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Gerard 't Hooft at Kitpi25

  1. Oct 9, 2004 #1

    marcus

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    Gerard 't Hooft at Kitp25

    Here is the audio etc for 't Hooft's Kavli25 talk, but it takes a few minutes to do the audio, so I am going to exerpt a few slides to give an idea of the general message.

    The title of the talk was The Future of Quantum Mechanics

    http://online.itp.ucsb.edu/online/kitp25/thooft/

    http://online.itp.ucsb.edu/online/kitp25/thooft/oh/02.html
    http://online.itp.ucsb.edu/online/kitp25/thooft/oh/03.html
    http://online.itp.ucsb.edu/online/kitp25/thooft/oh/04.html

    http://online.itp.ucsb.edu/online/kitp25/thooft/oh/06.html
    http://online.itp.ucsb.edu/online/kitp25/thooft/oh/07.html

    http://online.itp.ucsb.edu/online/kitp25/thooft/oh/13.html
    http://online.itp.ucsb.edu/online/kitp25/thooft/oh/16.html
    http://online.itp.ucsb.edu/online/kitp25/thooft/oh/18.html

    slides 2,3,4 say:
    ---exerpts from 't Hooft slides---
    From a pragmatic point of view there is nothing wrong with quantum mechanics...[But] the standard Hilbert space procedure for QM does not go well with gravitation, curved space-time, and cosmology.
    Attempting to reconcile General Relativity with QM leads to apparently insurmountable problems...
    (for anyone not happy with string theory: even with AdS/CFT, there can be
    no locality in 3+1 space-time.)

    Why should these problems NOT be related to the question of the foundation of quantum mechanics?

    this is a minority's view point, because:
    Theorem:"Hidden variables cannot be reconciled with locality and causality."
    ...
    Perhaps Quantum Gravity can be handled by thoroughly reconsidering Quantum Mechanics itself!
    ---end quotes from slides 2,3,4---

    Slides 6 and 7 depict two scientists who personify our expectations of Causality and Locality.

    ---exerpts from slides 13, 16, 18---

    Quantum Mechanics is NOT a theory that describes what is really going on, not an ONTOLOGICAL theory

    A key ingredient for an ontological theory: Information loss

    A Quantum state is defined to be an equivalence class of states which all have the same distant future...

    an apparently non-local, acausal definition!!

    What we call vacuum state may actually be a quantum superposition of many equivalence classes of ontological states.
    ---end quotes from 't Hooft---

    after the talk there was a panel, including Roger Penrose and James Hartle. this has their slides and audio of their commentary
    http://online.itp.ucsb.edu/online/kitp25/zee/
     
    Last edited: Oct 9, 2004
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  3. Oct 9, 2004 #2

    marcus

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    I wanted especially to check out what 't Hooft said yesterday at Kavli because Lubos Motl was at some pains to spin 't Hooft as a string supporter, when he posted yesterday on NEW
    http://www.math.columbia.edu/~woit/blog/archives/000089.html

    ---quote from Lubos---
    By the way, each string theorist is also using the name of 't Hooft roughly 3 times a day in average. We don't view him as a string theorist, but he would certainly joined the top ten if he described himself as a string theorist. ;-) 't Hooft is teaching a course on string theory and has extensive lecture notes, see

    http://www.phys.uu.nl/~thooft/lectures/string.html

    The Dutch are very good in our field, I would say, and of course support of their leaders is helpful.
    ---end quote---
     
  4. Oct 9, 2004 #3

    marcus

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    't Hooft spoke twice at this conference
    there was the 8 October 1:30 PM talk about the Future of Quantum Mechanics, slides of which are quoted here
    and also he was on the 9 October 9:30 AM panel, with Ashtekar, on the topic What is Quantum Gravity?---that came right after Witten's talk

    so as of now, today, Witten has given his 9:00 talk about the future of String and the panel (including Ashtekar and 't Hooft) has done its thing,
    probably short remarks by each of the members.

    With luck, the audio for these things should soon be appearing at
    http://online.itp.ucsb.edu/online/kitp25/

    But it hadnt yet, the last I looked
     
    Last edited: Oct 9, 2004
  5. Oct 9, 2004 #4

    marcus

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    Last edited: Oct 9, 2004
  6. Oct 9, 2004 #5

    selfAdjoint

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    Thanks Marcus for the links. Very interesting.
     
