- #71
Careful
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f-h said:
HUH ?? In classical GR, the METRIC gives me a notion of locality (the Alexandrov sets), nothing more is needed for that.
Where we stand-Baez talk at Luminy
- Thread starter marcus
- Start date
In summary, John Baez gave a talk on fundamental physics, discussing the search for a small set of laws that can explain everything in the universe. He also talked about the current state of theoretical physics, addressing issues such as the string theory controversy and the lack of progress in theoretical physics in the last 25 years. Baez suggests that the root of the problem may lie in the way physics students are trained to think and the deep emotional investment they have in their field. He also mentions the danger of clinging to old ideas and the need for physicists to be open to new and surprising data. Overall, Baez's talk offers a thought-provoking critique of the current state of theoretical physics and highlights the need for a shift
HUH ?? In classical GR, the METRIC gives me a notion of locality (the Alexandrov sets), nothing more is needed for that.
- #72
f-h
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I see what you are saying but you seem to be confusing a lot of different things. I recommend taking a step back and looking at this problem with all the different notions of (non)locality flying around again.
States in QM are nonlocal in some sense, but there are of course local observables on nonlocal states.
The existence of nonlocal states says nothing about the existence of local observables.
Also yes that's what I said, I can localize something else relative to the metric field, but the metric field is localized with respect to what? The question doesn't make sense. (at least in the naive sense of "local"). That is Kuchars objection to the possibility of constructing locally interpretable Observables in a gaugeinvariant theory/fully constrained theory (which you reinvoke quantum mechanically when you talk about "rigging" but which is not really related to superposition/entanglement).
One can study this problem and the formalism to overcome it in any toy model, no Master constraint program needed.
A more detailed response later.
States in QM are nonlocal in some sense, but there are of course local observables on nonlocal states.
The existence of nonlocal states says nothing about the existence of local observables.
Also yes that's what I said, I can localize something else relative to the metric field, but the metric field is localized with respect to what? The question doesn't make sense. (at least in the naive sense of "local"). That is Kuchars objection to the possibility of constructing locally interpretable Observables in a gaugeinvariant theory/fully constrained theory (which you reinvoke quantum mechanically when you talk about "rigging" but which is not really related to superposition/entanglement).
One can study this problem and the formalism to overcome it in any toy model, no Master constraint program needed.
A more detailed response later.
- #73
Careful
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**I see what you are saying but you seem to be confusing a lot of different things. I recommend taking a step back and looking at this problem with all the different notions of (non)locality flying around again.
States in QM are nonlocal in some sense, but there are of course local observables on nonlocal states. **
No, I am not confusing different things: you are. You speak from experience in *background dependent* QFT in which anyone knows that what you say is correct. Actually, it is easy to see where the virtues of that theory (local observables on entangled states) are destroyed in a ``background independent´´ approach whatever that may mean.
**
The existence of nonlocal states says nothing about the existence of local observables. **
Again, that is only true in a background dependent approach since you have an *a priori* notion of locality there.
**Also yes that's what I said, I can localize something else relative to the metric field, but the metric field is localized with respect to what? **
But it is the metric field which gives locality simply by measuring distances
**That is Kuchars objection to the possibility of constructing locally interpretable Observables in a gaugeinvariant theory/fully constrained theory (which you reinvoke quantum mechanically when you talk about "rigging" but which is not really related to superposition/entanglement).**
Well, my objections are MINE, I can only conclude that Karel is a clever chap. No smartusz, I just added rigging for constructing diff invariant states, otherwhise you might have complained that I work with ``unphysical´´ objects which I still do since my master/hamiltonian constraint is not solved.
**
One can study this problem and the formalism to overcome it in any toy model, no Master constraint program needed. **
Rubbish, all these toy models probably have the wrong classical limit.
States in QM are nonlocal in some sense, but there are of course local observables on nonlocal states. **
No, I am not confusing different things: you are. You speak from experience in *background dependent* QFT in which anyone knows that what you say is correct. Actually, it is easy to see where the virtues of that theory (local observables on entangled states) are destroyed in a ``background independent´´ approach whatever that may mean.
**
The existence of nonlocal states says nothing about the existence of local observables. **
Again, that is only true in a background dependent approach since you have an *a priori* notion of locality there.
**Also yes that's what I said, I can localize something else relative to the metric field, but the metric field is localized with respect to what? **
But it is the metric field which gives locality simply by measuring distances
**That is Kuchars objection to the possibility of constructing locally interpretable Observables in a gaugeinvariant theory/fully constrained theory (which you reinvoke quantum mechanically when you talk about "rigging" but which is not really related to superposition/entanglement).**
Well, my objections are MINE, I can only conclude that Karel is a clever chap. No smartusz, I just added rigging for constructing diff invariant states, otherwhise you might have complained that I work with ``unphysical´´ objects which I still do since my master/hamiltonian constraint is not solved.
**
One can study this problem and the formalism to overcome it in any toy model, no Master constraint program needed. **
Rubbish, all these toy models probably have the wrong classical limit.
- #74
masudr
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This is a very interesting discussion which I have been following for a while now. It also seems that others have been too, with an average of 163 views a day.
- #75
arivero
Gold Member
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Careful said:
Can I contradict it, then? Newtons conception of time and movement is a lot simpler than phase space and symplectic areas.
- #76
Careful
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Hehe, it is all yours for the takenarivero said:
- #77
Hurkyl
Staff Emeritus
Science Advisor
Gold Member
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Turnabout is fair play -- do you want to stop using words like "wrong", "rational", and "magic"?Careful said:
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Careful
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Wrong I can replace by extremely unlikely sometimes yes, master HurkylHurkyl said:
Magic is justified, unless you believe Harry Potter is actually around. Rational hmmm better replace it by classical rationality.- #79
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Careful said:
don't understand. there are QG approaches which have a boundary. there is an observer outside. what is studied is inside the boundary and is not the *entire*
Are you saying that these are invalid, or that they are not "background independent" (in your definition of background independence)?
- #80
Careful
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marcus said:
Well, you have to make a distinction between types of boundaries. If you have a timelike tube with as boundary two closed spatial hypersurfaces, then basically you are computing transition amplitudes between closed universes, that is fine but has no bearing upon the issue of local observables.
