Where we stand-Baez talk at Luminy

[Hurkyl]

I don't care for the moment about this too much, first concentrate on double slit, atomic physics and so on.

Since you brought it up, I might as well ask one of the issues I had been sitting upon:

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!

 

[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

 

[Careful]

Since you brought it up, I might as well ask one of the issues I had been sitting upon:

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!

A stable orbit is not simply circular or elliptic, the electron actually is performing a chaotic motion.

 

[Careful]

**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.

 

[Hurkyl]

A stable orbit is not simply circular or elliptic, the electron actually is performing a chaotic motion.

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?

 

[Hurkyl]

Rovelli and co have gone towards MWI, thereby withdrawing from iffy quantum mechanical issues; a ``solution´´ I find unacceptable.

Why is that unacceptable? Progress is often made by abandoning (the strictest form of) old concepts when they prove to be problematic.

In some sense, a problem of QM is that we demand classical answers from the quantum theory. So abandoning this demand seems to be the most natural thing to do.

Obviously you have a strong bias towards the classical, so you would naturally reject any attempt to head further away from the classical. Is there anything more to your opinion?

 

[Careful]

Ok f-h, now that I know in what subfield of QG you are, let me ask you a question (for a change). Karel Kuchar was already playing around with ideas concerning gauge fixing in the mid eighties. However, he is keen on manifest general covariance and insisted upon gauge fixing and at the same time securing the former. Hence, he needed to add the gauge conditions (in covariant form to the action) using Lagrange multipliers. What he found is that in this way some matter is introduced (I do not remember the type anymore) and of course the Hamiltonian constraint can be EXACTLY solved. This is in perfect agreement with the relativist idea that a preferred coordinate system needs to be dynanamically determined and coincides with a matter flow. It turned out however that the quantum version of the theory suffers from a breakdown of the gauge conditions.

Now, Dittrich and Rovelli, as you said introduce these gauge conditions *kinematically*, thereby dropping the issue of general covariance. Actually, apart from Thiemann, it seems almost anyone has given upon quantum covariance : (a) no strong closure of the Dirac algebra (b) I think one still needs to show in the master constraint programme how the Dirac algebra can be derived from the ``master algebra´´ (at the classical level). So, your quantum evolution operator is going to be explicitely gauge dependent, how can you reasonably argue that your classical limit is going to be GR?

 

[Careful]

**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? **

Ah, the other orbitals can only be maintained when the lower ones are (just as in QM these higher orbitals are unstable if only one electron is present). An electron in a higher orbital is going to resonate with a different component of the background field too, the electron-electron interactions are probably important here, about ``electron-spin´´ I am more reserved.

 

[Careful]

**Why is that unacceptable? Progress is often made by abandoning (the strictest form of) old concepts when they prove to be problematic.**

True, but it depends upon the scientist at hand what particular aspect is considered as problematic.

**
In some sense, a problem of QM is that we demand classical answers from the quantum theory. So abandoning this demand seems to be the most natural thing to do. **

Well, for me the most natural line of thinking is just the opposite.

**
Obviously you have a strong bias towards the classical, so you would naturally reject any attempt to head further away from the classical. Is there anything more to your opinion?**

Apart from the unbelievable philosophical implications this line of thinking has, there are indeed some serious issues left - some of which I partially adressed already. To name a few:
(a) As far as concerns the issue of consciousness, I DO think that the overall goal of QG demands that the measurement problem gets clarified (b) much more serious however are issues concerning the uniqueness of the dynamics (I do not expect the kinematics to be of much importance), a weakness which shows up already in attempts to quantize the Hamiltonian constraint and gets only worse in the more liberal spin network setup.
(c) the classical limit (you give up on quantum covariance so...?)
(d) the entire lack of ``economy´´, a good theory of nature should fit observation and be minimalistic, the degrees of freedom in such theory will be gigantic.
(e) the issue of self-consistency.
(d) there are fundamental aspects in cosmology I do not see adressed in LQG, ie the cosmological constant problem. This is a fundamental issue common to both GR and QFT nevertheless.

By the way, I did much more than expressing an opinion; I believe this discussion revealed quite well some of the aspects one is facing and which solution comes at what cost. Perhaps now, even more people will think Rovelli's approach is the correct one.

 

[jarek]

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.

:puke: why are you attacking me with your silly entropy all the time I was rather referring to point individuation, as that would be an example how you get "kinematical" background (i.e. manifold itself, of diff class rather) from dynamics (Einstein Eq's). I KNOW that such schemes are difficult, but no difficulty is too big to save the holy realism a la 19th century physics

I think Aharonov once tried something distantly similar, i.e. making kinematics part of dynamics, in QM. There are two articles: "Is the notion of time evolution correct in QM" or something like that.

best,
j

 

[Hurkyl]

Careful: your response doesn't make sense at all.

Merely abandoning the hypothesis that measurements must behave in a classical fashion does not change the classical limit, introduce additional degrees of freedom, introduce new chances for inconsistencies, or anything like that.

I think you've mentally substituted some particular theory of quantum gravity for what I actually said.

 

[f-h]

*But one problem is to show that this is consistent within a real dynamical framework (!)*

This is true classically as well. There is no theory of physics where we are actually able to describe observations as changes of the state of mind of a realistically modeled human brain. The problem of QM is not that it can not do that, but that it appears to be fundamentally incompatible with any such construction.

QG makes this problem no more or less urgent. If I treat part of the system under study as classical I can describe the rest by QM and get perfectly fine answers. There is nothing novel or unique here in this respect.

"Entanglement needs locality"

Wrong! Entanglement needs a Hilbertspace whose dimensionality is not a primenumber and a physically suggested split into two subspaces. I just saw a very nice talk by Terno where he points out that even in ordinary QFT such a split can not be Lorentz covariant.

*The 4 - geometries allowed by Einsteins equations ARE the metrics.*

Wrong. Geometries are metrics up to diffeomorphisms. Geometries know no points hence no locality, metrics do. Metrics are kinematical, before the implementation of the constraints, geometries are the gaugeinvariant dynamical entities.

* Well first of all, you cannot violate ``background independence´´ classically. *

Hu?

*Of course, you can choose some time T which you call evolution, a parameter which you treat *classically* I presume*

Nope, I work completely quantum mechanically. Time operator and all. Thiemann has a paper on reduced vs constrained Quantisation that addresses this point as well.

*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.*

So are you saying it's technically difficult or that it's conceptionally impossible?

Also one issue is locality. We have diffeoconstraints which mess up locality at first glance. We also have the issue that we have no external evolution parameters. Related but different issues.

*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.*

Not true. Self consistency is the whole point of Rovellis exercise. There's a student here in Marseille who is just working out a detailed description of EPR from Rovellis perspective.

*(I am wondering when Rovelli is going to write a paper about consciousness and zombies)*

Rovelli is about the last person to suspect of that. He deeply detests all talk of concioussnes and mysticism.
You are right, LQG is not trying to solve all problems of theoretical physicis at the same time. It's not a theory of everything, it doesn't solve the conceptional problems of Quantum mechanics and it can't cure the common cold. But it is a viable theory of Quantum Gravity. You are arguing that it can not succeed at solving the problems it is looking at, and you do repeat your opinion time and time again without producing arguments.

Some of the things you point out are real subtlties. But your attack on them illustrates my initial point in this thread. It's worth deeply and carefully considering the established theories to see what they have to tell us about the problems we face. In the case of the problems you point out they appear to be indeed solvable by this precise method, and your declarations of impossibility are premature and inconsistent.

*I do, it is just that we have very different ideas about the measurement problem. *

Wait so now your objection has transformed into "LQG doesn't solve the meassurement problem"? Well yes. I agree, it does not. Why should it? It seems to boil down to that you have a certain bias on what Quantum Gravity should be and LQG doesn't fit that bias. *shrugs* fair enough. We don't need to solve the meassurement problem or unification to do physics.

*Actually, apart from Thiemann, it seems almost anyone has given upon quantum covariance*

Dittrich was Thiemanns student when she wrote on complete Observables. Rovellis intention is precisely to construct covariant Observables with a local interpretation. The Observables themself are nonlocal in a technical sense, but they have a local interpretation.

On the technical side I'm very much a beginner, I do not know for sure but I believe that classically it has been shown that the Master constraint algebra is equivalent to the usual constraint Algebra. As usual I do not know what this issue could possibly have to do with the issue of local observables.

Most of what I do is not related to quantizing classical systems, but to interpreting quantum mechanical ones. We construct observables on the Physical Hilbertspace, where all constraints (including the "evolution constraint") are implemented already, so we don't have "quantum evolution Operators". There is no "evolution operator" anywhere in Rovelli/Dittrich. Your question doesn't make sense.

