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Questions tagged [quantum-gravity]

Any of the various explanations of gravity as a quantum theory, including string theory and loop quantum gravity.

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What's wrong with lattice quantum gravity?

Assume one can write the metric field on a lattice, so on each lattice point one has a value of $g^{\mu\nu}$. Similar to the way lattice QCD is formulated. Then later taking the distance between ...
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1answer
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What are the pros and cons of Causal Dynamical Triangulation (CDT)?

I understand that Causal Dynamical Triangulation (CDT) is a theory of quantum gravity, but i don’t hear it talked about nearly as often as say String Theory or Loop Quantum Gravity. What are it’s pros ...
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Time is the potential?

Perhaps time can be expressed as $$ t = \frac {Gh} {c ^ 4} \int \frac {dS} {r} $$ Where S is the entropy of entanglement of an arbitrary closed surface. r is the radius to the surface point. ...
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Can a no boundary proposal work without the big bang?

In Hawking's no boundary proposal he assumed that the the amplitude for a state A (such as a given 3-manifold) is the sum of all amplitudes for histories starting at a special big-bang-state (like the ...
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Does quantum cosmology work for open spacetime manifolds?

Hawkings no boundary proposal assumes a closed spacetime. But why would it not work with a infinite spacetime? With instead if periodic boundary conditions you might have asymptotic boundary ...
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27 views

Towards a quantum maximal supergravity?

Is there any evidence of ultraviolet quantumness and well defined behaviour in eleven dimensional supergravity? Of course, since it is one limit of M-theory, I see no good review of this topic in the ...
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1answer
44 views

Could the gravitational field be a cloud of particles not curved spacetime? [closed]

The more I've been looking into quantum gravity the more I've been coming to the conclusion that the graviton must be some sort of particle within a cloud of identical particles surrounding massive ...
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If gravity is a pseudoforce in general relativity, then why is a graviton necessary?

As far as I’m aware, gravity in general relativity arises from the curvature of spacetime and is equivalent to an accelerated reference frame. Objects accelerating in a gravitational field are in fact ...
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Does the Dirac sea have any mass or gravitational effect?

Dirac sea is a model for vacuum which considers the empty space as a sea full of negative-energy particles. Anti-particles are holes in this sea. Dirac sea is used to model Quantum field theory in the ...
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1answer
354 views

Is quantizing acceleration equivalent to quantizing gravity?

It's been said that gravity is locally equivalent to acceleration. It is also been said, that we need to quantize gravity. But doesn't that imply that this would have to be equivalent to quantizing ...
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What is so special about strings in string theory? [duplicate]

I mean to say that why have we kept ourselves stuck at strings. If strings explained stuff that particles couldn't a membrane explain more physical phenomena? I have just started learning about string ...
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Gravity-Induced Quantum State Reduction

Roger Penrose (along with two other co-authors) recently released a paper on the arXiv which proposes a test of quantum gravity with a Bose-Einstein condensate acting as a very massive 'particle'. The ...
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1answer
97 views

How to fuse quantum mechanics and general relativity?

I am very new to this topic but I have started reading Kevin Wray's lecture notes about string theory (PDF) and in the introduction he says: "Sometimes it is said that we don’t understand how to ...
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Question regarding the degeneracy of vacuum state in various spacetimes

In a talk, it was mentioned that if one attempts to do quantum gravity in the following spacetimes (of any dimension), then the vacuum state has the following degeneracy. In $AdS$ - There is a ...
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1answer
60 views

Emergent physics in the gap between the Planck scale and the “size” of quarks [closed]

The smallest probed length scale to date is around $\Delta x \approx 10^{-18} m$ (see here). If we compare this to the Planck scale of $\Delta x \approx 10^{-35} m$ we are left with almost 20 orders ...
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Why can't the Schwarzschild radius be larger than the Compton wavelength?

In the second paragraph of the third page of this paper, the author makes a statement The dynamical process must now involve, not the length scale $R_{s}$, but rather the universal length $L_{CS}$....
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Boundary bulk intuitive comparison between Planckian black holes and QCD jets

One of the key intuitions that we derive from trying to extrapolate QFT to the Planck scale is that when one tries to resolve features at those scales, you only get a larger uncertainty as your ...
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Why high energy physicists study complexity? [closed]

Recently, studying complexity is a trend in high energy physics. I have read there are two motivations behind these studies. First, Einstein-Rosen bridge behind the horizon keeps evolving even after ...
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28 views

In Causal Set Theory what constitutes a final state?

Causal Set Theory consists of a set of events and a partial time ordering. But what would constitute a final state? If you took as a final state simply a set of events which have no time-ordering, ...
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$\rho-3P$ for ultra-relativistic regime in early universe

I am trying to read this paper, in which they try to get dark energy by modifying the EFEs to its unimodular form. As interesting as it may be for someone here, I'm struggling to understand the ...
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1answer
75 views

Quantum state of a black hole

On page 8 of the paper "Soft Hair on Black Holes" the authors say: Let $|M\rangle$ denote the incoming quantum state of a black hole defined on $\mathcal{H}$. We take it to be formed with neutral ...
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1answer
154 views

Can LIGO be explained in terms of gravitons?

If electromagnetic waves from a star are so faint, all that can be detected are single photons on a photographic plate. For the LIGO experiment, the gravitational waves were so weak, I would have ...
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0answers
58 views

Do these particles actually reach future null infinity, and if not where they end up?

In this paper by analogy with a process $e^+e^-\to e^+e^-$ Andrew Strominger proposes that in black hole evaporation there is creation of soft photons and gravitons. This can be argued by infrared ...
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Why any theory of spacetime atoms would be wrong from start? What about causal sets?

