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Quantum Gravity for Dummies?

  1. Jul 8, 2014 #1
    I'm watching first and second semester undergraduate level quantum mechanics on-line (MIT Open Courseware). I am enjoying the lectures, but I'm not doing the homework or the math, just getting familiar with the concepts and tools. As I go through the open course ware, I'd really like to be taking a layman level look at what is going on conceptually in the field of quantum gravity at the same time.

    I'm a retired layman science enthusiast and don't expect to achieve any great level of understanding, but I am interested in quantum gravity lately from the standpoint that I wonder if there is some physical mechanics underlying it, not just the mathematics of spacetime. The same question comes to my mind about the unexplained aspects of QM. Shouldn't there be some physical mechanics going on when states are in superposition, or to explain the so called "spookiness"?

    Also, who would recommend how a retired non-science professional (accountant) could get an idea of the current work being done on Quantum Gravity Theory, in a form of science media that a layman might be comfortable with?
  2. jcsd
  3. Jul 8, 2014 #2


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    Science Advisor

    We do have a quantum theory of gravity at low energies, usually called gravity as an effective field theory. A major achievement is that this theory has made predictions that can be tested by current technologies, such as the tensor modes that the Planck mission may observe.

    Gravity as an effective field theory: http://arxiv.org/abs/1209.3511

    Tensor fluctuations: http://arxiv.org/abs/astro-ph/0303077v1

    Gravity as an effective field theory, however, fails at high energies. Here the leading candidate is string theory, because it is known to give the right low energy gravity, it has a perturbative expansion in which each term is finite, seems capanble of producing low energy particles similar to what we see, has produced microscopic calculations of the black hole entropy, and has a good candidate via the gauge/gravity duality for a non-perturbative definition of quantum gravity in some universes (not ours). Its problems are that we don't know how to formulate all sectors of the theory non-perturbatively, and we don't know whether it gives rise to the exact low energy phenomena we see.

    Gauge/gravity duality: http://www.pma.caltech.edu/~physlab/ph10_references/scientificamerican1105-56.pdf [Broken]

    Great set of lectures for the layman by Arkani-Hamed: http://www.cornell.edu/video/playlist/nima-arkani-hamed-on-future-of-fundamental-physics

    Other prominent approaches are Asymptotic Safety and Loop Quantum Gravity.
    Last edited by a moderator: May 6, 2017
  4. Jul 8, 2014 #3


    Staff: Mentor

    Atty got the situation correct.

    Older accounts will tell you that QM and GR are not compatible. But since then a lot of work in the area of sorting out issues with renormalisation has shed new light on the problem. Basically all our theories are only valid up to a cut-off in what's called the effective field theory approach to the problem of renormalisation (the great physicist Wilson got a Nobel prize for it) - that would be QED, QCD etc. Gravity is really no different. The key thing that separates gravity from the rest is the interesting physics for QED etc occurs before the cut-off, for gravity its beyond the cut-off. Physicists really want look behind that veil. In practical terms it does nothing to help with the issue of quantum gravity, but it does keep it in perspective.

  5. Jul 9, 2014 #4
    What I get is that at low energies, effective field theory has merit, but it is only part of the picture. Is the distinction between low energy and high energy anything to do with the difference between a quiet well oiled celetrial mechanics where the stars and planets are in their places and functioning smoothly, but when stars collapse and galaxies collide, effective field theory doesn't explain or predict the results?
    I liked the CMB/Inflation link, and have been interested and have read layman level material since I started my self-study of cosmology. In my "Dummy" state, lol, I put a lot of weight on the angular anisotropy of the CMB, and especially the wide angles. But that is another topic.
    Thanks for that link because I do want to get a feel for the quantum gravity landscape; I know I don't understand the holographic idea, but it opens up some interesting considerations. I hope to get a much better background in understanding the places that aren't quite so "different" first.
    I listened to the introduction to the Hamed series and a little of the first video. The introduction mentioned Arkani Hamed's ideas about gravity other dimensions, which reminds me of something that I have come across in my personal Googling. It involved the explanation for why gravity was so weak being related to "leaking" from other dimensions, where gravity was a much stronger force. I will keep the link, and as I go through the MIT Opencourseware, there may be an opportune time to go back and watch the videos, I'm sure they will be interesting and educational.
    I took a look at the Wiki on Asymptotic Safety. What I got was it is looking at the higher energies, without perturbations. I do like perturbations though, because as a layman, it gives me some reason to explain the complex galactic structure we observe, as opposed to a perfectly smooth inflation. I doubt if that is anywhere near the point of it, but it is a new piece of the puzzle for me. As for loop Quantum Gravity, I find myself going there from time to time, looking at it as a layman.

    Thank you very much for the words and links. It has been helpful and there is a lot there for me to investigate yet.

    Let me ask a question about rules and accepted practices. Am I allowed to come back and post additional questions on this thread, as I advance through the course work, and as I try to make sense of QM as it relates to the quantum gravity scene?
    Last edited by a moderator: May 6, 2017
  6. Jul 9, 2014 #5
    Thank you for that perspective. Can you give me a few words about what the "cut-off is, and the reasoning that says gravity is beyond the cut-off. Does that mean that QED, QCD, etc. address important physics, but don't extend into quantum gravity?

    Edit: Wait, I found your response to that question here
    Last edited: Jul 9, 2014
  7. Jul 9, 2014 #6


    Staff: Mentor

    Hope it helps because as I explained in that thread mathematically renormalisation is a tough subject. It took some of the greatest mathematical physicists that ever lived to sort it out. Its right at the limit of my math ability so explaining it to others non technically is a stretch. I suspect even if you were a mathematical whiz at it the jig is up explaining it in lay terms.

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