Thomas Larsson's post on LQG-String

[jeff]

Originally posted by marcus
Urs, I don't think you will get very far in clarifying this issue by phrasing it in terms of "honesty"
and, in effect, accusing other people of bad faith.
what you evidently have is a semantic issue where
two groups use some technical terms differently

Forget about what either side means when they say "canonical". This is not an issue of semantics. We can even put aside the issue of scientific integrity. What matters is that LQG quantization is fundamentally different from standard canonical quantization, something which LQG researchers apparently confirm when asked directly. However, I do think the question of why no one outside of the LQG camp knew this is worth asking.

 

[Urs]

The main problem is not that the terms are used differently. The problem is that one of the uses of this term refers to highly speculative physics.

And let me emphasize that by 'highly speculative' I mean something drastic. Of course every theory of quantum gravity in the absence of experiments has to be speculative. In string theory there is the single and obvious speculation that strings exist. Everything else follows. If they don't exist, they don't. Fine.

But in LQG the speculation is that at the Planck scale the quantum principle itself is radically different from everything we know so far. Maybe one can argue that the modified principle should still be called 'canonical'. Words are arbitrary. But it still refers to a concept drastically different from what is usually called canonical, outside the LQG-literature. You wouldn't claim that the LQG-like quantization of the 1d nonrelativistic particle in gr-qc/0207106 is 'canonical' would you? It's not canonical - it's weird!

I can say that with full confidence because if we know one thing for sure it is how the quantum theory of the 1d nonrel particle works. And it works very differently from the supposedly 'canonical' theory that is presented in gr-qc/0207106. Now from where comes the belief that applying this weird quantization to gravity gives something more reasonable?

If we just had a single hint that the quantum principle must be modified at the Planck scale. But do we have any? If the LQG-authors have such a hint then they at least have not published it. All that Thomas Thiemann said is that "experiment will show".

Right. Maybe Bohm trajectories are found at the Planck scale, or Jadczyks 'Event Enhanced Quantum Theory' or Smolins version of Nelson stochastics or nonunitary QM or whatnot. All this has been proposed. But all this is known and acknowledged to be highly speculative. Nobody would call a nonunitary version of QM a 'canonical' quantization. Unless, of course, he wants to risk to be misunderstood for over 10 years... :-)

But let us not get deeper in this kind of discussion. If there are any further technical issues to be discussed, in the vein of my discussion with Thomas Larsson on spr, then I am willing to participate. Otherwise there is little point in restating my assessment over and over again.

 

[Urs]

Jeff -

thanks, yes, that's my point.

Oh, and apparently I must clarify my use of the word 'honest'. It was surely not supposed to question the personal or scientific integrity of anyone. I was using this in the same sense as in, for instance 'Momentum eigenstates are not honest states.' or 'x is not an honest operator for a particle on the circle'.

So this is why I said the Hamiltonian constraint is an 'honest' constraint in LQG, because it is represented as an operator as usual for quantized constraints. This is not true for the other constraints, so they are not really Dirac constraints.

This is all I meant. I apologize if this was unclear. Honestly.

 

[jeff]

No problem.

 

[marcus]

Originally posted by Urs
...Otherwise there is little point in restating my assessment over and over again.

I think you are probably right about there not being much point.
Perhaps I had better take a turn and try to state my assessment instead. Or some of the others.
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. There is probably some breadth of interpretation as to what is expected and what is meant. I reject the idea that Ashtekar, Lewandowski were being dishonest or obtuse when they said what they were doing in 1995 was a canonical quantization.

It is not obvious that semantic issues can be resolved democratically---by a simple headcount---but I rather suspect that you (and the string theorist you have talked to about this) may be in the minority. I can understand that you must be tired of reiterating your position so many times in so many forums. You should not feel as if you are obliged to continue repeating your assessment.

 

[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.