Question about gravity

  1. I've been told three things.

    1. what we feel as gravity is really the bending of space time

    2. gravity (theoretically) has a messenger particle called a graviton

    3. at the very beginning of the big bang all the forces were condensed together into one super force.

    Either one of my ideas is wrong or i just don't understand something. If gravity is the bending of space time, how could gravity have a messenger particle and be condensed into a super force?
     
  2. jcsd
  3. No.1 idea is from general relativity and 2 is from quantum theory, there's no perfect compromise of these two theories, so it's no wonder that you cannot understand them both as one.As for no.3 I didn't hear that before, but I guess it's a untestified theory.
     
  4. jtbell

    Staff: Mentor

    The description of gravity in terms of space time curvature is of course part of general relativity, a specific theory whose predictions have been tested experimentally. So far, most of these tests have been successful. I have not heard of any that clearly contradict GR. Of course, not all the possible predictions have been tested!

    As far as I know, the description of gravity in terms of particle exchange (gravitons) is still largely speculative. There are many candidates for a theory of quantum gravity, using a variety of approaches. No specific theory of quantum gravity has made predictions that can be tested experimentally (again, as far as I know), or at least not enough of them to make it stand out from the rest of them.
     
  5. One thing is for sure. If you figure it out, you will be very famous because no one else has.
     
  6. DaveC426913

    DaveC426913 16,150
    Gold Member

    Yep. Arguably, it is one of the great unanswered questions of our time.

    To wit: How can general relativity's model of gravity as the curvature of spacetime and quantum theory's model of gravity as the transmission of a graviton particle be reconciled?
     
  7. general propositions in physics are theory. gravitons, as far as I know, are unsubstanciated theory.
     
  8. atyy

    atyy 10,100
    Science Advisor

    In some cases in classical general relativity, the bending of spacetime can be described as a gravitational wave propagating on flat spacetime - so loosely speaking, it would be a wave of spacetime curvature. In quantum theory, a particle is a wave, so the classical gravitational wave would be a state of gravitons, just like a classical electromagnetic wave is a state of photons. Or so it is hoped.
     
  9. Is "quantum theory's model of gravity as the transmission of a graviton particle" a necessity of that theory or an extension?

    The point being that if QT's model is "largely speculative" (jtbell's words) rather than a necessary consequence of the theory, then it's a bit early to be calling for reconciliation - especially if there is a raft of alternative QT gravity models.

    And, as jtbell also points out, these models suffer from the same problem as string theory, no experimental evidence nor any predictions on which experiments could be based. See blog article written by PF's own ZapperZ but note that he does not specifically mention the absence of predictions.

    cheers,

    neopolitan
     
  10. ah makes perfect sense now. My ideas were dealing with two separate theories, General relativity and quantum mechanics are totally different theories. But I've never heard of gravity as being waves in space time curvature, interesting.
     
  11. A possible unification of the two parts is the String theory. But how can this latter describe the "self-interaction"? In other words, a string is an one dimensional object and has positive mass. Then different parts of it may interact (for example) by gravity, mayn't it?

    Istvan
     
  12. All interactions in QM are via messenger particles such as the graviton.

    The model is well tested; gravitons have not.

    Much, but not all, of both GR and QT HAS been experimentally verified...parts have not and that is one reason black holes are of such interest because that is one place in the current universe where it is hoped a new combined theory can be developed...right now, neither theory works at the singularity....

    This occurs at very high temperatures. And it's not just forces but also all partricles. It's Higgs fields that theoretically breaks this supposedly symmetry among all forces and particles. An analogy be the snowflake which at freezing temperatures has an infinite variety of apparently different shapes, yet above freezing all appear merely as raindrops.
     
  13. Sorry for bumping this old thread but I have a question. Does the graviton only come into play under quantum or other extreme conditions? For example, would a bowling ball dropping on my foot need a force carrier/graviton?
     
    Last edited: Sep 14, 2009
  14. haushofer

    haushofer 969
    Science Advisor

    Gravitons enter the game as soon as you want to quantize the theory. Your bowlingball can perfectly be described by GR, and even by Newtonian gravity.

    In de EM-case you have the Maxwell equations, which are wave equations. These can be quantized, and the excitations of the EM field give you fotons.

    In the Einstein case, you can do a linear approximation, and these again give you wave equations, but this time for the metric. Again, if you want to quantize these linear equations, you get something which we call a "graviton".
     
  15. So, the graviton could explain the bowling ball but it would be overkill? It isn't neccessary but it could be explained by a graviton? The graviton is like a refinement for special conditions the way relativity is a refinement of newton's gravity? But the graviton is neccessary in some cases, but in most cases we can ignore it. Things only needed to be quanitized when they have quantum effects right? So we dont need the graviton until quantum effects are significant. Am I close?

    When you say the ball can be "perfectly" described by relativity, are you saying the graviton couldnt make it more accurate or fundamental?

    thanks.
     
  16. Everyone who said the graviton is from QM is wrong. The graviton is a masless particle that is predicted by string theory.
     

  17. What happens is that gravitons "attach" themselves to things. The more gravitons there are stuck to something, the more it will be attracted to the gravitational field. So it's not that your bowling ball needs gravitons, that would imply that it can be without gravitons which is absurd because if it were ever without gravitons it would float away.
     
  18. Thanks but I'm confused because I hear conflicting reports. This guy in the particle physics section says:

    "The first misconception is that gravitating objects emit streams of gravitons, and these streams are responsible for gravitation. This is not true. Massive objects don't emit gravitons just like electrically charged objects don't emit photons: charged objects don't glow just because they are charged."

    From this topic on gravitons.

    https://www.physicsforums.com/showthread.php?t=339160
     
  19. Ich

    Ich 1,931
    Science Advisor

    Just believe what haushofer says, and ignore ernestpworrel. Answer all your questions to haushofer's post with "yes".
     
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