Where we stand-Baez talk at Luminy

[Careful]

**Seems I struck a nerve, or maybe this is your normal attitude. **

I just always react like that when something that naive is said.

**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. Apart from the fact that every modern positivist will contradict your statement, even me as realist has to acknowledge that such persuit is logically flawed. GOD is the first reason for everything by definition - science is the art of finding a mechanism behind that piece of information we have access to. As I said before, science is natural philosophy, there does not exist something like truth in physics.
By the way, we are lightyears away from a theory of everything. No decent physicist will contradict that.

 

[f-h]

Marcus, thanks, yes that's precisely what I meant. As Rovelli recently explained in some detail in a seminar, Maxwells equations were taken to be effective equations of a more fundamental fluid mechanics until Einstein.

Careful, so what's the virtue of saying "if an electron scatters with a myon I get this and that cross section, if an electron scatters with a photon I get this and that cross section..." instead of saying "There are the following interaction vertices and propagators, and all Feynman diagrams contribute".

And actually, according to your mantra, I musn't stop there. I shouldn't talk about photons and so on which are part of nonperturbative noninteracting QFT (which IS understood quite well).

Also predicting "some" experiments isn't enough, you need to explain all the experiments that have been conducted before you. Now there is none that contradicts QM and very few at all that perhaps contradict GR (cosmological constant and dark matter don't contradict GR, and the pioneer anomaly is far to little understood to draw firm conclusions).

On the other hand there are millions of experiments that demonstrate these theories work. Of course you can *in principle* just go ahead and explain all of them in terms of some completely different theory. The sane option is to just show that your theory reduces/produces to GR/QM in some appropriate sense and thus automatically fulfills the experimental tests. (Just as GR becomes SR and SR becomes Newtonian, and QM becomes classical in the appropriate sense)

Knowing the range of validity of experimental tests is of crucial importance of course, see P and CP, which were assumed to be true but hadn't been tested.
Ignoring the theories that organize our knowledge of experiment isn't feasible though. Even the ugly Standard Model is a fantastic reductionist feat, explaining thousands of experiments and ridiculous amounts of data in terms of two dozen parameters (plus of course the millions more that were performed on Newtonian mechanics which is ultimately contained within it). In GR you have fewer experiments but only two parameters either.

The range of validity of our best theories exceeds the range of experiments conducted. Better theories or experiments will eventually tell us what the range of validity of current theories is, but they do subsume all we do know about reality.

With the important exception of cosmology. Which goes right back to the topic of this thread: Baez says "Hey look! Data (of sufficient precision to commence pondering it's detailed implications) that is beyond our current understanding of the world as expressed by GR/SM! Exciting!"

 

[Careful]

** As Rovelli recently explained in some detail in a seminar, Maxwells equations were taken to be effective equations of a more fundamental fluid mechanics until Einstein. **

And who says that was a correct move ? You do not seem to get my point that any view consistent with reality is fine, and that a wealth of views is a RICHNESS for physics.

**
Careful, so what's the virtue of saying "if an electron scatters with a myon I get this and that cross section, if an electron scatters with a photon I get this and that cross section..." instead of saying "There are the following interaction vertices and propagators, and all Feynman diagrams contribute". **

? Nowhere did I claim that *only* knowing particle phemenology is a virtue ! I said it was *sufficient* (modulo some other knowledge) to start doing original work in physics. Example : the preon models, they are not obviously related to the standard model at all ! Actually, no dynamics is presented yet; however to make progress, all you basically need to know is the classical action for the SM and some SM phemenology.

**
And actually, according to your mantra, I musn't stop there. I shouldn't talk about photons and so on which are part of nonperturbative noninteracting QFT (which IS understood quite well). **

Nah, only the non abelian cases are understood up to some level, QED is not. It all depends upon the level of rigor you want from a theory and QFT theorists are usually not very demanding what that is concerned.

**Also predicting "some" experiments isn't enough, you need to explain all the experiments that have been conducted before you. Now there is none that contradicts QM and very few at all that perhaps contradict GR (cosmological constant and dark matter don't contradict GR, and the pioneer anomaly is far to little understood to draw firm conclusions). **

Ahhhrr, when after 35 years the pioneer anomaly cannot be explained, at the moment that it becomes unlikely that dark matter can be predicted from GR, despite of recent admirable attempts, one should draw some conclusions. The cosmological constant is logically either a problem for GR, either for QFT or perhaps even for both. Concerning experiments which do not violate QM, well that is a delicate matter (all depends upon your prior bias and how you decide to make the statistics). By the way, experiments do not violate of confirm a theory so easily in the naive sense you are aiming at. Statistically, you can only express your *confidence* about a theory. The fact that the Higgs is still missing, that no perfect Bell experiment has been achieved, that supersymmetric particles have not been found yet and so on might also be considered as evidence against the theory (although stricly speaking the SM has the luxury to put these parameters wherever it wants to within reasonable bounds).