On the other hand, when you take a spatio temporal four dimensional cube, force classical boundary conditions and ``quantize´´ only the internal degrees of freedom, then of course you are doing a major cheat. Not only can you not make any local statements yet about what is happening inside the box, but worse: you have entirely ignored the problems of (a) entanglement versus environmental decoherence (b) how to define local observables.
Stricly speaking doing such act is illegal within pure quantum gravity, you might want to read what James Hartle has written about the quantum mechanics of closed systems. That some people do it is more a sign that they have run out of ideas than anything else. Since, if you allow for such travesty, then simply divide a classical universe UNIV into tiny boxes A, subdivide these further into boxes B. Imprint boundary conditions on A induced by UNIV and do quantization with fixed boundary conditions within A selecting wavelengths larger than B. The Feynman series will be finite and no need for an infinite number of counterterms. End of story: if this is all you mean by quantum gravity then we could have solved it 30 years ago by putting in physical regulators on the perturbation series (something which appears to be superior to LQG for now).
Another person who realizes this problematic very well is Roger Penrose who actually explicitly introduces a Newtonian time and a set of dynamically preferred Newton Schrodinger states.
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- #81
jarek
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Careful said:
Nah, Careful, please...Mackey put it in a very well defined form. If you don't accept other forms of statistics apart from the classical one (who was talking about thinking out of the box
), then sure entanglement is magic, local realism is lost (at least on paper) and we desperately need a genius who will restore it (at least on paper) Cheers,
jarek
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Careful
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Haha, I guess what Mackey did was to extend upon the work of Von Neumann and Shannon about quantum entropy no? The entire difficulty is of course to tell what your quantal degrees of freedom are : in the framework of dynamical geometry (such as quantum gravity), you do not even know where to choose themjarek said:Nah, Careful, please...Mackey put it in a very well defined form. If you don't accept other forms of statistics apart from the classical one (who was talking about thinking out of the box
Cheers,
jarek
(although there are some proposals by 't Hooft). Moreover, as I seem to remember, Bernard d'Espagnat has written some rather severe comments upon the conceptual difficulties quantum statistics faces in light of the measurement problem (I do not remember that well anymore) - my comment was intented in that direction not on the technical aspect of the matter (in cases where the degrees of freedom can be easily identified). Concerning your silly entanglement; IF such thing were observed (one day
) the easiest possibility is to allow for classical signals traveling > c, there is no law in nature which forbids that. The point is that for the undisputed QM experiments I have checked (double slit, black body, etc) I don't need QM at all Cheers,
Careful
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- #83
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Careful said:
I am not talking about A or B. the boundary does not consist of two spatial hypersurfaces, nor does it consist of a four dimensional cube.
I am imagining a boundary with no beginning or end. I think of it as a tube (your word) but it has no space-like initial/final pieces.
AFAIK I am not imagining something that is a major cheat or illegal.
"course you are doing a major cheat. Not only can you not make any local statements yet about what is happening inside the box,..."
As a rule the observer can never say what is happening inside the box, he has only observations at the boundary. this is not illegal or a cheat, it is normal, or so I think.
maybe there are other ways of handling the observer problem---I understand it is a connundrum for all quantum mechanics I believe (not just some specialized subject like LQG or some other QG) and doubtless lots of people have written about it.
I am impressed by how negative your views are, careful. You seem to always be arguing that nobody's approach can ever be successful. You already know at the outset that various works in progress, like LQG, are doomed to failure. I find it curious.
- #84
Careful
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**I am not talking about A or B. the boundary does not consist of two spatial hypersurfaces, nor does it consist of a four dimensional cube.
I am imagining a boundary with no beginning or end. I think of it as a tube (your word) but it has no space-like initial/final pieces. **
What is the damn difference? You still have to put asymptotic boundary conditions on future and past timelike infinity. Do you think that changing the classical boundaries are offering you a way out of my argument ??!
**
AFAIK I am not imagining something that is a major cheat or illegal.
"course you are doing a major cheat. Not only can you not make any local statements yet about what is happening inside the box,..." **
I am sorry to tell you that you did not understand the difficulties of quantum gravity then. BTW what you say is even complete heresy in QFT where you can still calculate expectation values of local observables in the box.
** As a rule the observer can never say what is happening inside the box, he has only observations at the boundary. this is not illegal or a cheat, it is normal, or so I think. **
See my previous comments.
**maybe there are other ways of handling the observer problem---I understand it is a connundrum for all quantum mechanics I believe (not just some specialized subject like LQG or some other QG) and doubtless lots of people have written about it.**
The problem of the observer becomes much more difficult in QG than in ordinary QM (any student of QG learns that).
**
I am impressed by how negative your views are, careful. You seem to always be arguing that nobody's approach can ever be successful. You already know at the outset that various works in progress, like LQG, are doomed to failure. I find it curious. **
What I find curious is that:
(a) nobody offers rational counterarguments to my reasonable no-go statement : I mean an LQG protege could friendly give me a reasonable physical mechanism why I should believe the contrary.
(b) actually, many prominent LQG'ers have left the field for similar reasons
(c) many good scientists outside the field think likewise (amongst others roger penrose)
Moreover, I find my comments far from negative, they clearly indicate where the stumblestones are : recognizing those and looking for plausible cures are necessary conditions for succes. These problems are with us from the real beginning Marcus and very little if no progress has been made on these issues by any of the traditional approaches. There has been written a lot of crap about it undoubtedly. Moreover, I have given a LOGICAL reason why defining local observables within a *background independent* quantum universe is IMPOSSIBLE; it seems to me that people like you have tremendous problems with logic.
Cheers,
Careful
I am imagining a boundary with no beginning or end. I think of it as a tube (your word) but it has no space-like initial/final pieces. **
What is the damn difference? You still have to put asymptotic boundary conditions on future and past timelike infinity. Do you think that changing the classical boundaries are offering you a way out of my argument ??!
**
AFAIK I am not imagining something that is a major cheat or illegal.
"course you are doing a major cheat. Not only can you not make any local statements yet about what is happening inside the box,..." **
I am sorry to tell you that you did not understand the difficulties of quantum gravity then. BTW what you say is even complete heresy in QFT where you can still calculate expectation values of local observables in the box.
