Luboš Motl's Criticism of LQG - What Does He Miss?

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I don't normally read Luboš's blog because I find his tendency to belittle others offputting to say the very least. It can also be difficult to find the value hidden in the pile of vitriol and condescension. Further, I find from my past experience that his level of arrogance has been an indicator of less intelligence than the bearer would have you believe. Extremely smart people, even geniuses, are nice people and argue with logic and not insults, from my experience. It is only the very smart who imagine themselves to be geniuses that resort to insults.

Nevertheless, I ran into this post when searching google as part of some research:

http://motls.blogspot.com/2010/10/richard-feynman-and-birth-of-quantum.html

wherein he states when describing Feynman's look at quantum gravity and the first attempts at quantization of classical GR:
Fine. So he knew that the tree-level amplitudes encoded the classical theory. It followed that if you go through all this formalism with first-class and second-class constraints, you translate all the things into the ugly Lorentz-symmetry-obscuring quantum formalism, and you compute the scattering amplitudes, you will only get data that are directly transformed from the classical behavior of gravitational waves according to classical GR!

If you actually want to predict the results of hypothetical or doable experiments, which is the ultimate application of any laws of physics, the calculations will give you nothing that you couldn't get from classical GR. (Recall that those other people didn't accept loop calculations so the tree-level amplitudes were everything that was possible or acceptable for them.)

By a relatively simple conceptual argument, Feynman was able to show that their "dreamed about" calculation of the scattering amplitudes in what they considered to be "quantum gravity" was just a tautological reshuffling of classical GR. From a physical perspective, it contained nothing new or valuable at all!
He ends with:
A huge portion of the community is still unable to learn the lessons that have been around for nearly half a century. They continue with their dumb canonical quantizations of GR, even though it's obvious that one can't learn anything about quantum gravity in this way, and they present their half-baked classical pictures how the Universe could have behaved around t=0 (or "before") even though it's obvious that such classical stories can't settle anything about quantum gravity and quantum cosmology, either.
Which indicates to me that he thinks the very criticisms that applied in the 1950s still apply today. He also seems to be talking about all non-string QG theories in the same vein, as if they suffer from the same conceptual weakness.

So three questions:

First, is Luboš criticism just that by quantizing GR by any means, you don't end up with new physics, you end up with quantized GR?

Second, has anything happened in the development of QG in the intervening years that Luboš misses?

Third, what is there about the quantization of GR using the latest spinfoam ideas that makes it give different predictions that can be tested? Is it just that having any quantum gravity theory allows one to make predictions (right or wrong) where quantum gravity dominates like the Big Bang, Black Holes, etc.?

Luboš clearly is extremely defensive about string theory, so it seems to me that one needs to take his comments with a grain of salt. But being an amateur on the steep learning curve, I want to understand what is wrong with his comments, as well as what is valid criticism.
 

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  • #2
MTd2
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Do not try to think to much about it. Just ignore everything and anything he says outside string theory unless Jacques Distler intervenes on the latter case. In this case, ingore the string theoretical part too.
 
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  • #3
marcus
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Luboš clearly is extremely defensive about string theory, so it seems to me that one needs to take his comments with a grain of salt. But being an amateur on the steep learning curve, I want to understand what is wrong with his comments, as well as what is valid criticism.
John Baez expressed his view as follows: "It's hard to ignore Lubos, but i find it always repays the effort."

Part of the trouble is boy-cry-wolf. L. has a history going back at least to 2003 on Usenet (well before his blog) of giving a priori reasons why LQG must fail which then turn out not to apply. At one point he had a list on Wikipedia of some 10 or 12 reasons LQG was inherently doomed.

By now after such a long track-record of unreliability, considering his current "gotcha" doesn't seem like an efficient way to spend one's time. He is definitely brilliant however, and the talented vituperation is entertaining.

Maybe someone else would like to pick apart whatever the current argument is.
 
  • #4
qsa
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John Baez expressed his view as follows: "It's hard to ignore Lubos, but i find it always repays the effort."

Part of the trouble is boy-cry-wolf. L. has a history going back at least to 2003 on Usenet (well before his blog) of giving a priori reasons why LQG must fail which then turn out not to apply. At one point he had a list on Wikipedia of some 10 or 12 reasons LQG was inherently doomed.

By now after such a long track-record of unreliability, considering his current "gotcha" doesn't seem like an efficient way to spend one's time. He is definitely brilliant however, and the talented vituperation is entertaining.

Maybe someone else would like to pick apart whatever the current argument is.
do you mean this one

https://www.physicsforums.com/showthread.php?t=2827
 
  • #5
Fra
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First, is Luboš criticism just that by quantizing GR by any means, you don't end up with new physics, you end up with quantized GR?
I didn't read his blog and while I don't care much to see his string view, but just guessing:

The importance of scaling theories, might mean that it's questionable to "quantize" a low energy theory in the way Rovelli suggest. As I noted inthe other thread, I don't think the way Rovelli applies QM smells right.

String theory does use a fixed background in the perturbative formulation, somehow this is natural because the background obviously comes from an observer. No background = no observer. The problem is not the bacgkround, the problem is that it's FIXED.

Similarly the problem is not the OBSERVER. The problem is that we don't understand how the observer itself enters the equations, and we dont' understand how to recover Observer democracy or "equivalence" in the equilibrium limit from such view.

The mechanism whereby the "excitations" feedback and revise the background is one of the key points ST has yet to work out. It seems the lack of such mechanism is also why there is a landscape of theories. This is to me a fatal problem rather than a blemish.