 

[f-h]

Also unless you sit down and describe in detail the precise language, assumptions and arguments of your "no-go theorems" and how they relate to what the host of people who have been working in this area have come up with, I'm out of this thread.
I hope it has been informative to some of the people following it, but it is entering the going round in circles phase now.

Also, my English sucks (especially above). Apologies.

 

[Careful]

**
I think Aharonov once tried something distantly similar, i.e. making kinematics part of dynamics, in QM. There are two articles: "Is the notion of time evolution correct in QM" or something like that.

**

I must confess I deliberately created some confusion with f-h in the hope he would learn something apart from Rovelli's credo (and actually see some weaknesses in them), but it seems I will have to be straightforward. The point identification schemes I was talking about serve geometrical path integral approaches to a unification of geometry, quantum mechanics (in the path integral formulation) AND matter (such as people try in causal sets). The idea goes back to Einstein :he was not very pleased about GR, the right hand -or left hand- side of the equation should not be there. That is, matter should be observable geometrical exitations or vice versa geometry should be the result of matter interactions, i.e. the metric as an effective observable and not as a dynamical variable. What I argued was that within the first scenario, when doing ``quantum mechanics´´ a la Feynman, a background metric is required in case you want to define local observables (in the second scenario that would probably be likewise). The second possibility is what the Sundance preon model could serve for, matter models interacting in a vacuum minkowski background creating physical measure sticks.

Now, Loop Quantum gravity, spin foam and all these models are of course not unifying matter and geometry. Moreover, the very fact that matter needs to be *different* from geometry in these models in order to define local observables can legitimatly be interpreted as a serious weakness, since the latter implies that there is no room within the quantised framework for further unification anymore. A further ramification of this procedure is that the number of states gets immense : one would definately need superselection rules to single out a very limited class of physical states.

It appears to me that there is a fundamental clash between background dependence, unfication of geometry, matter and QM on the one hand and a background independent quantization of geometry and matter on the other. Concluding : I am pretty much convinced that background independence is not the guiding principle for physics.

Cheers,

Careful

 

[Careful]

**
"Entanglement needs locality"

Wrong! Entanglement needs a Hilbertspace whose dimensionality is not a primenumber and a physically suggested split into two subspaces. I just saw a very nice talk by Terno where he points out that even in ordinary QFT such a split can not be Lorentz covariant. **

Of course, technically you can construct entanglement like that, but what does it do for me PHYSICALLY ?

**
*The 4 - geometries allowed by Einsteins equations ARE the metrics.*

Wrong. Geometries are metrics up to diffeomorphisms. Geometries know no points hence no locality, metrics do. Metrics are kinematical, before the implementation of the constraints, geometries are the gaugeinvariant dynamical entities. **

This is funny, by metrics I mean metrics up to diffeomorphisms (just language you know, I am a relativist so why would I speak of coordinate tensors?). Of course do geometries KNOW locality at the classical level (where did you get that wrong idea - there has been written plenty of stuff about that).


* Well first of all, you cannot violate ``background independence´´ classically. *

Hu?

Sure, background independence at the classical level means : solution of Einstein equations.


**
Not true. Self consistency is the whole point of Rovellis exercise. There's a student here in Marseille who is just working out a detailed description of EPR from Rovellis perspective. **

Of course that is the whole point, but he still needs to SHOW that, no ?

** *(I am wondering when Rovelli is going to write a paper about consciousness and zombies)*

Rovelli is about the last person to suspect of that. He deeply detests all talk of concioussnes and mysticism. **

Good, so then he should be consistent and avoid constructions which do lead to such exotism.

**
You are right, LQG is not trying to solve all problems of theoretical physicis at the same time. It's not a theory of everything, it doesn't solve the conceptional problems of Quantum mechanics and it can't cure the common cold. But it is a viable theory of Quantum Gravity. You are arguing that it can not succeed at solving the problems it is looking at, and you do repeat your opinion time and time again without producing arguments.**

No, I was hoping you to figure out that I was not speaking about LQG at all and at the same time to see that LQG has some irrepairable weaknesses (even if they would succeed in what they try to do).

**In the case of the problems you point out they appear to be indeed solvable by this precise method, and your declarations of impossibility are premature and inconsistent.**

No, they are pointing into the direction that a physical theory which leaves room for improvement is probably NOT background independent. The solution you presented was the very motivation for Einstein behind relativity : study matter configurations relative to other similar configurations.

** *I do, it is just that we have very different ideas about the measurement problem. *

Wait so now your objection has transformed into "LQG doesn't solve the meassurement problem"? Well yes. I agree, it does not. Why should it? It seems to boil down to that you have a certain bias on what Quantum Gravity should be and LQG doesn't fit that bias. *shrugs* fair enough.
*Actually, apart from Thiemann, it seems almost anyone has given upon quantum covariance* **

Ohw, this was just another thing I casually mentioned.

** (including the "evolution constraint") are implemented already, so we don't have "quantum evolution Operators". There is no "evolution operator" anywhere in Rovelli/Dittrich. Your question doesn't make sense. **

Sigh, evolution constraint versus evolution operator : If you have your constraint H, you are defining projection operators : P = limit(T -> infinity)1/2T integral(- T, T) dt exp( Ht) which is nothing but the Wick rotated version of limit of the operator : (T -> infinity) i/2T int(-T,T) exp(i H t) which contains the ``time evolution´´ exp(i H t).

I know that it is difficult to speak to people thinking differently but taking such a scolar ``definition oriented´´ attitude is not going to be of use.

Cheers,

Careful

 

[Careful]

**puke: why are you attacking me with your silly entropy all the time **

Ah, I was not sure where you were heading to...

** I was rather referring to point individuation, as that would be an example how you get "kinematical" background (i.e. manifold itself, of diff class rather) from dynamics (Einstein Eq's). I KNOW that such schemes are difficult, but no difficulty is too big to save the holy realism a la 19th century physics **

You don't have any idea how far I want to go for that But at least I am having good fun.

** I think Aharonov once tried something distantly similar, i.e. making kinematics part of dynamics, in QM. There are two articles: "Is the notion of time evolution correct in QM" or something like that. **

Thanks, for an orthomodular lattice guy you are pretty well informed about the interesting things too must have to do with your relativity past.

Cheers,

Careful

 

[f-h]

The entanglement is as physical as the split in the overall Hilbertspace you introduce. As I mentioned it's never even lorentz invariant in QFT, unless you take noninteracting systems.

*Of course do geometries KNOW locality at the classical level (where did you get that wrong idea - there has been written plenty of stuff about that).*

How? If you meant geometries when you said metrics my original question stands. The value g_mu,nu (x) of the metric tensor field is *meaningless*. It's notdiffeoinvariant. What are your local observables?
As a simple matter of fact (as I should have pointed out before) until Dittrichs construction there were only a handfull of observables of GR (and only for special cases) known at all. All of them global.
All calculations proceeded by introducing test systems and "fixing gauge". Of course it's not actually fixing gauge, gauge symmetries are not violated in nature.

Look a lot of what you say doesn't make any sense to me. If that's intentional then you're wasting my time, if not then perhaps you should go and polish your statements.

Ok, next thing. The natural thing in the quantum mechanical system is to introduce locality and what you erronously insist on calling gaugefixing AFTER the construction of the projection operator. THerefore my projection operator can not depend on them therefore your statements are just blatantly false.

It's obvious that you have thought about these issues for some time. But in the few areas where I have a rather good idea what's going on and could pin them down to something concrete your statements are just wrong, in those I have no clear grip on they seem just confused.

You are making up and dropping claims as you see fit, and screaming out extraordinary claims without a shred of argumentation. I usually call this behavior on the Internet trolling.

 

[Careful]

**The entanglement is as physical as the split in the overall Hilbertspace you introduce. As I mentioned it's never even lorentz invariant in QFT, unless you take noninteracting systems.**

But in background dependent QFT the split is *natural* since you can speak of local degrees of freedom (given a Lorentz frame).

**
As a simple matter of fact (as I should have pointed out before) until Dittrichs construction there were only a handfull of observables of GR (and only for special cases) known at all. All of them global. **

No, they are not, many invariants of the metric : Ricci scalar, Riemann scalar, gradients of the ricci scalar and so on allow you to put up a relational interpretation by amongst others measuring geodesic distances, this is old stuff really. And these invariants GENERICALLY allow you to construct local interpretations.

