What is Quantum gravity: Definition and 480 Discussions

Quantum gravity (QG) is a field of theoretical physics that seeks to describe gravity according to the principles of quantum mechanics, and where quantum effects cannot be ignored, such as in the vicinity of black holes or similar compact astrophysical objects where the effects of gravity are strong, such as neutron stars.
Three of the four fundamental forces of physics are described within the framework of quantum mechanics and quantum field theory. The current understanding of the fourth force, gravity, is based on Albert Einstein's general theory of relativity, which is formulated within the entirely different framework of classical physics. However, that description is incomplete: describing the gravitational field of a black hole in the general theory of relativity, physical quantities such as the spacetime curvature diverge at the center of the black hole.
This signals the breakdown of the general theory of relativity and the need for a theory that goes beyond general relativity into the quantum. At distances very close to the center of the black hole (closer than the Planck length), quantum fluctuations of spacetime are expected to play an important role. To describe these quantum effects a theory of quantum gravity is needed. Such a theory should allow the description to be extended closer to the center and might even allow an understanding of physics at the center of a black hole. On more formal grounds, one can argue that a classical system cannot consistently be coupled to a quantum one.The field of quantum gravity is actively developing, and theorists are exploring a variety of approaches to the problem of quantum gravity, the most popular being M-theory and loop quantum gravity.
All of these approaches aim to describe the quantum behavior of the gravitational field. This does not necessarily include unifying all fundamental interactions into a single mathematical framework. However, many approaches to quantum gravity, such as string theory, try to develop a framework that describes all fundamental forces. Such theories are often referred to as a theory of everything. Others, such as loop quantum gravity, make no such attempt; instead, they make an effort to quantize the gravitational field while it is kept separate from the other forces.
One of the difficulties of formulating a quantum gravity theory is that quantum gravitational effects only appear at length scales near the Planck scale, around 10−35 meters, a scale far smaller, and hence only accessible with far higher energies, than those currently available in high energy particle accelerators. Therefore, physicists lack experimental data which could distinguish between the competing theories which have been proposed and thus thought experiment approaches are suggested as a testing tool for these theories.

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  1. K

    I Conservation of energy in quantum gravity

    is conservation of energy present in our current theories of gravity? more specifically string theory and loop quantum gravity?
  2. Jim Kata

    I Does the graviton have to exist?

    The graviton is the helicity two particle one gets when quantizing gravity in a metric formulation. There are two reasons why I have this question. 1.) If you formulate gravity in a tetrad formulation you don't seem to have a helicity two particle just the tetrad and the connection which both...
  3. D

    A Has this experiment of "detecting" the quantum foam been conducted?

  4. K

    I Jonathan Oppenheim, "A postquantum theory of classical gravity?"

    in the news String Theory gets Competition: A New Attempt to Solve Physics' Biggest Mystery Sabine Hossenfelder based on Jonathan Oppenheim, "A postquantum theory of classical gravity?", Jonathan Oppenheim et al, "Gravitationally induced decoherence vs space-time diffusion: testing the...
  5. K

    A Are there any theorems restricting possible QG candidates?

    String theory, being such a theoretically successful theory as it is but with no experiment to single it out as THE model among alternatives, Im left to wonder, which models can be excluded? Are there essential no-go theorems regarding the physics of a theory for QG? Im aware of the...
  6. K

    I Three questions regarding QG theories

    Hi all! Im gonna try to keep it short but as detailed as possible. First of all, sorry for my lower level english, possible misunderstandings of the physics Im talking about and misusage of the forum, Im not sure if vague threads like this is allowed at all. Please feel free to just answer...
  7. K

    I Is loop quantum gravity dual to CFT?

    this paper arXiv:2310.02958 [pdf, other] What if Quantum Gravity is "just'' Quantum Information Theory? Aron C. Wall Comments: 6 pages, 2 figures. Additional references added to arxiv version Journal-ref: Proc. 28th Solvay Conf. Phys., ed. D. Gross, A. Sevrin, P. Zoller, World...
  8. Kinker

    I Does gravity cause quantum decoherence?

