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Physicsweb:'t Hooft Witten Dowker Davies

  1. Dec 5, 2005 #1

    marcus

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    http://www.physicsweb.org/articles/world/18/12/2/1

    speculative musings about quantum theory, gravity, spacetime
    in December issue of Physicsweb
    it's a fourway opinion piece featuring views of Gerard 't Hooft,
    Edward Witten, Fay Dowker, and Paul Davies.
     
  2. jcsd
  3. Dec 5, 2005 #2
    It is funny to see that 't Hooft is the only person who seeks for a underlying deterministic mechanism behind quantum theory although he denies local realism to be a plausible candidate (implying that his notion of reality is rather contrived) unlike I do (another possibity to violate the Bell inequalities, if experiment would force us to, is by locally giving up the arrow of time). Witten has IMO the correct worries (the nasty cat I teased quantum physicists with on the QM forum a lot of times) but he seems to be hoping that it will drop out somehow of (an improved version of) QM itself. Anyway, the main worries for QG in my view are: locality and causality (it seems to me that the most plausible way out is to look for a local realist space time alternative for quantum predictions at the appropriate scales).
     
  4. Dec 5, 2005 #3

    Kea

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    I liked 't Hooft's piece the best! Symmetry is no longer fundamental. He even used the word ontological. :smile:
     
    Last edited: Dec 5, 2005
  5. Dec 5, 2005 #4

    marcus

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    so glad you like 't Hooft's piece, Kea

    a propos ontological there is a recent paper of Rovelli et al where the authors explain that in a given particle physics experiment you cannot say how many particles are involved, or what the particles are, until you have specified the bounds of the apparatus and what is measured----their very number and existence depends on macroscopic considerations

    indeed what particles are there are contingent upon the observer, they have no independent existence. I believe this is a widely accepted fact and is not terribly "model dependent". For example Rovelli et al used a rather general framework---this was not a quantum gravity paper but more generally about quantum field theory IIRC.

    I will have to get the link. it is good of you to highlight ontological issues, now and then. (but a little philosophy can go a long way :smile: )

    BTW: Here is what I see as the key passage in what 't Hooft said:

    -------exerpt from 't Hooft Physicsweb------
    Quantum mechanics could well relate to micro-physics the same way that thermodynamics relates to molecular physics: it is formally correct, but it may well be possible to devise deterministic laws at the micro scale. However, many researchers say that the mathematical nature of quantum mechanics does not allow this - a claim deduced from what are known as "Bell inequalities". In 1964 John Bell showed that a deterministic theory should, under all circumstances, obey mathematical inequalities that are actually violated by the quantum laws.

    This contradiction, however, arises if one assumes that the particles we talk about, and their properties, are real, existing entities.my comment, of course they are not! they are contingent But if we assume that objects are only real if they have been precisely defined, including all oscillations as small as the Planck scale - and that only our measurements of the properties of particles are real - then there is no blatant contradiction. One might assume that all macroscopic phenomena, such as particle positions, momenta, spins and energies, relate to microscopic variables in the same way thermodynamic concepts such as entropy and temperature relate to local, mechanical variables. Particles, and their properties, are not (or not entirely) real in the ontological sense. The only realities in this theory are the things that happen at the Planck scale. The things we call particles are chaotic oscillations of these Planckian quantities. What exactly these Planckian degrees of freedom are, however, remains a mystery.
    ---endquote----
     
    Last edited: Dec 5, 2005
  6. Dec 5, 2005 #5

    marcus

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    Must warn you Kea that when discussion goes philosophical it can QUICKLY get over my head---and my eyes glaze. But to read 't Hooft's part of that opinion piece one MUST confront those issues if only briefly.

