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Newb graviton question

  1. Jul 6, 2005 #1
    Would a graviton theory replace the curved geometry of general relativity? If gravitons are the cause of the gravitational force, does that mean space and time aren't really curved as Einstein thought?.. :confused:

    And maybe an unrelated question, is it better to think of curved space-time as a model that explains the world very well, rather than the way the world actually is?
     
    Last edited: Jul 6, 2005
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  3. Jul 6, 2005 #2

    Haelfix

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    No, the two theories are equivalent, at least in the linearized form and too first order. Its one of the great oddities/mysteries actually, a quantum theory of a spin 2 particle reproduces exactly (minus a tiny observationally negligable discrepancy) the same equations as General relativity. Its quite beautiful really, the overlap of the geometry within gauge fields and the spacetime geometry of Einstein.

    However there are higher order quantum radiative corrections, and they are testable in principle (but not in practise), and there are rather deep conceptual questions within the field (like conformal anomalies, global topological problems, measurement problems, back reactions and the like) that have never really been understood. Supersymmetry was added to the mix in the eighties, and theorists almost thought they had the whole shebang solved, but then a rather unfortunate high order term proved that the theory was nonrenormalizable. That was then one of the principle motivations of string theory, as their formalism cured that divergence, otoh that theory has problems of its own, like excessive amounts of degrees of freedom and it gets really messy.

    Its a strange situation, and quite amazing that one can get so close yet remain so far away.
     
  4. Jul 6, 2005 #3

    marcus

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    better to think of the world really being curved spacetime (the model used to talk about stellar collapse, observed kind of black holes, big bang, inflation scenarios, dark energy/accelerated expansion etc.)

    I think it is is better to think of the "graviton" approximation as something that is typically useful in static almost linear situations. Haelfix indicates where the graviton approximation is applicable:

    at least, that is, where there is not too great a concentration of matter, or too great fluctuation in curvature. Then you can approximate spacetime by a fixed 4D spacetime like e.g. minkowski flat (which is the spacetime associated with zero matter and zero curvature) and you superimpose on top of that a little ripple or perturbation. To FIRST ORDER that gives an approximation of the real thing, namely your basic curved spacetime.

    you are approximating a dynamically curved thing by a static typically flat thing that you have added a little ripple or bump onto.

    this is called a "perturbative" approach (take a fixed standardized typically flat thing and perturb it slightly, calculate first order effects)

    conventional wisdom is that perturbative approach doesnt apply in regimes of high curvature and highly changeable geometry. It is a marvel (as Haelfix notes) that it works at all, even in the comparatively flat cases.


    No, I dont think so. It is a marvel that the graviton picture works so well as a perturbative first-order approximation of reality.

    Personally I doubt it is helpful to think of "gravitational force" bending lightrays as they pass by the sun by means of clouds of gravitons whizzing back and forth. It is more like those are just the geodesics in our curved spacetime. Nor does it help me to think of a black hole attracting another black hole by the two singularities sending clouds of gravitons whizzing back and forth between them---how do the gravitons escape from one to get to the other etc etc.

    So I consider gravitons as marvelous wonderful approximations in the static typically flat cases they apply, and as elegant mathematical constructs. But in answer to your question they are not the fundamental " cause of gravitational force" and NO it does not mean that "space and time aren't really curved as Einstein thought?"

    I'm following current research in non-perturbative quantum gravity, which has been making important progress recently. In the nonperturb. approaches to QG you do not use gravitons, you basically randomize the geometry of spacetime. so you have a quantum (curved) spacetime.
    Non-perturbative quantum gravity typically means the kind of stuff at the Loops 05 conference
    http://loops05.aei.mpg.de/
    the graphics there can give some idea

    if you do quantum gravity these days then most likely you think of spacetime as curved----only the curvature is uncertain as quantum things usually are.
     
    Last edited: Jul 6, 2005
  5. Jul 6, 2005 #4

    ohwilleke

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    There is considerably dispute in the quantum gravity community over whether space-time itself is discrete, or whether only a quantum particle like a graviton should be used to model the universe.