** As a rule the observer can never say what is happening inside the box, he has only observations at the boundary. this is not illegal or a cheat, it is normal, or so I think. **
See my previous comments.
**maybe there are other ways of handling the observer problem---I understand it is a connundrum for all quantum mechanics I believe (not just some specialized subject like LQG or some other QG) and doubtless lots of people have written about it.**
The problem of the observer becomes much more difficult in QG than in ordinary QM (any student of QG learns that).
**
I am impressed by how negative your views are, careful. You seem to always be arguing that nobody's approach can ever be successful. You already know at the outset that various works in progress, like LQG, are doomed to failure. I find it curious. **
What I find curious is that:
(a) nobody offers rational counterarguments to my reasonable no-go statement : I mean an LQG protege could friendly give me a reasonable physical mechanism why I should believe the contrary.
(b) actually, many prominent LQG'ers have left the field for similar reasons
(c) many good scientists outside the field think likewise (amongst others roger penrose)
Moreover, I find my comments far from negative, they clearly indicate where the stumblestones are : recognizing those and looking for plausible cures are necessary conditions for succes. These problems are with us from the real beginning Marcus and very little if no progress has been made on these issues by any of the traditional approaches. There has been written a lot of crap about it undoubtedly. Moreover, I have given a LOGICAL reason why defining local observables within a *background independent* quantum universe is IMPOSSIBLE; it seems to me that people like you have tremendous problems with logic.
Cheers,
Careful
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- #85
jarek
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Careful said:Haha, I guess all Mackey did was to extend upon the work of Von Neumann and Shannon about quantum entropy no?
Concerning your silly entanglement; IF such thing were observed (one day
Careful
No, I'm talking about orthomodular lattices more than about silly entropies
You will call it kinematics and say that it should be dynamically determined etc and I will agree. But if you need quantum statistics per se, understood here rather abstracly like you can speak of classical probability theory - here you are! Silly entanglement, I like the name
, has no connection whatsoever to superluminal signalling and you perfectly know that. C'mon, you know why Einstein called it spooky action at a distance and how he proposed to resolve it Cheers,
jarek
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What I am mainly challenging is what you said in post #70
what I said was
I still don't understand. Why do you think that " LQG or any background independent approach" necessarily treats the *entire* universe as one black box?
careful said:
what I said was
marcus said:
I still don't understand. Why do you think that " LQG or any background independent approach" necessarily treats the *entire* universe as one black box?
- #87
Careful
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**No, I'm talking about orthomodular lattices more than about silly entropies You will call it kinematics and say that it should be dynamically determined etc and I will agree. But if you need a quantum statistics, understood here rather abstracly, like you can speak of classical probability theory - here you are! **
Ah, indeed that is just kinematics I remember a joke about those following Piron et al : that they have forgotten there is something like the Schrodinger equation in QM
** Silly entanglement, I like the name , has no connection whatsoever to superluminal signalling and you perfectly know that. C'mon, you know why Einstein called it spooky action at a distance and how he proposed to resolve it **
Who says that I was talking about superluminal signalling (which we only know to be forbidden in free theories ), I was talking about waves which can go faster than c and transmit information between the particles. That is *not* forbidden, it does not necessarily lead to superluminal signalling. Although as I said, I don't care for the moment about this too much, first concentrate on double slit, atomic physics and so on. I feel that a natural solution for this (which I am working out now) will clarify the issue of EPR too.
na zdrowie
Careful
You will call it kinematics and say that it should be dynamically determined etc and I will agree. But if you need a quantum statistics, understood here rather abstracly, like you can speak of classical probability theory - here you are! **Ah, indeed that is just kinematics
I remember a joke about those following Piron et al : that they have forgotten there is something like the Schrodinger equation in QM ** Silly entanglement, I like the name
, has no connection whatsoever to superluminal signalling and you perfectly know that. C'mon, you know why Einstein called it spooky action at a distance and how he proposed to resolve it **Who says that I was talking about superluminal signalling (which we only know to be forbidden in free theories
), I was talking about waves which can go faster than c and transmit information between the particles. That is *not* forbidden, it does not necessarily lead to superluminal signalling. Although as I said, I don't care for the moment about this too much, first concentrate on double slit, atomic physics and so on. I feel that a natural solution for this (which I am working out now) will clarify the issue of EPR too. na zdrowie
Careful
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- #88
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Careful said:
first, please tell me what you mean by *background independent*
what I usually mean is that the theory does not explicitly assume a background metric on the manifold
but in discussions background independence is often used comparatively. one theory is MORE background independent than another (no theory is perfectly background independent, that might not even be meaningful)
what do you mean, exactly, by background independent?
- #89
jarek
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Careful said:Who says that I was talking about superluminal signalling (which we only know to be forbidden in free theories
Careful
Erm, by superluminal signalling I mean transmitting information faster than c. And this you cannot do with entangled pairs. You can invent whatever waves you like (Gisin and co once even put experimental bounds on their velocities) which propagate between entangled pairs, but information (meaning something you can access) carry they will not
Careful said:
I'm waiting especially for double slit :!)
Careful said:
Ah, that's something you say when you drink vodka (provided you can still speak
)jarek
- #90
Careful
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marcus said:
And I referred you to papers of James Hartle on closed quantum systems. Actually, there are people working in the foundations of quantum mechanics, trying to extend it in order to allow for ``classical components´´- the followers of Piron et al which I made a joke about to Jarek - so this is all kinematics for now. But your question is fairly basic quantum mechanics, which amounts to : where can we put the observer? In order to know that you have to be able to *dynamically* identify your classical components (Schroedinger cat problem, here she is again
) : you cannot just put it in by hand. Hence you notice immediatly a logical loophole : in order to identify dynamically classical localized subsystems you need local observables As a further example: anyone believing QG agrees that it was the dominant mechanism at the origin of the universe - unfortunatly you cannot put the observer in there in the way you see it.
If you want to know more about this, you can consult the road to reality of R. Penrose (not that I support everything he says there) - he explains it (in many pages) in a fairly entertaining way.
