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Who are typical young LQG people?

  1. Jun 14, 2005 #1


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    Always tricky to try to point to typical anything. Interesting discussion over at Not-Even-Wrong blogsite

    basically the issue is should young LQG people get more openings and support in the US. Thomas Larsson says no because (if I understand his reasoning) they are ignorant of something which he considers fundamental. Then there are other people in the discussion, one called "M" and one called anonymous, and so on. And also Peter Woit is sometimes talking, if I remember correctly.

    So I am thinking about a question basic to that kind of discussion which is WHO DO YOU MEAN WHEN YOU talk about young "LQG people"?

    Here, accordingly, are two I think are typical. They are at Perimeter Institute (PI) and Albert Einstein Institute (AEI) which are typical places to look if you want to find LQG people---especially young ones: grad students, postdocs, early-stage research fellows.
    Let us look at the kind of research they are doing lately.

    Laurent Freidel (PI and Uni.Lyon in France)
    Bianca Dittrich (AEI)

    These are the type of people that would get more postdocs and beginning faculty openings if you followed the proposals in Smolin's June 2005 "New Einstein" essay. And in regard to whom Thomas Larsson has faulted Smolin's essay because, as he says, it "more or less explicitly calls for more money" for this sort of young researcher. Larsson is a respectable guy, so let's look at the specifics.

    I will fetch links to Freidel papers and Dittrich papers of the past year or year-and-half.
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  3. Jun 14, 2005 #2


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    Freidel's recent work (2004 and 2005 papers)


    1. gr-qc/0506067
    A group field theory for 3d quantum gravity coupled to a scalar field
    Laurent Freidel, Daniele Oriti, James Ryan
    11 pages, no figures

    2. hep-th/0505016
    Group Field Theory: An overview
    Laurent Freidel (PI, ENS-Lyon)
    10 pages

    3. hep-th/0502106
    Ponzano-Regge model revisited III: Feynman diagrams and Effective field theory
    Laurent Freidel, Etera R. Livine
    46 pages

    4. hep-th/0501191
    Quantum gravity in terms of topological observables
    Authors: Laurent Freidel, Artem Starodubtsev
    19 pages

    5. gr-qc/0411117
    Canonical analysis of the BCEA topological matter model coupled to gravitation in (2+1) dimensions
    Laurent Freidel, R.B. Mann, Eugeniu M. Popescu
    17 pages, no figures

    6. gr-qc/0410141 [abs, ps, pdf, other] :
    Ponzano-Regge model revisited II: Equivalence with Chern-Simons
    Laurent Freidel, David Louapre
    27 + 23 pages, many figures

    7. hep-th/0401076
    Ponzano-Regge model revisited I: Gauge fixing, observables and interacting spinning particles
    Laurent Freidel, David Louapre
    Comments: 48 pages, 15 figures
  4. Jun 14, 2005 #3


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    Dittrich's recent work (2004 and 2005 papers)


    1. gr-qc/0506035
    Counting a black hole in Lorentzian product triangulations
    B. Dittrich (AEI, Golm), R. Loll (U. Utrecht)
    42 pages, 11 figures

    2. gr-qc/0411142
    Testing the Master Constraint Programme for Loop Quantum Gravity V. Interacting Field Theories
    Authors: Bianca Dittrich, Thomas Thiemann
    Comments: 20 pages, no figures

    3. gr-qc/0411141
    Testing the Master Constraint Programme for Loop Quantum Gravity IV. Free Field Theories
    Bianca Dittrich, Thomas Thiemann
    23 pages, no figures

    4. gr-qc/0411140
    Testing the Master Constraint Programme for Loop Quantum Gravity III. SL(2,R) Models
    Bianca Dittrich, Thomas Thiemann
    33 pages, no figures

    5. gr-qc/0411139
    Testing the Master Constraint Programme for Loop Quantum Gravity II. Finite Dimensional Systems
    Bianca Dittrich, Thomas Thiemann
    Comments: 23 pages, no figures

    6. gr-qc/0411138
    Testing the Master Constraint Programme for Loop Quantum Gravity I. General Framework
    Bianca Dittrich, Thomas Thiemann
    Comments: 42 pages, no figures

    7. gr-qc/0411013
    Partial and Complete Observables for Hamiltonian Constrained Systems
    B. Dittrich (Max Planck Institute, Potsdam and Perimeter Institute, Waterloo)
    38 pages
  5. Jun 14, 2005 #4


