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Why it is necessary to unite Gravity and other forces

  1. May 31, 2014 #1
    Our latest understanding of gravity is based on Einstein's general theory of relativity which says gravity is geometry not a force and matter curves the space around it. So why it is hard to find or believe that elementary particles can also do so. If they can't do so, why can't it be just that a system of particles curves space. And I know that gravity is considered as one of the four fundamental forces. And why it is a force while it is just a consequence of the curvature of space and it does not show any push-pull mechanism just like Newtonian gravity, which is quite convincing to think gravity as a force.
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  3. May 31, 2014 #2


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    Newtonian (classical) physics and GR take differing views of gravity. In limited circumstances, Newton works just fine but it fails eventually. GR ALSO fails, but far beyond where Newton fails. SO ... it would be nice to have a theory that didn't fail at all, don't you think?

    That theory will probably cover all the forces in some comprehensive way.

    Is is NECESSARY to have such a theory? Well, only if you want to really understand how the universe works. If you don't care, then it isn't necessary for you.
  4. May 31, 2014 #3


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    There's really no concrete reason, except perhaps a purely aesthetic one, that the forces should be united. The only indication we have of uniting forces is history. First with the uniting of the electric and magnetic forces through Maxwell, and then through the uniting of the weak and electromagnetic forces through the electroweak model. The next progression that people think of is to unite the strong force with the electroweak force to create a GUT (grand unified theory). But over 30 years of development have not given fruit to something more accurate than the standard model.

    That gravity should be finally united with a GUT is even one step farther than a GUT, and we don't know if this is possible or if this is how nature works.

    However, there is a separate issue of quantum gravity, which is separate from uniting all the forces. This, we would like to have, because we don't know how to make gravity work in the quantum scales. This is beyond current experimental capabilities; however, so it's really a theoretical problem at this point.
  5. May 31, 2014 #4
    So why can't an electron curve space?
  6. May 31, 2014 #5


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    There are a handful of convincing arguments as to why one would need to quantize gravity [1][2][3]. That being said there are still other approaches to gravity being taken; however these are definitely in the minority. Historically it started with QFT in curved space-time wherein one has the usual quantum fields propagating on a classical curved background and defined Einstein's equation in terms of a classical metric tensor equal to the expectation value of the quantized stress-energy tensor. However this eventually was not deemed an entirely satisfactory way of handling gravity with regards to quantum fields. Even in light of the arguments in favor of quantizing gravity, one also has approaches such as emergent gravity and more specifically entropic gravity in which, in a very loose sense, gravity emerges as a macroscopic result of more fundamental statistical microscopic phenomena sort of like how the thermodynamic coordinates, pressure and temperature, emerge out of statistical distributions [4].

    [1] http://tigger.uic.edu/~huggett/Nick/My Work_files/why.pdf
    [2] http://faculty.georgetown.edu/jmm67/papers/IsGravityNecessarilyQuantized.proofs.pdf
    [3] http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.47.979
    [4] http://arxiv.org/pdf/0911.5004v2.pdf
  7. May 31, 2014 #6
    Alright. Thanks everybody for replying to this topic and sharing your perspective and valuable info. I'll go deep in everybody's reply.
  8. May 31, 2014 #7


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    Quantization of gravity, and the unification of gravity with the other forces, are two different issues.

    Gravity can already be quantized to the point where it could be a source for observations made by Planck or BICEP2. Eg. http://motls.blogspot.com/2014/03/bicep2-primordial-gravitational-waves.html

    However, this quantization fails at high energies. In this respect, gravity is not different from quantum electrodynamics, which also fails (perturbatively) at high energies. There are two potential approaches to curing this failure at high energies. The first is to see if asymtotic safety can be made to hold for them. If asymptotic safety holds, then unification of gravity with other forces may not be needed. However, another possibility is that new degrees of freedom are important at high energies (ie. unification), and gravity is only a low energy manifestation of one aspect of these new degrees of freedom.
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