**
Look a lot of what you say doesn't make any sense to me. If that's intentional then you're wasting my time, if not then perhaps you should go and polish your statements. **

Well, you think it is a GOOD feature matter that it is necessary to define local observables. I argued why can look on this issue differently : I cannot help it if the ideas which I tell you sound strange or weird (they are actually very old and well known by specialists). For example: are you telling me you did not hear about the possibility of looking on gravity as
an *emergent* phenomenon coming particle interactions in Minkowski? Or vice versa, that a geometrization program of matter is one of the options?


**
Ok, next thing. The natural thing in the quantum mechanical system is to introduce locality and what you erronously insist on calling gaugefixing AFTER the construction of the projection operator. THerefore my projection operator can not depend on them therefore your statements are just blatantly false. **

I think again that we are missing each other's intentions here (there exist many models around each with different features so I can only guess what you are doing unless you SAY IT). Your gauge fixing is KINEMATICAL (right?) so I assume you start from something like causal spin networks which are foliated : each ``hypersurface´´ carrying a time label (am I still correct?). At time t_0 in the folation time you pick out a superposition of spatial spin networks each of which carry a copy of the particle at some vertex. Then you have some path integral formulation involving local amplitudes corresponding to elementary moves as well for the geometry of the spin network and for the particle (right ?). So now you can do a couple of things : you can ask about the probability that the local geometry at the node the particle is in at *foliation time* t is such and such. That I would call a gauge dependent construction (since you introduce a preferred foliation). Another thing which you could do is to introduce an external classical clock cl and ask about the expectation value of the time t at cl = 5 (here you could image allowing for processes which also go backwards in time). I agree that in BOTH cases the projection operator does NOT depend upon this issue (quite logical). I was merely pointing out that your DYNAMICAL rules do not necessary correspond to what you might expect the fully quantized Hamiltonian constraint to do.

**
It's obvious that you have thought about these issues for some time. But in the few areas where I have a rather good idea what's going on and could pin them down to something concrete your statements are just wrong, in those I have no clear grip on they seem just confused. **

They ARE not, I told you I was deliberatly misguiding you in the beginning. Moreover, you seem to be stuck into one type of language which makes it impossible to see for you that the other party might be suggesting something else than you think.

Cheers,

Careful

 

[f-h]

*No, they are not, many invariants of the metric : Ricci scalar, Riemann scalar, gradients of the ricci scalar and so on allow you to put up a relational interpretation by amongst others measuring geodesic distances, this is old stuff really. And these invariants GENERICALLY allow you to construct local interpretations.*

The Ricci scalar et al are invariants of the metric wrt local lorentz transformations, they are not gaugeinvariant wrt diffeomorphisms. They are not invariants of the GEOMETRY as they are not invariant under active diffeomorphisms. So when you say metric do you mean the metric field or the metric field up to equivalence under diffeomorphisms? You claimed before that it was the later, now you say something that indicates the former. It's hard to learn the language you are speaking in if you are constantly shifting.
Geodesic distance between what? The best thing you could do is say something like "the sum over all geodesic distances between places where the Ricci scalar field is 5". That's probably a gaugeindependent statement. I can do the same in LQG. That kind of statement has never been translated into an observable, and it can be argued that it's pretty nonlocal anyways.

Matter is what we usually do in classical GR for natural localized observables. But the matter might of course just be a peculiar (perhaps renormalized, perhaps open edge) excitation of the spin network. Just any identifiable featur relative to which we can localize.

- In QFT nothing that requires a Lorentz frame is natural.

I personally work with simpler toy models, and mostly on the conceptional framework, independent of a particular model, but what you describe is not at all what is suggested by Rovelli/Dittrich.
Assume for the moment that we have one constraint left to implement on some space K. The projector gives me a subspace of this space H which is annihalated by the constraint, the states of this subspaces are "nonlocal" in K relative to the usual operators (say with eigenvectors as specific spin network states). So any operator on this subspace H seems to be naturally nonlocal as well. Some of these operators correspond to local questions though (is a specific spin network in the "nonlocal" superposition? If so where's the particle on it?).
But I'm always talking about operators on H! So these operators are really gauge invariant questions! They get their *interpretation* through kinematical considerations, and this interpretation is not unique in any sense, but I'm manifestly nowhere breaking the gauge invariance of the observables or the states. I never even need to think about transitions between specific "timeslices" or foliations. This breaks gaugeinvariance no more or no less then the Newton Wigner operator breaks Lorentz invariance. You get different Newton Wigner operators for different frames but each individual operator has to be lorentz covariant.

One of the issues is that the spin network basis is not very well suited for intuitions regarding this. The physical projector acting on the spin networks does not produce a time orderd sequence of spin networks but a superposition of spin networks without any ordering. A spin network is not projected to a spinfoam or something like that.

Perez has written about the breakdown of this intuition in the case of LQG. Well worth checking out.

 

[Careful]

**
The Ricci scalar et al are invariants of the metric wrt local lorentz transformations, they are not gaugeinvariant wrt diffeomorphisms. They are not invariants of the GEOMETRY as they are not invariant under active diffeomorphisms. So when you say metric do you mean the metric field or the metric field up to equivalence under diffeomorphisms? You claimed before that it was the later, now you say something that indicates the former. It's hard to learn the language you are speaking in if you are constantly shifting. **

No, you just are not aware of the old relational constructions between events which are labelled by metric invariants : that is all. Ask to Rovelli about this, I guess he will explain you.

**
Geodesic distance between what? **

Between two events labelled by physical (metric) coordinates (a,b,c,d). Generically, there are only four of them: in special cases we have to add coordinates to avoid global ambiguities (of course in Minkowski you are lost - but there you cannot ask physical questions anyway). You know, matter curves geometry, this effect is visibe in the Ricci scalar, the Riemann tensor and so on...

**
The best thing you could do is say something like "the sum over all geodesic distances between places where the Ricci scalar field is 5". **

You can do MUCH better than that, but at least you start thinking.

**That's probably a gaugeindependent statement.**

Yes it is

**I can do the same in LQG. **

Of course you can

**
That kind of statement has never been translated into an observable, and it can be argued that it's pretty nonlocal anyways. **

It is not nonlocal at all if you understood differential geometry; actually the geodesic distance is the most natural variable to consider in GR, Synge has rewritten entire GR just based upon this.

** Matter is what we usually do in classical GR for natural localized observables. But the matter might of course just be a peculiar (perhaps renormalized, perhaps open edge) excitation of the spin network. Just any identifiable featur relative to which we can localize. **

Matter information can be GENERICALLY retrieved from metric invariants.


** - In QFT nothing that requires a Lorentz frame is natural.**

In flat spacetime for sure it is required : in curved spacetime QFT becomes much more complicated.

**I personally work with simpler toy models, and mostly on the conceptional framework, independent of a particular model, but what you describe is not at all what is suggested by Rovelli/Dittrich. **

But Smolin Markopolou work with that and many others provide similar models in such a spirit. Why should I only take into account the suggestions by Rovelli/Dittrich?

**Assume for the moment that we have one constraint left to implement on some space K. The projector gives me a subspace of this space H which is annihalated by the constraint, the states of this subspaces are "nonlocal" in K relative to the usual operators (say with eigenvectors as specific spin network states). So any operator on this subspace H seems to be naturally nonlocal as well. Some of these operators correspond to local questions though (is a specific spin network in the "nonlocal" superposition? If so where's the particle on it?). **

So, that is something similar to the projection operator for the constraint H I have written down before. Eeuh I would call the question whether a specific spin network is in a ``non local´´ superpostion a non local one. Could you mention why I shoud think of this as local?

**But I'm always talking about operators on H! So these operators are really gauge invariant questions! They get their *interpretation* through kinematical considerations, and this interpretation is not unique in any sense, but I'm manifestly nowhere breaking the gauge invariance of the observables or the states. I never even need to think about transitions between specific "timeslices" or foliations. This breaks gaugeinvariance no more or no less then the Newton Wigner operator breaks Lorentz invariance. You get different Newton Wigner operators for different frames but each individual operator has to be lorentz covariant.**

I think there was a confusion in our different use of language. My main concern is : what is your constraint and correspondingly H? How does it relate to the classical Hamiltonian constraint? How are all the four constraints treated and how should I think about the quantum Dirac algebra? So, it is quite obvious that you can abstractly look for gauge invariant statements. I did not deny that, my concerns are mainly of the above type.

**
One of the issues is that the spin network basis is not very well suited for intuitions regarding this. The physical projector acting on the spin networks does not produce a time orderd sequence of spin networks but a superposition of spin networks without any ordering. A spin network is not projected to a spinfoam or something like that. **

Ah ok, so you are really just modelling a constraint type of operator and looking for ``frozen´´ states. this was not entirely clear for me before and I was thinking more about spin foam, where you also easily ask such kind of questions.