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    I'll editorialize some. You can see that
    Dittrich has really paid her LQG dues bigtime and she unquestionably deserves to be considered a LQG person (the kind some of us think should not get jobs in US, and fault Smolin's essay on that account)

    I think it is darn lucky that the German research establishment is flexible enough that Dittrich can ALSO be working in Loll's CDT-type simplex gravity.
    In fact dittrich did her 2001 Phd thesis in CDT and co-wrote a 2003 paper with Loll. And CDT is dynamite and this recent Loll-Dittrich black holes paper is dynamite.

    so it is lucky that the German administrators have sense enough that Loll is not tied to one program or the other but can have the intellectual independence to contribute to various programs. she has been a real work-horse in Thomas Thiemann's Master Constraint program (what one might call "LQG proper", a brave determined and rare effort to carry out the canonical-LQG plan of the 1990s)

    the issues in Smolin's New Einstein essay are that the US is not getting good people because of being comparatively poor in (1)PROGRAM DIVERSITY and (2)INTELLECTUAL FREEDOM in the theoretical section of its major physics departments and research institutes.

    And these two things are what you see illustrated at AEI and PI and by the examples of Freidel and Dittrich. Smolin proposes that the US establishment get with it and have rival QG research agendas and programs represented, so students and postdocs have a choice. AND that it not tie its young researchers down to one program (like string) by giving the support THROUGH the program but give the support based on merit directly to the individual.

    And that I would guess is how Hermann Nicolai supports Dittrich, so she can use her mental independence and move around and work some with Thiemann and some with Loll-----some in canonical-LQG and some in CDT.

    It has got to be stupid to have a virtual monopoly by string on fundamental physics theory research. And that is what you do NOT see at AEI and PI. Those places you see several rival programs actively progressing and competing for the smart students.

    And then you get someone faulting Smolin's call for more program diversity in US, as "problematic" because it would lead to more money for LQG people like Dittrich!

    For cripes sake.

    A major US physics department should be so lucky as to get any of the good new LQG people.
    Last edited: Jun 14, 2005
  6. Jun 15, 2005 #5


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    I think you are fighting history, marcus. Turning the tide requires results. The burden falls upon the few and the proud to produce them. It's that simple in my mind. You won't turn any heads otherwise. Politicizing the argument, which I think Smolin has fallen prey to, is wasted effort [and I'm a huge fan of Smolin]. I say cut the crap and produce the observational evidence. The facts will speak volumes.
  7. Jun 15, 2005 #6


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    Partly my perspective is global or international-----partly my emotional identification is as an American (and yankee science has a proud tradition).

    I see things happening now in Germany, Holland, France, UK, Canada even forgoodnesssake Uruguay, that I wish the US had a part of the glory.

    I think Smolin is proposing ways to improve US physics departments that would be good for US science and would

    1. get them to be more like the places doing good interesting work, like Gerard 't Hooft institute in Utrecht

    2. restore them to more like the academic tradition I grew up with, where deans and department-heads consciously built in intellectual diversity.

    I think the european/canadian ITPs are more like what the US was like 30 years ago and I think they are benefitting from that and that we (or rather US theorists) are on a sidetrack.

    Also as a US taxpayer I dont like the waste of research funds.

    that is all pretty much independent of the issues of specific programs

    monopolies tend to be bad no matter what the company is---here it is a monopoly that gives the grad students and postdocs no choice, they have to work for the company------as workers they have no choice, as consumers of ideas they have no choice.

    It would be about equally oppressive if, say, 20 clones of Alain Connes were in charge of physics theory all over the US, and giving the postdocs a similarly limited intellectual choice to be indentured to serve in the cause of Non-Commutative Geometry. Or if everybody had to work on a hugely ramified Thomas Thiemann Master Constraint LQG programme. Or if every department chairman was an exact copy of Rodolfo Gambini and one could only do Consistent Discretizations. So it is not just string.

    It is my country's science establishment being wasted by a monopoly power situation. I've got a right to object.


    but I have an equally important internationalist side, Chronos, and from that perspective I am happy as a clam with how things are going! Why should it matter that the advances are not being made in the US? The US is only part of the picture.

    Most of the time it does not bother me at all that all the progress is abroad. I enjoy it greatly regardless. The papers are mostly in English which is a great blessing. We have the ARXIV (an American dividend left over from the glory days of string) so we get new postings almost immediately.

    There is a lot to be happy with, and I revel in it.

