Where we stand-Baez talk at Luminy

[Mike2]

**But I thought I was nothing more than restating the underlying permise of science - that there is a REASON FOR EVERYTHING. If this applies to everything to the very core of physical fundamental, then isn't that the same as saying that physics can be derived from logic? **

I am glad you reject QM, since this will tell you that the reason for everything is magic.

I don't know, there does seem to be something logically appealing about all possibilities being considered at once results in a single result. It has the touch of ultimate inevitability.

But if you are going to start with the premise that ultimate reality is not deducible, then I think you disqualify yourself from all debat. For you would then be denying the principles of reason that you would otherwise use. You cannot argue that there is no valid reason. That simply ends the debate with no answer.

 

[Careful]

**I don't know, there does seem to be something logically appealing about all possibilities being considered at once results in a single result. It has the touch of ultimate inevitability.**

Sure I would also like to know who God is, but Godel thought us we need another language than mathematics then.

**
But if you are going to start with the premise that ultimate reality is not deducible, then I think you disqualify yourself from all debat. For you would then be denying the principles of reason that you would otherwise use.**

Nah, I am never speaking about ultimate reality, only fools do that. All I speak about is a logically consistent theory based upon a minimal number of premises which predicts all experimental outcome known to mankind ``naturally´´. That is what I call reason and this is something different and way less ambitious than what you are aiming for.

 

[marcus]

**
this [Lecture #15] is where he quantizes general relativity----the earlier lectures develop various toy models and the classical theory, so this lecture is the first exposure to actual LQG. And it was different! the connection was not defined at points, with values in a Lie algebra, but was defined on curves, with values in the group. maybe that is familiar to you, but this is an introductory course. The treatment in the Lectures is different from what one would have gotten in the 1990s, or even a few years ago, I think.
**

General relativity is NOT quantized in LQG, that is simply false. LQC of Bojowald is an entirely different matter, but that is *not* quantized gravity.

In any case Lecture #15 is where Smolin quantizes the classical theory, that is what he says he is doing. You may mean something different by "quantize", or you may mean to say that he will not be SUCCESSFUL in the continuation (lectures #17 and 18 to come this week) where he treats dynamics.

But how could you know? You presumably have not gone into the future and watched #17 and 18 so you do not know if the treatment of the dynamics----the hamiltonian constraint---will be successful or not.

You may not even have watched #15, that we are talking about. So when you say that he is not doing what he says he is doing----i.e. what the lecture is about----there is some funny semantix bizness going on or else you are saying more than you know.

Of course you may be clairvoyant and the world authority about QG and the arbiter of what all the words mean----they may mean exactly how you want them to mean. But to me it sounds more like a love of contradiction and argument for its own sake.

 

[Hurkyl]

I have to ask.

What is "magical"? What makes QM magical, but not classical mechanics?Why is magical a bad thing?

The measurement and entanglement are magical. If you do not understand why or want to come up with some mathematical arguments why I should not care, save yourself the effort (I probably know it already). QM is the instantaneous reintroduction of Newtonian ideas after Einstein developped relativity.

You've managed to avoid answering my questions entirely.

I've seen the word "magical" most often used merely to describe something contrary to the speakers preconceived notion of how the universe should behave. Part of my intent is to give you a chance to clarify yourself, should you be something other than yet another person who automatically rejects anything contradicting his classical upbringing. (Yes, thus far that is how I've been interpreting these kinds of remarks that you've made)

It has also sounded like you may be criticizing QM becuase it doesn't explain "why" -- but any theory suffers from that problem.


I still have to guess at just what you mean from your remarks here. Measurement and entanglement are magical? What do you mean by magical? Why are they magical? What is bad about being magical?


Since you mention measurement, based on your final sentence, my guess is that you simply don't like the collapse postulate. I don't like it either, but that's not sufficient cause to call it wrong. I think you are pointing out that it violates (strict) relativity...

But I am not doubting the correctness of the Lorentz invariance of the *vacuum*, iit simply does not imply at all that our theories need to be formulated in a Lorentz invariant way.

but you've already admitted that's not sufficient reasion to reject a theory.


Since you mention entanglement, my guess is that you like... oh what's the right term? Observation independence? The one where if we have two spatially separated experiments, and we ask the "non-local question" about whether they give the same result, that we impose a mathematical condition on the probabilities involved.

But I've never seen a persuasive argument in favor of assuming statistical independence. In fact, I have foundational issues with the classical use of statistics.

QM is the instantaneous reintroduction of Newtonian ideas after Einstein developped relativity.

Are you merely asserting the collapse postulate violates (strict) relativity, or merely that the original QM was not relativity-friendly? Or something else entirely?

 

[masudr]

...we cannot even compute the classical pendulum hanging in a gravitational field exactly...

Elliptic integrals can help us write down the exact answer. And before you tell me that this is no good, save your breath. We cannot even write down the exact answer to y = sin(2.2424) all we can do is write down an algorithm and compute a finite number of steps of that algorithm that would give me a good enough answer. That doesn't mean we don't understand the sine function.

I could also write down an algorithm and compute a finite number of steps for the displacement-time function of a classical pendulum hanging in a gravitational field that would give me a good enough answer. Similarly, that doesn't mean we don't understand the classical pendulum. At least not in my definition of understand.

 

[Mike2]

**But if you are going to start with the premise that ultimate reality is not deducible, then I think you disqualify yourself from all debat. For you would then be denying the principles of reason that you would otherwise use.**

Nah, I am never speaking about ultimate reality, only fools do that. All I speak about is a logically consistent theory based upon a minimal number of premises which predicts all experimental outcome known to mankind ``naturally´´. That is what I call reason and this is something different and way less ambitious than what you are aiming for.

No, a fool makes claims he can't prove, nor intends to prove. So I would be the opposite of a "fool" because I would demand complete proof.

But if you are not looking for the most complete and reliable laws of nature, then you are just looking for engineering approximations for building better machines to sustain your meaningless existence.

 

[Careful]

Dear marcus,

LQG simply has no quantum dynamics yet which arises from any reasonable quantization. Of course they proposose some toy Hamiltonians (in which some quasi local terms of the classical Hamiltonian constraint are completely ignored and which are plagued with ambiguities) but it is very unlikely that these reproduce the correct classical limit (actually they have no idea so far about that yet) - moreover it is not even clear what the classical limit means in their framework is since there is a painful lack of physical observables (actually it is not even entirely clear what these observables are supposed to be/although there are *abstract* proposals a volonte of course ). I am pretty sure that when the day comes that the Hamiltonian constraint is solved, then we shall all hear about it.

