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Why do we need the graviton?

  1. Mar 11, 2004 #1
    I'm under the impression that general relativity with it's curved space-time as a description of the gravitational force is commonly accepted in the physics-community.

    However there is another interpretation of the gravitational force in terms of a particle, the graviton. This seems to me as a totally different mechanism. Is it true that only one of these descriptions can be right?

    So my question is: why do we need the graviton, when we've got this highly accurate theory of general relativity?
     
  2. jcsd
  3. Mar 11, 2004 #2
    If we want a quantum mechanical view of gravity, which I, atleast, would like to see in my lifetime, we would need to talk about the quanta of the gravitational field. Hence the graviton. General relativity is a classical theory, therefore gravity on small scales is not addressed. My understanding of GR is very crude, but I think that a theory will eventually come about which reconciles the geometric interpretation of curved backgrounds with the notion of a quantum field for gravity. Are we anywhere close to having quantum gravity... I guess it depends on how optimistic you are.
    Cheers,
    Norm
     
  4. Mar 11, 2004 #3

    Monique

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    Could someone at the same time explain the difference between gravitons and Higgs bosons, and why/how they give rise to gravitational force?
     
  5. Mar 11, 2004 #4

    Janitor

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    Call my knowledge on this topic very shallow.

    The Higgs is expected to be massive, electrically neutral, and spinless.

    The graviton is expected to be massless, electrically neutral, and spin-2.
     
  6. Mar 11, 2004 #5

    Monique

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    Re: Call my knowledge on this topic very shallow.

    I was just going to ask you the 'why' of

    "The Higgs is expected to be massive, electrically neutral, and spinless.
    The graviton is expected to be massless, electrically neutral, and spin-2."

    But I guess your title will explain more :wink:
     
  7. Mar 11, 2004 #6
    There is actually a fundamental difference between the two. The graviton is the particle which mediates the gravitational force, much like the vector bosons of weak interactions and the photon of electromagnetism or the gluons of the strong force. The Higgs is thought to be the field by which particles acquire mass. The idea is that through interaction with the field all particle acquire their specific masses. This is one of the most fundamental questions in physics... why does the proton have a mass of 938.280 MeV or any other particle for that matter?
    Cheers,
    Ryan
     
  8. Mar 12, 2004 #7
    ??

    It seems quite strange to me to have two different explanations of the gravitational force. One for the large and one for the small scale where it should obey QM.

    How can a theory of a force ever be taken seriously if there is such a seperation. Obviously gravity doesn't work by both curving space-time AND exchanging virtual messenger particles like the graviton, or does it...?
     
  9. Mar 12, 2004 #8

    ZapperZ

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    Re: ??

    But we still don't know if there is a "separation". That's why this is still an on-going research work. Maybe one is a generalization of the other, very much like SR and Newtonian laws.

    Besides, it isn't that strange to have more than one different "explanations" for the same thing. QM, for instance has several different types of formulation (Matrix, Schrodinger, path integral, 2nd quantization, etc, etc...). All of them appear to look different from each other. Even classical mechanics have two separate dichotomy in approach - Newtonian "forces" and Hamiltonian/Lagrangian/Least Action Principle. They all "meet" somewhere and agree on the outcome. I wouldn't be surprised if GR and QFT meet the same way (but I wouldn't hold my breath).

    Zz.
     
  10. Mar 12, 2004 #9

    Nereid

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    No matter how one 'interprets' a physical theory, it must account for experimental results, and make predictions which, when tested, are verified. Both QFT and GR do this, to some extraordinary degrees of accuracy (in some cases).

    However, QFT and GR, as they are currently formulated, cannot BOTH be correct - there are regimes where they produce conflicting predictions. Unfortunately (or maybe fortunately :wink: ), there are no nearby black holes for us to test those conflicting predictions (and we're not about to make a black hole in the lab anytime soon :frown: ).
     
  11. Mar 12, 2004 #10
    Nereid,

    Could you maybe comment on where they give conflicting predictions? Or direct to somewhere it has already been discussed?
    Thanks,
    Norm
     
  12. Mar 12, 2004 #11

    Nereid

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    I'll see what I can dig up, in detail. For now, Greene's "Elegant Universe" has a chapter or two which talk about this, and I'm sure many PF members - especially those who post to Strings&LQG - can contribute too.
     
  13. Mar 12, 2004 #12

    Haelfix

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    The graviton naively does produce general relativity.

    If you take a course in field theory, you can heuristically show that a spin 2 particle will output something that looks like Einsteins field eqns (up to a troubling constant).

    In essence, you will show that a graviton will output something known as the tetrad formulation of GR, a highly linearized version of GR.

    The problem is, this theory is nonrenormalizable... Hence the need for String Theory or something else.

    For a quick, simple and nontechnical (for field theory) look at this, I suggest reading A. Zee 'Quantum field theory in a nutshell'
     
  14. Mar 12, 2004 #13

    Monique

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    I'm going to attend a lecture by Brian Greene next week
    Maybe he'll go into this issue..
     
  15. Mar 12, 2004 #14

    Janitor

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    Last week I noticed Greene's "Fabric" book at a general-purpose warehouse store for $16.95. Has anybody here read that one?
     