Cheers,
Careful
- #91
Careful
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**Erm, by superluminal signalling I mean transmitting information faster than c. And this you cannot do with entangled pairs. You can invent whatever waves you like (Gisin and co once even put experimental bounds on their velocities) which propagate between entangled pairs, but information (meaning something you can access) carry they will not **
I know, but it is easy to get out of that one: you just give this wave a label indicating that it can only interact with the EPR pair (not with the apparatus of course). The EPR particles or ``photons´´ do not travel faster than c obviously - in this way each of the particles can know of the detectorfield of the other (which each of them feel 3 nanoseconds in advance ) It seems extremely unlikely that perfect entanglement exists so on long distances any such line of thinking is saved by the very low measurement rates (which is actually a prediction in SED and therefore far from conspirational). But let's not discuss this now.
**
I'm waiting especially for double slit :!)
**
Aha, I know this one is the golden key, just wait two or three months (the physical idea is there, and the math is following).
**
Ah, that's something you say when you drink vodka (provided you can still speak ) **
Haha, naaa zzdrrooowie
**I know, but it is easy to get out of that one: you just give this wave a label indicating that it can only interact with the EPR pair (not with the apparatus of course). The EPR particles or ``photons´´ do not travel faster than c obviously - in this way each of the particles can know of the detectorfield of the other (which each of them feel 3 nanoseconds in advance )
It seems extremely unlikely that perfect entanglement exists so on long distances any such line of thinking is saved by the very low measurement rates (which is actually a prediction in SED and therefore far from conspirational). But let's not discuss this now. **
I'm waiting especially for double slit :!)
**
Aha, I know this one is the golden key, just wait two or three months (the physical idea is there, and the math is following).
**
Ah, that's something you say when you drink vodka (provided you can still speak
) **Haha, naaa zzdrrooowie
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Careful said:
this is your post #84 on this thread.
I want to learn from you, careful, if you have something definite to teach me.
Tell me your LOGICAL reason, that you have given.
Please say what you mean by *background independent* (because people in different discussions mean different things by it)
and say what you mean by local observables
and prove that it is IMPOSSIBLE to define them.
Since you have already given your logical reason somewhere, this should not be difficult for you to do---I hope in just a sentence or two.
Please do not refer me to some other books and authors. Just give me the LOGICAL reason which you mentioned having given. I will appreciate it, I assure you.
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- #93
jarek
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Careful said:But your question is fairly basic quantum mechanics, which amounts to : where can we put the observer? In order to know that you have to be able to *dynamically* identify your classical components (Schroedinger cat problem, here she is again
I like a lot what you are saying re dynamics vs. kinematics. You got me once thinking with similar remark (all that orthomodular toys should really be dynamically determined, not rigid as they are right now). However the only theory with kinematics following from dynamics seems to be GR, as Bergmann and the followers, like Lusanna, showed. You have any other examples or clues? It seems that nobody ever though along this lines in mechanics (be it quantum or classical).
Cheers,
jarek
PS Re EPR - this is what I thought - "confined" sort of information, so multiplying ghosts and moving towards magic. People who attack EPR are sometimes very predictable
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- #94
Careful
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**
I want to learn from you, careful, if you have something definite to teach me.
Tell me your LOGICAL reason, that you have given. **
If you want to define a local observable, then I said that you have the following possibilities:
(a) no superposition of rigmapped spin networks (which is as good as classical)
(b) classical timelike boundaries (possibly combined with spatial caps) - but then you have no local information about the interior.
(c) figuring out a mechanism which gives relational information (more than just topological one !) between nodes in two different spin networks
(d) measuring expectation values of global observables which you try to fit to a Lorentzian manifold (not a classical solution to the vacuum Einstein equations in case you include matter)
Option (b) runs straight against quantum mechanics. Option (c) is tantamount to picking a background structure, option (a) is killing off superposition (something I like), option (d) is plagued with ambiguities like any black box modelling is.
**
Please say what you mean by *background independent* (because people in different discussions mean different things by it) **
By background independent I mean - in the concrete context of spin networks - there is no further relational data provided between spin networks than knotting information. More generally, in a covariant formulation, I mean that there are no identifications given between the different spacetimes (no gauge fixing).
**and say what you mean by local observables**
An example of a local observable is : the position of the moon relative to the Earth given axes determined by the sun, Jupiter and saturnus. But the no-go argument *precisely* consists in asserting that ANY definition of a local observable REQUIRES extra relational information of the type mentioned above. If you do not specify any further information then you are bound to limit yourself to global observables such as average volume, dimension and so on, in either then you need to see the entire universe as a black box or you have to kill off superposition.
For example the point of view in dynamical triangulations is that only global spatial observables - such as average volume, dimension, curvature and higher moments of those - can be measured. As such they indirectly claim that local observables do not exist.
**Please do not refer me to some other books and authors. Just give me the LOGICAL reason which you mentioned having given. I will appreciate it, I assure you. **
?? Well, well, you can only do that I presume...
So I define a local observable indirectly by summing up the kind of examples it should be able to cover (actually I should add more to the list). This is a sensible strategy if you want to find a new mathematical object, you start by telling what it should do. Note that f-h did not give a definition of a local observable either, he intuitively argued that these observables are somehow showing localized behavior at the *classical* level.
I want to learn from you, careful, if you have something definite to teach me.
Tell me your LOGICAL reason, that you have given. **
If you want to define a local observable, then I said that you have the following possibilities:
(a) no superposition of rigmapped spin networks (which is as good as classical)
(b) classical timelike boundaries (possibly combined with spatial caps) - but then you have no local information about the interior.
(c) figuring out a mechanism which gives relational information (more than just topological one !) between nodes in two different spin networks
(d) measuring expectation values of global observables which you try to fit to a Lorentzian manifold (not a classical solution to the vacuum Einstein equations in case you include matter)
Option (b) runs straight against quantum mechanics. Option (c) is tantamount to picking a background structure, option (a) is killing off superposition (something I like), option (d) is plagued with ambiguities like any black box modelling is.
**
Please say what you mean by *background independent* (because people in different discussions mean different things by it) **
By background independent I mean - in the concrete context of spin networks - there is no further relational data provided between spin networks than knotting information. More generally, in a covariant formulation, I mean that there are no identifications given between the different spacetimes (no gauge fixing).