**
Of course you may be clairvoyant and the world authority about QG and the arbiter of what all the words mean----they may mean exactly how you want them to mean. But to me it sounds more like a love of contradiction and argument for its own sake.**

No Marcus, I have just learned what does NOT work and please stop portreying me as such figure - it would be much better if we would stick to the content.

 

[Careful]

Hurkyl,

**
Since you mention measurement, based on your final sentence, my guess is that you simply don't like the collapse postulate. I don't like it either, but that's not sufficient cause to call it wrong. I think you are pointing out that it violates (strict) relativity... **

That the collapse violates strict relativity is quite obvious, that it cannot correspond to any physical process is also evident. My point is that it is completely obsolete: the major (original) experiments which should demonstrate the impossiblity of ``classical´´ realism can be *easily* explained by *well known* classical physics (I am writing a paper on that now, so you will have to be patient here).


** but you've already admitted that's not sufficient reasion to reject a theory. **

Indeed, and I started rejecting QM only when I saw the impossibility of quantum gravity (at confidence level of 90 percent). Before that, I had the same attitude as you: actually it is quite amusing to imagine superposed cats and so on, it is just that it does not belong to this world.

**
But I've never seen a persuasive argument in favor of assuming statistical independence. In fact, I have foundational issues with the classical use of statistics. **

Really, but quantum statistics is not even a well defined business yet. Concerning EPR, well there exist plenty of easy models to explain experimental outcome, one can only debate about their naturalness and their embedding in a larger theoretical framework in which case I would probably agree with you for now.


**Are you merely asserting the collapse postulate violates (strict) relativity, or merely that the original QM was not relativity-friendly? Or something else entirely? **

Well, there is no collaps mechanism in QFT (here I only know of (Patricks) MWI as a ``local explanation´´of long range EPR experiments), so it is definately violating special relativity.

Cheers,

Careful

 

[Careful]

Fine, my example here was poorly chosen (of course we understand the pendulum - I better would have mentioned the three body problem which is still providing surprises) : on the other hand I wanted to make clear that QFT has already a minimalistic axiomatic system - which leaves no room for simplification. It is just very hard (and until know too hard) to understand if these desirata are really satisfied (until now that has been done only for free theories) in our interacting theories. Progress in QFT is unlikely to come from simplification.

 

[Careful]

**No, a fool makes claims he can't prove, nor intends to prove. So I would be the opposite of a "fool" because I would demand complete proof.

But if you are not looking for the most complete and reliable laws of nature, then you are just looking for engineering approximations for building better machines to sustain your meaningless existence.**

You clearly do not understand the meaning of physics, the
MOST reliable laws of nature simply do not exist. That is just marketing pep talk for the public: the holy grail is much sexier than honest labor. I am looking for a *consistent* view and that is in some sense much more than what the holy grail chasers can legitimatly claim.

 

[f-h]

Careful, Hamiltons and Lagranges Mechanics are simpler then Newtons and enable us to understand things like Noethers Theorems.

In this sense I am sure that there are tremendous simplifications to be found in QFT. Our current mathematical and conceptual framework is not well suited to doing QFT.

Beyond that it seems (please correct me if I'm wrong) that your interprete our collective failure to get a good grip on all the revolutions of 20th century as an indication that these are spurious, temporary and shouldn't be taken to serious, that instead we need to disregard them and go "back to the basics".
You are convinced that a theory of Quantum Gravity is impossible in the conventional sense (care to elaborate?), the efforts of reconciling QM and GR are wrong headed because one or both of these are fatally flawed, which is expressed in our failure to unify them.

I think that from a historical viewpoint this conclusion is not warranted. At no point in the history of science was there a unified theory of all that was known at the time.

From a personal PoV I do not think that you can deduce physical insights from the fact that we are collectively to stupid to figure out a certain problem. As Bell said: What is proved by impossibility theorems is a lack of imagination.

Also what are you thinking about for QM? t'Hooft style discrete dispersive deterministic mechanics? Or the Bell experiment loopholes where you postulate complicated ocrrelations between macroscopic entities to explain simple microscopic correlations? (Conspiracy Theory Interpretations of QM: The Lord is subtle AND malicious).

 

[Mike2]

You clearly do not understand the meaning of physics, the
MOST reliable laws of nature simply do not exist. That is just marketing pep talk for the public: the holy grail is much sexier than honest labor. I am looking for a *consistent* view and that is in some sense much more than what the holy grail chasers can legitimatly claim.

What I may not understand is the campus politics that prevent us from even considering the goal of a complete theory.

But when you say a "consistent" theory, I assume you mean with every possible measurement that may be predicted from said theory. If that is the case, then I don't know that we disagree. For I suppose we may one day need a theory that does explain everything, spacetime, matter, energy, QM, GR, QG, and the number of dimensions in the universe. We may need to explain where spacetime itself came from. This might be required, for example, as boundary conditions that give us this vacuum from the landscape. And if a theory should give us how spacetime emerged from nothing (perhaps a singularity), then that would sound to me to be a theory from logic itself. For then you have some entity coming from nothing like a true proposition coming from a false proposition. All things derive from one point. How more logical can things get?

 

[Careful]

**Careful, Hamiltons and Lagranges Mechanics are simpler then Newtons and enable us to understand things like Noethers Theorems.**

Sure, I did not contradict that, I just said that it does not cover the full range of Newtonian physics and that it came from a NEW nontrivial view upon physics (minimization of energy instead of forces acting on...)

**
In this sense I am sure that there are tremendous simplifications to be found in QFT. Our current mathematical and conceptual framework is not well suited to doing QFT. **

But you will require a new physical idea for that.

**
Beyond that it seems (please correct me if I'm wrong) that your interprete our collective failure to get a good grip on all the revolutions of 20th century as an indication that these are spurious, temporary and shouldn't be taken to serious, that instead we need to disregard them and go "back to the basics". **

Well, I do not say you have to disregard them: you have to draw the right lessons from the wrong theories. They all contain useful information about nature, I am just afraid (actually convinced) that we have misinterpreted some crucial experiments in the sense that perfectly simple and rational explanations within classical physics were possible. Of course the Schrodinger wave contains some useful information : it was designed exactly to cover such experiments which should reveal particle wave duality.

