Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Graviton is out of favor

  1. Apr 16, 2004 #1

    wolram

    User Avatar
    Gold Member

    am i correct in thinking that the graviton is out of favor, along with
    other "things", that could explain gravity?
     
  2. jcsd
  3. Apr 16, 2004 #2

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    on the contrary, it is my impression that it is very much in favor
    with string theorists (who are a numerous tribe) and doubtless many others as well
    besides which, at least as an approximation to how the gravitational field behaves in common situations, it is an extremely useful analytical tool

    even more can be said in its favor, but I will leave that to other posters.

    the clearest posts about this that I can recall recently were by Haelfix
    in a thread about does string theory provide an explanation for gravity
    and Haelfix' views are reasonable and moderate (he doesnt preach one one side or the other of issues). so I will try to find them and copy them in.
    what he said was generally tolerant of the graviton, although
    not expressing a fanatical belief in it
     
    Last edited: Apr 16, 2004
  4. Apr 16, 2004 #3

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    this is from the "does string theory provide a quantum theory of gravity?" thread
    https://www.physicsforums.com/showthread.php?p=170235#post170235


    later he noted that he was restating an argument often made by others, not necessarily offering his own opinion
    https://www.physicsforums.com/showthread.php?p=171750#post171750
    there are some interesting points here which we might get into
     
    Last edited: Apr 16, 2004
  5. Apr 16, 2004 #4

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    wolram, as far as I know the "graviton" concept doesnt explain gravity any more than the einstein equation for the gravitational field

    In my view the field is at least as real a thing as a graviton!
    the equation tells how matter affects the field (the geometry)
    but does not provide a mechanism
    it says in what manner but not by what "means" (no microscopic hooks and levers)

    a "graviton" is a disturbance in the field
    and no mechanism is known by which a piece matter generates gravitons
    no mechanism, that is, which is any deeper than the original Einstein equations for the field

    so it is a false comfort, if one feels one somehow understands better having said "gravitons"---it is not more fundamental than the field and it does not explain things in any deeper way

    such is my opinion. there may be folk who passionately disagree and if so
    let us hear from them.
     
  6. Apr 16, 2004 #5
    Could you please elaborate?

    Is Haelfix saying that because manifolds are locally smooth, curvature will have negigible effect on the dynamics of the strings, and so background independence - while nice from a mathematical point of view - may not be strictly necessary?
     
  7. Apr 17, 2004 #6

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    It's something like that, I think. Better if Haelfix would elaborate than for me to try.
    I think in some of this thread he is reproducing a common argument used to justify stringy approaches, rather than stating his personal opinion.

    Anyway, I would paraphrase it much the same way you did. I would say, why bother with background independence? because it is difficult to implement and doesn't make much difference to the graviton [!]

    The picture I associate with this argument is that the graviton is bunched up in one place and small compared to the geometry, so the place where the graviton is imagined to be localized is approximately flat. It's not a terribly realistic or persuasive picture, to me, but the message it projects is that all gravitons are pretty much the same animal and cant tell the difference between one space and another, so lets just calculate everything in Minkowski space. :smile:
     
    Last edited: Apr 17, 2004
  8. Apr 17, 2004 #7

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    Last edited: Apr 17, 2004
  9. Apr 17, 2004 #8

    wolram

    User Avatar
    Gold Member

    the thing i dont understand is that, if gravity can be created by any form
    of energy, or condenced energy in the form of mass, these gravitons
    would have to be everywhere at, some constant density throghout the
    universe to give us consistent laws of gravity.
     
    Last edited: Apr 17, 2004
  10. Apr 17, 2004 #9
    Could gravitons give the universe most of its mass in the way that gluons give quarks most of theirs? A spin 2 graviton splitting into 2 spin 1 particles would be analagous to what gluons can do.
     
  11. Apr 17, 2004 #10

    selfAdjoint

    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    One difference between gluons and gravitons is that gluons are charged (with the charges of the strong force) and gravitons are not (the "charge" of gravity is mass, and gravitons are massless). So the energy in gluon-gloun interactions is much greater than that in graviton-graviton interactions. It is of course true that "gravity gravitates" but the contribution is relatively smal.
     
  12. Apr 17, 2004 #11
    If current gravity laws are wrong over big distances then it is possible in principle that the gravitational potential energy could be associated with a lot of mass -perhaps the missing mass of the universe?
     
  13. Apr 17, 2004 #12

    Haelfix

    User Avatar
    Science Advisor

    One has to be a little careful, there are some really subtle technicalities in quantum gravity that can throw your perturbation series to hell. (I think String theory has these problems too).

    You really need a metric that is asymptotically flat, and has enough killing vectors to be manageable.

    Theres a notion of in and out states, that underlies how we actually count particles in fock space (b/c the notion of a particle is very ambiguous in quantum gravity).

    What I don't know about, is how String theory's renormalizability transfers over to the graviton perturbation series (I asked that question on SpS recently). I don't know if geometry contributions are subject to these laws or not. If they are, indeed its a whole different ballgame.
     
  14. Apr 17, 2004 #13
    Yeah, it's almost done. I've had to revise my plan a little, basically to keep within the word limit and to keep it more in line with the course content. It's more about the ontology of spacetime described in quantum gravity, but a lot of that has links to the background (in)dependence of the various theories.
     
  15. Apr 18, 2004 #14

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    speaking of ontology
    there is this piquant quote from Einstein (1916)

    "...the requirement of general covariance takes away from space and time
    the last remnant of physical objectivity..."

    from "Grundlage der allgemeinen Relativitaetstheorie" Annalen der Physik vol 49 page 769 ff.

    My experience of you is you often have a completely different take on things from me so that I would be interested to know if you touch on this (kind of ontological) saying of A.E. in your paper and what you make of it and do you have a way of deflecting it or disposing of it or keeping it at bay. :smile:

    maybe we have discussed this quote before but if so I dont remember how you handled it. it is a bit radical, how he puts it.

    -------------------
    added when I saw your response:

    What you say in following post seems to me soundly reasoned, concise and clear.
    So rather than complicate things by replying to it I am simply
    applauding "in advance"----the paper you are writing about this should
    be a good one, perhaps you will post it and give us a link in case others
    want to have a look
     
    Last edited: Apr 19, 2004
  16. Apr 19, 2004 #15
    I take general covariance to remove any ontological significance from the manifold. If the values with physical meaning do not depend on which smooth manifold we take (up to diffeomorphism) then the points on the smooth manifold have no intrinsic identity. I think that we can only ascribe physical meaning to points when there is a metric describing them, since this metric represents relations that are supervenient upon a real physical entity - the gravitational field.

    So I would say that general covariance removes physical objectivity from space and time in that we can't remove space and time from matter. There is no clear distinction between the two, as there is in, say, Newtonian physics.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Graviton is out of favor
  1. Gravitons ? (Replies: 18)

  2. Absorbing gravitons (Replies: 2)

Loading...