  16. Mar 13, 2004 #15

    Monique

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    I've heard mixed reviews: most people complain it is too general, too many analogies, repetitive compared w/ his previous book and other authors in the field. But still a very good read, especially when you haven't read his previous book in a long time (as I have :P).

    I've recently ordered it, so I'll find out soon what new things he had to say :D
     
  17. Mar 13, 2004 #16

    Janitor

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    Monique,

    Thank you for the information.
     
  18. Mar 13, 2004 #17

    Monique

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    Are you still going to buy the book? I think you should. People were disappointed because they wanted a physics/mathematic rigorous book, instead they got an interesting read w/ many inspiring and identifiable analogies..

    What Brian himself says about his two books:
     
  19. Apr 1, 2004 #18
    General relativity does describe the nature of spacetime and the resulting gravitational effects quite well, though you have to keep in mind that any theory (except by definition a fundamental theory from which all else follows) has its regime of applicability, beyond which it will give nonsense results.
    Take as the prototype: Newtonian vs. Qauntum mechanics. QM is supposed to be the more fundamental theory, which it is, from which Newtonian physics should arise, which it does. No one would doubt (except cranks perhaps) that Newtonian physics works fine for everyday "human" scales...and even though QM is supposed to be more fundamental theory, this doesn't mean we go out and apply it to describe planes in flight. The reasons: (1) Describing the system in terms of the consituents is too complicated...also, talking about the protons, neutrons, and electrons making up the plane and surrounding air in motion is not very useful to describe the macroscopic object. (2) Newtonian mechanics is a very good approximation when talking about a collective object like a plane, so using QM and talking about all the particles making up the plane would be overkill in accuracy anyway.
    The same discussion applies to general relativity, where this now plays the role of the effective theory describing macroscopic scales quite nicely. However, the theory does not apply below a certain distance scale: for example, it can't handle extreme spacetime curvatures well (singularities are the signal here). Therefore, it is commonly believed in the physics community that there is a more fundamental theory describing the nature of spacetime, and that general relativity should emerge from it at the distance scales where it is a good approximation. In addition, spacetime is dynamical, and we would expect quantum mechanics to apply to it as well...why should only electrons, light, and everything else *other than* spacetime behave fundamentally qauntum mechanically?
    Gravitons arise as a description that tries to mirror what was done for *all the other known constituents of the universe*: by using quantum field theory. The idea here is that gravitons arise as quantum excitations of spacetime, but in a way that is still an approximation: we consider only fluctuations of spacetime about a given background spacetime that is smooth down to the smallest distance scales. There are arguments that the reason this description doesn't work as a truly fundamental theory of spacetime is that spacetime structure is *not smooth* down to all distance scales...that the geometry of spacetime is quantized...it, too, has fundamental constituents!
    Some physicists work on such an attempt at quantizing spacetime starting with the classic general relativistic theory. Gravitons would arise as a more macroscopic (albeit a tiny macroscopic) approximation to what's really going on. This approach is called "Qauntum Geometry" to distinguish it from any other qauntum gravity theories.
    A larger group of physicists work in string theory, which is any possible way to find a fundamental theory of spacetime (and with it, everything else tied together!). More ambitious, no? This is the camp that gets more attention...like Nova specials and magazine articles.
    In the end, one must be careful to remember that neither of these paths (quantum geometry and string theory) have any testable predictions yet, and so they are hypotheses for the way things are...so talking about gravitons and strings as if they are known to exist is misleading. The Standard Model of particle physics, on the other hand, has been (and continues to be) tested and though it is still not the most fundamental description of things, it is a beautiful thing.
    On that note, good night.
     
  20. Apr 1, 2004 #19

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    Javier, you are on a roll!

    I got home today and found several interesting and informative posts by you. You wrote, "neither of these paths... have any testable predictions yet..."

    Do you think it is possible that some day some approach to gravity may actually allow us to derive Newton's gravitational constant G as the (macroscopic at least) coupling constant? So far, are theorists inserting G into their theories as a given parameter instead of trying to allow it to come to the surface naturally, so to speak?
     
  21. Apr 2, 2004 #20
    Good question

    So far, it is not clear why the value of Newton's constant (and the speed of light, etc) is what it is. In "fundamental theories", there are a very few (maybe one or hopefully someday zero) free parameters, and things like Newton's constant can be expressed in terms of them...but we leave them free to see if there is a mechanism that fixes these parameters such that G=what it should.
    There have been similar issues explored in the successful standard model of the weak and electromagnetic unification: masses and couplings of things we detect are determined in terms of a fewer number of more fundamental couplings. However, in the Standard model of particle physics, there are still a relatively large number of parameters whose values remain unexplained. In models like GUTs, where a unification of strong with electroweak interactions is attempted, the values of couplings and masses are determined by yet a smaller set of parameters (the couplings and masses would be determined via group theory: when a gauge group breaks into a number of other groups, like SU(5)-->SU(3)xSU(2)xU(1), the new fields and interactions are given masses and couplings dictated by the group theory of how the decomposition occurs).
    In my opinion, Newton's constant will be determined in terms of a more fundamental theory...but there is no guarantee!
     
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