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