Cheers,

Careful

 

[f-h]

**I can do the same in LQG. **

- Of course you can.

Then what did you say I can not do in QG again? I thought your point was somehow that this amounted to unacceptable gaugefixing?

Yes when I say gaugeindependent in a fully constrained system that means frozen. Dittrichs observables are frozen observables on frozen states.

We started out talking about locality in Quantum mechanical systems without a background structure. So now you agree that in LQG I can play the same old relational space time scalar game.
This is of course one of the predecessors for (or a special case of) Rovellis game. You use a nongauginvariant quantity (the Rici scalar field) and relate it's value to another quantity that is nongaugeinvariant too in such a way that you get a gauge invariant quantity (Dittrich basically replays that game in the new language in her second paper on Observqbles in GR, and to my knowledge that is the first time the space time scalar game has actually been translated into real Dirac observables). Of course that's not the whole story since this does not reflect how we do real GR predictions. This is a chief concern, to make conceptionally clear how the actual predictions we make can be formulated in a background independent formalism.

*Matter information can be GENERICALLY retrieved from metric invariants.*

Are you talking about Kuchar's constructions now? Could you expand on this a bit?

---
What are you actually saying I can not do in LQG?
You made very general points about what is impossible in Quantum Gravity. I point out that in LQG I can do these things so I provide a counterexample to your initial claims of incompatibility, no?

Now you are saying that we don't have a good Quantum algebra, that the quantization of GR has only partially succeeded so far. Well I would have to agree since Thiemann says the same thing as well. But what has that to do with your initial claims that background independence and locality are incompatible in Quantum mechanics?

"So, it is quite obvious that you can abstractly look for gauge invariant statements." So if I can do that, and these have a local interpretation in an appropriate sense of the word local (as you seem to agree for spacetime scalars for example which Dittrich treats with the same methods), then there is no fundamental conceptional incompatibility between Background independence, locality and QM, right?

So then there is no reason to conclude that background independence as suggested by GR is a red herring Quantum mechanically.

 

[Careful]

**
- Of course you can.

Then what did you say I can not do in QG again? I thought your point was somehow that this amounted to unacceptable gaugefixing? **

Sorry, but I told you EXPLICITELY in my previous message what my DIRECT concerns are vis a vis your toy models. I explained you why this idea of how to construct local observables is LIMITED, no room for improvement any more. The lack of further unification possibilities leads to a pleithorea of physical states, so you will have to tell us why nature is so selective (a kind of initial value problem if you want to).


** We started out talking about locality in Quantum mechanical systems without a background structure. So now you agree that in LQG I can play the same old relational space time scalar game. **

Sure, but you did at the same time not make any effort to dig deeper into the measurement problem either. But you still do not understand here what I am telling you.

**
This is of course one of the predecessors for (or a special case of) Rovellis game. You use a nongauginvariant quantity (the Rici scalar field) and relate it's value to another quantity that is nongaugeinvariant too in such a way that you get a gauge invariant quantity (Dittrich basically replays that game in the new language in her second paper on Observqbles in GR, and to my knowledge that is the first time the space time scalar game has actually been translated into real Dirac observables).**

Yes and no, I told you in the beginning already that some old knowledge was rewritten into the ADM framework. No, because if you add sufficiently many invariants then you can give an event a diffeomorphism invariant meaning even if you would naively expect the diffeo's to shift the point around in the manifold. You are still not fully comprehending the magnitude of how far you can gow with this idea.


** Of course that's not the whole story since this does not reflect how we do real GR predictions. This is a chief concern, to make conceptionally clear how the actual predictions we make can be formulated in a background independent formalism. **

Very simple, take ONE point with physical coordinates (a,b,c,d) and call that ``I now´´. The worldline will be generically determined by the dynamics and we can make observations like we do them with sattelites and so on. ADDENDUM : of course you might say : well, we need to say where in the four manifold *all* people are alive, so we need a kinematical gauge to do that. You might do that in this way, but I have pointed out on the philosophy forum once that you can also choose phyiscal globally defined ``lifetime´´ functions to achieve that. Moreover in *classical* GR any such construction is NOT changing anything to the way we observe the universe, this still happens by sending/recieving signals from distant galaxies (the lifetime function being more like a philosophical completion of GR). However in QM, this DOES change the physics in the sense that it provides a natural basis for defining entanglement. It *changes* the phyiscs because entanglement gives you non-local correlations which CANNOT be explained by local mechanisms unless you add ghost fields which violate causality or so. People usually think that they did nothing wrong because entanglement is just ``kinematical´´ : that is how dangerous empty words can be.

**
*Matter information can be GENERICALLY retrieved from metric invariants.*

Are you talking about Kuchar's constructions now? Could you expand on this a bit? **

Just read about the genericity conditions for spacetimes, I do not remember if it was Kuchar in particular, Bergmann, Ehlers and others have done a lot of work on constructing many different ways to interpret GR just on basis of the metric (and light signals).

**
---
What are you actually saying I can not do in LQG?
You made very general points about what is impossible in Quantum Gravity. **

The general point I make about *background independent* quantum gravity is that in principle NO further unification between matter and geometry is possible (the way I argued about this is indeed through the very notion of local observable). Moreover, since you still work with the metric field as dynamical variable, I am afraid that you won't come to a satisfying unification with electromagnetism either. En plus, you do not change anything substantial to QFT and neither to GR, so I do not see why you should solve the cosmological constant unless you can somehow kill off the vacuumenergy in QFT by a natural mechanism.

**
I point out that in LQG I can do these things so I provide a counterexample to your initial claims of incompatibility, no? **

You still do not understand that I was trying YOU to DEDUCE that these ideas are all quite limited and produce an extreme high number of degrees of freedom.

**
Now you are saying that we don't have a good Quantum algebra, that the quantization of GR has only partially succeeded so far. Well I would have to agree since Thiemann says the same thing as well. But what has that to do with your initial claims that background independence and locality are incompatible in Quantum mechanics? **

Ahhhhrrrr, I explained you already that background independence and locality are not incompatible PROVISO that you do not care about solving other problems floating around. And it is silly to agree with me because Thiemann says the same, perhaps we are both demented (who knows ?? )

**
So then there is no reason to conclude that background independence as suggested by GR is a red herring Quantum mechanically. **

I just said that to you in the beginning hoping that you would try to look for something else (but no, just defensively shutting down).

My main concern about all this business is that - apart from the very scarse ``progress´´ which has been made in the last 20 years - there is a very sharp *small* possibility for further improvements to be made. I tried to make you see a glimpse of that by going over to a construction where matter would be ``geometrical exitations´´ but it seems I failed miserably (I still underestimate the defensive reflex of people).

By the way, Smolin seems bend over to the geometry as an observable of an interacting matter ensemble too, but of course still has to say that you can do this topologically (in either background independent). That is where I disagree and I have offered a series of arguments why you can expect a background to be necessary in the case of matter from geometry.

Cheers,

Careful

 

[Careful]

To wrap up: the very limited goals set up by LQG and similar approaches are a weakness since other theories outside QM and GR (say EM) are making us aware that something else is happening (I have pointed this out several times in my posts) - at least when you *really* try to unify them. So, even in the event one will succeed in finishing LQG or whatever spin foam in a natural way at some point in time (and everything points into the direction that this is unlikely), then still have you constructed a theory which is too narrow if you want to do a good job incoorporating other forces. That is a good reason why one should look for new physics, no ??

Cheers,

Careful

 

[jarek]

FYI: I´m nowhere even close to orthomodular guys , I don't even know any in person

Here are the Aharonov papers:

IS THE USUAL NOTION OF TIME EVOLUTION ADEQUATE FOR QUANTUM-MECHANICAL SYSTEMS .1.
PHYSICAL REVIEW D 29 (2): 223-227 1984

IS THE USUAL NOTION OF TIME EVOLUTION ADEQUATE FOR QUANTUM-MECHANICAL SYSTEMS .2. RELATIVISTIC CONSIDERATIONS
PHYSICAL REVIEW D 29 (2): 228-234 1984

I like the second one much more Anyone aware if his description of evolution in QM (surfaces rather than time parameter, much like ADM) has been further developed?

Cheers,
j

 

[Careful]

**FYI: I´m nowhere even close to orthomodular guys , I don't even know any in person **

My sincere apologies ()

**
Here are the Aharonov papers:

IS THE USUAL NOTION OF TIME EVOLUTION ADEQUATE FOR QUANTUM-MECHANICAL SYSTEMS .1.
PHYSICAL REVIEW D 29 (2): 223-227 1984

IS THE USUAL NOTION OF TIME EVOLUTION ADEQUATE FOR QUANTUM-MECHANICAL SYSTEMS .2. RELATIVISTIC CONSIDERATIONS
PHYSICAL REVIEW D 29 (2): 228-234 1984

**

Thanks, I will certainly take a look at it.