**
On the other hand there are millions of experiments that demonstrate these theories work. Of course you can *in principle* just go ahead and explain all of them in terms of some completely different theory. The sane option is to just show that your theory reduces/produces to GR/QM in some appropriate sense and thus automatically fulfills the experimental tests. (Just as GR becomes SR and SR becomes Newtonian, and QM becomes classical in the appropriate sense) **

Well one would indeed expect that to be sane (although I firmly disagree that QM has been shown to become classical in any appropriate sense), it is just that this does not work out ! You know, when you have been thinking about fundamental issues for ten years, you probably either:
(i) stop asking some annoying questions and go on with the business (ii) you try something entirely new.

**
With the important exception of cosmology. Which goes right back to the topic of this thread: Baez says "Hey look! Data (of sufficient precision to commence pondering it's detailed implications) that is beyond our current understanding of the world as expressed by GR/SM! Exciting! **

Right, now start thinking about that, really TRY to find a rational answer to the problems and then see after 10 years how you really think about our theories. By the way Baez probably did not mention that our theories do not present a consistent view upon reality - tsssss.

But again, what is your point ? The engineers, those who come up with practical applications, do you really think they make their inventions using QFT or QM or relativity rigorously? Nah, they all use classical or ad hoc calculations. Personally, it seems to be a contradiction to claim to be a physicist while not being hard-core interested in fundamental issues; become engineer then and indeed use just some tools of QFT if you really need it (which is doubtful).

Cheers,

Careful

 

[arivero]

Marcus, thanks, yes that's precisely what I meant. As Rovelli recently explained in some detail in a seminar, Maxwells equations were taken to be effective equations of a more fundamental fluid mechanics until Einstein.

Er... but they are. Abelian U(1) gauge equations, aka Maxwell's equations, are effective equations of broken SU(2)xU(1).

Of course the point is that the new nonabelian gauge equations are still relativistically invariant ones.

 

[Careful]

Er... but they are. Abelian U(1) gauge equations, aka Maxwell's equations, are effective equations of broken SU(2)xU(1).

Of course the point is that the new nonabelian gauge equations are still relativistically invariant ones.

Invariant under the Lorentz group yes, but why see that as such a virtue (only the vacuum needs to be Lorentz invariant strictly speaking)? You know the problems the minimal coupling principle brings along when you try to get relativity in. Therefore, it could be entirely reasonable to construct non relativistic theories formulated in some preferred Lorentz frame, which *are* invariant under the Lorentz group when averaged over the SO(3) degrees of freedom (actually, this is what Holland does). Gravity then would have to be formulated in such a way that the metric is not the dynamical variable.

 

[arivero]

Time ago I read an argument for Lorentz group in terms of stability/unstability. The point being that c->infinity is sort of an "unstable" theory, where any finite value of c is preferable. In the same sense quantum mechanis is more reasonable, more "stable" than a h->0 theory as classical mechanics.

 

[Careful]

Time ago I read an argument for Lorentz group in terms of stability/unstability. The point being that c->infinity is sort of an "unstable" theory, where any finite value of c is preferable. In the same sense quantum mechanis is more reasonable, more "stable" than a h->0 theory as classical mechanics.

Ah, but the velocities are still bounded in this fluid model as far as I remember (don't panic ); in the same way one might consider the de Broglie mass problem for the complex KG field not as a ``real´´ issue. Concerning the stability issue for QM, let's say I agree that a quantized theory can be reasonably *expected* to be more stable (after all, that was the purpose no - although stability analysis is often not done yet) than the *corresponding* classical counterpart. I disagree of course that no realist stable alternative exists, actually it is easy to understand how stable Hydrogen and Helium can be obtained by making suitable compton scale modifications of EM.

 

[f-h]

Nitpick: You missed my noninteracting.

Careful, we are clearly in very different buisnesses, you seem to be trying to come up with a complete consistent explanaition of everything. You are outraged that people are working with incomplete and inconsistent explanaitions.

I'm personally just mostly interested in understanding nature to the best of my abilities. My time and abilities being severly constrained by me being human I thus simply take the theories that all these other people have constructed to the best of their abilities to describe how nature behaves and work from there, incomplete and inconsistent as they may be, they combine and contain everything we know about nature so far (minus cosmology).
Luckily for me all we know about nature is formulated in two conceptionaly very different theories. So for a conceptional thinker there is a reasonable case to be made that by studying the conceptional dichotomy and seeing if it does imply true incompatibility or if it suggests some subtle change to the concepts we use to do physics.