**and say what you mean by local observables**
An example of a local observable is : the position of the moon relative to the Earth given axes determined by the sun, Jupiter and saturnus. But the no-go argument *precisely* consists in asserting that ANY definition of a local observable REQUIRES extra relational information of the type mentioned above. If you do not specify any further information then you are bound to limit yourself to global observables such as average volume, dimension and so on, in either then you need to see the entire universe as a black box or you have to kill off superposition.
For example the point of view in dynamical triangulations is that only global spatial observables - such as average volume, dimension, curvature and higher moments of those - can be measured. As such they indirectly claim that local observables do not exist.
**Please do not refer me to some other books and authors. Just give me the LOGICAL reason which you mentioned having given. I will appreciate it, I assure you. **
?? Well, well, you can only do that I presume...
So I define a local observable indirectly by summing up the kind of examples it should be able to cover (actually I should add more to the list). This is a sensible strategy if you want to find a new mathematical object, you start by telling what it should do. Note that f-h did not give a definition of a local observable either, he intuitively argued that these observables are somehow showing localized behavior at the *classical* level.
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- #95
Careful
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**I like a lot what you are saying re dynamics vs. kinematics. You got me once thinking with similar remark (all that orthomodular toys should really be dynamically determined, not rigid as they are right now). However the only theory with kinematics following from dynamics seems to be GR, as Bergmann and the followers, like Lusanna, showed. You have any other examples or clues? It seems that nobody ever though along this lines in mechanics (be it quantum or classical). **
Pfew, that is a difficult one (I guess you are somehow referring to this discussion about dynamical entropy, no?). I doubt it if you can find a general prescription for such thing, even in concrete examples such as the amount of information stored on the black hole horizon, it gets very difficult if the horizon itself is non stationary.
**
PS Re EPR - this is what I thought - "confined" sort of information, so multiplying ghosts and moving towards magic. People who attack EPR are sometimes very predictable **
Haha, this was just the most obvious scenario which came to my mind in a few minutes.
Pfew, that is a difficult one (I guess you are somehow referring to this discussion about dynamical entropy, no?). I doubt it if you can find a general prescription for such thing, even in concrete examples such as the amount of information stored on the black hole horizon, it gets very difficult if the horizon itself is non stationary.
**
PS Re EPR - this is what I thought - "confined" sort of information, so multiplying ghosts and moving towards magic. People who attack EPR are sometimes very predictable
**Haha, this was just the most obvious scenario which came to my mind in a few minutes.
- #96
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I appreciate your efforts here. I am not entirely satisfied because I understood you to say you had a proof of a more general fact (not tied to spin networks). I will have to think and see if it generalizes in some obvious way.
The statement you claimed IIRC was that it is logically impossible to define local observables in an
background independent theory.
The usual meaning of background independent is that that theory does not require a fixed background metric on the manifold to be established in advance.
If I don't see, from your post, how to make good your "no-go" claim, I will get back to you.
Thx.
The statement you claimed IIRC was that it is logically impossible to define local observables in an
background independent theory.
The usual meaning of background independent is that that theory does not require a fixed background metric on the manifold to be established in advance.
If I don't see, from your post, how to make good your "no-go" claim, I will get back to you.
Thx.
Careful said:**
I want to learn from you, careful, if you have something definite to teach me.
Tell me your LOGICAL reason, that you have given. **
If you want to define a local observable, then I said that you have the following possibilities:
(a) no superposition of rigmapped spin networks (which is as good as classical)
(b) classical timelike boundaries (possibly combined with spatial caps) - but then you have no local information about the interior.
(c) figuring out a mechanism which gives relational information (more than just topological one !) between nodes in two different spin networks
(d) measuring expectation values of global observables which you try to fit to a Lorentzian manifold (not a classical solution to the vacuum Einstein equations in case you include matter)
Option (b) runs straight against quantum mechanics. Option (c) is tantamount to picking a background structure, option (a) is killing off superposition (something I like), option (d) is plagued with ambiguities like any black box modelling is.
**
Please say what you mean by *background independent* (because people in different discussions mean different things by it) **
By background independent I mean - in the concrete context of spin networks - there is no further relational data provided between spin networks than knotting information. More generally, in a covariant formulation, I mean that there are no identifications given between the different spacetimes (no gauge fixing).
**and say what you mean by local observables**
An example of a local observable is : the position of the moon relative to the Earth given axes determined by the sun, Jupiter and saturnus. But the no-go argument *precisely* consists in asserting that ANY definition of a local observable REQUIRES extra relational information of the type mentioned above. If you do not specify any further information then you are bound to limit yourself to global observables such as average volume, dimension and so on, in either then you need to see the entire universe as a black box or you have to kill off superposition.
For example the point of view in dynamical triangulations is that only global spatial observables - such as average volume, dimension, curvature and higher moments of those - can be measured. As such they indirectly claim that local observables do not exist. **Please do not refer me to some other books and authors. Just give me the LOGICAL reason which you mentioned having given. I will appreciate it, I assure you. **
?? Well, well, you can only do that I presume...
So I define a local observable indirectly by summing up the kind of examples it should be able to cover (actually I should add more to the list). This is a sensible strategy if you want to find a new mathematical object, you start by telling what it should do. Note that f-h did not give a definition of a local observable either, he intuitively argued that these observables are somehow showing localized behavior at the *classical* level.
- #97
Careful
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Ah, but the arguments are not tied to spin networks at all : for example they also apply to causal sets (I just presented them in a form suitable for spin networks for clarity). Joe Henson writes a lot about this issue in pretty much the same way as I speak about it: that is how points in different spacetimes could be ``the same´´ which is just gauge fixing in disguise IMO. Even if *you* believe some mighty clever construction might avoid my argumentation and still satisfy our intuition, try in good spirit yourself to figure out how it could work (you will see you end up in (a), (b), (c) or (d))
Ah, concerning the background metric : the argumentation here is a bit more difficult - I will try to be as clear as possible. In order to add this extra relational information without introducing a background metric you have to look for kinematical ``comparison mechanisms´´ depending only upon the intrinsic structures of the spin networks, causal sets or whatever. Apart from the fact that any choice of such ``way of comparing´´ is highly non-unique and quite complicated, any particular choice gives no unique answer either, which leads to further ambiguities. The main problem furthermore is that such ``identification mechanisms´´ are not transitive and neither symmetric, meaning that if I compare p_1 in U_1 with p_2 in U_2 and p_2 in U_2 with p_3 in U_3, then it is generically not like that that the same mechanism compares p_1 with p_3, and p_2 does not necessarily need to be compared with p_1. That is: the points in the different spacetimes do not form a chain. If you consistenly apply this weakness then you end up with the conclusion that any point in any spin network will be included in your definition of ``one point´´. The only way to avoid this ``diffusion of points´´ is to pick out one spin network which serves as a reference; this is your background.
