Thomas Larsson's post on LQG-String

[marcus]

the history of quantum theory since 1925 or so
has been one of drastic innovations
with people occasionally getting the Nobel prize
for coming up with drastically different ways to
solve persistent nagging problems.

You have shifted from saying that LQG is not
"canonical" to suggesting that LQG is "drastically different"
in its style of quantizing the gravitational field.
That is flattering to LQG. But I guess it could be an overstatement, I mean from an historical perspective---compared with
some other major steps in quantum theory the extent of innovation may be less radical---but innovative steps are
hard to measure.

It occurs to me that a lot of physicists have not been paying
attention to LQG and are only begining, some of them, to take notice.
The newness in the Loop approach has certainly not been concealed!
Rovelli, for instance, has stated loudly several ways he considers Quantum Gravity to be a radical departure. And given arguments to the effect that they are necessary. If anyone did not notice differences it is their own fault for being inattentive.

Someone who has so far only noted one "difference", may possibly not have been listening.

Urs you keep mentioning a 2002 paper I haven't read by someone who is somewhat of an out-lying figure. He strikes me as being enough on the margin that I can't decide whether he does recognizable LQG or not. I could be wrong but I should think any criticism you wanted to make would connect with reality better if you would direct it to widely-read papers by core people.

 

[Urs]

You mean gr-qc/0207106? That's by Abhay Ashtekar.

 

[marcus]

Originally posted by Urs
You mean gr-qc/0207106? That's by Abhay Ashtekar.

Urs,

Please, when you refer to a paper do not just give a code number.
If you are mentioning it for the first time in a post,
have the courtesy to give a clue as to what the paper is
for those like me who can't remember the arXiv number of
all the papers you reference.

That should make an improvement in how easily we
can communicate.

Thanks, I know the Ashtekar/Fairhurst/Willis paper you meant now.

 

[Urs]

Marcus -

I tell you what:

Since I have mentioned this paper for quite a while now and many times, here, on s.p.r. and at the Coffee Table, have written summaries and critical discussions of this paper in these three groups, have compared its techniques to those used by Thomas Thiemann, have pointed out where Thomas Thiemann's approach is even more non-standard then this paper, since all this has happened without you noticing, I now step back for a while and give you time to have a second look at all that has been said.

Especially I suggest you have a look at the last dozen or so entries of the 'Amazing bid' thread where the content of this paper was part of an 'exercise' which was jointly analyzed by several participants. Try to read these posts and see if you get the point. The basic issue is discussed there in terms of very elementary examples.

When you are sure you have read them and tried to understand them and tried a second time to understand them and still feel that you have questions, then feel free to ask again. But please, before asking, make sure that you really have read what has already been said.

 

[jeff]

G

Originally posted by marcus
I would like to determine what it is that people generally understand by canonical quantization and Dirac's program of quantizing a classical theory with constraints.

Excellent idea! (Though you really should've done this before delving into it's applications in QGR)

Originally posted by marcus
It occurs to me that a lot of physicists have not been paying attention to LQG...If anyone did not notice differences it is their own fault for being inattentive.

I've made this point. Anyway, there's plenty to be learned by LQG die-hards like you by exploring from this new perspective the question of LQG's potential as a source of good ideas and plausibility.

 

[Haelfix]

Having played around with some of the toy problems, I think I see the main issues now. Again, its mathematically consistent (at least to first guesstimate), but physically weird.

In the LQG approaches discussed so far, gravity really is, simply different. Now, I'd love to see the person who came up with that quantization scheme in the first place. A priori, I am wondering what the motivation was, since that crucial step could have been done in the usual way, without necesarily spoiling the fundamental tenets of LQG.

 

[jeff]

Originally posted by Haelfix
...crucial step could have been done in the usual way, without necesarily spoiling the fundamental tenets of LQG.

If so, it would've been done that way.

 

[selfAdjoint]

According to Velhinho's new paper, the logic was set by the magnetic densitized tetrads. Other options might have been available for the connection variables, but the magnetic variables had to go along to provide coordinates in the phase space, since they were conjugate to the connections. And they could only be consistently quantized in this one way.

If this is so, then the original Ashtekar "new variables" are the reason.

BTW Jeff, have you cleared out your PM mailbox yet? You sent me a message and I want to reply.

 

[jeff]

In the standard quantization of string theory, the conformal anomaly imposes einsteins equations on the background. Such a condition can't make sense in a theory that aims to be background-independent, so perhaps logical consistency requires one use a non-standard quantization. The string example suggests that LQG quantization loses the extra classical information needed to produce GR.

selfAdjoint,

There's something wrong with the pm system since my message box contained only one message. This happens to me quite often. so I've pm'ed greg about this. Anyway, I just wanted to know if you've had fun interacting with urs as he's struggled to nail down what's special about LQG.

 

[selfAdjoint]

Originally posted by jeff

selfAdjoint,

There's something wrong with the pm system since my message box contained only one message. This happens to me quite often. so I've pm'ed greg about this. Anyway, I just wanted to know if you've had fun interacting with urs as he's struggled to nail down what's special about LQG.

Did you check your sent items folder? That's what hangs me up sometimes. There's a folder selection box over toward the right of your PM screen.

Yes I have enjoyed the give and take with Urs. I am not skilled enough to discuss with him on an even level, but I can follow the articles, and do the integrals. I think I learn more, or maybe better, this way than in either self teaching or online study groups.