For this change in concepts new ideas certainly are valuable, and there are many many around. BUT there are some concepts that are at the core of the best theories we have: QM and GR. These theories are unique in the simplicity with which they reproduce an incredible number of experiments, other theories and concepts might be able to do so to, but (for those I have seen) they are *all* *significantly* more complicated then the theories built on these concepts.

Now GR is instructive, it says location is relational wrt dynamical entities. This was the philosophical point of view held at the time of Newton, against which Newton put his principia.
To Newtons contemporaries absolute time and space made no sense. Then for a couple of centuries we got used to it until Einstein realized that the old concepts were right, that there is no absolute space and time, just the local configuration of the gravitational field, and that the conceptional advance of Newton to introduce absolute space time was fruitfull because we live in a particular largely non dynamical gravitational field the physics of which can be absorbed into the background structure of our theory.

Newtons conceptional advance therefore was *real* significant and included deep information about physics, which were only fully revealed centuries later, and only after Maxwells equations revealed additional structures of nature.

Yes some of the concepts underlying GR/QM are likely wrong. But the conceptual advances of GR/QM are still the deepest knowledge about physics we have. So I take it as a starting point. If I realize they are not fundamental, I will in any case need to explain why they are effective, since it's a simple empirical fact that they are.

So I don't sound like I'm chaneling Rovelli completely I'll close with quoting Wittgenstein (emphasize mine):

http://www.kfs.org/~jonathan/witt/t634en.html"

So too the fact that it can be described by Newtonian mechanics asserts nothing about the world; but this asserts something, namely, that it can be described in that particular way in which as a matter of fact it is described. the fact, too, that it can be described more simply by one system of mechanics than by another says something about the world.

 

[Careful]

**Nitpick: You missed my noninteracting.**

Did I ? Who cares about noninteracting theories, these are not physical anyway.

** Careful, we are clearly in very different buisnesses, you seem to be trying to come up with a complete consistent explanaition of everything. You are outraged that people are working with incomplete and inconsistent explanaitions. **

No, I am not. I just react as a difficult person to (a) those who are expected to do fundamental physics and are just messing around in the conservative pool of inconsistency (b) those who are not doing fundamental physics but still claim that everything is fine (c) those who label intelligent people who really try something different - for good reasons - as crackpots. I understand that lab physicists are happy with the Feynman toy, but then just don't attribute anything more to it.

** I'm personally just mostly interested in understanding nature to the best of my abilities. My time and abilities being severly constrained by me being human I thus simply take the theories that all these other people have constructed to the best of their abilities to describe how nature behaves and work from there, incomplete and inconsistent as they may be, they combine and contain everything we know about nature so far (minus cosmology). **

Come on, don't react like this. All of us are just human, Einstein, Schwinger and the rest included... But reading from your attitude (not everyone is the same) why don't you stick to a conservative topic which is certainly worthwhile like particle phemenology, or something in statistical mechanics or solid state physics ? You want to spend your time on LQG which is most probably not a good investment ...


**For this change in concepts new ideas certainly are valuable, and there are many many around. BUT there are some concepts that are at the core of the best theories we have: QM and GR. These theories are unique in the simplicity with which they reproduce an incredible number of experiments, other theories and concepts might be able to do so to, but (for those I have seen) they are *all* *significantly* more complicated then the theories built on these concepts. **

QM is a conceptual disaster, and GR is certainly not easy at all : the non-linearities involved are not well understood and only very special classes of solutions are known. But I told already somewhere that GR and EM are also not unified yet (even at the classical level); so there are many more problems around than just QM and GR (which is what some people want you to believe).

**Now GR is instructive, it says location is relational wrt dynamical entities. This was the philosophical point of view held at the time of Newton, against which Newton put his principia.
To Newtons contemporaries absolute time and space made no sense. Then for a couple of centuries we got used to it until Einstein realized that the old concepts were right, that there is no absolute space and time, just the local configuration of the gravitational field, and that the conceptional advance of Newton to introduce absolute space time was fruitfull because we live in a particular largely non dynamical gravitational field the physics of which can be absorbed into the background structure of our theory. **

Of course - as relativist I understand this beauty - but is also entails lots of difficulties even just in combination with EM. The problem is the right hand side of your equations and in particular the minimal coupling principle used to construct the energy momentum tensor. Really, take a look at Weyl gravitation and Finsler approaches to the unification of GR and EM and you will see that they either contradict observation or either violate causality (the same for Kaluza Klein btw).

**
Newtons conceptional advance therefore was *real* significant and included deep information about physics, which were only fully revealed centuries later, and only after Maxwells equations revealed additional structures of nature. **

Sure, physics was just philosophy before, he introduced the math and the dynamical laws.