    Does gravity cause quantum decoherence? In the microscopic world, gravity seems to act weakly, but in the macroscopic world, it seems to act strongly. Is this the boundary between the microscopic world and the macroscopic world? So a phenomenon like quantum tunneling can occur in the microscopic...
  9. K

    I Witten’s paper: "A Note On The Canonical Formalism for Gravity" LQG

    Submitted on 16 Dec 2022 (v1), last revised 26 Jun 2023 (this version, v2)] A Note On The Canonical Formalism for Gravity Edward Witten Download PDF We describe a simple gauge-fixing that leads to a construction of a quantum Hilbert space for quantum gravity in an asymptotically Anti de...
  10. atyy

    B Quanta article about Renate Loll

    "Renate Loll has seen universes that would give Doctor Strange nightmares." https://www.quantamagazine.org/renate-loll-blends-universes-to-unlock-quantum-gravity-20230525/
  11. Bertin

    Other Choosing one's Master's thesis

    Dear all, I open this thread hoping you can give me some advice on the situation that follows. I am ending my master degree in Physics, with a strong curiosity—and almost complete ignorance of—quantum gravity. It is relevant here to let you know that the reason I am attracted to this/these...
  12. Bertin

    Studying Comparing Candidate Theories of Quantum Gravity

    Hi, you all, I have been for a couple of semesters interested in quantum gravity as a problem, but truth is I never have been properly introduced to any of the candidate theories. Actually, there are multiple candidates and I would like to compare them. The question then is the following: do...
  13. TerranIV

    B Shouldn't quantum gravity be an interaction between mass and spacetime?

    Einstein showed (via general relativity) that spacetime is curved by mass, mass moves in relation to this curvature, and that gravitation arises as secondary effect. Why then are we looking for quantum gravity as some sort of mass<->mass interaction? Aren't the fundamental interactions better...
  14. G

    I At which point is gravity inconsistent with quantum mechanics?

    I'd like to understand how gravity does not combine with quantum mechanics. At least there is no accepted theory of quantum gravity, so I assume it is not solved? I'm only starting to learn QFT and eventually GR. Maybe, someone can already outline where those theories fail to combine and comment...
  15. TheHeraclitus

    I Carlo Rovelli, loop quantum gravity and point particles

    I am reading a popular-science book Reality Is Not What It Seems by Carlo Rovelli, one of the founders of loop quantum gravity. He writes: and and So basically, space (spacetime) is just another quantum field like all the others, and the quanta of this field is the nod. Nods have volume...
  16. ohwilleke

    I Can Core Theory Be Derived From Nine Lines?

    Christoph Schiller, "From maximum force to physics in 9 lines -- and implications for quantum gravity" arXiv:2208.01038 (July 31, 2022). This paper asserts that nine propositions can be used to derive the Standard Model and GR and can point the way to quantum gravity, although he cheats a bit...
  17. H

    A Loop quantum gravity and General relativity

    Hi PFs, I am reading this paper written by carlo Rovelli: https://arxiv.org/abs/1010.1939 there are many things that i fail to understand, but i would like to begin with a simple thing. Rovelli write that: It is locally Lorentz invariant at each vertex, in the sense that the vertex amplitude...
  18. lindberg

    I Can Shape Dynamics Be Tested Against Quantum Mechanics?

    Shape Dynamics implements nicely Mach's principles. But how well does it fare when it comes to Quantum Mechanics? How can it be experimentally distinguished from other theories?
  19. bbbl67

    B Is gravitized quanta the solution rather than quantum gravity?

    Usually we hear about people working on a theory of quantum gravity, in order to avoid the singularity in the center of a black hole for example. But what if it's the other way around to some extent as well? What if it's gravity keeping quantum objects from doing their greatest reality-defying...
  20. ohwilleke

    I Is the Continuum Limit of Spin Foam Dynamics Equivalent to Massless Gravitons?

    The spin-foam approach to quantum gravity is part of the class of approaches, that also include loop quantum gravity and a variety of other methods, that sets out to quantize space-time rather than the gravitational force itself. But, according to a new paper, it turns out that "the continuum...
  21. Question69

    B Unitarity in GR+QFT: Obeying Full QG Model?

    Does unitarity have to be obeyed in a full quantum gravity model?
  22. M

    I Is quantum gravity still an immensely popular field?