Cheers,
Careful
Ah, concerning the background metric : the argumentation here is a bit more difficult - I will try to be as clear as possible. In order to add this extra relational information without introducing a background metric you have to look for kinematical ``comparison mechanisms´´ depending only upon the intrinsic structures of the spin networks, causal sets or whatever. Apart from the fact that any choice of such ``way of comparing´´ is highly non-unique and quite complicated, any particular choice gives no unique answer either, which leads to further ambiguities. The main problem furthermore is that such ``identification mechanisms´´ are not transitive and neither symmetric, meaning that if I compare p_1 in U_1 with p_2 in U_2 and p_2 in U_2 with p_3 in U_3, then it is generically not like that that the same mechanism compares p_1 with p_3, and p_2 does not necessarily need to be compared with p_1. That is: the points in the different spacetimes do not form a chain. If you consistenly apply this weakness then you end up with the conclusion that any point in any spin network will be included in your definition of ``one point´´. The only way to avoid this ``diffusion of points´´ is to pick out one spin network which serves as a reference; this is your background.
Cheers,
Careful
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- #98
f-h
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Careful, you are throwing a lot of independent problems together and mixing them up. Each individually can be addressed.
There are two issues of locality. Entangled states and background independence. These are conceptionally different and it would help if you would stop throwing them together.
One is the observer within the QM system, wavefunction of the universe style. Nobody knows how to do this (except perhaps for Hartle). It's a well known problem, and not specific to QG. However if we declare a part of the system to be the observer we have a working interpretation relative to that observer Everett style. No classical boundary information needed. (morally that's how Rovelli get's a propagator, notice that the classical boundary conditions drop out, there is a quantum mechanical state to which the question is relative, a semiclassical one, a superposition of many spinnetworks, but a real quantum mechanical state)
Classical GR, no matter. What are the local observables? If I construct them via Dittrich it is crucial to realize that Kinematical information (before implementing the diffeoinvariance) supplies the notion of locality.
Without that, talking purely about the 4-geometries allowed by Einsteins equations and not about metrics we don't have a notion of locality and the question becomes meaningless.
The notion of locality obtained by going kinematical refers to the possibility of auxiliary systems with the same kinematics being coupled to the background independent theory relative to which we ask local questions. Test particles.
Your spinfoam objection has a precise analogue in classical GR and is resolved by Rovellis/Dittrichs work. There is nothing specifically Quantum mechanical about your objection.
Freidel has constructed 2+1 background independent QFTs with testparticles. If I have a testparticle on a superposition of spin networks that is a sollution to the Hamiltonian constraint I can of course ask local questions with respect to it's location on the spin network. Just like I use testparticles in classical theory relative to which I can ask about the local state of the geometry (which I can't sensibly if I *only* work with the allowed geometries)
Can we define a sensible notion of relative locality of Quantum mechanical systems? Yes, if I take a spin state and couple it to a Quantum mechanical system I can say the coupling is local in time or space (as operators in the kinematical Hilbertspace!) or whatever other partial observable I cook up in the kinematics.
Is Quantum mechanics nonlocal? Not really. It leads to no nonlocal effects at least. That's one of the points of Rovellis relational QM which makes good sense if taken as an epistemology of QM rather then an interpretation.
So far I see no argument in anything you say that comes close to substantiating your very strong dogmatic claims which you have repeated several times now.
There are two issues of locality. Entangled states and background independence. These are conceptionally different and it would help if you would stop throwing them together.
One is the observer within the QM system, wavefunction of the universe style. Nobody knows how to do this (except perhaps for Hartle). It's a well known problem, and not specific to QG. However if we declare a part of the system to be the observer we have a working interpretation relative to that observer Everett style. No classical boundary information needed. (morally that's how Rovelli get's a propagator, notice that the classical boundary conditions drop out, there is a quantum mechanical state to which the question is relative, a semiclassical one, a superposition of many spinnetworks, but a real quantum mechanical state)
Classical GR, no matter. What are the local observables? If I construct them via Dittrich it is crucial to realize that Kinematical information (before implementing the diffeoinvariance) supplies the notion of locality.
Without that, talking purely about the 4-geometries allowed by Einsteins equations and not about metrics we don't have a notion of locality and the question becomes meaningless.
The notion of locality obtained by going kinematical refers to the possibility of auxiliary systems with the same kinematics being coupled to the background independent theory relative to which we ask local questions. Test particles.
Your spinfoam objection has a precise analogue in classical GR and is resolved by Rovellis/Dittrichs work. There is nothing specifically Quantum mechanical about your objection.
Freidel has constructed 2+1 background independent QFTs with testparticles. If I have a testparticle on a superposition of spin networks that is a sollution to the Hamiltonian constraint I can of course ask local questions with respect to it's location on the spin network. Just like I use testparticles in classical theory relative to which I can ask about the local state of the geometry (which I can't sensibly if I *only* work with the allowed geometries)
Can we define a sensible notion of relative locality of Quantum mechanical systems? Yes, if I take a spin state and couple it to a Quantum mechanical system I can say the coupling is local in time or space (as operators in the kinematical Hilbertspace!) or whatever other partial observable I cook up in the kinematics.
Is Quantum mechanics nonlocal? Not really. It leads to no nonlocal effects at least. That's one of the points of Rovellis relational QM which makes good sense if taken as an epistemology of QM rather then an interpretation.
So far I see no argument in anything you say that comes close to substantiating your very strong dogmatic claims which you have repeated several times now.