** You are convinced that a theory of Quantum Gravity is impossible in the conventional sense (care to elaborate?), the efforts of reconciling QM and GR are wrong headed because one or both of these are fatally flawed, which is expressed in our failure to unify them **

They are probably both wrong headed : GR in the minimal coupling principle and QM in the radical, unnecessary departure from local realism with as a consequence the need for a collapse of the wavefunction. Here you have one reason why QM and GR won't get married in a background independent way : it is logically impossible to define LOCAL observables in quantum gravity without introducing a preferred vacuum metric (or background frame) unless you have a mechanism which kills off superposition or put in by hand classical boundaries to a piece of ``observable space time foam´´.


**I think that from a historical viewpoint this conclusion is not warranted. At no point in the history of science was there a unified theory of all that was known at the time.**

Nah, sure there was: this was the great feeling at the end of the 19'th century. People thought they knew it all until they found out that an electron radiates, oeeps

**
From a personal PoV I do not think that you can deduce physical insights from the fact that we are collectively to stupid to figure out a certain problem. As Bell said: What is proved by impossibility theorems is a lack of imagination. **

Well Bell was speaking clearly about his own NO GO theorems then : I appreciate your idea that local realism is not for a waste yet. In my opinion, people had a lack of imagination when they saw black body radiation for the first time and thought about the Young double slit experiment in the beginning of the 20'th century. Let me tell you : if your QM professor would tell you that for all these experiments there exists a perfectly rational explanation, no student would accept QM anymore.

**Also what are you thinking about for QM? t'Hooft style discrete dispersive deterministic mechanics? **

Something like that yes, but I do not start from the weird deterministic quantum systems he uses (neither do I need information loss), moreover he has no clear answer yet how to avoid the measurement problem as far as I know. But I do admire the spirit of his curiosity and honesty.

**Or the Bell experiment loopholes where you postulate complicated ocrrelations between macroscopic entities to explain simple microscopic correlations? (Conspiracy Theory Interpretations of QM: The Lord is subtle AND malicious).**

There are also other possibilities which are by far not as malicious as people want you to believe (most of them are actually pretty simple toy models), but I would certainly like to avoid constructions involving backwards causation, yes. *IF* you come up with a theory which predicts non trivial correlations beyond the lightcone (in either when you break special relativity) you better do it in a natural and clear cut way (that is actually what 't Hooft is doing).

 

[Careful]

The point Mike is that perhaps you do not need to do all of these things you mention, partially because some of them don't ``melt´´ together and partially because some of the insights coming from each of these theories are obsolete and incorrect. I stress that we do not need to see these theories as immutable : you have to come up with some insight which captures the essence of all of them. (a) QM tells you that particles and waves interact on the microscale (b) GR gives you the strong equivalence principle and so on. For example : (b) does *not* logically imply that the ``metric field´´ has to be the dynamical variable for gravitation, neither does it tell you that the minimal coupling principle needs to be valid (ie. light could travel at speed > c in curved spacetime).

Again, take a deep look at geometric unification attempts for GR and EM and you might better understand my point of view.

Cheers,

Careful

 

[CarlB]

I've been following the fascinating discussion with great interest and entertainment. I find myself agreeing with careful on the need for extreme variation in approaches to the foundations of physics. Uh, I guess I should mention that I was recently awared "Crank of the Day" at http://www.crank.net .

Carl

 

[f-h]

Now we're talking, there are specific assumptions within the overall conceptional framework that seem unjustified/amendable. Trying to find a *physically* motivated replacement for them is certainly a very good idea. Trying to replace them just for the heck of it, certainly isn't.

I happen to think that LQG has a lot of potential in relation to matter coupling (and I don't mean Sundance et al). And John Baez has an intriguing paper out that suggests, citing TQFT, that maybe we will only get a good grasp of (at least some of) the strangeness of QM if we view it from the PoV of QG.

But this is a world from claiming that GR/QM are fundamentally "wrong headed". In fact this approach must take each theories core claims extremely serious to possible cure some of the defects of the other theory.

BTW your claim why we can't marry GR and QM is wrong, see Rovelli/Dittrich partial/complete observables.
The Dirac Observables are of course highly "nonlocal" (whatever that may mean in this context) but there are classes of them which have a clear local interpretation neithertheless.

 

[Careful]

BTW your claim why we can't marry GR and QM is wrong, see Rovelli/Dittrich partial/complete observables.
The Dirac Observables are of course highly "nonlocal" (whatever that may mean in this context) but there are classes of them which have a clear local interpretation neithertheless.

Entirely false. These observables are all *classical* and the constructions involved are based upon old ideas of Karel Kuchar (and involve good old fashioned gauge fixing ), hardly worthwhile calling something new. Classically, the problem is still tractable of course (albeit very messy and technically complicated), it is just that it becomes impossible *quantum mechanically*. It is clear you did not understand my comments and did not fully comprehend these papers either.

You might for a change think about what I say. The issue of matter coupling in LQG is very disputable : where are the usual anomalies which reflect *physics* ? - actually ``anything goes´´ is a disadvantage in that respect - there has been quite some discussion about that involving Jacques Distler et al.

 

[f-h]

Of course the ideas are not new, they are a reaction to Kuchars ideas but they are exactly critical of his views. They say that gauge fixing is not needed (or if you want to insist on calling it gauge fixing, it is also needed in non relativistic QM). The correct physical motivation is in the "What is observable in GR" paper. They are the correct conceptual interpretation of the actual calculations done in GR.

If you interprete them as nothing but a fancy way to do gauge fixing I guess they don't make much sense quantum mechanically, but that's not Rovellis point.
Of course Dittrichs papers are classical, but there are people working on the Quantum Mechanics of this ideas, and it turns out it's not completely untractable. Far from it.

Even if it was technically untractable Dittrichs work proves that classically there are Dirac Observables with good local interpretations, so why shouldn't there be Dirac Observables like that in QM? If you have objections that are specific to QM I'd love to hear them.