Cheers,

Careful

 

[CarlB]

I explained you why this idea of how to construct local observables is LIMITED, no room for improvement any more. The lack of further unification possibilities leads to a pleithorea of physical states, so you will have to tell us why nature is so selective (a kind of initial value problem if you want to).

I think that this is the heart of the matter. If I understand you, in terms of QFT, it boils down to the Coleman-Mandula theorems. If so, I would like to point out that these theorems assume perfect Poincare symmetry, and if an underlying theory violates this, it will allow more possibilities for unification.

In particular, the Koide mass formula is pretty good evidence that there is a unification out there:

The strange formula of Dr. Koide
Alejandro Rivero, Andre Gsponer
We present a short historical and bibliographical review of the lepton mass formula of Yoshio Koide, as well as some speculations on its extensions to quark and neutrino masses, and its possible relations to more recent theoretical developments.
http://www.arxiv.org/abs/hep-ph/0505220

For a list of the problems that the above formula gives to the standard model, and also to the usual ways of unification, see Dr. Koide's comments here:

Challenge to the Mystery of the Charged Lepton Mass Formula
Yoshio Koide
Why the charged lepton mass formula m_e +m_\mu +m_\tau = 2/3\; (\sqrt{m_e}+\sqrt{m_\mu} +\sqrt{m_\tau})^2 is mysterious is reviewed, and guiding principles to solve the mystery are presented. According to the principles, an example of such a mass generation mechanism is proposed, where the origin of the mass spectrum is attributed not to the structure of the Yukawa coupling constants, but to a structure of vacuum expectation values of flavor-triplet scalars under Z_4 \times S_3 symmetries.
http://www.arxiv.org/abs/hep-ph/0506247

Carl

 

[arivero]

I think that this is the heart of the matter. If I understand you, in terms of QFT, it boils down to the Coleman-Mandula theorems.

And to the puzzling detail of CKM mixing between generations; I still do not understand fully how the mixing can happen under Coleman-Mandula conditions; the whole electroweak group should commute with Poincare, and the mass eigenstates should be charge eigenstates too, shouln't?

 

[CarlB]

And to the puzzling detail of CKM mixing between generations; I still do not understand fully how the mixing can happen under Coleman-Mandula conditions; the whole electroweak group should commute with Poincare, and the mass eigenstates should be charge eigenstates too, shouln't?

This smells very much like the windmill what JC Yoon has been tilting at recently. Well the holy grail on which I'm "working", though I really don't know how to do it, is to get the MNS matrix into the same form as the Koide mass formula. There is a bit of a hint by JC Yoon that I do not understand. Wish I did. I put comment #6 here on the question of whether the CKM / MNS matrices imply that charge is not conserved:

https://www.physicsforums.com/showthread.php?t=106583

My guess is that if you argued that Coleman Mandula was contrary to mixing angles, the response would be identical to the response JC Yoon got when he claimed that the standard model is not perfectly Lorentz symmetric. That is:
https://www.physicsforums.com/showthread.php?t=108277

Carl

 

[arivero]

Well I have no problem with the Standard Model; the Lagrangian of the Standard Model does not have mass terms, all the particles are nicely massless. Masive particles are incompatible with Nonabelian Gauge fields, we are taught in the kindergarten.

But I have a respect for Wigner classification scheme and Coleman-Mandula theorem (nice and short paper, btw) so I wonder what happens when particles get mass and must transmute from the massless to massive representations.

 

[marcus]

To get back to the initial post on this thread

a simple rather beautiful talk

http://math.ucr.edu/home/baez/where_we_stand/mentioned in yesterday's TWF #227
http://math.ucr.edu/home/baez/week227.html

Here is my take on it: what I think the talk by Baez at Luminy means.
Luminy and Perimeter are the two capitals of LQG, Baez was talking to the LQG community plus a wider audience.

LQG is beginning to strive for a background independent unified theory that should explain dynamical geometry AND matter.

Baez was saying something simple and possibly profound. namely he says look at these mysteries

black hole
dark matter
cosmological constant
curious neutrino behavior
messiness of the standard model

He is saying, I think, that one should find a fundamental QG theory that
explains dark matter, consmological constant, what happens inside black holes, neutrino astonomy observations, and simplifies the standard model.

He was suggesting, I think, that if one could find the fundamental degrees of freedom describing dynamical geometry that could also very likely turn out to describe matter. Such a theory could, I think, be local because matter defines locality

a place ultimately is always referred to a material event or object----a tabletop, a rock, a tree, a measuring machine. Baez does not mention locality in his talk, but I think he is evoking the idea of a unified theory (unifying matter and geometry) and such a theory has matter to establish place.

Baez paper that he released about the same time as he posted his "where we stand---fundamental physics" talk has an essentially TOPOLOGICAL character.
It is about 4D beef theory. Where matter is discovered as some topological tangles in the beef.

there are some microscopic defects in the spacetime that create the snarly-gnarly of matter----stuff in the geometry that doesn't go away easily.

That is enough, or already too much, to say about the new Baez paper on "4D BF theory" but it does provide a little context for "where we stand" because they appeared about the same time.

Here's the 4D beef paper.
http://arxiv.org/abs/gr-qc/0603085
Here is one of the PF threads about it
https://www.physicsforums.com/showthread.php?p=945369#post945369

 

[marcus]

I think if we really want to have a discussion in this thread, which is actually a discussion of Baez "where we stand" Fundamental Physics talk, then we need to begin by looking at the SIX MYSTERIES that form the backbone of his talk.

http://math.ucr.edu/home/baez/where_we_stand/where_we_stand.pdf

I think LQG is entering a new stage where it strives to provide a unified theory (of spacetime and matter) and if it can explain matter then for example it should be able to explain MASS which is closely connected to the geometry of spacetime. This is one of Baez 6 mysteries.

I will quote the mysteries and also, in parens, comments he makes in between to help make them understandable:

===quote Baez===
Mystery 1. What is making the expansion of the universe accelerate? Does the vacuum have negative pressure?

Mystery 2. Does the Higgs really exist? What is the origin of mass?

Mystery 3. Why do these 18 numbers have the values they do? ...(Where can we find defects in the Standard Model? In the heavens.)

Mystery 4. Do neutrino oscillations fit into a slightly modified Standard Model – now requiring 25 dimensionless numbers – or must the theory be changed more drastically?

Mystery 5. What happens to things when they fall into a black hole?

(... there are not enough stars to explain the mass of the Virgo Supercluster! This ‘missing mass problem’ is also evident in other ways:
• Galaxies rotate faster than can be explained by all understood forms of mass.
• Our theories of galaxy formation don’t work without positing ‘cold dark matter’.
• Fluctuations in the microwave background radiation fit a model with cold dark matter, not a model without. We need at least 5 times more cold dark matter than normal matter! Or perhaps something more radical: e.g., general relativity is wrong.)

Mystery 6. What is cold dark matter – or what else explains what this hypothesis tries to explain?

(...But meanwhile, experiments and observations continue, showing that we live in a universe that is far from understood, even at the simple level of fundamental physics. This is not bad. It merely leaves more fun for our children and grandchildren. . . if we leave them a world in which they can afford to study such questions.)

===end quote===

 

[Careful]

Everyone in physics knows these questions.

**
He was suggesting, I think, that if one could find the fundamental degrees of freedom describing dynamical geometry that could also very likely turn out to describe matter. Such a theory could, I think, be local because matter defines locality **

The problem which such line of thinking is the *enormous* amount of freedom in the number of states the universe can be in (by the way, I argued that such line of thinking will always involve a kind of ``preferred geometry´´). Basically, you still have to tell which piece of spacetime 1 corresponds to which piece of spacetime 2 and so on - complete freedom in such choice introduces ambiguities growing faster than n! where n is the number of ``events´´ in your spacetime. So, by this kinematical labelling procedure you can retrieve ``locality´´ - this is an old story, say in causal sets (where people really try to do better than that and still use Minkowski as reference background ). You know, it is possible to repair just any shortcoming by increasing the number of degrees of freedom in your construction - that is an ancient trick. It is just that by doing this, you are simply ignoring the fact that you did not understand at all what is going on (even worse: success in such approach might even convince you that this was the right path - that is what happened with quantum mechanics). Moreover, any such theory is far removed from LQG which is still more or less directly ``quantized gravity´´.

