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Does LQG Do Nothing Cool Even If It Is Right?

  1. Oct 8, 2014 #1

    ohwilleke

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    In "Loop quantum gravity and observations" by A. Barrau and J. Grain at http://arxiv.org/abs/1410.1714 the bottom line seems to be that the phenomenological implications of LQG relative to existing predictions from GR are so extremely subtle that they have virtually no practical effects that can be discerned without ultrahigh precision instruments. It makes the tweak to the orbit of Mars predicted by GR relative to Newtonian gravity look veritably spectacular and vivid by comparison.

    Am I missing something, or is the correct conclusion that LQG, if proven right, is basically just wrapping up loose ends that would otherwise lack a mathematically rigorous treatment?
     
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  3. Oct 9, 2014 #2

    Demystifier

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    You are missing the fact that the same can also be said about even more revolutionary theories of the 20'th century, such as theory of relativity and quantum theory. Compared to the 19'th century theories, the 20'th century theories make little difference for phenomena seen under normal everyday-life conditions. And yet, they make a very big difference under certain very extreme conditions, such as velocities close to the velocity of light (special relativity), strong gravitational fields (general relativity), and small distances (quantum mechanics). Likewise, LQG makes a very big difference under extreme conditions of a sufficiently high energy concentrated within a sufficiently small volume (Planck scale).
     
  4. Oct 19, 2014 #3

    marcus

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    I agree with Demystifier about the importance of understanding phenomena that involve extremely high energy density.

    Also I wouldn't knock "wrapping up loose ends" i.e. resolving paradoxes and inconsistencies between theories.

    As a comment on your shorthand "LQG, if proven right, is basically..." I'd like to suggest Wieland's July 2014 paper and September ILQGS talk as exemplifying the current development of the theory and ask (if that extension of Spinfoam QG is "proven right" and gains acceptance, which is assuming a lot!) what do you think that would entail?

    For one thing, unless I'm mistaken, since it joins Spinfoam QG with the Cortês-Smolin energetic causal set (ECS) picture, it addresses issues such as the passage of time, and the experience of the present moment---but does so in a polychronistic manner rather than the monochronistic one of conventional quantum mechanics and quantum field theory---suggesting a fundamental alternative to the block universe picture of the world.

    You've probably already checked out the relevant papers by Wieland and by Cortês-Smolin. For convenience, I'll get links. They're on Part II of the third quarter MIP poll and in some other threads as well.
    Both abstracts are here, scroll down, they are #9 and #10 on the list:
    https://www.physicsforums.com/threa...portant-qg-paper-part-ii.773590/#post-4867362
    New action for simplicial gravity in four dimensions
    Wolfgang M. Wieland

    Spin foam models as energetic causal sets
    Marina Cortês, Lee Smolin

    One of the cool things about Wieland's paper is that he extends LQG in such a way that it makes ECS able to connect with it.
     
    Last edited: Oct 19, 2014
  5. Oct 19, 2014 #4

    marcus

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    Wieland's "New Action" paper does some other cool things as well. It resolves a conceptual tension around the Hamiltonian in Spinfoam QG and arrives at a new way of looking at the 4d simplicial complex. Here is a quote from page 1:

    "...We will propose such a theory, and prove an intriguing correspondence: The entire simplicial complex represents a system of free particles propagating in a locally flat auxiliary spacetime, with every tetrahedron representing one of those particles, the volume-weighted time-normals representing the particles’ four-momenta, and the entire discretized action turning into an integral over the worldlines of those auxiliary particles..."

    Just to keep it handy, here's the link to the July 2014 "New Action..." paper:
    http://arxiv.org/abs/1407.0025
    The number is 1407.0025 is fairly easy to remember in any case.

    This might be the Loop theory paper currently best to focus on
    (theory, that is, as contrasted with observations/phenomenology where a lot else is happening).
     
    Last edited: Oct 20, 2014
  6. Oct 20, 2014 #5

    julian

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    "just wrapping up loose ends that would otherwise lack a mathematically rigorous treatment?"

    According to the no hair theorem for black holes in classical GR the entropy of a black hole is zero and so seems to be in contradiction of physical arguments that it should have entropy. In LQG it is realized that the entropy of a black hole is accounted for by purely quantum mechanical degrees of freedom of the gravitational field.
     