  7. Oct 10, 2004 #6

    marcus

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    my pleasure! Peter Woit has commented on Witten's talk http://www.math.columbia.edu/~woit/blog/archives/000090.html

    what I noticed about Witten's talk was that it was explicitly aimed at a non-string audience

    he explicitly said this at the outset, and even warned people that since it was going to be for outsiders that the questions afterwards should be in keeping with this----he was eliciting non-technical questions (instead of those his string colleagues would want to be asking)

    so Witten's message, as I take it, was "this is what we ought to be telling people who are not themselves doing research in our field but who may influence how our field is seen by the general public, the funding agencies, the academic and institutional support people, the scientific community".

    As such Witten's talk doubtless served a useful purpose, indeed provided an essential piece the picture for the people there. If they were wondering what line to take with non-string academics and NSF committees etc, or with non-string colleagues at the faculty club or the dean's cocktail party, well this is it.

    So it was a useful and essential talk although people may be right in saying it didnt give information from later than 1997 or whatever---original thought or big news.

    the key word I heard from both Steven Weinberg and from Edward Witten at this conference on "The Future of Physics" (or as witten talk was titled, "The Future of String Theory", and there may be a tendency to equate)

    the key word was "ONLY"

    "the one best hope for unifying the forces"
    "the only reasonable way to extend particle physics, or to go beyond the standard model"

    these are not exact quotes, but give the sense of the key point they were emphasizing. IT MAY NOT (quite) WORK BUT IT IS THE ONLY HOPE

    one obvious trouble with adopting that position is that it commits you to deprecate the investigation of other possible approaches-----for various reasons alternatives must be treated with condescension or simply not admitted to exist.

    the silence after Ashtekar's panel talk was so remarkable, contrasted with the loud applause for the next speaker, that the panel moderator could not ignore the difference and had to make an extenuating remark.

    it was at that point that I felt the wagons were in a circle and everyone outside, even Ashtekar, the token outsider, was a potential Commanche.
     
  8. Oct 10, 2004 #7

    selfAdjoint

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    Well, Witten's point that superstrings was the only valid generalization of field theory was actually on the money, unless you consider supersymmetric GUTs to be generalizations. The usual argument is that you can't generalize the standard model significantly with the same tools that were used to build the standard model. This is the Coleman-Mandula theorem. Supersymmetry is the one tool beyond the standard model that field theorists are comfortable with (Frank "Tony" Smith's careful constructions with Lie Algebras are not accepted by the community). And string theory is the only generalization of what field theory talks about - from point particles to one dimensional, and now to k-dimensional objects (branes).

    Plus string physics is the only accepted physics that even looks at representing both gravity and the standard model (or some GUT). LQG does none of this and follows another path. If it truly accepts non-unitarity as basic then that path is going to be long and lonely.

    "Wide is the path that leads to perdition, and many are they who take it, but narrow ans steep is the path that leads to salvation, and few are they who follow it."
     
    Last edited: Oct 10, 2004
  9. Oct 10, 2004 #8

    marcus

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    getting away from unitary time-evolution---something David Gross mentioned in his final talk summarizing questions and future directions---may be a steep and narrow path.

    But it is certainly not one that loop gravitists will be traveling alone.

    Just in the recent KITP25 conference I heard it mentioned by several speakers
    (and you can bet they werent talking about Loop specifically!)

    Gross was especially graphic and empatic about it----talking about a physics "without time" and time being 'emergent' from some more basic timeless model.
    He motivated this by remarking that in Gen Rel time is such a "tricky" concept. And he made a point of how radical a step it was because to a large extent physics is "about" this unitary time evolution and predicting the future. Time in some sense more basic to physics than space.
    So he had his own point of view, but what he actually said was not really very different from what Rovelli says about physics sans time (or at least without an absolute time and unitary evolution by an ideal clock).
     
  10. Oct 10, 2004 #9

    marcus

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    http://online.itp.ucsb.edu/online/kitp25/witten/oh/08.html

    "It may well be that string theory is the only way to reconcile gravity
    and quantum mechanics (as Weinberg suggested Thursday)..."

    there is that ONLY WAY theme
    it gets harped on in various forms and contexts,
    and here is another important theme:

    http://online.itp.ucsb.edu/online/kitp25/witten/oh/10.html

    "One thing I can tell you, though, is that most string theorists suspect
    that 'spacetime' is an
    'emergent phenomenon'...


    so again the idea recurs that time, fundamentally, does not exist
    or at the very least, one would suppose, unitary evolution according
    to the absolute time told by an ideal clock would not exist.