    Here is Rovelli and Colosi

    http://arxiv.org/abs/gr-qc/0409054
    Global particles, local particles
    21 pages

    "The notion of particle plays an essential role in quantum field theory (QFT). Some recent theoretical developments, however, have questioned this notion; for instance, QFT on curved spacetimes suggests that preferred particle states might not exist in general. Furthermore, a certain tension derives from the fact that QFT's particle states are intrinsically nonlocal, while experiments are localized. These considerations have lead some to suggest that in general QFT should not be interpreted in terms of particle states, but rather, say, in terms of eigenstates of local operators. On the other hand, it is not completely obvious how to reconcile this view with the empirically-observed ubiquitous particle-like behavior of quantum fields. We observe here that already in flat space there exist --strictly speaking-- two distinct notions of particles: globally defined n-particle Fock states and local particle states. The last describe the physical objects detected by the real finite-size particle detectors and are eigenstates of local field operators. In the limit in which the particle detectors are large compared, say, to the Compton wavelength, global and local particle states converge in a weak topology defined by physical measurements (but not in norm). This observation reconciles the two point of view and provides a local definition of particle state that remains well-defined even when the conventional global particle states are not defined. This definition could play an important role in quantum gravity, when asymptotic regions may not be available."

    Maybe nothing new here to you. But at least it can serve to substantiate what the 't Hooft discussion is about.

    Kea, I really like what I blued in the 't Hooft quote. It feels like the lifting of a weight off my mind. How obvious!
    -------exerpt from 't Hooft Physicsweb------
    Quantum mechanics could well relate to micro-physics the same way that thermodynamics relates to molecular physics.... One might assume that all macroscopic phenomena, such as particle positions, momenta, spins and energies, relate to microscopic variables in the same way thermodynamic concepts such as entropy and temperature relate to local, mechanical variables.
    -----endquote----

    Such an evocative analogy! quantum mechanics EMERGES from something at an even smaller scale

    down at planckian scale, particles do not even exist, there is something else which we have to discover
    imagine a micromicro web of relationships----out of collective behavior of tens thousands of linkages in this web a gross macroscopic collective behavior called "PARTICLE" has to emerge.
    this collective coarsegrain behavior we identify as "particle" and assign "position" and "momentum" is analogous to the phonons of vibration in a crystal----it is a particular dance that the more elementary entities do.

    so the historical analogy and perspective that 't Hooft points us to is like the emergence of the atomic theory of matter---which many people did not accept even in 1905. They had overall laws of how a gas behaved (or a liquid) without supposing that the gas (or liquid) was made of many separate molecules.

    I must say I find this very congenial, and reminiscent of what Sundance Bilson-Thompson was doing at the blackboard near the end of his 16 November Perimeter talk.
    he was making a blurred chalky mess of it, because he kept rubbing out and redrawing, but he was trying to imagine "moves" by which interconnections in the micro-cobweb could change and from which the ILLUSION of particle interactions could arise (he did not say it quite that way :smile: )
     
    Last edited: Dec 6, 2005
  7. Dec 5, 2005 #6

    marcus

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    Careful, would you care to have a look at this?

    http://streamer.perimeterinstitute....rType=WM64Lite&mode=Default&shouldResize=true

    it intrigues me and i would be interested to know your reaction.
    some of what is discussed near the end is tentative-----work just being being done by Bilson-Thompson in collaboration with Smolin.

    sometimes that link doesnt work the first time, but I try it again and it always seems to work the second time
     
  8. Dec 5, 2005 #7

    Kea

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    Dear Marcus

    Yes, I remember the Rovelli et al paper. He has a talent for identifying and analysing interesting issues, but none of the discussion is philosophical in the sense of 't Hooft's picture, that you go on to describe.

    I know I often point this out....but once again: Heisenberg said that one of his greatest insights was the realisation, following some comments of Einstein, that particles were not fundamental. But then the issue with your statement above becomes what do you mean by scale? In a world where measurement is paramount an understanding of scale is not given a priori.

    :smile:
     
  9. Dec 5, 2005 #8
    Can you reduce, in scale, a definite "real" particle to a fundemental limit?..lets define a particle from the observer point of reference as the smallest 'particulate' observable by a Human observer, without the aid of mechanical detectors, lets say a speck of dust. The speck of dust comprises of a "LARGE" number of atoms which renders it visable by an observer.