    Am also very pleased by the exoplanet search, where incidentally US teams ARE among the leader. and all the whole observational astronomy show. come to think of it this is an example of US science establishment promoting diverse projects in a highly successful strategy----exoplanets, CMB astronomy, X-ray astronomy, gammarayburst astronomy, neutrino astronomy, infrared, huge gamut of variety.
    Last edited: Jun 15, 2005
  8. Jun 15, 2005 #7


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    I do not like the monopoly argument. After all, there are reasons for scientific monopoly, when a theory is reasonable enough to become part of the language while agreeing with the observed reality (this caution is implicit: which language do not agree?). If we are looking for Market Theory metaphors, the question is more in the side of "market herding"

    EDITED: this website points to some papers on herding or crowding effects.
  9. Jun 15, 2005 #8


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    the use of an analogy depends on context. I do not like the analogy of monopoly in the case of theories which have been tested and where nature has something to say about it. It would not occur to me to use a market analogy about established models.

    where I would use the market analogy is confined to the theory wing of a physics department where people are working on untested ideas.
    then the young researchers are somewhat like "investors" who invest their time in learning about one approach or another and then gamble their careers on that approach by working for some years with that approach.

    An investor is somewhat like a consumer, as he looks over the possibilities and chooses the most attractive, and somewhat like a worker (and the grad students and postdocs certainly get worked hard and even exploited by the established academics)

    I would like these "investors" to have a range of choice. Partly this is for moral reasons----if there is only one company to invest in and one company to work for, and one cannot start up one's own company, then both workers and investors are likely to be exploited. But also there is a practical reason---it is good to have the benefit of the intelligent choices of investors who are on the scene. like the stock market, it is a way of using the intuition and hunches of informed people and of dealing with risk.

    THE CAREER OF DITTRICH is a pointer for me, I learn from watching her try out working for Thiemann and then go work on CDT with Loll, I believe she is smarter than me (or better informed because she is there on the ground) and I can learn from watching her make free choices and gamble with her career.

    I want her to be free to choose among interesting theory gambles because IF SHE HAS ONLY ONE OPTION THEN I DONT LEARN ANYTHING.

    So it is not merely a moral consideration, not merely for Dittrich sake. It is for my sake as a watcher and for the reason of making efficient use of the intuition and informed guesses of people like her who are gambling. that is, it is good for science (which is partly an intuition machine involving risky decisions).

    From all the decisions of all the fundamental theory grad students in the US, every year I learn ZERO because they have only one choice, string or nothing. All those possible informed choices that could be accumulating information for me are doing nothing because there is no choice. It is like a stock market where there are smart well-motivated investors but with only one company's stock for sale.

    Certainly none of this applies to empirically tested theories. Once one can do experiments and make observations it is no longer the same situation.

    Probably the grad student/stock market analogy is not worth arguing about though. If you do not like it, that is fine with me. I can simply not use that particular analogy.
  10. Jun 15, 2005 #9


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    I started thinking about who are typical of the young crop of LQG people because of the Smolin essay
    [published in June 2005 Physics Today, a monthly of the American Physical Society]

    In case you dont have hardcopy of the Smolin essay, Motl provided this link to the text
    http://waltf007.mindsay.com/ [Broken]

    And then there was the discussion about the essay at Not Even Wrong,
    and Smolin's clarifying post there, which selfAdjoint refers to


    sA says:
    <<Smolin's comments are a must-read for anyone interested in several things that have been posted here in the past. Such items as Thiemann's LQG quantization of the closed string and the assertion that LQG quantization must be wrong because it doesn't produce an anomaly as "it is known" to be required. Smolin tracks this down to the difference between quantizing with a fixed background and in a background free environment. And Smolin says, in response to a request for a blanket term to cover everything from Thiemann to Loll by way of Rovelli and Smolin himself with all their coworkers, that he uses background-free approach to quantum gravity. I propose that WE declare Background Independent Quantum Gravity (BIQG) to be the term we use here on PF and that you, Marcus get behind it as you have so we can really sell it.

    Would it be possible to get a copy of Smolin's comment posted here?>>

    And I agree Smolin comment at Peter's blog is good reading and has some valuable insights. Why not post it here, as sA suggests? So I will. It fits in more with this thread discussion of diversity/lack of diversity in US departments. Choices grad students have, what their options are here in US and abroad especially in Europe.

