Exploring Quantum Gravity: Rovelli's New Book

[jeff]

In the forward to rovelli's new book "Quantum Gravity", james bjorken states quite plainly that the effective field theory approach to quantum gravity correctly taught us that GR must be viewed as just an effective field theory, and in fact this is the universally shared view.

The problem for lqg is that the central construct in lqg, spin networks, only makes sense if GR is in fact exactly correct. I don't see why it would make sense for an author to allow a forward to be written by someone else, that contradicts the basic premise of the book.

In fact, rovelli defends lqg by stating...

"But the modification of the notions of space and time has to do with the diffeomorphism invariance and the background independence of the action, not with it's specific form."

In other words, it is inaccurate to view lqg as a genuine candidate quantum gravity theory, and thus as a rival of string theory. Rather, lqg is just a toy theory serving as a laboratory to explore a small number of fundamental issues in quantum gravity.

 

[marcus]

This "admission" is not anything Rovelli explicitly says, or which one can logically infer from any of his statements. What you offer as Rovelli's admission is your own statement.

Let's actually quote generously and not mix short quotes out of context with our own (in this case clearly rather strained) interpretation. LQG as we see it in Rovelli's new book is supportive of the widely shared idea that, as it says in Bjorken's Foreword on page ix:
"...the Einstein-Hilbert action is no doubt only the first term in an infinite series..."
I know of hardly anyone besides you Jeff who supposes that LQG is inseparably wedded to one form of the Einstein-Hilbert action and is inherently incapable of accomodating higher-order terms.

I think you are simply mistaken about the next point, which I will distinguish by boldface:

...

The problem for lqg is that the central construct in lqg, spin networks, only makes sense if GR is in fact exactly correct. I don't see why it would make sense for an author to allow a forward to be written by someone else, that contradicts the basic premise of the book.

Really I think it is a nice custom to have someone else do the Forward to one's book. Bjorken is generous with his praise of the book and points to some unique ways the book is valuable. It is a friendly Forward and does not attack any "basic premise" of LQG or of Rovelli's book!

Your problem is you think, as you say, that spin networks don't make sense unless one has decided to use one particular form of the Einstein-Hilbert action! That is pretty far-fetched. Spin networks are for setting up the kinematic Hilbert space, before you get around to dynamics.

Jeff as a bit of sympathetic advice, I see you as too much into Attack-Defense stuff, like when you first appeared at PF wearing the nickname "Steinitz" who was a chess champion.
Bjorken, in his nice forward, is not attacking, and yet you interpret what rovelli says on page 6 (written long before Bjorkens contribution) as a
defense of LQG:

In fact, rovelli defends lqg by stating...

"But the modification of the notions of space and time has to do with the diffeomorphism invariance and the background independence of the action, not with it's specific form."
...

that is not a defense of LQG, which doesn't need defending. If anything it points to deficiencies in the Stringy approaches which are mostly NOT diffeo invariant/background independent.

So then you draw the totally unjustified conclusion that what I just quoted from Rovelli means that he is "admitting LQG is not correct". There is no logic to this at all, that I can see.

In other words, it is inaccurate to view lqg as a genuine candidate quantum gravity theory, and thus as a rival of string theory. Rather, lqg is just a toy theory serving as a laboratory to explore a small number of fundamental issues in quantum gravity.

No this is not what Rovelli is saying. It is what YOU are saying.

Since you seem eagerly interested in this passage around page 6 of Rovelli's book, i will take the trouble to type in a longer exerpt, so that anyone who cares to can see what Rovelli is actually saying, instead of just having brief sentences out of context.

 

[marcus]

---quote from Rovelli page 6---

...According to this [particle theorist] opinion, GR should not be taken too seriously as a guidance for theoretical developments.

I think that this opinion derives from a confusion: the confusion between the specific form of the Einstein-Hilbert action and the modifications of the notions of space and time engendered by GR.

The Einstein-Hilbert action might very well be a low-energy approximation of a high-energy theory. But the modification of the notions of space and time does not depend on the specific form of the Einstein-Hilbert action. It depends on its diffeomorphism invariance and its background independence. These properties...are most likely to hold in the high-energy theory as well.

One should not confuse the details of the dynamics of GR with the modifications of the notions of space and time which GR has determined. If we make this confusion, we underestimate the radical novelty of the physical content of GR.

The challenge of quantum gravity is precisely to fully incorporate this radical novelty into QFT. In other words, the task is to understand what is a general-relativistic QFT, or a background-independent QFT.
---end quote---

Jeff you see it is not a defense (as you were determined to call it) but a bold challenge that is almost a Quantum Gravity manifesto. In this view little details of one particular action formula are not what matters. What matters is a fully relativistic quantum physics.

that means a thoroughly background-independent quantum physics, since background independence (with it's radically different take on space and time) is the distinguishing feature of general, as opposed to merely special, relativity.

This passage is pointing to exactly where, in Rovelli's view, string needs to change, and where particle theorists (inasmuch as they work with fixed backgrounds) miss the boat. In his view of course, probably not yours since you have pretty much absorbed a particle theorist perspective.

BTW I have to congratulate setAI on the comparison with an angry radio personality, which is amazingly apt.