/Fredrik
 
  • #6
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Do not try to think to much about it. Just ignore everything and anything he says outside string theory unless Jacques Distler intervenes on the latter case. In this case, ingore the string theoretical part too.
I see PW on NEW lists Distler along with Lubos. What's the dig on Jacques Distler? Lubos 2.0?
 
  • #8
MTd2
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I see PW on NEW lists Distler along with Lubos. What's the dig on Jacques Distler? Lubos 2.0?
What's PW and NEW?
 
  • #10
MTd2
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I got it. PW=Peter Woit and NEW = Not Even Wrong. My comments are almost always not approved by Peter Woit, so, that is a place that doesn't stand out on my mind.
 
  • #11
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I see PW on NEW lists Distler along with Lubos. What's the dig on Jacques Distler? Lubos 2.0?
That's Peter Woit and his blog http://www.math.columbia.edu/~woit/wordpress/" [Broken].

I subscribe to the RSS feed for Not Even Wrong and I still didn't get the reference, duh. Perhaps I too dense for this physics stuff :smile:

EDIT: I'm even too dense to notice that I was replying to an old listing and missed the above posts clarifying. Ooops.
 
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  • #12
atyy
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The basic conceptual problem with LQG is that it equates background independence (curvature of spacetime is completely specified by matter and matter is completely specified by the curvature of spacetime) with general covariance. Problems: 1) There are classical generally covariant Lagrangians that correspond to flat spacetime theories, so how do we know LQG has anything to do with background independence? 2) How can one have background independence without matter - John Barrett once said that a theory without matter is not really a theory of gravity. These are internal problems with LQG between its philosophical basis and its mathematical formulation - that does not mean LQG's mathematical formulation is not actually very interesting. Various versions of LQG are connected with condensed matter ( http://arxiv.org/abs/0907.2994 , and via AdS/CFT to string?) and to group field theory ( http://arxiv.org/abs/1008.0354 ). I am also wondering if KKL's version of LQG will connect up with Asymptotic Safety.
 
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  • #13
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Actually Lubos is a very good physicist and in general it's not him who misses something but it is often rather the other way around. If he is occasionally not the most polite and diplomatic person, it doesn't mean that he is wrong.

For many experts in the field it is pretty clear that LQG has a load of severe problems, and apart from philosophical ones (which may be debatable) there are also concrete and technical ones; this is the reason why LQG is not taken seriously by many. See the very clear and unbiased review of Alexandrov and Roche
http://www-spires.dur.ac.uk/cgi-bin/spiface/hep/www?eprint=arXiv:1009.4475
I would recommend this article to anybody who is seriously interested in these matters.
 
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  • #14
atyy
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A further technical critique of LQG is provided by http://arxiv.org/abs/1010.1939 in which a key equation (26) is pointed out to be a likely source of divergences.
 
  • #15
MTd2
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Actually Lubos is a very good physicist and in general it's not him who misses something but it is often rather the other way around.
This has convinced me that it is indeed not the way around:

http://golem.ph.utexas.edu/~distler/blog/archives/002199.html#c032759

The rest of the discussion makes me confident of that. This is a subject that supposedly was matter of his expertise. Mind that not even Garrett got that treatment.
 
  • #16
Fra
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See the very clear and unbiased review of Alexandrov and Roche
http://www-spires.dur.ac.uk/cgi-bin/spiface/hep/www?eprint=arXiv:1009.4475
I would recommend this article to anybody who is seriously interested in these matters.
"Although this is not a priori impossible, this is also one of the main points of criticism of these approaches: it is not clear why one should believe that an action describing a classical theory in infrared remains valid also in ultraviolet."
-- http://arxiv.org/PS_cache/arxiv/pdf/1009/1009.4475v1.pdf

A good point.

Rovelli comments this general point

"Others, on the other hand, and in particular some hard–core particle physicists, do not accept the lesson of GR. They read GR as a field theory that can be consistently formulated in full on a fixed metric background, and treated within conventional QFT methods. They motivate this refusal by insisting than GR’s insight should not be taken too seriously, because GR is just a low–energy limit of a more fundamental theory. In doing so, they confuse the details of the Einstein’s equations (which might well be modified at high energy), with the new understanding of space and time brought by GR. This is coded in the background independence of the fundamental theory and expresses Einstein’s discovery that spacetime is not a fixed background, as it was assumed in special relativistic physics, but rather a dynamical field."
-- http://arxiv.org/PS_cache/gr-qc/pdf/0604/0604045v2.pdf as

From the other thread we briefly discussed this as well, and at least to speak for myself I think that the LESSON from GR could be different from what rovelli thinks. I made the effort to try to characterize is as EQUIVALENCE of observers vs DEMOCRACY of observers.

I think GR has deep lessons, but we must not forget that there is at least usually OBSERVER attached to the spacetimes. The way howto recover objectivity (respect observer democracy) without discarding he notion of observer seems ambigous.

/Fredrik
 
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  • #17
Fra
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"...In doing so, they confuse the details of the Einstein’s equations (which might well be modified at high energy), with the new understanding of space and time brought by GR. This is coded in the background independence
My attempt to expose different ways to pull a core lesson from GR, addressed exactly rovelli's point - HOW the BI is encoded. That point is independnt of exactly how einsteins equations look. It was a general point concerning specifically the meaning of BI, in terms of an observer context.

/Fredrik
 

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