    So I will fetch a copy and post it here.
    Last edited by a moderator: May 2, 2017
  11. Jun 15, 2005 #10


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    Dear Peter and colleagues,

    I am grateful for the attention given to my essay. I only want to emphasize a few points here. The main thing is that the essay is carefully written. It does not advocate more funds to LQG or any other program. It explicitly advocates more support and positions for young, ambitious theorists pursuing their own research programs who are unaffiliated with any larger program. Several proposals are made for how to accomplish this. I would hope that the focus of the discussion could be on these proposals.

    -String theory is criticized in the essay mainly because it is currently sociologically dominant, and so subject to the problems mentioned. It was necessary to do so as many readers of physics today will be unfortunately unaware that there are any problems with string theory, or any viable alternatives. Anyone with a long enough memory will know that the sociological issues in high energy theory predate string theory, and have hurt physics in the past, i.e. in the case of S-Matrix theory.

    -I hope I don’t have to say that I am not anti-string theory. My current last paper on the ArXiv is a technical paper in string theory, and I have 14 more in past years, plus 8 papers on related topics such as the landscape. I wouldn’t have written these papers if I didn’t think there was a good chance string theory is relevant to nature. The fact that someone like me who contributes sometimes, but not exclusively, to string theory, is not considered “a string theorist” is part of the sociological problems my essay criticizes. Similarly, the fact that one can elicit angry responses, and be called “anti-string” for carefully and correctly recounting the actual status of various conjectures is a sign of an unhealthy sociology. No one calls someone anti-LQG or anti-QCD when they do a similarly honest summary of what is known and not known in those fields.

    -I would claim that the sociological issues mentioned in the essay have hurt string theory even more than they have hurt the alternative programs, because they greatly limit the range of ideas worked on, and because people with a lot of imagination and intellectual independence are either selected out or choose themselves to work within communities which are more friendly to diversity and imagination. As a result, key issues such as the question of a background dependent formulation, or perturbative finiteness, don’t get a lot of attention, in spite of their centrality for the whole program.

    -I was grateful that someone noted the range of subjects at the LQG meetings. This was not planned, it is a natural outcome of the more open and curious atmosphere among people who work on the subject. We don’t believe we should have a meeting without inviting people from alternative and rival programs to report to us what they are doing, as well as to serve as critics. At the meeting in Marseille last May we even invited a persistent critic of LQG-Ted Jacobson-an early contributor who is now very critical of the subject-to give a talk to lay out his criticisms. I think it would be very good for string theory if the organizers of their meetings took a similar attitude.

    -Someone asked for a blanket term for LQG, CDT, causal sets etc. We use background independent approaches to quantum gravity. There is a lot of interchange of ideas, techniques and people among these programs, and many of us have contributed to more than one. There is a very different intellectual climate, in which diversity, creativity and independence are strongly encouraged.

    -Someone is asking for what is “LQG proper?” But the fact is that a lot of different things are now going on roughly under the name of or related to LQG. After all, this is now a community of > 100 people and there is no orthodoxy and no one trying to control what people work on. We agree generally on what has been achieved and what problems remain open, but not much beyond that. There is a healthy variety of approaches and attitudes towards the open problems. If there is one thing we all agree on it is that no approach is likely to achieve the right theory that is not background independent at its foundations. Come to the meeting and see what is happening.

    -While the point of my essay was not to advocate more funding to any particular direction, if you ask me I will of course say that I think that people working on background independent approaches to quantum gravity deserve much more support. Among them are Loll and Freidel, that I am glad someone mentioned, but there are many others.

    -I did not, as Lubos implies, advocate funding a large number of people who do nothing but think about the foundations of quantum theory. What I do advocate is much more support for the kind of person who might be inclined to work on foundational issues. These are deep and independent thinkers who believe that the road to progress in physics is confronting the hard problems directly. But there is no need to argue about whether more funding for foundations of quantum mechanics would be fruitful. The experiment has been done. For decades there was no support at all, and slow progress. Then, because of the possibility that quantum computers could break codes, there has been a lot of support for the last few years. And a lot of progress has been made, both experimentally and theoretically on aspects of foundations of QM.

    -Although this essay was not written to advocate LQG, since it is attacked in response I should try to clear some things up. Someone asks for an accounting of the present status of the field. I among others, have given one in hep-th/0408048, shortly to be updated.