    We really should quote Weinberg's ONLY WAY statement, since it was the opening address and made this keynote point most radically and emphatically.
     
  11. Oct 10, 2004 #10

    marcus

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    On my RealOne player the point in Weinberg's talk is called
    35:00/47:46

    you use the mouse to drag the button along the line until it reads
    that it is in minute 35 of the 47 minute talk by Steven, then you press play.

    he says:
    35:15 "In other words string theory is what it is because it is the ONLY WAY of combining gravity and the standard model in 4 dimensions..."

    also back at 34:25 and up ahead at 36:08 and 36:24
    in this passage he uses the phrase "only way" some 5 or 6 times
    probably more but I stopped counting
    basically for about 2 minutes he is just repeating the main message
    "only way only way only way..."


    36:08 "Maybe that's all that string theory is, maybe string theory is just the only way of doing physics that includes gravity, and quantum mechanics. And if there is some other way---it that's wrong---then we all ought to try hard to find what other way there is..."

    36:24 "So my guess is that string theory IS the only way, and that may be what string theory is..."
     
    Last edited: Oct 10, 2004
  12. Oct 10, 2004 #11

    selfAdjoint

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    I want to pick up a point you made in a post or two back. When quantum field theorists speak of emergent time and remark that this is a problem for unitary time evolution, that's different from developing a theory that doesn't have unitarity from the gitgo. LQG doesn't fail unitarity just because it quantizes spacetime, it does so because it's development is non-unitary, at least according to Larsson, and I believe Urs is on board with this thought too.

    BTW I agree with Weinberg's 36:08 comment in your post #10, just above. "..we all ought to try hard to find what other way there is...". Most of the "other way" candidates, like Smith's, are at first glance beyond the pale. Somebody should grit her/his teeth and look into these rejecta.
     
  13. Oct 11, 2004 #12

    marcus

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    there is a great quote from Feynman re how important it is to do this---where he also recognizes the serious career risk of
    riding the dark horse

    the way the academic ladder is set up (maybe in physics more than mathematics) can apparently offer someone a choice between boarding the bandwagon and languishing in genteel obscurity

    and yet he says it is necessary, absolutely essential to progress in theoretical physics, and he urges people to do it despite the possible hardships

    -----------
    my hunch is that whether or not it is to young researchers advantage to check out alternatives to the bandwagon---well, that depends a lot on how the establishment is set up and who the administrators are. I have a hunch
    that, say, Hermann Nicolai is making it worth some young people's while to hazard a look into alternatives. My impression is that he understands better than some others the strategy of diversifying the search and crossing lines
    --------

    I think it is pathetic that Harvard does not have a couple of world class young Loop Gravitists on the faculty----since they have first rate string theorists.

    for one thing it would help keep Lubos in bounds :smile:
    there would be someone with the authority to contradict
    overthetop partisan assertions.

    If someone like Nicolai were chairman of harvard physics department, there would be Loop counterparts of Lisa Randall Nima Arkani-Hamed
     
  14. Oct 11, 2004 #13

    marcus

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    I guess you saw that this month's Americas conference on Quantum Gravity has a talk by Hanno Sahlmann on doing string theory using LQG methods

    that is a sample of what goes on at Perimeter--where they have both stringists and loop gravitists in residence and host both string and LQG workshops (not simultaneously! not combined! but at least they do both)
    and Sahlmann is talking on the loop-string.

    http://www.perimeterinstitute.ca/activities/scientific/PI-WORK-2/participants.php

    BTW at the same conference Smolin's talk is about deriving "physics from loop gravity"-----work was bound to get started on that, sometime or other.

    I think Steven Weinberg said "only way" about 6 times in 2 minutes.

    I went and got the link to the PF post about this:
    https://www.physicsforums.com/showthread.php?p=336733#post336733
    and the exact titles of their talks. Curious what theyll say.
    Sahlmann: String Theory with LQG methods

    Smolin: Physics from Loop Quantum Gravity
     
    Last edited: Oct 11, 2004
  15. Oct 11, 2004 #14

    selfAdjoint

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    I will especially want to hear/read Sahlmann's talk. He has seemed to me to be the deepest and most careful theorist in that group. Smolin, well he is in the business of throwing out challenges.

