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Question about Gravity

  1. Sep 30, 2011 #1
    hey, just have a general question about gravity; hopefully it isn't too basic a question. Also, please excuse the crudeness of my descriptions.

    Am I right in saying that there are 4 main "forces" in the universe; the weak & strong atomic forces; electromagnetism; and gravity? Am I also right in saying that the weak and strong atomic force, as well as electromagnetism, are seen to be, in some resepct at least, manifestations of the same force?

    I'm just wondering is gravity seen as a manifestation of this same force?
  2. jcsd
  3. Sep 30, 2011 #2
    Yes, there are 4 fundamental interactions.

    Yes, it's the GUT force (see http://en.wikipedia.org/wiki/Grand_Unified_Theory)

    No, gravity is not included into GUT. Scientists hope that GUT and gravity are manifestations of a common interaction but the so called theory of everything is still under construction.
  4. Sep 30, 2011 #3
    cheers for that. I was familiar with GUT alright, but wasn't aware of whether gravity was seen, or even hoped to be, a manifestation of a common interaction; it seems pretty logical that it is - although I know that is not how science works!

    is the theory of everything the attempt to unify Quantum Mechanics and Relativity?
  5. Sep 30, 2011 #4
    Is gravity seen as being an electromagnetic force according to relativity?
  6. Sep 30, 2011 #5


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  7. Sep 30, 2011 #6
    cheers cepheid & dr.stupid.

    how is gravity seen according to relativity?

    I know the analogy of the bowling ball on the rubber sheet, but is there a reason why it isn't seen as a manifestation of the same force?
  8. Sep 30, 2011 #7


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    Nope. In Relativity gravity is a consequence of spacetime geometry. In this respect it is quite different from the other forces which are mediated by the exchange of particles (quantum theory).

    This is one of the reasons why trying to unify quantum theory with General Relativity into a Grand Unified Theory is such a difficult problem.
  9. Sep 30, 2011 #8


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    General Relativity (GR) describes gravity not as a force, but rather as a result of the curvature of spacetime. The idea in GR is that mass/energy curves spacetime, and the curvature of spacetime tells objects how to move. So, freely-falling objects, which classically we refer to as "accelerating" under the influence of some "force" are, in fact, just following the most direct path through spacetime that they can. These paths that the particles "naturally" follow are referred to as geodesics.

    To my mind, this seems to set gravity apart as being fundamentally different from the other three interactions, all of which seem to be explained in particle physics as being mediated by other particles that get exchanged between the two particles that are doing the interacting. However, I admit that my knowledge of particle physics is lacking.

    In situations/regimes where both quantum mechanics and general relativity are applicable (such as in a black hole, or in the very early universe), physics tends to break down, and these two theories become irreconcilable or mutually inconsistent. We need a quantum theory of gravity that unifies or what general relativity says about gravity with what quantum theory says about the rest of the fundamental interactions in nature. That's my understanding of the situation.

    EDIT: beaten by gneill! At least we are saying the same thing. That is always a good sign...
  10. Sep 30, 2011 #9
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