    As to the issue of anomalies, i.e. the claim that we ignore the established knowledge that “INFINITE-DIMENSIONAL CONSTRAINT ALGEBRAS generically acquire anomalies on the quantum level...” is simply false. It is contradicted by rigorous existence and uniqueness theorems in LQG. As a few people do nevertheless take this seriously let me start from a point we can agree about and see if we can clear this up for good. I would hope we can all agree that:

    1) The approach to quantization of constrained systems is different in string theory and LQG. The former approach depends on a gauge fixing that refers to a fixed background metric. It results in the construction of a Fock space. The latter is background independent and involves no background metric, no gauge fixing and results in a state space unitarily inequivalent to a Fock space.

    2) There is a body of rigorous results that support each kinds of quantization. Hence it cannot be a question of which is correct mathematically. Both are correct, within their contexts. It is a question only of which construction is appropriate for which theories and which describes nature.

    3) The treatment of constraints in string theory depends on certain technical features of 1+1 dimensional theories, particularly the fact that there is a gauge in which L_0 plays the role of a Hamiltonian and therefore should, in that gauge, be quantized so as to have a positive spectrum. The anomalies are not generic, as asserted above, rather they depend on the additional condition that L_0 should be a positive operator. There are other reps of Diff(S^1 ) that are non-anomalous but in which L_0 is not positive. So a choice is made in the standard quantization of string theory, which his motivated by the physics. This does not mean it is the right choice for all physical theories.

    4) Conversely the existence and uniqueness theorems which support the LQG quantization work only in 2+1 dimensions and above for the reason that gauge fields don’t have local degrees of freedom in 1+1 dimensions. The existence theorems tell us that there are quantizations in 2+1 and higher of diffeo invariant gauge theories that have unitary, anomaly free realizations of diffeo invariance. The uniqueness theorem tells us that the resulting state space we use in LQG is unique.

    5) Now it is true that Starodubstev and Thiemann have found it an interesting exercise to apply the LQG techniques to free string theory. Not surprisingly they get a theory that is unitarily inequivalent to the usual one. This does not mean that the usual quantization of string theory is wrong, nor does it mean that the LQG techniques are wrong when applied to other problems, where the existence and uniqueness theorems together with a large number of results prove their worth. All we learn is that the two quantizations are inequivalent, which was to have been expected.

    6) With regard to the non-standard quantization, in which holonomies, but not local field operators are well defined, it is of course true that when applied to standard systems this leads to inequivalent results. “This apparently leads to unphysical consequences, such as an unbounded spectrum for the harmonic oscillator.” But, give me a break, do you really think someone is proposing to replace the standard quantization of the harmonic oscillator with the alternative one? What is being proposed is that the quantization used in LQG is well suited to the quantization of diffeo invariant gauge theories.

    In case it is not obvious, let me emphasize that harmonic oscillators are not relevent here, and can play no role in a background independent quantum theory, precisely because the division of a field into harmonic modes requires a fixed background metric. Thus, the physics of the problem REQUIRES an alternative quantization.

    The detailed motivation is, I think, well argued in the papers, and are supported by the results as well as the existence and uniqueness theorems. First, is well known that a complete coordinatization of the gauge invariant configuration space for a non-Abelian gauge theory requires the holonomies. Second, using them gives rise to the unitary non-anomolous reps of the spatial diffeomorphisms.

    Nor is anyone proposing using non-seperable Hilbert spaces for the full theory, the point is that when one mods out by the piecewise smooth spatial diffeos one is left with a seperable Hilbert space.

    I am frankly puzzled why someone who claims to know the literature well would throw up examples like the harmonic oscillator up in this context. I can try to understand their point of view, but it certainly reads as if they either are choosing to ignore the basic point, which is that background independent quantizations cannot use fock space, or they are looking to make debating points to impress ignorant outsiders. They must know comments like this are not going to influence experts, because they are, after all, taken from our own papers, written precisely because we wanted to clarify the difference between the new and standard quantizations and the limits of the applicability of each.

    With regard to the sociology of the string-loop division, “Roughly speaking, string theorists are fundamentally particle theorists with a strong understanding of quantum theory, whereas loop people are gravitists with a background in GR”, this is a myth. Rovelli, myself and many other people in LQG were trained as particle physicists, myself at Harvard in the late 70’s. Most of the physical motivation for LQG comes directly from ideas about formulating gauge theories in terms of loops that were studied by Polyakov, Wilson, Migdal, Mandelstam, Neilsen and others. LQG is squarely an outgrowth of their intellectual tradition. The only thing we added was to correctly treat the diffeomorphism invariance exactly in the quantum theory. This led to new results just as the exact treatment of gauge invariance in lattice gauge theory led to new results. I would claim that we made progress in LQG precisely because we had a very good grounding in QFT.