    I was just wondering how much work is being done in the field of quantum gravity nowdays. Is there still a huge volume of research published on the topic? Are we closer to a "solution" nowdays than we were a few years ago? And also, what exactly would constitute a solution to such problem?
  23. A

    A Loop Quantum gravity or String theory?

    I understand that string theory has almost no testable predictions, however loop quantum gravity is an enticing candidate for only quantum gravity and it doesn't explain much of symmetry, constants, mixing angles etc in Standard model. There is obviously not enough evidence to create a full...
  24. Haorong Wu

    A Does quantizing metric fields mean quantum gravity?

    (I am not sure which forum this post belongs to. Hope someone kindly helps me move it to a proper forum.) In papers, for example, here, here, and here, the authors start from the Lagrangian for matters and gravitational fields, then Dirac's constrained canonical quantization is used. They...
  25. Haorong Wu

    I Current progress in quantum gravity?

    Hi, there. I am interested in quantum gravity. But I am not sure how to find review papers about its recent developments. I tried "quantum gravity" on google scholar. But I could not find a suitable review other than that about loop quantum gravity. I am not sure what are the other approaches to...
  26. ohwilleke

    I Rovelli on Quantum Gravity

    Rovelli points to three pieces of existing observational evidence that should guide future quantum gravity research. Bottom line: * abandon Lorentz invariance violating quantum gravity theories, * abandon supergravity and string theory, and * stop working on the anti-deSitter/conformal field...
  27. AfonsoDeAlbuquerque

    A Triangulating Hamiltonian Constraint in LQG

    Im trying to obtain regularized (and triangulated) version of Hamiltonian constraint in the LQG. However, one step remains unclear to me. I am starting with the Euclidean Hamiltonian:$$H_E=\frac{2}{\kappa} \int_\Sigma d^3 x N(x)\epsilon^{abc} \text{Tr}(F_{ab},\{A_c,V\}) $$ Now i have to...
  28. J

    A Charge conjugation in quantum gravity

    The charge associated with gravitational interactions is the mass. In the Standard Model, charge conjugation is the "flippin" of all kinds of charges (electric, color, etc). So, if we were to, say, incorporate quantum gravity in a beyond the Standard Model theory, what would the full charge...
  29. T

    A Quantum theory without classical time: quantum gravity and unification

    It came to my attention yesterday this, from my ignorant point of view, amazing paper that describes what it looks as another Theory of Everything: https://arxiv.org/abs/2110.02062 If I didnt understand incorrectly, from first principles / a pre quantum theory (Trace Dynamics, 8D octonionic...
  30. J

    A Looking for the earliest articles about Quantum Gravity

    I am looking for first articles of quantum gravity in the history which before Matyevei. What is earliest articles of quantum gravity and how can ı find them?
  31. TheHeraclitus

    I Ontology of Causal Dynamical Triangulation

    Also, what does Sabine mean in this video by the sentence: (5:51 in the video) Thank you!
  32. M

    A Weinberg's gauge-fixed quantum gravity

    In this 1965 paper by Weinberg, https://journals.aps.org/pr/abstract/10.1103/PhysRev.138.B988, he describes a quantum field theory of the graviton in a Coulomb-like fixed gauge, where the free graviton has only space-space components and is traceless. This of course makes the field dynamics...
  33. G

    I Flat arena for quantum gravity?

    In a textbook, which is not in Englisch language unfortunately, I found a passage saying that intrinsic curvature of spacetime is just a specific definition. The alternative definition is that spacetime is flat, whereas clocks and rods have variable lengths - which is just Feynman’s bug...
  34. A

    Philosophical question about Quantum Gravity

    I imagined all possible quantum gravity theories in one set, of which only one is correct, and further divide these theories into testable and non-testable. Question: how do we know if the correct theory is currently testable? If it is testable, then we need to come up with new theories until...
  35. A

    I Momentum cutoff, Lorentz violation and the vacuum state

    Hi all - related to a question I asked some time ago: If one introduces a momentum cutoff, the result in the most basic case is Lorentz violation. That is, some form of preferred frame must be introduced. I'm wondering what this does to the vacuum state? That is, how does one keep the vacuum...
  36. ProtonNeutron

    A What are the recent advancements in quantum gravity?