    Last edited: Oct 20, 2014
  7. Oct 20, 2014 #6

    julian

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  8. Oct 20, 2014 #7

    julian

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    Lee Smolin:

    … someone might earn a Clay prize by rigorously constructing quantum Yang-Mills within LQG. It will certainly not be me, but there are people working on exactly that program. The conjecture is that background independent QFTs are more likely to exist rigorously in 3+1 dimensions than Poincare invariant QFTs.
     
  9. Oct 23, 2014 #8

    marcus

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    I think the relation of the LQC bounce to inflation is one of the remarkably cool things it does. And LQC has been derived from LQG so the cosmological tests serve to test the full LQG theory, not only the LQC application of it.
    The Barrau Grain paper that Ohwilleke cited actually outlines this the predictions for cosmology and astrophysics observations (cool if you happen to be interested in cosmology and astrophysics, various unexplained signals in the gammaray and microwave and radio wavelength sky, understanding the start of expansion, the CMB etc). And the Barrau Grain paper has references if anyone wants to pursue such coolness further : ^)

    But what currently intrigues me is this, particularly the new version of causal sets that results from Cortes-Smolin putting ECS together with Spinfoam QG. I think it can lead to an improved understanding of the passage of time and the experience of the present moment .
    I'll try to explain that.
     
  10. Oct 23, 2014 #9

    marcus

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    The causal set idea of the passage of time is well described in this talk by Fay Dowker which summarizes work by Rafael Sorkin and collaborators:
    google "dowker passage arxiv" to get the arxiv copy.
    http://arxiv.org/abs/1405.3492
    The birth of spacetime atoms as the passage of time
    The idea is that there is no 3D instantaneous version of reality, what we experience is a confused disorderly process of causation at the frontier of the past where space-time cells are proliferating and the order in which they appear is pure gauge that is physically meaningless.

    Any experience of reality I may have involves this current process of causation. Say I want to see if the wall is firm so I put the computer down and walk over and push on it. At every step this experience involves the birth of space-time atoms and the occurrence of events. Which go to build up the past, a kind of crystal of dead events which can no longer be influenced by our actions or experienced.
    Best if you have a look at Dowker's paper, it is short and written for a wide audience, not just for physicists.

    The relevance is that Cortes-Smolin have devised a version of causal sets in which the space-time cells or atoms are 4-simplices and they are undergoing Pachner moves . This means that the authors can ascribe dynamics to the growth of the past (and this disorderly proliferation we call the present) based on Wieland's action .
     
    Last edited: Oct 24, 2014
  11. Oct 24, 2014 #10

    marcus

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    That May 2014 paper by Fay Dowker used brief dialog to point out the need for a change in our theory of space-time to include the experience of the present and the passage of time. She compares the present situation to an anomaly in Newtonian gravity. Newtonian gravity was at odds with experience in that it suggested that gravity was a force. But we do not feel the force of gravity the way we feel other forces. People put up with that curious anomaly for 300 years and then in 1915 it was discovered that gravity was not a force after all, it was the curvature of space-time geometry. By the principle of equivalence one can zero it out. It is not felt in free fall.
    So one had a mismatch of theory with experience that eventually required a fundamental change in the theory. Here's the dialog:
    ==quote Dowker http://arxiv.org/pdf/1405.3492.pdf ==

    ... challenges the view that quantum gravity effects can only show themselves as phenomena in regimes far beyond our immediate reach. The idea is that we may have access, through our intimate experience, to a physical phenomenon that is not present in General Relativity but is part of a more complete theory of spacetime. Lest this seem far-fetched, let me introduce an example from history ...

    Every day in the centuries between Newton’s discovery of the Law of Universal Gravitation and Einstein’s discovery of General Relativity, every human being was making an observation with no explanation within the Newtonian theory but which correlates perfectly with GR. This observational fact, was, throughout that time in history, “hidden in plain sight.” ...

    17th Century Scientist: There is a physical force of weight on you. Look at all the data, celestial mechanics, etc. The Newtonian theory of gravitation accounts for all that data.

    17th Century Sceptic: But I don’t experience this gravitational force of weight whereas I can feel mechanical forces of comparable magnitude. Why?

    Scientist: The force of weight is physical. So your sense-experience of no force must be an illusion. Neurology, psychology, the way the mind and body work to produce sense-experience must be responsible for this illusion of lack of gravitational force of weight.

    Sceptic: Maybe. But maybe this is telling us to look again at our theory, with the lack of gravitational force of weight as a heuristic.

    The following is a parallel conversation that may, in the future, make similar sense:

    21st Century Blockhead: There is no physical passage of time. Look at all the data, celestial mechanics, etc. The theory of General Relativity with spacetime as a Block perfectly accounts for all that data.