    String theory, as it is practiced, makes much more contact with the general relativity tradition, especially the once discredited tradition of extending general relativity to add dimensions and degrees of freedom in the search for a unified field theory. You are much more likely to read a paper which studies solutions to a generalizationsof the Einstein equations, with hbar=0, by a string theorist than by someone working on a background independent approach to quantum gravity.

    This of course does not mean that string theory is wrong. But I believe it does mean that by enforcing a narrowly restrictive notion of what constitutes good work, the community of string theorists has hampered progress in string theory by excluding from consideration the lessons learned by attempts to do what string theory must do eventually if it is to be a real theory: which is to find a background independent formulation of a quantum theory of spacetime.

    Posted by: Lee Smolin at June 14, 2005 12:34 PM

    ------------end quote-------------
    above is from the Not Even Wrong blog
    see also roughly around post #75 of this other thread
    Last edited: Jun 15, 2005
  12. Jun 15, 2005 #11


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    As I understand the direction Smolin is going he is basically arguing for making the distribution of non-string theorists more diffuse. This is fundamentally the wrong direction. Creative advance is strongly tied to the existence of communities of scientists, often in the same place. The movie industry thrives because it is contentrated in Hollywood. Einstein had an edge because he was working as a patent clerk in a city where there was a thriving traditional scientific community. He sends in an article and Planck reads it. He has problem with tensor math, and there's someone at hand to pick up the slack. Concentration and not diffusion is what is needed, along with an open community. Arvix is going to produce the next Einstein.
  13. Jun 15, 2005 #12


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    lot of excellent points.
    but I suspect the QG string-free approaches are ready for some exponential growth and could use a little wider diffusion.

    main thing is what they have in Europe is more diversity in the institutes and the amount of dispersion appears to be kind of self-adjusting

    and an atmosphere of allowing different approaches lets people in existing departments strike out for themselves, so you dont necessarily take people away from one place to seed a department some other place.

    I agree with the value of a traditional scientific community but I am not too worried about QG spread too thin. (they always network anyway)

    the principles of program diversity and intellectual freedom for young researchers are so important, as I see it, that it trumps considerations of what is temporarily good or bad for one particular program like LQG.
  14. Jun 23, 2005 #13


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    marcus, is this truly a relevant question (in the title) in an informative forum like this, no one cares about the people. the theories and their connection to reality is and should be your first priority, the question raises only political agenda from your behalf (who is working on what and where).
    what may be your next topic issue?
    how much do lqg people get paid for their theories?!
  15. Jun 23, 2005 #14


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    Maybe you don't, but many of us do. The idea that you can isolate the theories from the people and communities that produce them is just one step above the popular idolization of Einstein and Feynmann, and in the end it's just as fallacious. There is an exciting dynamic today in the theoretical community that is producing advances in several different directions, and "you can't tell the players without a program."
  16. Jun 23, 2005 #15


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    I think it is a relevant science policy question. I'm happy if you simply ignore the thread if you don't like the topic of discussion.

    Actually instead of saying "LQG people" I would now prefer to say "QGATS" for quantum gravity alternatives to string. It is more descriptive, the other can be interpreted too narrowly so that it isnt fully representative

    Smolin proposed that research be made more diversified in US institutions, more like in Europe and Canada. he gave various reasons I thought were good. One has to look at the practical consequences. If physics theory research were more diverse what kind of grad students and postdocs would one be seeing that one does not see in US institutions now?
    What kind of young researcher would be getting offers of fellowships in the US, where they now locate outside the US? Or beginning faculty position offers.

    the field of QGATS (quantum grav. alt. to string) is now broader than just strictly interpreted canonical LQG. got to go, back soon
    Last edited: Jun 23, 2005
  17. Jun 23, 2005 #16


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    Post #10 in this thread was a copy of Smolin's 14 June response on N.E.W. to comment made there on his essay.
    Then there were further comments by others---if you want to see them follow the links to N.E.W.
    then Smolin responded further. Just for completeness I will copy his 18 June and 21 June responses on Peter's blog.

    ----quote Smolin 18 June Woit's blog---
    Thanks again for all the insightful comments. Perhaps I can add something to a few of the threads of discussion.

    -On LQG and discrete structure. First, do we agree that even though electrons move in space the spectrum of the hydrogen atom being discrete means that quantum mechanics of the atom has discrete structure? In a very similar sense, since all the geometric observables including volume, area (and yes length) have discrete spectra, corresponding to a discrete basis (of diffeo classes of embeddings of labeled graphs) then the quantum geometry of space has become discrete. The key point is that the discreteness scale-roughly L_Planck, cannot be taken to zero, otherwise black hole entropy comes out wrong, and semiclassical states do not correspond to classical metrics.