    Also, I'm inclined to think that FIRST ORDER and linearized is a bit of an understatement. Newtonian gravity gets you there.
     
  6. Jul 6, 2005 #5

    marcus

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    hi Ohwilleke!

    let's focus on the nonperturbative QG community
    this conference practically speaking defines that community
    http://loops05.aei.mpg.de/

    here is the list of invited speakers
    http://loops05.aei.mpg.de/index_files/Programme.html

    you will be teaching me something I'd like very much to know if you would please tell me which of these listed researchers thinks that a
    graviton should be used to model the universe.

    you may know something about one of these people that i don't, probably you do. Please give me an arxiv link to a paper by one of them that illustrates this.

    when I read non-perturb QG papers I rarely if ever see mention of graviton (except as an approximation to variable geometry in almost flat or vacuum situations)

    I also do not see spacetime modeled as a collection of PARTICLES. I see examples of discrete structure (discrete spectra of area and volume operators) but the structure is implemented with extended objects like networks or triangulations or spinfoams or manifolds. I dont see anything in the research that I would picture as made of point-particles or wavelets in some static medium.

    You must have something "particular" in mind----some "particly" picture of spacetime in somebody's research article. I'd like to see it :smile:

    For me it's a question of what are the right words and mental images to use in discussing current nonperturbative QG directions.
     
  7. Jul 6, 2005 #6

    marcus

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    I am tryng to make sense of what you say here

    Well in Loop approach and the related spinfoam approach they use SPIN NETWORKS (a spin network extends throughout all space) and SPIN FOAMS (a spin foam is co-extensive with spacetime). These things serve to represent a state of the gravitational field, or a state of spacetime geometry. They have a certain combinatorial aspect because you can in principle describe them by listing data.

    and using spin networks leads to area and volume operators with discrete spectra.

    but I have never heard anyone describe a spin network or a spinfoam as a PARTICLE.

    I never read where Ashtekar said that a spin network was "like a graviton".

    That is just one sample approach, LQG, and one person. I am trying to see what you might mean.

    ohwilleke, if there is "considerable dispute" then does that include the non-perturbative QG crowd?

    If you would, look down the list of representative QG people, the invited speakers, and tell me who would dispute with whom, about whether spacetime should be described as made of something "like gravitons" or some kind of point particle. You may be right! I dont know the work of everybody on the list, far from it.
     
  8. Jul 6, 2005 #7

    wolram

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  9. Jul 6, 2005 #8

    marcus

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  10. Jul 6, 2005 #9

    marcus

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    If this is a newb question then newb questions can be a big help. This question gets me to see something.

    The thing that distinguishes the NonString QG people is that they treat GRAVITY AS GEOMETRY.
    For people who do geometrical quantum gravity, gravity is not some force being propagated by some particle analogous to a photon or a virtual photon which is traveling through an inert static space.

    If there were a static inert space and gravity were a force mediated by some particle, call it a graviton, then the job would be QUANTIZE THE PARTICLE.

    But for people who do geometrical quantum gravity, the job is to QUANTIZE THE GEOMETRY, make a hilbert space that describes the uncertain shape of the universe, the imperfectly known shape of spacetime.
    Because for them gravity is not a force mediated by a spray of little graviton-particles, for them gravity is geometry.
     
  11. Jul 6, 2005 #10

    marcus

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    A couple of weeks ago selfAdjoint was saying how do we describe the non-string QG people.

    It used to be we would just say "Loop Quantum Gravity" but that is no longer precise enough or general enough. there are a bunch different approaches.

    We were trying different terms out, like B.I.QG (background independent QG) because for all of those people they dont start with a fixed background geometry. the model has a changing dynamic geometry.
    And we also were saying QGATS (quantum gravity alternatives to string) because it is the non-string QG people we are trying to give a name to.