- #99
f-h
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Basically you do not understand the conceptional set up of background independence, and the nature of local physical statements in a background independent theory.
Really have you read Rovellis "What is observable in Classical and Quantum Gravity."?
C. Rovelli, What is observable in classical and quantum gravity?, Class Quant Grav 8 (1991) 297. G/A
This does not neccessarily represent our current best understanding of these issues but it lays some of the important conceptional groundwork from which to see them as the apparent nonsubstantial problems they are.
Really have you read Rovellis "What is observable in Classical and Quantum Gravity."?
C. Rovelli, What is observable in classical and quantum gravity?, Class Quant Grav 8 (1991) 297. G/A
This does not neccessarily represent our current best understanding of these issues but it lays some of the important conceptional groundwork from which to see them as the apparent nonsubstantial problems they are.
- #100
f-h
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arivero said:
I can explain the basic ideas of Lagrangian mechanics to someone who has never had any formal physics education a lot quicker then Newtons ideas.
Familiarity is not simplicity is not naturality.
- #101
Hurkyl
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Since you brought it up, I might as well ask one of the issues I had been sitting upon:Careful said:
The paper you mentioned in a previous thread seemed to say that in the presence of the background EM field acts in just the right way to corral electrons into a stable orbit. But at face value, this seems to reject the possibility of any other sort of orbit!
- #102
Careful
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**
There are two issues of locality. Entangled states and background independence. These are conceptionally different and it would help if you would stop throwing them together. **
In order to speak about an entangled state within the context of *background independent* quantum gravity, you have to be able to identify the local degrees of freedom (hence you need a background independent notion of locality) - something which is a priori given in background dependent QFT. In LQG you can only speak about superposition of rigged spin network states since you cannot identify the local degrees of freedom. If you throw in matter, let's say point particles, then these objects will quickly diffuse on the spin networks (as is known by QM), so how can you speak about local geometry when a single particle is smeared out over the distance of half a meter - say ? In that case you must be playing around with projection operators of the type : ``particle is on a vertex with such intertwiner, so many ingoing and outgoing edges and such spin labels´´ or ``This is the geometry, where is the particle?´´. But where is the OBSERVER ??
**
One is the observer within the QM system, wavefunction of the universe style. Nobody knows how to do this (except perhaps for Hartle). It's a well known problem, and not specific to QG. **
I did not claim it was specific to QG, I said QG makes the question more urgent. By the way, I always have seen THAT problem as the main one to be solved, we observers form part of the universe and cannot be separated from it. This has to be a dynamical result and not an assumption.
**However if we declare a part of the system to be the observer we have a working interpretation relative to that observer Everett style. No classical boundary information needed. (morally that's how Rovelli get's a propagator, notice that the classical boundary conditions drop out, there is a quantum mechanical state to which the question is relative, a semiclassical one, a superposition of many spinnetworks, but a real quantum mechanical state)**
But one problem is to show that this is consistent within a real dynamical framework (!) - that is you need to adress the issue of decoherence properly. Here Rovelli just argues that *background dependent* QM *practice* has thought us it is like that; sweeping lots of the difficulties from the table like that.
**Classical GR, no matter. What are the local observables? If I construct them via Dittrich it is crucial to realize that Kinematical information (before implementing the diffeoinvariance) supplies the notion of locality. **
Like I said, that is a gauge dependent construction. But it does not need to be like that at all you know : most classical relativists would argue that observers actually are nothing but matter flows themselves. Observables are diffeomorphism invariants which you can construct from matter and the metric. The spirit of Einsteins theory is that everything is dynamical included observation itself - of course this is one of the clashes between GR and QM which are not just solved by some local gauge fixing.
**
Without that, talking purely about the 4-geometries allowed by Einsteins equations and not about metrics we don't have a notion of locality and the question becomes meaningless.**
? The 4 - geometries allowed by Einsteins equations ARE the metrics.
**
The notion of locality obtained by going kinematical refers to the possibility of auxiliary systems with the same kinematics being coupled to the background independent theory relative to which we ask local questions. Test particles.**
Well first of all, you cannot violate ``background independence´´ classically. Second, when you pick out a gauge, it needs to be dynamically determined and this goes at the cost of adding Lagrange multipliers in your action principle. This is actually something Karel Kuchar has written a lot about in his papers on the problems of quantisation in the Gaussian gauge.
**
Your spinfoam objection has a precise analogue in classical GR and is resolved by Rovellis/Dittrichs work. There is nothing specifically Quantum mechanical about your objection. **
Of course there is something specifically quantum mechanical about my objections unless you separate the observer from an isolated subsystem of the universe (an option which I mentioned already). By the way when you pick out isolated subsystems that is tantamount putting on ``classical boundary conditions´´ - you have to be of bad will if you do not want to understand that.
** Freidel has constructed 2+1 background independent QFTs with testparticles. If I have a testparticle on a superposition of spin networks that is a sollution to the Hamiltonian constraint I can of course ask local questions with respect to it's location on the spin network. Just like I use testparticles in classical theory relative to which I can ask about the local state of the geometry (which I can't sensibly if I *only* work with the allowed geometries)**
I have heard about this: apart from the salient feature that there is no gravitation in 2+1 dimensions (the theory is just topological), it of course fairly obvious that you can ask local questions about the state of the geometry - like the ones I mentioned in the beginning. The particle, being in more geometries at once of course, and at the same time in more places at once in the same spin network. Of course, you can choose some time T which you call evolution, a parameter which you treat *classically* I presume (time should *also* be a quantum observable, no ?) and ask for the expectation value of the volume the particle is occupying or even a specific probability about the local geometry itself. It is just that for one realistic particle of dimensions of 10^{-18} meters you will have an immense number of states to consider.
**Is Quantum mechanics nonlocal? Not really. It leads to no nonlocal effects at least. That's one of the points of Rovellis relational QM which makes good sense if taken as an epistemology of QM rather then an interpretation.**
I am not going to nag about terminology here: Rovelli just does not address the issue of self consistency (of his relational QM) AFAIK by appealing to an argument that *experience* shows that it works consistently.