    -But it is true that if you derive a version of LQG from a strict quantization of GR, there is a fixed background, which is the bare differential manifold. There is no background metric but there is a background topology and differential structure, defining the diffeo classes of embeddings of the spin networks.

    -Hence, Markopoulou followed by Freidel and others, proposed dropping the embedding and basing the theory just on combinatorial spin networks. These models are then discrete in a stronger sense. There are some advantages to this (reformulation in terms of a matrix model, cleaner relation to causal sets) but one can non longer claim the theory is a precise result of a quantization of GR. Both frameworks, with and without embeddings, continue to be studied.

    -The discreteness of length was shown in T. Thiemann, gr-qc/9606092, J.Math.Phys. 39 (1998) 3372-3392. Angles also have disrete spectra: S. Major, Class.\ Quant.\ Grav.\ {\bf 16}, 3859 (1999) gr-qc/9905019; gr-qc/0101032.

    -On spin foam models and discreteness. There are several different spin foam models under study. In all of them a history is a discrete labeled combinatorics structure (for example branched 2-complex.) In some of them the label sets are continuous because they come from the rep theory of Lorentz or Poincare and areas are not discrete. But these have not been shown to correspond to evolution amplitudes for canonical states. Others (Reisenberger, Markopoulou, etc) do give evolution amplitudes for spin networks and have discrete areas.

    - M asks, is there a suitable correspondence principle where known physics can be recovered? The answer is yes. There are several results that show that excitations of certain LQG states reproduce, for momenta small in Planck units, the spectra of conventional QFT’s including gravitons, photons etc on flat space or de Sitter spacetime. Some are cited in section 4.4 of my review hep-th/0408048. See also hep-th/0501091. See recent papers by Freidel, Livine and others that show in full detail how standard Feynman perturbation theory emerges from a spin foam model for gravity coupled to matter in 2+1 when G_Newton goes to zero.

    As to what people in non-string approaches to quantum gravity are doing, I agree, why not look at the conferences? Here are some recent ones, some with talks available.

    http://www.ws2004.ift.uni.wroc.pl/flash.html [Broken]
    (talks at: http://www.ws2004.ift.uni.wroc.pl/html.html [Broken])
    http://www.perimeterinstitute.ca/activities/scientific/PI-WORK-2/ [Broken]

    -Several of the comments ask, why quantize as in LQG? Why not quantize with another approach (such as one that uses anomalous reps?)

    I do not see how there can be an apriori reason to prefer one quantization scheme over another one. Our job is to construct candidate quantum theories of gravity, compare their results and learn from them. In LQG there are existence and uniqueness theorems that prove that the approach exists, and theorems that guarantee uv finiteness. Thus, the approach leads to a structure that mathematically exists and within which computations can be done. Many computations have been done.

    We are thus no longer at a stage where it is interesting to ask why do or why not do questions. There are now a different class of questions which include: Does the theory make predictions? How do they compare with experiment? What properties have been shown? What remains to be shown? There are certainly several key open issues to discuss, and we are not shy to discuss them.

    No one is claiming that we know LQG is the right theory of nature. We are claiming that it is a well developed approach, that gives an apparently consistent answer to what we think is a necessary question, which is how to construct a diffeo invariant QFT in the absence of a fixed background metric. This gives a rich arena with many open problems and many things to do either to understand it better, make predictions, or as a jumping off point for the invention and study of new theories.

    So the attitude is rather different from other approaches. Some string theorists admit they do not know what string theory is, but they nevertheless are sure it is right. In LQG we study well defined theories, which have many good properties, but most of us feel no need to “believe in them” pending experimental confirmation.

    So our attitude is if someone like Thomas Larsson has a different approach that’s great. We know what its like to be starting something new other people don’t understand or support, and we will support you, so long as you don’t waste your and our time attacking us on a priori grounds. We suggest you should try to develop your ideas to at least the point where we can compare the results.