**
a place ultimately is always referred to a material event or object----a tabletop, a rock, a tree, a measuring machine. Baez does not mention locality in his talk, but I think he is evoking the idea of a unified theory (unifying matter and geometry) and such a theory has matter to establish place. **

Sure, that is what we do in practice, but the question is whether the theory can do this in a elegant and minimalistic manner.

**
Baez paper that he released about the same time as he posted his "where we stand---fundamental physics" talk has an essentially TOPOLOGICAL character.
It is about 4D beef theory. Where matter is discovered as some topological tangles in the beef. **

That is possible in principle but *extremely* difficult. Causal sets is trying to do this for about 20 years now and the lesson to be learned from this is the following: one is only still at the very beginning in understanding how to characterize dynamical geometry without reference to the continuum. Now, you might say : ``ah but this is beautiful, plenty of open problems for the future generation´´. I would agree with this if the benifit would be a better understanding of quantum mechanics and the measurement problem : I mean if you want to go that far in constructing a theory which is unlikely to give any substantial predictions in 50 years, then you better make sure it is worthwhile, no ? I can only observe that one is still looking for a right formulation of ``quantum´´ in that context, so also causal set researchers do not really know very well how to deal with QM.

The painful contradiction in this story is that really fundamental research in QM is frowned upon and almost forbidden; so far your liberty in academics

Cheers,

Careful

 

[marcus]

The painful contradiction in this story is that really fundamental research in QM is frowned upon and almost forbidden; so far your liberty in academics

Cheers,

Careful

You sound very disappointed, Careful. Have you tried yourself to do really fundamental research in QM and found this by your own experience that it is not approved?

 

[marcus]

I think if we really want to have a discussion in this thread, which is actually a discussion of Baez "where we stand" Fundamental Physics talk, then we need to begin by looking at the SIX MYSTERIES that form the backbone of his talk.

http://math.ucr.edu/home/baez/where_we_stand/where_we_stand.pdf

I think LQG is entering a new stage where it strives to provide a unified theory (of spacetime and matter) and if it can explain matter then for example it should be able to explain MASS which is closely connected to the geometry of spacetime. This is one of Baez 6 mysteries...

for continuity, here are the 6 mysteries Baez mentioned, in condensed paraphrase:

Mystery 1. Acceleration? (What dark energy is supposed to explain)

Mystery 2. Mass? (Inertia and gravitational mass: the interaction between matter and spacetime)

Mystery 3. Model parameters. (Why these particular numbers?)

Mystery 4. Neutrino oscillations?

Mystery 5. Black holes. (What happens to things? The infall.)

Mystery 6. Missing mass? (Explain what cold dark matter was invented to explain...or else say what it is.)


(Baez comment about #6... there are not enough stars to explain the mass of the Virgo Supercluster! This ‘missing mass problem’ is also evident in other ways:
• Galaxies rotate faster than can be explained by all understood forms of mass.
• Our theories of galaxy formation don’t work without positing ‘cold dark matter’.
• Fluctuations in the microwave background radiation fit a model with cold dark matter, not a model without. We need at least 5 times more cold dark matter than normal matter! Or perhaps something more radical: e.g., general relativity is wrong.)

======================

Baez new paper "4D BF theory" suggests that matter is made of the same stuff as space. Figuratively, matter is just the tangles in it.
And time is a homotopy.

This is clearly inviting bids for a unified theory. And it is not an isolated instance. there are papers not just by Baez Wise and Cran but also by Oriti and Ryan, Krasnov, Freidel, Livine, Smolin, Sundance, Starodubstev and so on.

So by all appearances it looks like the LQG community is getting ready for a push for some unified theories.

And a unified theory (a theory of space time and matter) ought to address some of Baez six puzzlesSO HERE WE HAVE THE TOPIC of the thread in the form of a nice DEBATE ISSUE. Some people will immediately want to give reasons they have thought up of why IT WILL NEVER WORK and why whatever the LQG community comes up with along the lines of these recent papers is bound to FAIL. Or they say it is a waste of taxpayer money, or they say they know the only right way to approach the problem but nobody will listen to them.

And the other side of the debate is YES IT MIGHT FAIL but it is only a few people--- the LQG people are a small research community with at most a few dozens active in this line---and it is a very SMALL EXPENSE of taxpayer money---and it MIGHT SUCCEED. They might actually get closer to a new understanding of spacetime and matter!

=============================

here's another PF thread discussing the direction LQG is going with links to some recent papers
https://www.physicsforums.com/showthread.php?t=115289

 

[Careful]

You sound very disappointed, Careful. Have you tried yourself to do really fundamental research in QM and found this by your own experience that it is not approved?

Fundamental research in QM only gets accepted:
(a) when your name is 't Hooft, Biley, ... or when you are directly ``protected´´ by them either in the sense that you work together with one of those people, or you are working on 't Hooft quantum mechanics.
(b) when you elaboratly confirm in the first 2 pages of your paper how great quantum mechanics is, that it was the most supreme human intellectual achievement of all times
(c) when you explicitely mention in your paper that it is merely your intention to humbly probe deeper into the conceptual framework of QM
(d) depending on whether (a) is satisfied or not, people will ask you to present a solution for ALL well known problems at once (in one paper).

Of course, Marcus I am intensively working on QM (I am a persistent person when I found something *worthwile* to do). Obviously, my ideas shifted already a great deal since I started doing this, they got entirely classical. I am actually still shocked that most physicists take Bell's theorem seriously, it is so easy to bypass it while still securing Lorentz invariance in the necessary way. You have no idea how religious rational people can be about their pet theory.

Cheers,

Careful

 

[Careful]

**
SO HERE WE HAVE THE TOPIC of the thread in the form of a nice DEBATE ISSUE. Some people will immediately want to give reasons they have thought up of why IT WILL NEVER WORK and why whatever the LQG community comes up with along the lines of these recent papers is bound to FAIL. **

Well marcus, that is an essential part of the debate, no ?? You simply seem to forget that these objections require answers (the critics are usually not stupid you know). The key issue in these presumed ``background independent´´ approaches is LOCALITY. The ``solutions´´ presented by the LQG community are well known for a few decades and do no not contribute really to our understanding. They merely replace the locality problem for the question why God placed his pin into the enormous landscape of possibilities in order to generate our universe (by the way, this does not mean yet that the locality problem is solved - the right dynamics still has to decide about that.). Moreover, locality or not, there are plenty of ambiguities left in LQG - the quantization of the Hamiltonian constraint seems to be plagued by that already. BTW, that is exactly the reason why I boldly state that locality has not been solved: I do not like pseudo solutions.

** And the other side of the debate is YES IT MIGHT FAIL but it is only a few people--- the LQG people are a small research community with at most a few dozens active in this line---and it is a very SMALL EXPENSE of taxpayer money---and it MIGHT SUCCEED. They might actually get closer to a new understanding of spacetime and matter! **

No, they actually did not. What you are presenting now (matter from geometry) has little or nothing to do with what has been tried out between 1985 and 2000 when people were rather exclusively looking for a quantum version of the constraint algebra (because that was matter + geometry) and spin networks were known from the days Penrose launched them (around 1975 I guess). By the way, I think Hossi also asked you why this should be only part of the LQG community, there are plenty of other less problematic approaches which can account for that.

Cheers,

Careful

 

[marcus]

The painful contradiction in this story is that really fundamental research in QM is frowned upon and almost forbidden; so far your liberty in academics

Cheers,

Careful

You sound very disappointed, Careful. Have you tried yourself to do really fundamental research in QM and found this by your own experience that it is not approved?

Fundamental research in QM only gets accepted:
(a) when your name is 't Hooft, Biley, ... or when you are directly ``protected´´ by them either in the sense that you work together with one of those people, or you are working on 't Hooft quantum mechanics.
(b) when you elaboratly confirm in the first 2 pages of your paper how great quantum mechanics is, that it was the most supreme human intellectual achievement of all times
(c) when you explicitely mention in your paper that it is merely your intention to humbly probe deeper into the conceptual framework of QM
(d) depending on whether (a) is satisfied or not, people will ask you to present a solution for ALL well known problems at once (in one paper).

Of course, Marcus I am intensively working on QM (I am a persistent person when I found something *worthwile* to do). Obviously, my ideas shifted already a great deal since I started doing this, they got entirely classical. I am actually still shocked that most physicists take Bell's theorem seriously, it is so easy to bypass it while still securing Lorentz invariance in the necessary way. You have no idea how religious rational people can be about their pet theory.