    If you want to look at who those people are and what they call themselves you just go to the Loops 05 website
    http://loops05.aei.mpg.de/

    and you see that what they say right up front is
    "....the annual international meeting on non-perturbative/background independent quantum gravity takes place...."

    they dont mean just loop LQG, or spinfoams, or causal sets, or CDT, or Laurent Freidel's stuff, or Martin Reuter stuff, or whatever.

    BUT THE NEWB WHO ASKS THE QUESTION PROBABLY DOES NOT HAVE A CLEAR IDEA OF " non-perturbative/background independent quantum gravity".

    It suddenly dawns on me that what we should have been saying all along was GEOMETRIC quantum gravity.

    That is, the approaches to QG that take seriously the idea that gravity is geometry and you are not going to get a satisfactory theory of spacetime by fixing on a static inert spacetime and letting graviton ripplets ripple across it.

    Geometric QG (whatever the several approaches) always assumes that it is not going to work to quantize the particles or the superimposed ripples. Geometric QG takes for granted that you have to quantize the shape of spacetime itself.

    Now this is maybe something which says it in ordinary language, and communicates, the way
    "non-perturbative/background independent" does often not communicate.
     
  12. Jul 6, 2005 #11

    ohwilleke

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    I am using the word quantum gravity is a broad sense. Any theory which includes a graviton is a quantum gravity theory. LQG is also a quantum gravity theory.

    If you look at the big picture, long term history, people coming from the astro-physics direction (black holes and what not), have tended to look for quantatizing the geometry approaches, while people coming from the particle physics direct have tended to look for a spin-2 graviton.

    My point is that there is not consensus on whether geometry or a particle is the right way to go in the quantum gravity world.

    Note, incidentally, that one could have a discrete Minkowksiesque 4D space time, while still having GR gravitational effects mediated via a graviton.
     
    Last edited: Jul 6, 2005
  13. Jul 6, 2005 #12

    marcus

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    OK ellipse, now maybe I can reply.
    I wont say which is better. Some people prefer to think of a fixed spacetime with gravitons going around mediating gravity force. Some people prefer to say "gravity is geometry". Each of them thinks or talks as if their idea is more fundamental.

    I can tell you my personal opinion is that Einstein geometrical model of gravity is more precise, makes more accurate predictions, more widely applicable---so I think of it as more fundamental, more like what the world really is like. But people can have whatever opinion.

    Now when it comes to quantizing, which is the premier world-class quest in theoretical physics today, there are two groups:

    there are the STRING people who like the graviton picture of something moving in a fixed geometry, typically flat background but can be special curved cases too, and

    there are the GEOMETRIC quantum gravity people.

    The latter (maybe we can call them GQG) are especially fun to watch, in my view, because they have made extensive progress in just the past couple of years. So their conference this year is apt to be rather lively. they are going to have to discover whether they fight it out between competing GQG approaches or whether some will merge.

    keep tabs on this site
    the full program for October is supposed to be posted sometime this month
    http://loops05.aei.mpg.de/
     
  14. Jul 6, 2005 #13

    wolram

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    BY Marcus.
    I am more interested in hearing about some potential dispute among the Non-String QG people (I dont think the string approach has much future to it so what interests me is differences among the Non-String folk.)
    So its polyester for Marcus? and meat and two veg for me :biggrin:
     
  15. Jul 6, 2005 #14

    marcus

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    just thinking about meat and two veg reminds me I haven't had lunch yet.
    will be back in a while.

    maybe i will start a thread about this "geometrical QG" descriptor.

    I'm looking for a way of saying in clear ordinary language what that cluster of research lines is
    that they call "nonperturbative background independent approaches to QG"
     
  16. Jul 6, 2005 #15

    wolram

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    Marcus, please come up with something interesting and not to Mathy,to give
    some sort of incentive, for us plebs
     
  17. Jul 6, 2005 #16
    Thanks, everyone, that was very helpful. I am currently reading The Elegant Universe and thought ST was the best approach to unification so far (it sure gets a lot of publicity!), so I'm glad there are other things being actively pursued. And as far as math goes, I've just started working through MTW, so the only QM stuff I know is pop-sci. This GQG (I like that name, btw :biggrin: ) approach sounds like something I'd like better than ST, since I really love GR and the geometric approach. Are there any pop-sci books out on some of these other, non-string approaches?