    For example, someone asks, “What's wrong with anomalies? Sure, it turns first class constraints into second class constraints, but Dirac showed us how to deal with that.” Fine, we only insist that this is not the only way. The LQG results and theorems show that you can find diffeomorphism invariant states through a different procedure, involving only first class constraints, which is

    a. Construct a kinematical Hilbert space, which is a rep of a Poisson algebra that coordinatizes the phase space, which carries a unitary and non-anomalous rep of the spatial diffeo’s.

    b. Use that non-anomalous unitary rep to construct explicitly another Hilbert space, which is the space of diffeomorphism invariant states.

    c. Compute many observables of interest representing diffeo invariant classical quantities as finite operators on this space, leading to predictions of physical interest, an ultraviolet finite theory etc.

    There are by now so many rigorous results supporting this construction that the burden of proof is on the other side: given that this procedure works and leads to a well defined finite physical theory, why not explore its consequences as a possible quantum theory of gravity?

    So when Urs says, “It seems to me that the reason to drop weak continuity in the quantization of gravity in 3+1 dimensions is that it makes an otherwise intractable problem tractable - but possibly at the cost of having oversimplified a hard problem,” fine, but lets discuss the results. Does this lead to a space of states with enough physical states and with a well defined dynamics? YES. Are some states interpretable as semiclassical states? YES. Does that dynamics have all the properties we require for a quantum theory of gravity? YES to some questions such as uv finiteness, other questions are still open, such as a proof that the ground state is semiclassical.

    -Aaron says, “It is, in fact, a radically different approach to quantization that, when applied to current theories, gives experimentally incorrect answers.” Thomas Larsson argues that “I find it very disturbing that LQG methods yield the wrong result for the harmonic oscillator.” I don’t understand the logic of their arguments at all. Yes, it is a different quantization, i.e. one based on representations of the algebra of Wilson loops and electric flux’s rather than local field operators. Yes, it is unitarily inequivalent to Fock space. That is good, as Fock space knows about a particular fixed background metric. If a background independent Hilbert space, which quantizes the whole space of metrics, were unitarily equivalent to a Fock space based on a single fixed metric, something would be wrong.

    The claim is precisely that this is a new class of QFT’s which is available to quantize diffeo invariant gauge theories in 2+1 dimensions and above, and which has novel features and leads to novel results. So long as the resulting theory is well defined, I don’t see the force of an argument from a priori grounds. that experience shows Fock type quantizations must be right in all cases because they only work when there is a fixed background metric, while the whole point of the new quantization is that it provides an answer to the question of how to construct a well defined QFT in the absence of any background metric.

    -If you still want to have an argument on a priori grounds as to why representations of non-canonical algebras will be required to have a background independent quantum theory of gravity, please go back to the papers of Chris Isham from the late 70’s and 80’s where he made a detailed and convincing case for this. These papers, together with the work of Polyakov, Wilson, Midgal etc on formulating quantum gauge theories directly in terms of Wislon loops were the major motivation for LQG. What we did was construct the non-canonical algebras Isham called for from Wilson loops. Also, please note that lattice gauge theory is not based on Fock space.

    -Finally, I am not a director of PI, just one of the scientists, so PI is very far from “Smolin’s institute”. Also, when I am defending LQG I try to discuss the whole research program, not my own personal work, which departs in some papers quite a bit from that of many of my friends.

    Posted by: Lee Smolin at June 18, 2005 07:35 AM
    ---end quote---

    ---quote Smolin 21 June Woit's blog---
    Re the last comment, on recovering ordinary QFT from LQG, let me stress again that there are explicit known semiclassical states, and ordinary QFT is recovered at long wavelengths by studying excitations of them. Hence, we know that the physics of flat, or DeSitter spacetime is in the theory.

    The problem is to go beyond these results to
    i) show that the ground state, subject to some appropriate boundary or asymptotic conditions, is such a state, ii) show whether classical spacetime emerges from a generic physical state and iii) show whether lorentz invariance is preserved, broken or deformed by Planck scale corrections in the ground state, and hence predict what should be seen in AUGER, GLAST, ICECUBE and other upcoming experiments.

    In recent papers, Freidel et al show how deformed Poincare invariance arises as the limit of LQG coupled to matter in 2+1. See also hep-th/0501091 for an admittedly heuristic argument that this is true also in 3+1.

    For these see sections 4.4 and 5 of hep-th/0408048 and the references provided there.

    As to whether it might be easier to obtain certain results in a different formulation with anomalous reps of the spatial diffeo's, perhaps, and this could be worth trying, but only if one does not put in what is to be shown, which would be the case if those reps are constructed with reference to a background metric.

    Posted by: Lee Smolin at June 21, 2005 10:37 AM
    ---end quote---
    Last edited by a moderator: May 2, 2017
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