    I am currently doing my masters in theoretical physics and I would like for my phd to be something quantum gravity oriented. However it seems hard to find what's "hot" in the field those past few years and I only know the basics of these topics (though I believe this can be improved). What are...
  37. T

    B Problems on the making of a quantum gravity theory

    Hello. What are the problems specifically or mathematically or physically that physicists find difficulty in solving to make a theory of quantum gravity? Thank you.
  38. T

    A What happens after a theory of quantum gravity is found?

    Hello there. What will physicists do after a theory of quantum gravity is found?Will they ask, if it is found ,more questions about it and try to develop it?What other questions will they make probably?Thank you.
  39. I

    B Black Hole singularities in Quantum Gravity

    If singularities don't exist in QG then what prevents particles from just collapsing falling further until they collapse into a singularity? Is there a repulsive force in QG ? Is time infinitely stretched near a singularities? What else could be happening?
  40. Lutz-F

    B Theory of Loop Quantum Gravity

    From the german Version of Carlo Rovellis book "La realtà non è come ci appare. La struttura elementare delle cose" I have learned about the theory of Loop Quantum Gravity that space and time arise through the interactions of gravitational quanta, the space quanta have discrete volume spectra...
  41. Quantum23

    A What is gravity in the superfluid vacuum model of quantum gravity?

    It has been hypothesized by quantum field theory that the vacuum is not empty due to the energy time uncertainity relationship. Instead it is filled with a sea of virtual particles popping in and out of existence and renormalized to the observed value we see today in experiments like the casimir...
  42. ChrisVer

    A Search for literature on Quantum Gravity

    Hi all, I am looking for a pedagogical (or maybe also historical) paper that describes the attempts of quantizing gravity and the problems that appear along the lines. It is quite interesting to hear from public talks that "when we try to canonically quantize gravity as we did EM we get...
  43. ohwilleke

    I Trouble for Loop Quantum Gravity?

    A new preprint establishes that almost all models implementing loop quantum gravity violate the principle of general covariance, and can only satisfy a more limited condition. Is this the huge problem for loop quantum gravity theory that it appears to be? Or is the less strict condition that can...
  44. P

    I Necessity of Quantum Gravity given Planck scales for nuclear physics

    In the solutions (page 6, points ii) and iii)), https://ocw.mit.edu/courses/physics/8-04-quantum-physics-i-spring-2013/assignments/MIT8_04S13_ps1_sol.pdf, it is mentioned that given that the Planck mass is about 20 orders of magnitude larger than a proton and that the Planck length is about 20...
  45. K

    I Is there some agreement on properties of a theory of quantum gravity?

    Having read many times that there is no theory of quantum gravity, yet physicists at Physics Forums must have some ideas of what a theory of quantum gravity will contain. Is it allowed to discuss these questions at Physics Forums? Wikipedia does allow some current theoretical work to be...
  46. G

    How Does an Old Physicist Relearn and Gain New Skills?

    Trying to burnish some lost skills and pick up some new ones.
  47. Meruem

    B Quantum Gravity and Special Relativity

    is there a equation that links quantum gravity and special relativity?
  48. P

    Other Submission to Classical and Quantum Gravity: Awaiting Decision After 1 Month

    I submitt a paper to journal Classical and Quantum Gravity. The status "awaiting referee reports" lasted one mont. However, now , the paper is on status "awaiting decision" two weaks up to now. Is a ill signal the delayed on the status "awaiting decision"?
  49. A

    A What topics in Quantum Gravity don't require QFT or string theory?

    Full quantization of gravity is a big issue, but that's not what I'm asking here. I'm asking about quantum effects that involve any form of gravitation (Newtonian or GR) but that don't require a full quantization of GR or anything like that. Things like gravitational neutron interference or the...
  50. J

    A Which is the quantized variable in quantum gravity?

    In Quantum Mechanics, the position (or momentum) variable is quantized. I define "quantization" as promoting a variable into a probability distribution. For example, with the double slit experiment, the classical assumption that the position/path of a particle is "unique" cannot explain...