 

[f-h]

Of course in a background independent theory the adjective "local" becomes highly ambigious anyways. Local with respect to what? If you have a second field interacting with the first you can talk about observables of the first local relative to the second. That's the strongest statement that is well defined, and it's the notion of loaclity essentially captured in Rovellis approach.

 

[Careful]

**Of course the ideas are not new, they are a reaction to Kuchars ideas but they are exactly critical of his views. They say that gauge fixing is not needed (or if you want to insist on calling it gauge fixing, it is also needed in non relativistic QM). The correct physical motivation is in the "What is observable in GR" paper. They are the correct conceptual interpretation of the actual calculations done in GR. **

I know of Rovelli's points of view as well on relativity as on QM. The latter - that is his relational QM - I firmly disagree with (just a sequence of words), his GR view on the other hand is ``from the old school´´, solid and beautiful. Please, do you want to drop the word CORRECT - something physicist use too often when they don't fully understand what they are talking about.

**
If you interprete them as nothing but a fancy way to do gauge fixing I guess they don't make much sense quantum mechanically, but that's not Rovellis point. **

But they ARE nothing but gauge fixing (I don't remember all the details but that was certainly a very shared feeling at the time).

**
Of course Dittrichs papers are classical, but there are people working on the Quantum Mechanics of this ideas, and it turns out it's not completely untractable. Far from it. **

Of course, in principle you can try to quantize these observables (Hamiltonian constraint or master constraint - did not hear about that for a while ?? you cannot speak about an observable before solving these issues), the point is that *quantum mechanically* they have no *local* meaning whatsoever.

**Even if it was technically untractable Dittrichs work proves that classically there are Dirac Observables with good local interpretations, so why shouldn't there be Dirac Observables like that in QM? **

But that was already known for about at least 40 years (!), relativists had plenty of candate observables in their pocket. It is just that these work only LOCALLY on the classical level (for example when you try to find a dynamical arrow of time). This is a problem which they face too, but do not adress so far - exactly this issue killed off the work of Kuchar and company. What Dittrich and Rovelli did was rewriting it in the ADM Hamiltonian framework. Concretely, you can say in this framework : I can label star X by metric invariants (a,b,c,d) and this coordinate labelling will be unique in the neighborhood of X and probably provides you with an arrow of time and some other physical coordinates. However, star Y can have the same labels, then you need to add a new one and so on and so on. In the end you might end up with 25 labels to characterize all stars in the universe. This indeed gives you *classically* information about *localized* curvature hence *perhaps* matter (you know Einsteins theory violates the Mach principle).

**
If you have objections that are specific to QM I'd love to hear them. **

I told you that already: in LQG or any background independent approach you treat the *entire* universe as a quantummechanical black box. Anyone who has learned about the dirty word entanglement knows that you cannot split it up into subsystems (decoherence is something you have to show, not to assume), so what can you do ? Aha, the only thing which is left is to promote the above coordinates to operators and evalute them on all (unknown ) physical states. So *all* information you have quantum mechanically acces to is just this, nothing else. Now, if your universe would be one ``riggmapped´´ spin network (no superposition) each of whose subnetworks are eigenvectors of your preferred observables, there would not be any problem since this is actually analogous to the classical situation: you can map events to mathematical points here. But what if your universe is a superposition of ``riggmapped´´ spin networks ? How do you define then the action of a *localized* observable without knowing how the nodes in the different spin networks are relatively ``positioned´´? Topological information like knotting of spinnetworks in not enough here : *localization* is a metrical issue ! You cannot do it without an artificial labelling which is tantamount to choosing a background frame. Another possibility would be to restrict to global observables and calculate expectiation values of them and then try in good faith - with an infinite number of ambiguities - to fit a Lorentzian manifold to the data. The latter procedure certainly makes you wonder about the ambiguities in perturbative quantum gravity (these are actually not that bad) and is perpendicular to the way WE make measurements in the universe. The last possibility I see is to put classical boundaries which is a somewhat more liberal version of the labelling procedure.

Cheers,

Careful

 

[Careful]

Of course in a background independent theory the adjective "local" becomes highly ambigious anyways. Local with respect to what? If you have a second field interacting with the first you can talk about observables of the first local relative to the second. That's the strongest statement that is well defined, and it's the notion of loaclity essentially captured in Rovellis approach.

HUH ?? In classical GR, the METRIC gives me a notion of locality (the Alexandrov sets), nothing more is needed for that.

 

[f-h]

I see what you are saying but you seem to be confusing a lot of different things. I recommend taking a step back and looking at this problem with all the different notions of (non)locality flying around again.
States in QM are nonlocal in some sense, but there are of course local observables on nonlocal states.
The existence of nonlocal states says nothing about the existence of local observables.

Also yes that's what I said, I can localize something else relative to the metric field, but the metric field is localized with respect to what? The question doesn't make sense. (at least in the naive sense of "local"). That is Kuchars objection to the possibility of constructing locally interpretable Observables in a gaugeinvariant theory/fully constrained theory (which you reinvoke quantum mechanically when you talk about "rigging" but which is not really related to superposition/entanglement).
One can study this problem and the formalism to overcome it in any toy model, no Master constraint program needed.

A more detailed response later.

 

[Careful]

**I see what you are saying but you seem to be confusing a lot of different things. I recommend taking a step back and looking at this problem with all the different notions of (non)locality flying around again.
States in QM are nonlocal in some sense, but there are of course local observables on nonlocal states. **

No, I am not confusing different things: you are. You speak from experience in *background dependent* QFT in which anyone knows that what you say is correct. Actually, it is easy to see where the virtues of that theory (local observables on entangled states) are destroyed in a ``background independent´´ approach whatever that may mean.

**
The existence of nonlocal states says nothing about the existence of local observables. **

Again, that is only true in a background dependent approach since you have an *a priori* notion of locality there.

**Also yes that's what I said, I can localize something else relative to the metric field, but the metric field is localized with respect to what? **

But it is the metric field which gives locality simply by measuring distances

**That is Kuchars objection to the possibility of constructing locally interpretable Observables in a gaugeinvariant theory/fully constrained theory (which you reinvoke quantum mechanically when you talk about "rigging" but which is not really related to superposition/entanglement).**


Well, my objections are MINE, I can only conclude that Karel is a clever chap. No smartusz, I just added rigging for constructing diff invariant states, otherwhise you might have complained that I work with ``unphysical´´ objects which I still do since my master/hamiltonian constraint is not solved.