** Yes some of the concepts underlying GR/QM are likely wrong. But the conceptual advances of GR/QM are still the deepest knowledge about physics we have. So I take it as a starting point. If I realize they are not fundamental, I will in any case need to explain why they are effective, since it's a simple empirical fact that they are. **

Ah, I challange you - just for fun - to try to find classical mechanisms (short scale modification of known theories are allowed - nobody checked them anyway beyond a micrometer) behind the fundamental experiments which claim that QM is necessary. I expect you at least to come up with a few experiments where you can tell : hey this I can explain without magic.

Really, no hard feelings, why do you want to do LQG ? I feel you are somehow interested in fundamental issues, but on the other hand you don't know what to do (don't worry if I knew what I know now, I would not have done 1/5'th of what I have done in the past )

Cheers,

Careful

 

[Hurkyl]

I have to ask.

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

 

[f-h]

Oh but I'm happy to be a conservative fundamentalist. Uhmmm... in terms of physics that is.

The paragraph "All I want to do..." wasn't meant entirely jokingly. I think we just disagree what's most "fundamental" or "deep".

Why LQG? I have always maintained that (up to a certain point where the subject becomes to young) fundamental physics is one of the few things that get simpler the further you go. There are less and less concepts to contend with, computational complexity is replaced by conceptional subtility. Complexity is emergent and the simplicity is hidden by the concepts that seem natural only from the incidental perspective of creatures whoes concepts were formed to deal with the particular pocket/scale of stability in the emergent chaos which they occupy.

Michelson Morley is a strange experiment, Lorentz and co did a lot of complicated stuff to describe it, and who knows, perhaps they could have succeeded, but Einstein realized that there is a much simpler way if we change the fundamental concepts.

Sure you could do all of gravitational physics in terms of PPN gravity, but again, Hubbles findings will look strange and complicated.

Sure I can construct non local guiding waves or even more natural models for the double slit experiment, but again, the concepts of QM, self contradictory as they may be, are simpler.

The standard model and QFT are unfinished. They are not simple yet. Which is why i was saying before that this is perhaps the msot important thing to do at the moment: Figure out what the proper concepts are that render the SM+QFT simple.

String theory has given up on simplicity.
But LQG on the other hand is a great example for simplicity. The spin network theory they arive at is extremely simple. As Smolin said in one of the lectures, "You wouldn't believe you could do physics with that, would you?"

That's of course just a teaser. There will need to be more and greater simplicity.

But if there is no simplicity in fundamental physics I'm not interested. If it's going to be complicated fundamentaly then I'd do statistical physics/complexity/emergent phenomena. Not as sexy but great conceptional challenges as well and the added bonus that one actually knows one is talking about reality.

 

[marcus]

as a side remark. In my experience when people criticize LQG they often have something static in mind---something according to some book or some article which they have read and say "ahah! that is LQG" but it is changing. the whole field is a number of moving targets.

I was reminded of this yesterday while watching Smolin lecture #15. this 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.

maybe teaching something also helps to make it simpler---the challenge of having to explain to new people

BTW I liked very much some of what f-h said about evolution of human's understanding of nature

 

[Careful]

** Michelson Morley is a strange experiment, Lorentz and co did a lot of complicated stuff to describe it, and who knows, perhaps they could have succeeded, but Einstein realized that there is a much simpler way if we change the fundamental concepts. **

Haha, even Einstein's theory of general relativity contains an ether like aspect. But I am not doubting the correctness of the Lorentz invariance of the *vacuum*, it simply does not imply at all that our theories need to be formulated in a Lorentz invariant way.

**
Sure I can construct non local guiding waves or even more natural models for the double slit experiment, but again, the concepts of QM, self contradictory as they may be, are simpler. **

Think deeper.


**
The standard model and QFT are unfinished. They are not simple yet. Which is why i was saying before that this is perhaps the msot important thing to do at the moment: Figure out what the proper concepts are that render the SM+QFT simple. **

That is just not possible, we cannot even compute the classical pendulum hanging in a gravitational field exactly. You are fallen for oversimplification.

**
String theory has given up on simplicity. **

Yes, that is the best evidence that something is deeply wrong.

**
But LQG on the other hand is a great example for simplicity. The spin network theory they arive at is extremely simple. As Smolin said in one of the lectures, "You wouldn't believe you could do physics with that, would you?" **

But Smolin is correct : it hasn't anything to do with physics yet! You seem to have no idea whatsoever of the very DIFFICULT problems any such approach is facing, but I am sure Smolin will only speak about that once you can play around with spin networks (and even then).

**
But if there is no simplicity in fundamental physics I'm not interested. If it's going to be complicated fundamentaly then I'd do statistical physics/complexity/emergent phenomena. Not as sexy but great conceptional challenges as well and the added bonus that one actually knows one is talking about reality. **

Sure, I agree that simplicity as well as conceptual clarity and beauty are fundamental requirements; but I am afraid that LQG does not pass the test.

 

[Careful]

**
this 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.

 

[Careful]

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.

 

[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