    21st century Sceptic: But I don’t experience a Block. I experience a sequence of moments. Why?

    Blockhead: The Block is physical, the passage of time is not physical. So your sense-experience of time passing must be an illusion. Neurology, psychology, the way the mind and body work to produce sense-experience must be responsible for this illusion of the passage of time.

    Sceptic: Maybe. But maybe this is telling us to look again at our theory, with a physical passage of time as a heuristic.
    ==footnote==

    5 If there is only a force upwards on you from your chair and no weight acting down on you then you must be accelerating upwards, away from the earth. And so must someone sitting on a chair on the other side of the planet. For everyone on the surface of the earth to be stationary and yet also accelerating away from the centre of the earth requires that the spacetime around the earth be curved.
    ===endquote===
     
    Last edited: Oct 24, 2014
  12. Oct 25, 2014 #11

    marcus

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    I think it's a safe conclusion that whatever physical world-theory arises from the current confusion will definitely have a place in it for the experience of the present and the passage of time.

    ==quote Dowker http://arxiv.org/pdf/1405.3492.pdf ==

    Blockhead: The Block is physical, the passage of time is not physical. So your sense-experience of time passing must be an illusion. Neurology, psychology, the way the mind and body work to produce sense-experience must be responsible for this illusion of the passage of time.
    Sceptic: Maybe. But maybe this is telling us to look again at our theory, with a physical passage of time as a heuristic.

    ===endquote===

    So this is a big deal. Major change in store. Physics has so far been using a Block universe, or basing models on simple flat Lorentz spacetime--even on a naive global time variable.
    This provides a way to respond to Ohwilleke's question that started the thread: What's the potential importance of Spinfoam QG, assuming it passes astrophysical tests described in Barrau Grain http://arxiv.org/abs/1410.1714?

    What's the potential importance? It's different from the Block universe world trajectory--the past has a frontier where it is taking shape. A frontier which (if I understand Wieland's presentation) does not correspond to a reading on anyone's physical clock.

    Because the ORDER one imagines new space-time simplices being added to the past is pure gauge.
    There's a swarm of tetrahedra moving on time-oriented edges that form a branched manifold and the evolution parameter is not a physical time. Where paths come together, and where they branch out into several, are vertices corresponding to spacetime simplices. Cells from which the spacetime past is built, or by which it grows. there is no synchronicity about the weaving worldliness of this swarm of tets. Spacetime arises from their interactions (Pachner moves). But you can't specify a state at some particular instant.
    This accumulation of interactions ( = pentachorons = 4-simplices) does not take place IN time because there is no time. One only gets proper time for the individual path of a particular tetrahedron after the fact.

    Maybe I can make this clearer. I'll try to say it better. I'm trying to paraphrase from the two papers:
    (google "cortes causal spin foam" and "wieland new action")
     
    Last edited: Oct 25, 2014
  13. Oct 27, 2014 #12

    ohwilleke

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    I don't agree that this is true for the 20th century modern physics discoveries.

    SR & GR implied: (1) time can be slowed down by moving something very fast, (2) light is influenced by gravity, (3) time is influence by gravity, (4) E=mc^2 which is at the heart of nuclear weapons and nuclear energy, (5) black holes exist and indeed turn out to be central of the structure of galaxies, and (6) interstellar travel during human length lifetimes faces fundamental barriers different in kind from those involved in breaking the sound barrier (i.e. change in velocity takes more additional momentum at higher velocities and nature has a speed limit).

    QM implied: (1) quantum tunnelling which makes transistors possible, (2) QM makes it possible to understand and predict superconductivity, (3) QM provides an easily accessible example of non-deterministic behavior in the physical world, (4) radioactive decay is essentially perfectly understood in principle and critical to nuclear fission power generation, (5) QM provides an easily demonstrable example of non-local phenomena (entanglement), (6) phenomena from heat to X-rays are fundamentally the same, (7) lead can be changed into gold (in principle), (8) the last "1% of phenomena" have an immensely elaborate explanation, (9) pure energy can give rise to matter and routinely does, (10) anti-matter exists, and (11) there is an arrow of time in the laws of physics.
     
    Last edited: Oct 27, 2014
  14. Oct 28, 2014 #13

    Demystifier

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    Ohwilleke, excluding (11) (because it is very questionable that QM implies arrow of time), which of these phenomena can you see in normal everyday-life conditions?
     