**
One can study this problem and the formalism to overcome it in any toy model, no Master constraint program needed. **

Rubbish, all these toy models probably have the wrong classical limit.

 

[masudr]

This is a very interesting discussion which I have been following for a while now. It also seems that others have been too, with an average of 163 views a day.

 

[arivero]

**Careful, Hamiltons and Lagranges Mechanics are simpler then Newtons and enable us to understand things like Noethers Theorems.**

Sure, I did not contradict that, I just said that it does not cover the full range of Newtonian physics and that it came from a NEW nontrivial view upon physics (minimization of energy instead of forces acting on...)

Can I contradict it, then? Newtons conception of time and movement is a lot simpler than phase space and symplectic areas.

 

[Careful]

Can I contradict it, then? Newtons conception of time and movement is a lot simpler than phase space and symplectic areas.

Hehe, it is all yours for the taken

 

[Hurkyl]

Please, do you want to drop the word CORRECT - something physicist use too often when they don't fully understand what they are talking about.

Turnabout is fair play -- do you want to stop using words like "wrong", "rational", and "magic"?

 

[Careful]

Turnabout is fair play -- do you want to stop using words like "wrong", "rational", and "magic"?

Wrong I can replace by extremely unlikely sometimes yes, master Hurkyl Magic is justified, unless you believe Harry Potter is actually around. Rational hmmm better replace it by classical rationality.

 

[marcus]

...I told you that already: in LQG or any background independent approach you treat the *entire* universe as a quantummechanical black box...

don't understand. there are QG approaches which have a boundary. there is an observer outside. what is studied is inside the boundary and is not the *entire*

Are you saying that these are invalid, or that they are not "background independent" (in your definition of background independence)?

 

[Careful]

don't understand. there are QG approaches which have a boundary. there is an observer outside. what is studied is inside the boundary and is not the *entire*

Are you saying that these are invalid, or that they are not "background independent" (in your definition of background independence)?

Well, you have to make a distinction between types of boundaries. If you have a timelike tube with as boundary two closed spatial hypersurfaces, then basically you are computing transition amplitudes between closed universes, that is fine but has no bearing upon the issue of local observables.

On the other hand, when you take a spatio temporal four dimensional cube, force classical boundary conditions and ``quantize´´ only the internal degrees of freedom, then of course you are doing a major cheat. Not only can you not make any local statements yet about what is happening inside the box, but worse: you have entirely ignored the problems of (a) entanglement versus environmental decoherence (b) how to define local observables.

Stricly speaking doing such act is illegal within pure quantum gravity, you might want to read what James Hartle has written about the quantum mechanics of closed systems. That some people do it is more a sign that they have run out of ideas than anything else. Since, if you allow for such travesty, then simply divide a classical universe UNIV into tiny boxes A, subdivide these further into boxes B. Imprint boundary conditions on A induced by UNIV and do quantization with fixed boundary conditions within A selecting wavelengths larger than B. The Feynman series will be finite and no need for an infinite number of counterterms. End of story: if this is all you mean by quantum gravity then we could have solved it 30 years ago by putting in physical regulators on the perturbation series (something which appears to be superior to LQG for now).

Another person who realizes this problematic very well is Roger Penrose who actually explicitly introduces a Newtonian time and a set of dynamically preferred Newton Schrodinger states.

 

[jarek]

Really, but quantum statistics is not even a well defined business yet.

Careful

Nah, Careful, please...Mackey put it in a very well defined form. If you don't accept other forms of statistics apart from the classical one (who was talking about thinking out of the box ), then sure entanglement is magic, local realism is lost (at least on paper) and we desperately need a genius who will restore it (at least on paper)

Cheers,
jarek

 

[Careful]

Nah, Careful, please...Mackey put it in a very well defined form. If you don't accept other forms of statistics apart from the classical one (who was talking about thinking out of the box ), then sure entanglement is magic, local realism is lost (at least on paper) and we desperately need a genius who will restore it (at least on paper)

Cheers,
jarek

Haha, I guess what Mackey did was to extend upon the work of Von Neumann and Shannon about quantum entropy no? The entire difficulty is of course to tell what your quantal degrees of freedom are : in the framework of dynamical geometry (such as quantum gravity), you do not even know where to choose them (although there are some proposals by 't Hooft). Moreover, as I seem to remember, Bernard d'Espagnat has written some rather severe comments upon the conceptual difficulties quantum statistics faces in light of the measurement problem (I do not remember that well anymore) - my comment was intented in that direction not on the technical aspect of the matter (in cases where the degrees of freedom can be easily identified).

Concerning your silly entanglement; IF such thing were observed (one day ) the easiest possibility is to allow for classical signals traveling > c, there is no law in nature which forbids that. The point is that for the undisputed QM experiments I have checked (double slit, black body, etc) I don't need QM at all

Cheers,

Careful

 

[marcus]

Well, you have to make a distinction between types of boundaries. [A] If you have a timelike tube with as boundary two closed spatial hypersurfaces,

On the other hand, when you take a spatio temporal four dimensional cube, force classical boundary conditions...

I am not talking about A or B. the boundary does not consist of two spatial hypersurfaces, nor does it consist of a four dimensional cube.

I am imagining a boundary with no beginning or end. I think of it as a tube (your word) but it has no space-like initial/final pieces.

AFAIK I am not imagining something that is a major cheat or illegal.

"course you are doing a major cheat. Not only can you not make any local statements yet about what is happening inside the box,..."

As a rule the observer can never say what is happening inside the box, he has only observations at the boundary. this is not illegal or a cheat, it is normal, or so I think.

maybe there are other ways of handling the observer problem---I understand it is a connundrum for all quantum mechanics I believe (not just some specialized subject like LQG or some other QG) and doubtless lots of people have written about it.

I am impressed by how negative your views are, careful. You seem to always be arguing that nobody's approach can ever be successful. You already know at the outset that various works in progress, like LQG, are doomed to failure. I find it curious.

 

[Careful]

**I am not talking about A or B. the boundary does not consist of two spatial hypersurfaces, nor does it consist of a four dimensional cube.

I am imagining a boundary with no beginning or end. I think of it as a tube (your word) but it has no space-like initial/final pieces. **

What is the damn difference? You still have to put asymptotic boundary conditions on future and past timelike infinity. Do you think that changing the classical boundaries are offering you a way out of my argument ??!