 

[setAI]

it's weird- the more that LQG grows and flourishes- the more that it shows the way forward- and the way toward understanding the REAL role of string theory as part of a deeper theory- the more we get these die hard string-religion Believers coming out and making up outlandish criticisms of LQG!

it's really pathetic- supposed genius level physicists with IQs approaching my own who are so livid yet illegitamate that they cannot even fashion a proper attack! look at Lenny Susskind and his sad attempt at responding to Smolin's Anthropic paper- it was so infantile and full of "all the smart people agree with me and not you nya-nya-nya!" that he- a giant in the field- couldn not get it published on arXiv!

and Lubos is basically been reduced to something like an angry conservative radio show personality with his immature ranting and childish epithets- basically using attacks like "you are a retard"-


you guys should just lock yourselves away with Billy Cottrell in his prison cell- you can put your hands over your eyes and ears and ignore the world and keep telling yourselves that strings are not just AN area of physics- but "THE area of physics" :rofl:

 

[jms]

Indeed, people working with string theory definitely have a very strange attitude to criticism. Outsiders are often just making the point that it might be better to keep other options open and not give young students the impression that string theory is the only reasonable thing to do. Giving this impression just kills creativity in both ends of the spectrum, junior and senior, no matter what's the status of the theory.

It's very discouraging to see that even the brightest ones have hard time with keeping their attitudes somewhat reasonable. There seems to be too many people doing theoretical physics these days. Unfortunately, intelligence, let alone creativity, is not additive... Lots of vegetables make a soup.

 

[jeff]

setai and jim,

Did either of you actually check the book, or draft I quoted? I don't see any evidence of this or of any physical arguments in your answers. Despite marcus's usual word games and twisted meanings, what I wrote is simple to check, understand, and in fact, correct.

You must understand that the revelation is not that the people who do lqg believe that GR is exactly correct (in fact, they don't and never have) even though though they know that lqg requires for it's definition that GR be treated as if it we're.

The revelation is that carlo has indicated this in his book, my point being that when I explain this to marcus, he responds with remarks about my own bad attitude or whatever.

This time I tried to avoid all that by simply referring marcus to the beginning of carlo's book.

 

[jeff]

people working with string theory definitely have a very strange attitude to criticism.

Huh? Where are you getting this from?

 

[jeff]

it's weird- the more that LQG grows and flourishes- the more that it shows the way forward- and the way toward understanding the REAL role of string theory as part of a deeper theory- the more we get these die hard string-religion Believers coming out and making up outlandish criticisms of LQG!

it's really pathetic- supposed genius level physicists with IQs approaching my own who are so livid yet illegitamate that they cannot even fashion a proper attack! look at Lenny Susskind and his sad attempt at responding to Smolin's Anthropic paper- it was so infantile and full of "all the smart people agree with me and not you nya-nya-nya!" that he- a giant in the field- couldn not get it published on arXiv!

and Lubos is basically been reduced to something like an angry conservative radio show personality with his immature ranting and childish epithets- basically using attacks like "you are a retard"-


you guys should just lock yourselves away with Billy Cottrell in his prison cell- you can put your hands over your eyes and ears and ignore the world and keep telling yourselves that strings are not just AN area of physics- but "THE area of physics" :rofl:

What the f**k are you talking about?

 

[selfAdjoint]

He is talkin about present reality, my friend. If you want any more of the chapter and verse than he has already given, go read the comments on Peter Woit's blog. I didn't actually see Lubos use "retard", but I did see "monkey brains".

 

[jeff]

He is talkin about present reality

Who gives a sh*t about what lubos thinks? What he said doesn't have anything to do with what I'm talking about. If you have a problem with lubos, email him. My initial post is correct.

 

[jeff]

I've emailed rovelli the following:

Hi Carlo,

Congratulations on your book, which is better bound (and perhaps even better smelling, I think) than other books in that series.

Now, my understanding is that the construction of lqg requires that General Relativity must be treated as if it's exactly correct. But you write that even if it's not, it still makes sense to pursue lqg for the reasons you give. I appreciate this, but I'd still like to know whether you agree with Bjorken's statement in the forward to the effect that General Relativity must in fact necessarily be only a low energy effective theory, and if you don't agree, why?

Thanks,
jeff

I'll post his response the moment I receive it.

 

[Wave's_Hand_Particle]

I've emailed rovelli the following:

Hi Carlo,

Congratulations on your book, which is better bound (and perhaps even better smelling, I think) than other books in that series.

Now, my understanding is that the construction of lqg requires that General Relativity must be treated as if it's exactly correct. But you write that even if it's not, it still makes sense to pursue lqg for the reasons you give. I appreciate this, but I'd still like to know whether you agree with Bjorken's statement in the forward to the effect that General Relativity must in fact necessarily be only a low energy effective theory, and if you don't agree, why?

Thanks,
jeff

I'll post his response the moment I receive it.

If its anything like the time He visited these forums, and came across your acrid tongue egotistical ramblings, I guess the reply will amount to the equivilent information of a word-doc, zero-point-field :tongue2:

 

[jeff]

If its anything like the time He visited these forums, and came across your acrid tongue egotistical ramblings, I guess the reply will amount to the equivilent information of a word-doc, zero-point-field :tongue2:

Why on Earth did you think it was a good idea to post this?

 

[Wave's_Hand_Particle]

Why on Earth did you think it was a good idea to post this?

It my opinion that you can't take any sort of critism, its your ego, like I said when I saw your photo some years ago, Monkey's can laugh at themselves..why can't you?