Cheers,

Careful

I wish you would give us links to your arXiv articles, so we can sample some of your research

 

[f-h]

*people will ask you to present a solution for ALL well known problems at once (in one paper).*

It's of course perfectly reasonable to expect the same from the LQG community with respect to the tricky conceptional issues involved. ;)

 

[Careful]

*people will ask you to present a solution for ALL well known problems at once (in one paper).*

It's of course perfectly reasonable to expect the same from the LQG community with respect to the tricky conceptional issues involved. ;)

No, there is a very distinct difference you know. If you manage to find a deeper realist theory behind quantum mechanics then many fundamental problems are solved at once, especially the conceptual ones. BTW: the LQG community already had 20 years to ``settle´´ the conceptual issues in a proper way - fundamental research into QM only revived very recently by very few people. I have the impression that researchers somehow got the idea that conceptual issues will be magically solved if enough technical progress is made (in the wrong conceptual framework).

I will spell out the details behind my own ideas on a realist QM in proper time (as I promised before + I mentioned this will take still around two months). For now, my only intention is to try to convey to you (marcus) that a physical theory usually starts out from a very clear conceptual picture (of the mechanisms at hand); then and only then the technical aspects follow. My conclusion in this game is that a thorough reformulation of QM and GR are necessary in order to restore the conceptual picture and that these problems are certainly *not* going to be solved by playing around with toy models. There is nothing wrong or negative when I present some of my arguments why I think LQG will not succeed in that in a satisfactory way: it is up to you to respond to them.

Cheers,

Careful

 

[marcus]

Careful what I would like from you is an arxiv link to one or several of your research papers, please. You have made your own research rejection by academia an issue. I want to know what it is.

I don't especially want to discuss the way that scientific theories are supposed to develop (whether from initially playing around with toy models or NOT doing that) in this thread.

I think of LQG as what the people in the LQG research community do---several lines of investigation in progress and shifting----I am not quite sure what your definition is, or how relevant your discussion of it is to this thread.

the topic of this thread is Baez talk on the current situation in fundamental physics and looking ahead----the role of research pursued by the LQG people at Luminy is also relevant since Baez was addressing those people, he knows their research, and he has just brought out a 4D BF theory paper himself at about the same time. (4D BF is one of those things being studied under the general rubric of LQG these days, as per the Freidel Starodubstev paper last year---yes I know putting BF with LQG is untidy, active research is not arranged in neat logical compartments)

 

[CarlB]

(d) depending on whether (a) is satisfied or not, people will ask you to present a solution for ALL well known problems at once (in one paper).

An example of this would be someone having you explain what "measurement" means when the standard model clearly doesn't have a clue.

Carl

 

[Careful]

**Careful what I would like from you is an arxiv link to one or several of your research papers, please. You have made your own research rejection by academia an issue. I want to know what it is. **

Oh no, Marcus I did not make that an issue at all : my background (as a person) here is entirely unimportant (and many of my papers were well received actually), let's stick to the content right ? I thought on this forum we can discuss physics and not be merely royalty watchers. When I spoke about realist papers I did not speak about myself in the first place. I am offering some specific general discussion points about background independent approaches and I get no response from you on these concrete fundamental issues.

**
the topic of this thread is Baez talk on the current situation in fundamental physics and looking ahead **

So, that means we can speak about fundamental issues in physics, no? Good, you don't like what I say, so please argue. In case you find yourself unable to do so, then we indeed better quit this discussion which has clearly no use then. For a change, can I ask you why you are so LQG oriented, you clearly are not aware of the details of many works, so how can you so confidently defend it ? Because you simply don't like the monopoly of string theory and you are in for some alternative ideas ?

Cheers,

Careful

 

[Careful]

An example of this would be someone having you explain what "measurement" means when the standard model clearly doesn't have a clue.

Carl

You got it

 

[marcus]

The painful contradiction in this story is that really fundamental research in QM is frowned upon and almost forbidden; so far your liberty in academics

Cheers,

Careful

You sound very disappointed, Careful. Have you tried yourself to do really fundamental research in QM and found this by your own experience that it is not approved?

Fundamental research in QM only gets accepted:
(a) when your name is 't Hooft, Biley, ... or when you are directly ``protected´´ by them either in the sense that you work together with one of those people, or you are working on 't Hooft quantum mechanics.
(b) when you elaboratly confirm in the first 2 pages of your paper how great quantum mechanics is, that it was the most supreme human intellectual achievement of all times
(c) when you explicitely mention in your paper that it is merely your intention to humbly probe deeper into the conceptual framework of QM
(d) depending on whether (a) is satisfied or not, people will ask you to present a solution for ALL well known problems at once (in one paper).

Of course, Marcus I am intensively working on QM (I am a persistent person when I found something *worthwile* to do). Obviously, my ideas shifted already a great deal since I started doing this, they got entirely classical. I am actually still shocked that most physicists take Bell's theorem seriously, it is so easy to bypass it while still securing Lorentz invariance in the necessary way. You have no idea how religious rational people can be about their pet theory.

Cheers,

Careful

I have to repeat. Your apparent disatisfaction with the reception of your research is an issue. You have raised the business of academic freedom and suppression of approaches that arent somebody's (like 't Hooft's) pets. and you keep suggesting that you have the right approach and that whatever the QG community at large is pursuing is doomed to fail. It is too obvious that if your ideas can't be accepted then everybody else has to be wrong.

You clearly have an ax to grind. So show us the ax.
Show us the research which is presumably on the right track---since you act like you've shown all these other people to be on the wrong track.

All the time you have been here at PF there is only one person that I can remember whose research you have approved----it is Johan Noldus. Since I don't have your papers, I will use his as an example.

IIRC it was in the "List of new initiatives" thread, I will get the link. Your approval even in this case was very lukewarm, I must say. I would like to have a sample of your own, but lacking that I will go fetch some that I recall your speaking positively about.

================
here we are

https://www.physicsforums.com/showthread.php?p=852574#post852574

Now I see that this PF forum can actually sometimes serve as an OUTLAW CAFE in some of its threads. We can help compensate for deficiencies in the system.
...
...
6. Johan Noldus (just noticed)

http://arxiv.org/abs/gr-qc/0508104
Towards a fully consistent relativistic quantum mechanics and a change of perspective on quantum gravity
17 pages, submitted to CQG

"This paper can be seen as an exercise in how to adapt quantum mechanics from a strict relativistic perspective while being respectful and critical towards the experimental achievements of the contemporary theory. The result is a fully observer independent relativistic quantum mechanics for N particle systems without tachyonic solutions. A remaining worry for the moment is Bell's theorem."can anyone express why we should dismiss the ideas of ... Noldus? Or, conversely, does anyone LIKE what [he has] to say and find it interesting?

... In the paper of Noldus, it is never claimed that there is a new approach to quantum gravity, actually many physicists have gone along the same path before him. The author simply wants to investigate what can happen to quantum mechanics when you consistently apply the principle of general covariance and the reality assumption that all processes describing the outcome of an experiment are real and happen in spacetime (although the latter is not explicitly stated, it is clear from the context). Therefore, if you do not dispose of the wave function, you need to attach a reality to it (the author sees measurement as part of that same physical process, so no U/R split). Assuming this, he basically comes to some form of the self field approach to quantum mechanics, which was derived by other means previously by A.O. Barut and J.P. Dowling and is known not to give the correct predictions (although the know differences are rather small, within a bound of 5%), as the author clearly states. Logically, in such a viewpoint, the author is forced to revise the theory of spin (see it really as a spinning particle) which, as he claims, is not excluded to be possible within the framework of of classical Einstein Maxwell theory (and he provides a nice reference which indeed suggests this). So, the question is now, what next? The author seems even to want to go further and see the wavefunction itself as an effective description of a classical chaotic locally causal process (i.e. the wave function is a thermodynamical ``averaged´´ description of a chaotic, locally causal, deterministic process. The locally causal aspect of this approach (which is somehow suggested by its spacetime character) IS the DARING aspect of this approach since it requires that no perfect Bell experiment is ever to be performed. However, IMO, a Bell test which kills off local realism, does not need to kill off such approach; one might consider adding nonlocality scales at that moment in time.

To wrap up: this paper is just (as said in the abstract) the report of an exercise made by the author. Most physicists who think about QM for some while write a paper about it (however, most of these papers are boring and simply discussing some personal ontological viewpoints). It remains to be seen what comes next (and that will be the real test).

even here you are not very generous with your approval, Careful. But in the next you seem to warm up some:
https://www.physicsforums.com/showthread.php?t=103750

This last link here is the closest thing to praise from you of someone's research that I have seen.
If you think everybody else work is so hopeless (and to me your arguments seem forced, as if by a need to show it) then I would really like to see what you do yourself.