**
AFAIK I am not imagining something that is a major cheat or illegal.

"course you are doing a major cheat. Not only can you not make any local statements yet about what is happening inside the box,..." **

I am sorry to tell you that you did not understand the difficulties of quantum gravity then. BTW what you say is even complete heresy in QFT where you can still calculate expectation values of local observables in the box.


** As a rule the observer can never say what is happening inside the box, he has only observations at the boundary. this is not illegal or a cheat, it is normal, or so I think. **

See my previous comments.

**maybe there are other ways of handling the observer problem---I understand it is a connundrum for all quantum mechanics I believe (not just some specialized subject like LQG or some other QG) and doubtless lots of people have written about it.**

The problem of the observer becomes much more difficult in QG than in ordinary QM (any student of QG learns that).

**
I am impressed by how negative your views are, careful. You seem to always be arguing that nobody's approach can ever be successful. You already know at the outset that various works in progress, like LQG, are doomed to failure. I find it curious. **

What I find curious is that:
(a) nobody offers rational counterarguments to my reasonable no-go statement : I mean an LQG protege could friendly give me a reasonable physical mechanism why I should believe the contrary.
(b) actually, many prominent LQG'ers have left the field for similar reasons
(c) many good scientists outside the field think likewise (amongst others roger penrose)

Moreover, I find my comments far from negative, they clearly indicate where the stumblestones are : recognizing those and looking for plausible cures are necessary conditions for succes. These problems are with us from the real beginning Marcus and very little if no progress has been made on these issues by any of the traditional approaches. There has been written a lot of crap about it undoubtedly. Moreover, I have given a LOGICAL reason why defining local observables within a *background independent* quantum universe is IMPOSSIBLE; it seems to me that people like you have tremendous problems with logic.

Cheers,

Careful

 

[jarek]

Haha, I guess all Mackey did was to extend upon the work of Von Neumann and Shannon about quantum entropy no?

Concerning your silly entanglement; IF such thing were observed (one day ) the easiest possibility is to allow for classical signals traveling > c, there is no law in nature which forbids that. The point is that for the undisputed QM experiments I have checked (double slit, black body, etc) I don't need QM at all

Careful

No, I'm talking about orthomodular lattices more than about silly entropies You will call it kinematics and say that it should be dynamically determined etc and I will agree. But if you need quantum statistics per se, understood here rather abstracly like you can speak of classical probability theory - here you are!

Silly entanglement, I like the name , has no connection whatsoever to superluminal signalling and you perfectly know that. C'mon, you know why Einstein called it spooky action at a distance and how he proposed to resolve it

Cheers,
jarek

 

[marcus]

What I am mainly challenging is what you said in post #70

... in LQG or any background independent approach you treat the *entire* universe as a quantummechanical black box...

what I said was

don't understand. there are QG approaches which have a boundary. there is an observer outside. what is studied is inside the boundary and is not the *entire*

Are you saying that these are invalid, or that they are not "background independent" (in your definition of background independence)?

I still don't understand. Why do you think that " LQG or any background independent approach" necessarily treats the *entire* universe as one black box?

 

[Careful]

**No, I'm talking about orthomodular lattices more than about silly entropies You will call it kinematics and say that it should be dynamically determined etc and I will agree. But if you need a quantum statistics, understood here rather abstracly, like you can speak of classical probability theory - here you are! **

Ah, indeed that is just kinematics I remember a joke about those following Piron et al : that they have forgotten there is something like the Schrodinger equation in QM

** Silly entanglement, I like the name , has no connection whatsoever to superluminal signalling and you perfectly know that. C'mon, you know why Einstein called it spooky action at a distance and how he proposed to resolve it **

Who says that I was talking about superluminal signalling (which we only know to be forbidden in free theories ), I was talking about waves which can go faster than c and transmit information between the particles. That is *not* forbidden, it does not necessarily lead to superluminal signalling. Although as I said, I don't care for the moment about this too much, first concentrate on double slit, atomic physics and so on. I feel that a natural solution for this (which I am working out now) will clarify the issue of EPR too.

na zdrowie

Careful

 

[marcus]

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

first, please tell me what you mean by *background independent*

what I usually mean is that the theory does not explicitly assume a background metric on the manifold

but in discussions background independence is often used comparatively. one theory is MORE background independent than another (no theory is perfectly background independent, that might not even be meaningful)

what do you mean, exactly, by background independent?

 

[jarek]

Who says that I was talking about superluminal signalling (which we only know to be forbidden in free theories ), I was talking about waves which can go faster than c and transmit information between the particles. That is *not* forbidden, it does not necessarily lead to superluminal signalling.

Careful

Erm, by superluminal signalling I mean transmitting information faster than c. And this you cannot do with entangled pairs. You can invent whatever waves you like (Gisin and co once even put experimental bounds on their velocities) which propagate between entangled pairs, but information (meaning something you can access) carry they will not

Although as I said, I don't care for the moment about this too much, first concentrate on double slit, atomic physics and so on. I feel that a natural solution for this (which I am working out now) will clarify the issue of EPR too.

I'm waiting especially for double slit :!)

na zdrowie

Ah, that's something you say when you drink vodka (provided you can still speak )


jarek

 

[Careful]

What I am mainly challenging is what you said in post #70
what I said was
I still don't understand. Why do you think that " LQG or any background independent approach" necessarily treats the *entire* universe as one black box?

And I referred you to papers of James Hartle on closed quantum systems. Actually, there are people working in the foundations of quantum mechanics, trying to extend it in order to allow for ``classical components´´- the followers of Piron et al which I made a joke about to Jarek - so this is all kinematics for now. But your question is fairly basic quantum mechanics, which amounts to : where can we put the observer? In order to know that you have to be able to *dynamically* identify your classical components (Schroedinger cat problem, here she is again ) : you cannot just put it in by hand. Hence you notice immediatly a logical loophole : in order to identify dynamically classical localized subsystems you need local observables


As a further example: anyone believing QG agrees that it was the dominant mechanism at the origin of the universe - unfortunatly you cannot put the observer in there in the way you see it.

If you want to know more about this, you can consult the road to reality of R. Penrose (not that I support everything he says there) - he explains it (in many pages) in a fairly entertaining way.

