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Unifying GR and QM?

  1. Dec 29, 2015 #1
    I am new to this forum and relatively new to learning physics itself. I have a very basic question.

    What is the motivation for looking for a unified theory of quantum gravity? Other than the aesthetics of having a unified understanding of reality, do we have any physical or mathematical reason to believe they can be/should be unified?

    Both are standing strong for almost a century of real/thought experiments with probably zero error. We have not seen any deviations in the predictions of both. I can see a small problem in gravity in flat galaxy rotation curve. But honestly that doesn't look like a GR failure. GR is better than that. :) QM is 100% good, AFAIK.

    So someone could naturally think to leave them alone as 2 seperate laws of nature.

    Given all this I am sure there must be reasons to believe in a unity. But i would really honestly appreciate if someone could explain it, mathematically or otherwise.

    Thanks for reading.
  2. jcsd
  3. Dec 29, 2015 #2


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  4. Dec 29, 2015 #3


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    Let me ask you a question right back. What theory is it that you think explains reality at the center of a black hole? (Do you think GR does it? Do you think QM does it?)
  5. Dec 29, 2015 #4


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    We don't necessarily have to have a conflict with observation in order to provide a reason to do theoretical work.

    But anyway, there is at least one quantum gravity effect that I think we have access to, and that's the fluctuations in the cosmic microwave background. There are claims that measurements are getting good enough to be sensitive to effects from inflation, and those effects require at least a restricted form of quantum gravity (semiclassical gravity) for their explanation.
  6. Dec 29, 2015 #5


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    This topic comes up frequently, so the forum search will give many useful results.
    We know there are systems where both are relevant at the same time, and none of them individually can be used to describe those systems accurately. The early universe is a very prominent example, black holes are one, extremely high-energetic particle collisions are also examples.

    There should be some consistent descriptions including effects of both frameworks at the same time.
  7. Dec 30, 2015 #6
    Because human beings are curious creatures. Also, whenever we discover new physics, it usually leads to new technology. We wouldn't have our modern society if we'd decided that Newton's laws, which also worked almost perfectly for more than 100 years.
  8. Jan 25, 2016 #7

    How would this be done? I have seen the standard model Lagrangian before. Would the standard model Lagrangian arise out of some sort of symmetry breaking from this original A-field Lagrangian?
  9. Jan 25, 2016 #8


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    I don't know. I have only seen speculative remarks along these lines but no substantive work ( maybe because I don't look for it).
  10. Jan 25, 2016 #9


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    The short answer is that GR and QM are mathematically inconsistent with each other (QM does incorporate special relativity, however). For example, point particles are singularities in GR. Fortunately, GR effects are so tiny in contexts where we usually do QM, and QM effects tend to be irrelevant in lots of the places where we do GR. But, both are relevant, for example, in figuring out questions of cosmology and black holes.
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