Look f-s, we clearly have different views here, when I speak about QG, I mean wave function of the universe, a unification between the observed and the observer. That is something LQG has stopped adressing, instead it took the more pragmatic road which is clear from the ideas behind relational QM (I am wondering when Rovelli is going to write a paper about consciousness and zombies).
Bedtime now.
Cheers,
Careful
There are two issues of locality. Entangled states and background independence. These are conceptionally different and it would help if you would stop throwing them together. **
In order to speak about an entangled state within the context of *background independent* quantum gravity, you have to be able to identify the local degrees of freedom (hence you need a background independent notion of locality) - something which is a priori given in background dependent QFT. In LQG you can only speak about superposition of rigged spin network states since you cannot identify the local degrees of freedom. If you throw in matter, let's say point particles, then these objects will quickly diffuse on the spin networks (as is known by QM), so how can you speak about local geometry when a single particle is smeared out over the distance of half a meter - say ? In that case you must be playing around with projection operators of the type : ``particle is on a vertex with such intertwiner, so many ingoing and outgoing edges and such spin labels´´ or ``This is the geometry, where is the particle?´´. But where is the OBSERVER ??
**
One is the observer within the QM system, wavefunction of the universe style. Nobody knows how to do this (except perhaps for Hartle). It's a well known problem, and not specific to QG. **
I did not claim it was specific to QG, I said QG makes the question more urgent. By the way, I always have seen THAT problem as the main one to be solved, we observers form part of the universe and cannot be separated from it. This has to be a dynamical result and not an assumption.
**However if we declare a part of the system to be the observer we have a working interpretation relative to that observer Everett style. No classical boundary information needed. (morally that's how Rovelli get's a propagator, notice that the classical boundary conditions drop out, there is a quantum mechanical state to which the question is relative, a semiclassical one, a superposition of many spinnetworks, but a real quantum mechanical state)**
But one problem is to show that this is consistent within a real dynamical framework (!) - that is you need to adress the issue of decoherence properly. Here Rovelli just argues that *background dependent* QM *practice* has thought us it is like that; sweeping lots of the difficulties from the table like that.
**Classical GR, no matter. What are the local observables? If I construct them via Dittrich it is crucial to realize that Kinematical information (before implementing the diffeoinvariance) supplies the notion of locality. **
Like I said, that is a gauge dependent construction. But it does not need to be like that at all you know : most classical relativists would argue that observers actually are nothing but matter flows themselves. Observables are diffeomorphism invariants which you can construct from matter and the metric. The spirit of Einsteins theory is that everything is dynamical included observation itself - of course this is one of the clashes between GR and QM which are not just solved by some local gauge fixing.
**
Without that, talking purely about the 4-geometries allowed by Einsteins equations and not about metrics we don't have a notion of locality and the question becomes meaningless.**
? The 4 - geometries allowed by Einsteins equations ARE the metrics.
**
The notion of locality obtained by going kinematical refers to the possibility of auxiliary systems with the same kinematics being coupled to the background independent theory relative to which we ask local questions. Test particles.**
Well first of all, you cannot violate ``background independence´´ classically. Second, when you pick out a gauge, it needs to be dynamically determined and this goes at the cost of adding Lagrange multipliers in your action principle. This is actually something Karel Kuchar has written a lot about in his papers on the problems of quantisation in the Gaussian gauge.
**
Your spinfoam objection has a precise analogue in classical GR and is resolved by Rovellis/Dittrichs work. There is nothing specifically Quantum mechanical about your objection. **
Of course there is something specifically quantum mechanical about my objections unless you separate the observer from an isolated subsystem of the universe (an option which I mentioned already). By the way when you pick out isolated subsystems that is tantamount putting on ``classical boundary conditions´´ - you have to be of bad will if you do not want to understand that.
** Freidel has constructed 2+1 background independent QFTs with testparticles. If I have a testparticle on a superposition of spin networks that is a sollution to the Hamiltonian constraint I can of course ask local questions with respect to it's location on the spin network. Just like I use testparticles in classical theory relative to which I can ask about the local state of the geometry (which I can't sensibly if I *only* work with the allowed geometries)**
I have heard about this: apart from the salient feature that there is no gravitation in 2+1 dimensions (the theory is just topological), it of course fairly obvious that you can ask local questions about the state of the geometry - like the ones I mentioned in the beginning. The particle, being in more geometries at once of course, and at the same time in more places at once in the same spin network. Of course, you can choose some time T which you call evolution, a parameter which you treat *classically* I presume (time should *also* be a quantum observable, no ?) and ask for the expectation value of the volume the particle is occupying or even a specific probability about the local geometry itself. It is just that for one realistic particle of dimensions of 10^{-18} meters you will have an immense number of states to consider.
**Is Quantum mechanics nonlocal? Not really. It leads to no nonlocal effects at least. That's one of the points of Rovellis relational QM which makes good sense if taken as an epistemology of QM rather then an interpretation.**
I am not going to nag about terminology here: Rovelli just does not address the issue of self consistency (of his relational QM) AFAIK by appealing to an argument that *experience* shows that it works consistently.
Look f-s, we clearly have different views here, when I speak about QG, I mean wave function of the universe, a unification between the observed and the observer. That is something LQG has stopped adressing, instead it took the more pragmatic road which is clear from the ideas behind relational QM (I am wondering when Rovelli is going to write a paper about consciousness and zombies).
Bedtime now.
Cheers,
Careful
- #103
Careful
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A stable orbit is not simply circular or elliptic, the electron actually is performing a chaotic motion.Hurkyl said:
- #104
Careful
- 1,670
- 0
**Basically you do not understand the conceptional set up of background independence, and the nature of local physical statements in a background independent theory. **
I do, it is just that we have very different ideas about the measurement problem. Rovelli and co have gone towards MWI, thereby withdrawing from iffy quantum mechanical issues; a ``solution´´ I find unacceptable.
I do, it is just that we have very different ideas about the measurement problem. Rovelli and co have gone towards MWI, thereby withdrawing from iffy quantum mechanical issues; a ``solution´´ I find unacceptable.
- #105
Hurkyl
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Thus I said it's being corraled into its orbit.
The point is if that the background field is believed to act by pushing the electron towards this particular orbit, then by what phenomenon would an electron manage to maintain any other sort of orbital?