 

[Kea]

My, my, haven't we all been busy? First, please permit me to define a few broad terms, as I believe they are commonly used.

Quantum Gravity: a very loose term meaning any theory capable of completely reproducing computational SM plus GR. This, of course, rules out conventional Strings, LQG and a large number of other proposals.

Background Independent: a property of a physical theory or model that must incorporate (1) independence of spacetime coordinates, and (2) a lack of any form of aether. This second condition is far more stringent. Even GR fails this test.

Logic: a not-necessarily-classical use of syntax with accompanying semantics involving truth-valued propositions and operations such as AND, OR, NOT, implication and quantifiers.

(Local) Observable: a mathematical construct representing the semantics of measurable numerical quantities. Clearly any theory that does not address the problem of measurement cannot hope to have a complete description of such operators.

Magic: entertainment involving well-trained tigers, hoops, mirrors, saws and people in shiny costumes.

In response to a couple of Careful's comments:

Moreover, I have given a LOGICAL reason why defining local
observables within a *background independent* quantum universe is IMPOSSIBLE.

Not according to the definitions that I have given above. You have been using a more restrictive set of definitions, in response to the comments of others, but some readers reasonably appear to be finding this loose use of terms confusing.

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.

I think I more or less or agree, although you have not been clear, but the fact that QM needs alteration does not necessarily mean that we need to throw QM intuition out the window. I wonder if it has occurred to you that your conclusions re the necessity of a classical direction here might in fact be wrong. The alternatives involve enlarging one's sphere of logic - seemingly dire perhaps - but alternatives in the spirit of QM all the same.

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)

An article I like by d'Espagnat is Quantum logic and non-separability in the volume The Physicist's Conception of Nature, edited by Mehra for Dirac's 70th birthday.

 

[Careful]

** You have raised the business of academic freedom and suppression of approaches that arent somebody's (like 't Hooft's) pets. and you keep suggesting that you have the right approach and that whatever the QG community at large is pursuing is doomed to fail. It is too obvious that if your ideas can't be accepted then everybody else has to be wrong. **

Ah marcus, you clearly did not understand how research works. When you are a young researcher who wishes to really solve conceptual problems, you have to sidestep for some amount of time with the consequence that one stops publishing. Those who do publish, are very much aware of these shortcomings I raised too (otherwise you do not deserve PhD in front of your name), they merely content themselves with the knowledge that one of their future heroes will solve it.

**
All the time you have been here at PF there is only one person that I can remember whose research you have approved----it is Johan Noldus. Since I don't have your papers, I will use his as an example.
IIRC it was in the "List of new initiatives" thread, I will get the link. Your approval even in this case was very lukewarm, I must say. **

Well, clearly this guy was only at the beginning of adressing the conceptual flukes in QM at that time - the retour to realism does not necessarily imply a negation of the Bell tests.

**If you think everybody else work is so hopeless (and to me your arguments seem forced, as if by a need to show it) then I would really like to see what you do yourself. **

As I said, career safety desicions often inforce a more or less conventional research topic upon you - unless you work in a math institute, then you basically can commit a reasonable amount of heresy. Your hero smolin has also adressed this problem several times.

But, again, tell me why you are such a LQG believer ??

Cheers,

Careful

 

[f-h]

Welcome to the fry. ;)

Background Independent: a property of a physical theory or model that must incorporate (1) independence of spacetime coordinates, and (2) a lack of any form of aether. This second condition is far more stringent. Even GR fails this test.

What do you mean by aether? Especially in the context of classical GR? All LQG people I know seem to consider classical GR to be BI. So your definition seems to be stricter then general usage.

(Local) Observable: a mathematical construct representing the semantics of measurable numerical quantities. Clearly any theory that does not address the problem of measurement cannot hope to have a complete description of such operators.

Hmmmm... I think I disagree, we can construct localized observables for QM, we do it all the time. Every Heisenberg Operator is an observable with a localisation in time. That is possible because, while we do not have a consistent description of meassurement there are many things we do know about the process and how it appears in the formalism. This allows us to do physics *effectively*. The operationalists never had a problem with QM after all.

 

[Careful]

**
Background Independent: a property of a physical theory or model that must incorporate (1) independence of spacetime coordinates, and (2) a lack of any form of aether. This second condition is far more stringent. Even GR fails this test.**

That is not correct Kea (but it is funny to notice that the meaning of ``background independence´´ causes confusion ) : background independent means any construction which does not depend upon the choice of a background *metric*. Now, if you would choose (1) as axiom, then I can dynamically prefer Minkowski as well as geodesic coordinates (without giving up on covariance !). (2) is probably unphysical, this is something which goes back to Maxwell, through what medium do the waves travel ? Actually, the background metric version can also be criticised similarly : as is well known, the lapse and shift function in the ADM formulation of gravity serve as lagrange multipliers, so why would it be forbidden to add a background metric through lagrange multipliers? As I once commented, background independence is not so clear cut (as our conversation proves again ). Really, it is instructive to read Kretchmann's comments upon the issue of general covariance and how even Newtonian physics can be reformulated in a (ugly) covariant way.


**Logic: a not-necessarily-classical use of syntax with accompanying semantics involving truth-valued propositions and operations such as AND, OR, NOT, implication and quantifiers. **

Clearly a mathematician, but then you can go any side you want to with, say, fuzzy logic.

** (Local) Observable: a mathematical construct representing the semantics of measurable numerical quantities. Clearly any theory that does not address the problem of measurement cannot hope to have a complete description of such operators. **

Well, a *local* observable is rather more than that no.

**Magic: entertainment involving well-trained tigers, hoops, mirrors, saws and people in shiny costumes. **

Nah, these things can all be explained by local physics, the measurement problem cannot unless you do it in MWI.

**Not according to the definitions that I have given above. You have been using a more restrictive set of definitions, in response to the comments of others, but some readers reasonably appear to be finding this loose use of terms confusing.**

Ah, then I was still teasing f-h (what I clearly stated later on).

**
I think I more or less or agree, although you have not been clear, but the fact that QM needs alteration does not necessarily mean that we need to throw QM intuition out the window. **

Well, I have added somewhere the ``deduction´´ where the background *metric* would appear (so although I started off from a more restrictive definition, I think they are effectively equivalent). I agree that it was not crystal clear - in the sense that it is not written in elaborate mathematical form (which would consume more than 20 pages) - but if you would care about following the details (by trying out each particular step yourself) then you will see the conclusion I made there is hard to escape.

**
I wonder if it has occurred to you that your conclusions re the necessity of a classical direction here might in fact be wrong. **

Of course I have considered this. The point is that a classical attempt can logically solve the ``big´´ problems at hand in a conceptually clear intuitive way. I have no crystal ball in which I can predict if one can recover all experimental output in a *nice* way, but my private attempts so far give me any reason to believe that it will; neither is there any historical record about the implausible character of such enterprise.

**
The alternatives involve enlarging one's sphere of logic - seemingly dire perhaps - but alternatives in the spirit of QM all the same.
**

I agree (with both statements).

Cheers,

Careful

 

[Careful]

**
What do you mean by aether? Especially in the context of classical GR?
All LQG people I know seem to consider classical GR to be BI. So your definition seems to be stricter then general usage. **

What does it mean that GR is BI ?? All this can mean is that GR is a reparametrization invariant theory (that is defined independent of coordinates), this has *nothing* to do with the meaning of BI. The ``idea´´ behind BI orginated from the observation that perturbative quantum gravity is not perturbatively renormalizable around Minkowski. So either, this means that GR as a QFT is ill defined (a conclusion string theorists draw) or it means that Minkowski is just a bad background to do perturbation theory around and that it might be that GR as a QFT around a *different* background could be non-perturbatively renormalizable (you still have to explain then why it goes wrong when perturbing around minkowski of course).

**
Hmmmm... I think I disagree, we can construct localized observables for QM, we do it all the time. Every Heisenberg Operator is an observable with a localisation in time. That is possible because, while we do not have a consistent description of meassurement there are many things we do know about the process and how it appears in the formalism. This allows us to do physics *effectively*. The operationalists never had a problem with QM after all. **

Sure, the ``working´´ interpretation of QM is perfectly fine, it just does not solve anything (you still have to go to MWI if you do not wish to improve upon it).

If I were you f-h, I would start by properly learning *classical* GR before you wish to attend a barbecue in my honor. It is clear that I wish to maintain my identity as Careful, please respect that and argue scientifically.


Cheers,

Careful

 

[f-h]

Most people, I think, go by the way it's defined, for example, in Wald: General covariance and no nondynamically defined objects (aether). That's how I understood Kea. But then of course GR is Background Independent.