    Also, is it possible that the other forces could be thought of in terms of geometry, rather than particles.. kind of like Kaluza-Klein, I suppose?
     
    Last edited: Jul 6, 2005
  18. Jul 6, 2005 #17

    marcus

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    popular-written sources on geometric QG approaches

    for LQG:
    Carlo Rovelli "Loop Quantum Gravity" article in physics world (online, will get link)
    http://cgpg.gravity.psu.edu/people/Ashtekar/articles/rovelli03.pdf
    Lee Smolin "Atoms of Space and Time" in SciAm January 2004 (not online, must find in library)
    Lee Smolin "Three Roads to Quantum Gravity" book (not online, I haven't read but am told it is a good general audience book)
    other semi-popular online articles at Abhay Ashtekar's website
    http://cgpg.gravity.psu.edu/people/Ashtekar/articles.html


    for Renate Loll's CDT:
    here is a short notice by science writer Adrian Cho from the American Physical Society's "Focus" newsmagazine
    http://focus.aps.org/story/v14/st13


    there are some links in the nearby thread "Quantum Graffiti"
    https://www.physicsforums.com/showthread.php?p=588926#post588926
    I translated an article that was in a popular Dutch periodical, into English, and posted my rough translation in the "Quantum Graffiti" thread.

    CDT (causal dynamical triangulations) is comparatively new, invented in 1998, involves having a computer assemble half a million or so triangle-like 4D building blocks in thousands of different random formations with quantum weighting. Add up all the ways spacetime can be, all possible geometries, to get a quantum weighted sum. From this, or from a statistical sampling, measure "average" geometric properties, it's a Feynmanesque path integral (add up all the ways how spacetime can get from this to that spatial configuration). CDT only last year reached the point where it could generate normal-looking 4D spacetime that at large scale on the average obey Einstein equation of GR. Microscopic behavior (at planck scale) is not classical. It is new enough that there is not much popular writing about it!


    we will keep our eyes out for more wide audience stuff on any type "GQG". there are several other approaches I didnt mention.
     
    Last edited: Jul 6, 2005
  19. Jul 6, 2005 #18
    Thanks, also, another question, just for clarification. Is it true that, in string theory, the graviton is the reason for gravity, rather than space-time curvature? Obviously, string theory does allow for space-time curvature, because the excess dimensions have to be curved in order to be so small, right? So we're not just talking about extending flat Minkowskian geometry to include extra dimensions.. But as far as gravity is concerned is it all due to the graviton, the same way the standard model attributes EM to the photon? That sounds kind of strange, because wasn't GR's approach to gravity the reason physicists began thinking of the possibility of space-time being curved? But if gravity, as far as string theory is concerned, is no longer due to space-time curvature, then the reason space-time can be curved has been replaced. It must now be curved so that string theory can include extra dimensions..

    Do any of these "GQG" theories include excess dimensions?
     
    Last edited: Jul 6, 2005
  20. Jul 7, 2005 #19

    selfAdjoint

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    The difference between (most) string theory and GR is not flat versus curved, but background dependent versus background independent. In GR, spacetime is a dynamic component of the physics, there is no "stage" upon which the physics takes place, rather the scenery transforms and is transformed by the action. By contrast string theory has strings and branes moving around in a fixed geometry. The compact dimensions are curved, yes, but they are assumed to have constant geometry - torus, Calabi-Yau manifold, or whatever, and just act as a background for the physics in the foreground.
     
  21. Jul 7, 2005 #20
    Thanks Marcus for these posts here. Always have been confused by my pop-sci books that first explain gravity as different from all other forces but then some chapters later they talk about the yet-to-discover gravitons that transmit gravity just like the bosons of all the other forces. That never made sense to me.
     
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