Cheers,

Careful

 

[Careful]

**Erm, by superluminal signalling I mean transmitting information faster than c. And this you cannot do with entangled pairs. You can invent whatever waves you like (Gisin and co once even put experimental bounds on their velocities) which propagate between entangled pairs, but information (meaning something you can access) carry they will not **

I know, but it is easy to get out of that one: you just give this wave a label indicating that it can only interact with the EPR pair (not with the apparatus of course). The EPR particles or ``photons´´ do not travel faster than c obviously - in this way each of the particles can know of the detectorfield of the other (which each of them feel 3 nanoseconds in advance ) It seems extremely unlikely that perfect entanglement exists so on long distances any such line of thinking is saved by the very low measurement rates (which is actually a prediction in SED and therefore far from conspirational). But let's not discuss this now.

**
I'm waiting especially for double slit :!)
**

Aha, I know this one is the golden key, just wait two or three months (the physical idea is there, and the math is following).

**
Ah, that's something you say when you drink vodka (provided you can still speak ) **

Haha, naaa zzdrrooowie

 

[marcus]

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

this is your post #84 on this thread.

I want to learn from you, careful, if you have something definite to teach me.

Tell me your LOGICAL reason, that you have given.

Please say what you mean by *background independent* (because people in different discussions mean different things by it)

and say what you mean by local observables

and prove that it is IMPOSSIBLE to define them.

Since you have already given your logical reason somewhere, this should not be difficult for you to do---I hope in just a sentence or two.

Please do not refer me to some other books and authors. Just give me the LOGICAL reason which you mentioned having given. I will appreciate it, I assure you.

 

[jarek]

But your question is fairly basic quantum mechanics, which amounts to : where can we put the observer? In order to know that you have to be able to *dynamically* identify your classical components (Schroedinger cat problem, here she is again ) : you cannot just put it in by hand.

I like a lot what you are saying re dynamics vs. kinematics. You got me once thinking with similar remark (all that orthomodular toys should really be dynamically determined, not rigid as they are right now). However the only theory with kinematics following from dynamics seems to be GR, as Bergmann and the followers, like Lusanna, showed. You have any other examples or clues? It seems that nobody ever though along this lines in mechanics (be it quantum or classical).

Cheers,
jarek

PS Re EPR - this is what I thought - "confined" sort of information, so multiplying ghosts and moving towards magic. People who attack EPR are sometimes very predictable

 

[Careful]

**
I want to learn from you, careful, if you have something definite to teach me.
Tell me your LOGICAL reason, that you have given. **

If you want to define a local observable, then I said that you have the following possibilities:
(a) no superposition of rigmapped spin networks (which is as good as classical)
(b) classical timelike boundaries (possibly combined with spatial caps) - but then you have no local information about the interior.
(c) figuring out a mechanism which gives relational information (more than just topological one !) between nodes in two different spin networks
(d) measuring expectation values of global observables which you try to fit to a Lorentzian manifold (not a classical solution to the vacuum Einstein equations in case you include matter)

Option (b) runs straight against quantum mechanics. Option (c) is tantamount to picking a background structure, option (a) is killing off superposition (something I like), option (d) is plagued with ambiguities like any black box modelling is.

**
Please say what you mean by *background independent* (because people in different discussions mean different things by it) **

By background independent I mean - in the concrete context of spin networks - there is no further relational data provided between spin networks than knotting information. More generally, in a covariant formulation, I mean that there are no identifications given between the different spacetimes (no gauge fixing).

**and say what you mean by local observables**

An example of a local observable is : the position of the moon relative to the Earth given axes determined by the sun, Jupiter and saturnus. But the no-go argument *precisely* consists in asserting that ANY definition of a local observable REQUIRES extra relational information of the type mentioned above. If you do not specify any further information then you are bound to limit yourself to global observables such as average volume, dimension and so on, in either then you need to see the entire universe as a black box or you have to kill off superposition.

For example the point of view in dynamical triangulations is that only global spatial observables - such as average volume, dimension, curvature and higher moments of those - can be measured. As such they indirectly claim that local observables do not exist.


**Please do not refer me to some other books and authors. Just give me the LOGICAL reason which you mentioned having given. I will appreciate it, I assure you. **

?? Well, well, you can only do that I presume...

So I define a local observable indirectly by summing up the kind of examples it should be able to cover (actually I should add more to the list). This is a sensible strategy if you want to find a new mathematical object, you start by telling what it should do. Note that f-h did not give a definition of a local observable either, he intuitively argued that these observables are somehow showing localized behavior at the *classical* level.

 

[Careful]

**I like a lot what you are saying re dynamics vs. kinematics. You got me once thinking with similar remark (all that orthomodular toys should really be dynamically determined, not rigid as they are right now). However the only theory with kinematics following from dynamics seems to be GR, as Bergmann and the followers, like Lusanna, showed. You have any other examples or clues? It seems that nobody ever though along this lines in mechanics (be it quantum or classical). **

Pfew, that is a difficult one (I guess you are somehow referring to this discussion about dynamical entropy, no?). I doubt it if you can find a general prescription for such thing, even in concrete examples such as the amount of information stored on the black hole horizon, it gets very difficult if the horizon itself is non stationary.


**
PS Re EPR - this is what I thought - "confined" sort of information, so multiplying ghosts and moving towards magic. People who attack EPR are sometimes very predictable **

Haha, this was just the most obvious scenario which came to my mind in a few minutes.

 

[marcus]

I appreciate your efforts here. I am not entirely satisfied because I understood you to say you had a proof of a more general fact (not tied to spin networks). I will have to think and see if it generalizes in some obvious way.

The statement you claimed IIRC was that it is logically impossible to define local observables in an
background independent theory.

The usual meaning of background independent is that that theory does not require a fixed background metric on the manifold to be established in advance.

If I don't see, from your post, how to make good your "no-go" claim, I will get back to you.

Thx.