    This speck of dust cannot be reduced in scale any smaller, and remain visible,(local), the speck of dust has Two paramiters converging>> Quantum to Macro. From the MACRO observer's frame it is a finite limit, as small as it gets before it goes "hidden". From a Quantum frame, it has a LARGE value, a "EXCESS" number of atoms?...excess number because there is an interaction of the
    observer, therefore it "MUST" be overdense.

    Now interestingly if one starts to seperate (as oppossed to reduction ) the particulate further, then the observer is taken out of the equation, and the process becomes "mechanical", reliant
    upon the device of seperation, the particulate quantity now conforms to Quantum "scale", and the local density of the particulate has a variable signature, ie it is unstable if interactions
    occur.

    Now again, if one treats the particulate as being an observer quantity, then it is one side of an equilbirated system, any reduction OR seperation will render it 'observationally', undetectable. Consequently, if one treats the Quantum System likewise (equilibirated), or below the "observable-limit", then it actually can be extended "globally" across fields, and still be below the observable limit with respect to an observer, the speck of dust has collapsed/reduced out of visible(local) frame, but not out of global frame, the quantum level.

    The implications of reduced system analysis often skip straight to the planck scales, without regard to observer defined limits?

    P.S the "other" converging paramiter is of course Time!..local time is set according to observers, global time is observer free.
     
    Last edited: Dec 5, 2005
  10. Dec 6, 2005 #9

    marcus

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    it is a fascinating question what do you mean by scale?

    just as a shot in the dark I'd conjecture that the fundamental scale has a QUALITATIVE meaning. This would be the scale at which prepositions are no longer significant----at which there are no longer clear relationships such as under, over, between, beside, outside.

    being able to measure lengths, or distances between things, might be something that gradually establishes at scales considerably larger than the fundamental one. indeed "things" only emerge and become identifiable at increased scale, as one zooms out.

    but since this is so vague, the question remains as intriguing as before---what is meant by scale, when one zooms in so close that distinct entities are no longer possible to identify?

    when one zooms into where there might only be a web of spatial relation and pre-matter merely the twisting and braiding of the strands of that web----like any twist or braid not perfectly localized.

    one only recognizes the twisting and braiding as matter as one steps back.

    oops it is after midnight, your question started me musing. should turn in.
     
    Last edited: Dec 6, 2005
  11. Dec 6, 2005 #10
    Hi marcus, I have a dreadful connection so could you refer me to a paper? But first tell me why I should find it interesting: as far as I seem to remember this preon model was a unification (in the old QFT tradition) between the weak and strong interactions; quarks consist of preons as well as electrons do (did I remember that correctly?). In contrast to most people, I do think that QFT *itself* needs serious modifications. Nevertheless, the Rovelli paper seems very interesting although it does not try to modify QFT I guess. I myself am advocating already for years that we do need a quasi local particle notion for QFT (such as the notion of a null bifurcation horizon is a quasi local concept replacing the event horizon - of a black hole say.). I do not know if they do that (did not check up the paper yet) - but if they do - I would be glad to hear about it.
     
    Last edited: Dec 6, 2005
  12. Dec 6, 2005 #11

    marcus

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    Hi Careful, thanks for the question. Here is the most recent paper by Bilson-Thompson
    http://arxiv.org/abs/hep-ph/0503213
    A topological model of composite preons
    Sundance O.Bilson-Thompson
    9 pages, 3 figures

    "We present a modification of the preon model proposed independently by Shupe and Harari. A basic dynamics is developed by treating the binding of preons as topological in nature and identifying the substructure of quarks, leptons and gauge bosons with elements of the braid group B3. Topological considerations and a straightforward set of assumptions lead directly to behaviour consistent with much of the known phenomenology of the Standard Model. The preons of this model may be viewed as composite in nature, and composed of sub-preons, representing exactly two levels of substructure within quarks and leptons."

    there are several reasons you might find it interesting

    1. it is different from the 1970s preon model---so the objections to its antecedent do not necessarily apply