  15. Oct 28, 2014 #14
    Barrau and Grain wrote (on Page 4 of their paper, see Post # 1 )

    "In bouncing cosmologies, either from the loop approach or any other, the question
    of anisotropies is very important for a clear reason: the shear term varies as
    a^(-6) where a is the "mean" scale factor of the Universe . When the
    Universe is contracting, the shear term becomes more and more important and
    eventually drives the dynamics.", so for them shear is an important factor. By way of emphasis, they add, on
    Page 6:

    "The phase and shear are the initial variables to set." but conclude onPage 7:

    that "An important issue however remains: what would be a "natural" initial value for the shear? "

    Shear is a spatially relative displacement gradient associated with spatial inhomogeneities like vortices in a finite liquid, or structural
    defects in a finite continuum solid (see V. Volterra Ecol. Norm. Sup., 324 (1907), pp. 401––517) How could shear possibly arise on a non-localised bouncing cosmological scale, that includes by definition all that exists (whether the geometry be open or closed) and serve as an important issue in the present context?
     
  16. Oct 28, 2014 #15

    marcus

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    This is an interesting topic in part because it points to the fact that our PERSPECTIVE on quantum gravity research may be changing. As interest in string theorizing declines, more people have been looking at the alternatives, including LQG-SF, but also Causal Sets, CDT, Shape Dynamics, AsymptoticSafetyQG...
    and may have been revising their perspectives.
    So what Ohwilleke is prompting is an examination of LQG, how has it been progressing? How has it been changing? What can we expect from it, if a version of LQG is tested observationally and checks out well enough to gain provisional acceptance?

    There was just recently (26 October) a brief review of the situation consisting of a few comments by Lee Smolin made in Peter Woit's block. He gave a number of links to papers touching on these issues. It might be helpful here to quote.
    Lee began the discussion by commenting on a recent narrowly-focused conference as follows:

    Lee Smolin says:
    October 24, 2014 at 2:53 pm
    Dear Peter,

    If the conferees want an answer to the question “Are there any plausible alternatives to string/M-theory as a fundamental theory of physics?” they ought to invite some of the people contributing to the development of such alternatives and have an honest examination of the question. They have plenty to choose from, they could start with the speakers at Loops 13, which 200 people attended: http://www.pirsa.org/C13029 [Broken].

    Fifteen or twenty years ago it was possible to say that string theory was more promising than its alternatives, and indeed 15 years ago I switched my research from LQG to M theory. But I think that any objective evaluation of the evidence has to credit much more substantial progress has taken place since concerning the alternatives; especially spin foam models, but also other background independent approaches including shape dynamics, CDT, group field theory and tensor models, etc.

    To support this I would point to the fact that much more is known from spin foam models about the challenges it faced 15 years ago, including substantial recent results on the emergence of GR in the semiclassical limit, the entropy and temperature of generic black holes, the elimination of cosmological and black hole singularities, finiteness, etc.

    So, I would urge my string theory friends who think they know the answer to the question to have another look. There is a whole generation of brilliant young theorists working on alternatives to string theory you should meet!

    Thanks,

    Lee
    =======================
    The following series of comment then ensued:
    • Jesper says:
      October 25, 2014 at 5:27 pm
      Dear Lee
      could you provide links/references to the recent results you mention on emergence of GR in a semi-classical limit?
      Thanks
    • Bob says:
      October 26, 2014 at 1:03 pm
      Dear Lee,
      Thanks for those paper references. They look interesting. I have a question for you. LQG, spin foams, etc., are characterized as a quantum theory of pure gravity, without matter and matter interactions. Can you discuss the ways in which Loop Quantum Gravity addresses the criticisms that quantum gravity needs to unify gravity with the other interactions because gravity cannot be decoupled at Planck scales from all other interactions (if there are any) at Standard Model, and higher, energy scales?
      Thanks very much!
    • Lee Smolin says:
      October 26, 2014 at 2:16 pm
      Dear Bob,
      Actually there is a moderate sized literature on matter coupling and unification within LQG. To summarize, let me emphasize that there are several different questions.

      1) It is just not true that “LQG, spin foams, etc., are characterized as a quantum theory of pure gravity, without matter and matter interactions.” From very early in its development, it was worked out in detail how to include gauge fields, fermions and scalars in LQG. For example, see Thomas Thiemann’s book.

      2) Extension to supergravity is also straightforward, at least for N=1, including D=11. See papers by Yi Ling and myself as well as recent papers by Thiemann and collaborators,

      3) Hence, even if LQG doesn’t constrain the matter or gauge field content of what it is coupled to, LQG provides a framework in which matter gauge fields and gravity are all coupled together.

