for the possibility of information-exchange. And RQM doesn’t allow us to assume that data can just be stored in the “state of a system” that persists over time, because the state of a system “in itself” is not well-defined. I imagine that even stable properties, like the mass or charge of a particle, should be understood as invariants in the information communicated through the interaction-web rather than as static facts that are simply given. After all, the only empirical significance of “mass” or “charge” is in the structure of the communicated data we observe.
In this context, it’s interesting that the charge of an electron in QED involves such complex self-interaction with the e/m field that we have to resort to renormalization – essentially, plugging in by hand the empirical charge that corresponds to the sum of an infinite series of interaction terms, that we can’t actually calculate. And QCD seems to imply that a far more complex set of interactions underlies the mass of protons and neutrons, etc.
Clearly a lot more is going on here than is represented in basic QM. I think the importance of RQM is in shifting from a paradigm focused on defining the characteristic of basic realities – particles, fields, branes or what have you – to one focused on how information becomes meaningful – observable, determinable – in the context of other information that is also observable in the context of other information. I takes only a few steps down that path, but I think they’re potentially important steps.
On another note – if and when you get a chance, I’d be interested in what you think of Zurek’s treatment of the physical basis probability in Section III or the paper cited above (at the end of the OP). He seems to think it’s something very new.
Thanks – Conrad
|May24-09, 11:51 PM||#12|
But overall his take on probability does not address all the issues I have with it.
In my view, the notion of degree of freedom itself, is relative. This is why I don't think it makes sense with universal microstructures, where the observer views are simple deterministic transformations. I don't think such a simple model makes sense. The transformations and symmetries themselves, are merely emergent and I don't believe in universal level of determinisim like has been suggested by others. The alternative in my view, are that of evolving law.
I think that the physical constraints of a bounded observer, and how these interact, will give expectations on WHICH physical laws that are likely to emerge. This would include also which symmetries that are emergent.
If such a scheme can be found, and that scheme would predict the symmetries of the standard model and GR in some appropriate limit it would be bingo of course; it would then not only provide unification of interactions, it should also provide unification of the classical and quantum logic (which is normally taken as starting points).
|communication, observer, relational, rovelli|
|Similar Threads for: Where can we go with Relational QM?|
|Relational QM||Quantum Physics||12|
|Relational theory II||General Discussion||1|
|Relational theory II||Set Theory, Logic, Probability, Statistics||0|
|Relational theories||Set Theory, Logic, Probability, Statistics||7|
|Relational theories||General Discussion||6|