    2. the preons are a small number of elements (realized topologically with braids and twists) which combine according to simple rules. they combine to form quarks leptons photons etc. of the right types and numbers, and apparently undergo the right sorts of decays and reactions. conserved numbers can be defined topologically

    3. this would remain simply a scheme that reproduces the right menu of Std. Mdl particles, unless the strands, braids, twists could be supposed to actually be in some representation of the quantum state of spacetime---but they can be. A spin network is a possible model of spacetime into which the topological preons of B-T can be introduced.
    One can have twists and braids in the strands of a network.

    Point 3 was not discussed in the paper, it is the subject of current work by Smolin and B-T, which was briefly discussed in the recorded talk by B-T and also in a conference talk given by Smolin around 12 October.
    Apparently the B-T topol. preon model was new in March this year, and the possible implementation in spin networks was noticed for the first time later this year---so far nothing published about it.
     
    Last edited: Dec 6, 2005
  13. Dec 6, 2005 #12
     
  14. Dec 6, 2005 #13

    Kea

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    Marcus, you're becoming a category theorist! Seriously, if you agree that the logic of measurement is paramount I don't see where else you can go.

    :smile:
     
    Last edited: Dec 6, 2005
  15. Dec 6, 2005 #14

    marcus

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    one catch, sundance preons don't have a manifold. they arent embedded in anything-----they're just a scheme of braided strands

    it makes his system of preons seem wanting----somehow incomplete and provisional.

    the spin-networks smolin is using aren't necessarily embedded either, so if they succeed at implementing sundance preons in the context of a network (which expresses a quantumstate of the gravitational field) then there will still be no manifold in sight.

    puzzling business, the gradual dissolution and dispersal of the manifold "idea" from quantum gravity-----one sees it in Loll's triangulation gravity, CDT, as well
     
    Last edited: Dec 6, 2005
  16. Dec 6, 2005 #15
    I realized that :smile: I am well aware of the manifold departure in QG, but it seems that one has difficulties in manifold recovery :rofl: (also in CDT!)
    It is easy to break a leg, to heal it is an entirely different matter.
     
  17. Dec 6, 2005 #16

    marcus

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    nice idea: to go where the logic of measurement is paramount

    well there are several places like that where I'd like to go, such as Classical Greece and Renaissance Florence. as a bird you must have a lot of frequent flyer miles, so why don't you consider coming along?
    :smile:
     
  18. Dec 6, 2005 #17

    Kea

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    Yes, lots of frequent flyer kilometres, but alas no time machine! :smile:
     
    Last edited: Dec 6, 2005
  19. Dec 6, 2005 #18

    marcus

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    Smolin calls that the "Inverse Problem". Once you depart from the manifold it is hard to recover----or to find conditions under which you get a manifold back. IIRC he discussed that problem in a recent paper called "the case for background independence" as it arises in several QG approaches

    it's admittedly a formidable problem

    but maybe one should not try to get a manifold back, at least not a differentiable manifold------but only to approximate such a thing at large scale (which CDT shows signs of being able to do)

    maybe at small scale down at some fundamental level spacetime is a churning compound fracture mess.

    you impress me as knowledgeable so that I would expect you already to be familiar with my notions as I voice them. How about telling me what CDT paper you base your view of CDT on. then maybe I can talk more efficiently----both of us looking at the same page of the book so to speak---or anyway less vaguely.
     
    Last edited: Dec 6, 2005
  20. Dec 6, 2005 #19

    Kea

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    By the way, Sundance points out clearly some large potential departures from the Standard Muddle, such as no Higgs boson.
     
  21. Dec 6, 2005 #20

    marcus

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    I think that's a great opportunity for the network in which the sundance strands live to play a role and give things mass and gravity
    so my intuition is that a higgs and a graviton are exactly what one does not want----refer to functions of the network

    you are very right, kea, no Higgs boson!
    I'm amazed at what he gets by way of quarks and leptons though
     
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