**
I want to learn from you, careful, if you have something definite to teach me.
Tell me your LOGICAL reason, that you have given. **

If you want to define a local observable, then I said that you have the following possibilities:
(a) no superposition of rigmapped spin networks (which is as good as classical)
(b) classical timelike boundaries (possibly combined with spatial caps) - but then you have no local information about the interior.
(c) figuring out a mechanism which gives relational information (more than just topological one !) between nodes in two different spin networks
(d) measuring expectation values of global observables which you try to fit to a Lorentzian manifold (not a classical solution to the vacuum Einstein equations in case you include matter)

Option (b) runs straight against quantum mechanics. Option (c) is tantamount to picking a background structure, option (a) is killing off superposition (something I like), option (d) is plagued with ambiguities like any black box modelling is.

**
Please say what you mean by *background independent* (because people in different discussions mean different things by it) **

By background independent I mean - in the concrete context of spin networks - there is no further relational data provided between spin networks than knotting information. More generally, in a covariant formulation, I mean that there are no identifications given between the different spacetimes (no gauge fixing).

**and say what you mean by local observables**

An example of a local observable is : the position of the moon relative to the Earth given axes determined by the sun, Jupiter and saturnus. But the no-go argument *precisely* consists in asserting that ANY definition of a local observable REQUIRES extra relational information of the type mentioned above. If you do not specify any further information then you are bound to limit yourself to global observables such as average volume, dimension and so on, in either then you need to see the entire universe as a black box or you have to kill off superposition.

For example the point of view in dynamical triangulations is that only global spatial observables - such as average volume, dimension, curvature and higher moments of those - can be measured. As such they indirectly claim that local observables do not exist. **Please do not refer me to some other books and authors. Just give me the LOGICAL reason which you mentioned having given. I will appreciate it, I assure you. **

?? Well, well, you can only do that I presume...

So I define a local observable indirectly by summing up the kind of examples it should be able to cover (actually I should add more to the list). This is a sensible strategy if you want to find a new mathematical object, you start by telling what it should do. Note that f-h did not give a definition of a local observable either, he intuitively argued that these observables are somehow showing localized behavior at the *classical* level.

 

[Careful]

Ah, but the arguments are not tied to spin networks at all : for example they also apply to causal sets (I just presented them in a form suitable for spin networks for clarity). Joe Henson writes a lot about this issue in pretty much the same way as I speak about it: that is how points in different spacetimes could be ``the same´´ which is just gauge fixing in disguise IMO. Even if *you* believe some mighty clever construction might avoid my argumentation and still satisfy our intuition, try in good spirit yourself to figure out how it could work (you will see you end up in (a), (b), (c) or (d))

Ah, concerning the background metric : the argumentation here is a bit more difficult - I will try to be as clear as possible. In order to add this extra relational information without introducing a background metric you have to look for kinematical ``comparison mechanisms´´ depending only upon the intrinsic structures of the spin networks, causal sets or whatever. Apart from the fact that any choice of such ``way of comparing´´ is highly non-unique and quite complicated, any particular choice gives no unique answer either, which leads to further ambiguities. The main problem furthermore is that such ``identification mechanisms´´ are not transitive and neither symmetric, meaning that if I compare p_1 in U_1 with p_2 in U_2 and p_2 in U_2 with p_3 in U_3, then it is generically not like that that the same mechanism compares p_1 with p_3, and p_2 does not necessarily need to be compared with p_1. That is: the points in the different spacetimes do not form a chain. If you consistenly apply this weakness then you end up with the conclusion that any point in any spin network will be included in your definition of ``one point´´. The only way to avoid this ``diffusion of points´´ is to pick out one spin network which serves as a reference; this is your background.

Cheers,

Careful

 

[f-h]

Careful, you are throwing a lot of independent problems together and mixing them up. Each individually can be addressed.

There are two issues of locality. Entangled states and background independence. These are conceptionally different and it would help if you would stop throwing them together.

One is the observer within the QM system, wavefunction of the universe style. Nobody knows how to do this (except perhaps for Hartle). It's a well known problem, and not specific to QG. However if we declare a part of the system to be the observer we have a working interpretation relative to that observer Everett style. No classical boundary information needed. (morally that's how Rovelli get's a propagator, notice that the classical boundary conditions drop out, there is a quantum mechanical state to which the question is relative, a semiclassical one, a superposition of many spinnetworks, but a real quantum mechanical state)

Classical GR, no matter. What are the local observables? If I construct them via Dittrich it is crucial to realize that Kinematical information (before implementing the diffeoinvariance) supplies the notion of locality.
Without that, talking purely about the 4-geometries allowed by Einsteins equations and not about metrics we don't have a notion of locality and the question becomes meaningless.
The notion of locality obtained by going kinematical refers to the possibility of auxiliary systems with the same kinematics being coupled to the background independent theory relative to which we ask local questions. Test particles.

Your spinfoam objection has a precise analogue in classical GR and is resolved by Rovellis/Dittrichs work. There is nothing specifically Quantum mechanical about your objection.

Freidel has constructed 2+1 background independent QFTs with testparticles. If I have a testparticle on a superposition of spin networks that is a sollution to the Hamiltonian constraint I can of course ask local questions with respect to it's location on the spin network. Just like I use testparticles in classical theory relative to which I can ask about the local state of the geometry (which I can't sensibly if I *only* work with the allowed geometries)

Can we define a sensible notion of relative locality of Quantum mechanical systems? Yes, if I take a spin state and couple it to a Quantum mechanical system I can say the coupling is local in time or space (as operators in the kinematical Hilbertspace!) or whatever other partial observable I cook up in the kinematics.

Is Quantum mechanics nonlocal? Not really. It leads to no nonlocal effects at least. That's one of the points of Rovellis relational QM which makes good sense if taken as an epistemology of QM rather then an interpretation.

So far I see no argument in anything you say that comes close to substantiating your very strong dogmatic claims which you have repeated several times now.

 

[f-h]

Basically you do not understand the conceptional set up of background independence, and the nature of local physical statements in a background independent theory.

Really have you read Rovellis "What is observable in Classical and Quantum Gravity."?

C. Rovelli, What is observable in classical and quantum gravity?, Class Quant Grav 8 (1991) 297. G/A

This does not neccessarily represent our current best understanding of these issues but it lays some of the important conceptional groundwork from which to see them as the apparent nonsubstantial problems they are.

 

[f-h]

Can I contradict it, then? Newtons conception of time and movement is a lot simpler than phase space and symplectic areas.

I can explain the basic ideas of Lagrangian mechanics to someone who has never had any formal physics education a lot quicker then Newtons ideas.

Familiarity is not simplicity is not naturality.