    Marcus, I wish you could get off the bandwagen for LQG as the alternative to String physics. It's not. LQG has no specific LQG account of particle physics, only a proposal to interface the standard model to their quantum spacetime. If you want to account for the unexplained parameters in the standard model you have to look elsewhere.
     
  16. Oct 11, 2004 #15

    marcus

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    What proposal do you mean?
     
  17. Oct 11, 2004 #16
    Hi,

    Do you have the reference of who and why says that LQG is non-unitary?
     
  18. Oct 11, 2004 #17

    marcus

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    How is it different?

    I suppose one could go back and do a historical search to find out how many years QG theorists have been saying that unitary time evolution may indicate a model is unphysical.

    My perception is that Loop gravitists are not in the business of criticizing string and rarely point this out---but on one occasion last year Rovelli did speak out about it. He said that the fact that string models have unitary time evolution was grounds for suspecting something was wrong with them.
    Coming from GR that's not what one expects.

    As far as I know Loop has always been frankly nonunitary---but I would have to go back and read old papers to nail that down. It certainly was explicit in Rovelli's "Dialog" back in October 2003, and it was hardly news then but just something taken for granted.

    With all respect to Urs, Thomas, whoever else, they seem to think they discovered something when they learned that LQG was nonunitary!
    It seems they made a big fuss about discovering something that
    quantum gravitists had been already frankly pointing out for some time.

    Last year Rovelli's book has a big discussion of "physics without time", much of a chapter is devoted to discussing time, the different notions of time, why not to expect a fundamental theory to be one of time-evolution
    (and that wasnt especially new, he has earlier papers about that, but it was part of the book because the book tries to be comprehensive) and now in October 2004 we have DAVID GROSS talking about "physics without time."

    He presents it as if it is a new cutting edge recognition, looking ahead to the future of string theory. Welcome to the club :smile:
    So my question is How is it different?
     
  19. Oct 11, 2004 #18

    marcus

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    I think you may know the literature better than I, so please throw in any ideas, references etc.

    I will find the place in Rovelli's "Dialog" and paste it in. then there is chapter 2 of Rovelli's book.

    I would ask you. Do you know any basic treatment of LQG where it says that there is unitary time-evolution? I was under the impression that a fundamental theory could not have this, but I could easily be mistaken.

    (the business is different with Urs and Thomas Larsson "discovering" that LQG was nonunitary and thinking that it was somehow inadvertent and a mistake of the quantum gravitists! That is fairly recent and I have it
    only second hand from selfAdjoint)

    I will fetch that bit of dialog

    Rovelli
    http://arxiv.org/hep-th/0310077.
    October 2003

    ---exerpt from page 18 of "Dialog"---
    Simp - ... Is loop gravity unitary?
    Sal – No, as far as I understand.

    Simp – This is devastating.
    Sal – Why?
    Simp – Because unitarity is needed for consistency.
    Sal – Why?
    Simp – Because without unitarity probability is not conserved.
    Sal – Conserved in what?
    Simp – In time.
    Sal – Which time?
    Simp – What do you mean “which time?”. Time.
    Sal – There isn’t a unique notion of time in GR.
    Simp – There is no coordinate t?
    Sal – There is, but any observable is invariant under change of t, therefore everything is constant in this t just by gauge invariance.
    Simp – I am confused.
    Sal – I know, it is always confusing. . . Nonperturbative GR is quite different from physics on Minkowski . . .
    Simp – Do we really need to get in the conceptual complications of GR?
    Sal – Well, if we are discussing the theory that is supposed to merge GR and QM...
    Simp – String theory merges the two without these complications.
    Sal – This is why I think that string theory does not really merge GR and QM.

    ---end exerpt---
     
    Last edited: Oct 11, 2004
  20. Oct 11, 2004 #19

    selfAdjoint

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    It is NOT just about time, time is a side issue. The nonunitary theory does not conserve probability, the sum of the probabilities over a complete set of alternatives doesn't add up to 100%.

    And Rovelli may talk about the absence of time in his book, but it does not, at least the draft I saved does not, contain any of the words unitary, unitarity, nonunitary or nonunitarity. I repeat, these are two different issues.
     
  21. Oct 12, 2004 #20

    marcus

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    The Dialog here was originally part of Rovelli's book---in October/November drafts---but in the December 31 draft, which I saved the Dialog was removed. I understand that a publisher had asked for it and Rovelli decided to let it be published separately.

    I will look for some other things that are in the 31 December draft for you.
     
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