      4) There are issues that need to be addressed with fermion doubling which are the subject of work in progress with Jacob Barnett.

      5) You can ask in addition whether, “quantum gravity needs to unify gravity with the other interactions”. This from a LQG point of view is an open question.

      6) But you can ask if LQG gives a compelling framework for unifying the different interactions? Here the answer appears to be yes. The simplest possible extension of the theory, gotten by extending the gauge group of gravity, which is the Lorentz group, to a larger group, G, yields a natural extension of Einstein-Yang Mills. This comes about from a built in spontaneous breaking of the gauge group G to the product of the Lorentz group with a compact factor H, which becomes the Yang-Mills gauge group. See http://arxiv.org/abs/0712.0977, older papers by Peldan and later papers by Krasnov et al. See also http://arxiv.org/abs/1212.5246 which explores implications for a unification of the electroweak interactions with gravity.

      7) Going beyond all these solid results are indications that the chiral fermions of the standard model may emerge from topological excitations of spin network states, http://arxiv.org/abs/hep-th/0603022.

      Clearly there is much still to do in this direction, but these are sufficient to assure us that LQG provides a framework within which to describe matter and gauge fields coupled to quantum gravity which can suggest hypotheses as to further unification.

      Lee
     
    Last edited by a moderator: May 7, 2017
  17. Oct 28, 2014 #16

    marcus

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    The analogies with GR and QM may be more pronounced than you suggest. You mention a MISCELLANY of significant commercial applications (e.g. transistors) and implications (e.g. antimatter exists, speed limited by growing inertia) which impress astonish fascinate normally curious people.
    Most of the things you mention are things we do not actually SEE in everyday life. We see transistors but we do not actually see quantum tunneling or antimatter or nuclear fission.

    "(9) pure energy can give rise to matter and routinely does..." The reverse would be a possible application of QG. We suspect that very small black holes could convert ordinary matter (like sand or water) completely into pure energy. This is a QG process and small BH are QG systems par excellence. But we don't really understand BH do we? There are a lot of puzzles still. So if you want to imagine a commercial application.

    Actually it's hard to foresee commercial applications for really new theoretical developments and it WOULD HAVE BEEN hard in the cases of 1915 GR and 1925 QM. The motivation of the original researchers is probably more likely to have been philosophical, that is "to tie up conceptual loose ends." To understand geometry better, or time, or inertia, or energy, or atoms, or the thermodynamics of light....

    The comparison with the two major developments (GR and QM) of the 20th century is apt. As I suggested, the original motivation for them was, more likely than not, philosophical---to understand the concepts better, in effect "tie up loose ends".

    So what about the problem of "now" and the passage of time? Is our experience of the specialness of the present not an effect of quantum space-time geometry? Classical theories only give us a Block Universe worldview in which time does not pass and there is no representation of the present moment. which we constantly experience.

    For discussion's sake, I propose that spacetime is constantly being made from the interactions of Planck-scale quanta of geometry. And it does not exist for us because (since we cannot interact with it) we cannot experience it. It is the past--something we know about but do not experience. We do experience the present---the interactions of quanta of geometry and matter which are forming the past, as we speak : ^) this is a QG phenomenon.
    The present moment is a QG phenomenon which we constantly experience and which to understand is one of the jobs of LQG and other theoretical attempts at QG.

    Seriously Ohwilleke, you toss out a miscellany of commercial applications on one hand and impressive fascinating IMPLICATIONS on the other. I think it is not hard to imagine, in the case of QG (i.e. quantum space-time geometry and how it interacts with quantum matter) applications and implications that are just as fascinating and impressive as those of 1915 GR and 1925 QM.
    : ^)
     
    Last edited: Oct 28, 2014
  18. Oct 28, 2014 #17

    marcus

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    It occurs to me to suspect that when you said "Am I missing something, or...?" you were playing "duffle's advocate" in this thread,
    and were hoping that your challenge would provoke just the sort of responses that Demystifier, others, and I have been offering. ; ^))
     
  19. Oct 29, 2014 #18

    Demystifier

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    Marcus, what is a "duffle's advocate"? oo)
     
  20. Oct 29, 2014 #19

    marcus

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  21. Oct 29, 2014 #20

    MathematicalPhysicist

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    Don't let the people that invest the money for this research know that there's nothing spectacular about it...
     
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