After watching this public talk by Nina Arkani-Hamed titled "The doom of spacetime", I can't fully understand what he said in the answer to an audience question about gravity. He said we can't simply ...
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37 views

Could every particle encode the distances to all other particles?

I was thinking about quantum gravity and pre-geometry and wondering this question: "If space does not exist. How does a particle know how far away it is from another particle?" i.e. there are no ...
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Paper similar to Loop quantum gravity (Living review) by Rovelli but for string theory

So basically I want to know whether there exist any paper similar to the above mentioned one but for string theory which discusses the central achievement, main idea and open problems of the theory. ...
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0answers
87 views

Spacetime geometry of black hole formed by the scattering of two gravitons

In the paper "Breakdown of predictability in gravitational collapse" Stephen Hawking mentions the following on section 5: Consider, for example, the scattering of two gravitons. In this case the ...
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1answer
122 views

In string theory, what replaces the black hole singularity?

In string theory, what replaces the black hole singularity? I know that it is possible for a single string to have arbitrarily large energy. Is a black hole just a single string?
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1answer
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Constructing Gravitational waves with gravitons

Suppose I want to construct a gravitational wave as a coherent sum of many gravitons. It's easy to think of what the frequency distribution of the gravitons should be, as all the LIGO discoveries more ...
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1answer
62 views

Is there any threat to the results of our effective field theories from unknown higher energy theories?

We use renormalization arguments (and experiments) to change the couplings of a theory and suppress the higher energy physics (saying things like “whatever the fundamental theory, this will be true of ...
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1answer
66 views

Faster than light in quantum gravity?

Imagine there's two objects a light second apart in a space with a certain metric. So no signal can reach the other under a second. But in quantum gravity where we sum over metrics, there may be a ...
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92 views

Does the existence of graviton contradict gravity being spacetime curvature? [duplicate]

Is gravity a property-curvature of space-time it's self as descriped in GR? Or the notion of 'graviton' is necessary in order to embed the 'classical GR theory' to the quantum's mechanics 'world' ...
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1answer
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In theoretical physics, is the Graviton related to the fundamentals behind the $F = ma$ relationship?

If the Graviton transmits the gravitational force to nuclei within atoms, and an object with mass gains energy by being accelerated (change velocity / change time), does that imply that $F = ma$ is ...
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1answer
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What happens to time component of metric in Wheeler-de-Witt equation?

In the Wheeler-DeWitt equation, space-time is "foliated" and the metric $g_{\mu\nu}$ is decomposed into a metric on the surface of a 3D slice of space-time. The Wheeler-DeWitt equation is then written ...
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1answer
57 views

The theater, the actors and the 'graviton' [duplicate]

In my perception of the universe, there's the theater which is the 'spacetime' and the actors meaning the 'particles'. If i got it right, GR claims that the 'actors' effect the 'theatre' by 'bending' ...
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Correlation function late time decay and information loss

In the perturbative treatment of, let's say, a scalar field theory on AdS spacetime, the correlation functions decay exponentially at late times, indicating, as people say, information loss. I guess ...
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1answer
200 views

String theory and one idea of “quantum structure of spacetime”

First of all, I do recognize that I haven't studied string theory up to this point. I'm actualy just getting started with it. So my question here is as follows: Einstein's theory of General ...
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How can Asymptotic Safety solve its problem with Gribov Ambiguity?

Jacques Distler clearly explains how asymptotic safety (AS) suffers from the Gribov Ambiguity problem. https://golem.ph.utexas.edu/~distler/blog/archives/002833.html What can be done to solve this ...
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1answer
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Anti matter of gravity [duplicate]

We all know that the fundamental particles to be controlling the electric and magnetic forces in our universe are obtained in pairs(electrons and protons, north pole and south pole.) Likewise I guess ...
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0answers
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Non-local field theory (of Jaffe type) vs strings

As it is well known expectation values of fields are distributions. One usually works with tempered distributions or even with Jaffe fields (https://doi.org/10.1103/PhysRev.158.1454), all defining ...
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Is the Causal Set Theory a QFT of gravity?

It's hard for me to tell if the Causal set theory is a quantum field theory of QG. There really isn't that much information on the subject.
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1answer
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How do “gravitons” work? [closed]

I believe that there are no "gravitons". A mass warps SpaceTime. Anything moving past the mass has it's direction changed by that warping not by the mass. "Freeze" SpaceTime & remove the mass &...
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2answers
107 views

Actuality of black hole singularity

With the completion of finite theory of physics, even if the theoretical black hole singularity is completely eliminated, how can we be sure that it does not exist in nature? Physics theory is ...
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1answer
2k views

Could we have already discovered the graviton and not noticed?

Just a random, baseless question I'm throwing out there; but how do we know that we haven't discovered the graviton? How do we know we haven't just labeled it as something else? i.e. how do we know ...
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What are Tom Banks's arguments against a QFT of QG cont..? [duplicate]

What are Tom Banks's arguments against a QFT of quantum gravity? I do not feel that the answer given here includes all of his arguments. The answer given in the other question only included his ...
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0answers
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Folded strings or hidden preons as black hole interior: signals with gravitational waves?

Suppose the black hole interior (non-kerr, kerr-like,...) is made of some kind of subsubatomic stuff (e.g., folded strings or superstrings, folded branes, buches of preonic pregeometry or whatever). ...
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1answer
171 views

What are Tom Banks's arguments against a QFT of quantum gravity? [closed]

https://arxiv.org/abs/1007.4001 Pages 1-9 What are his arguments here against a QFT of QG, can someone provide a simpler explanation of these? And to what specific theories of QG do these arguments ...
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1answer
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Does asymptotic safety violate the holographic principle?

As a local QFT, is asymptotic safety compatible with the holographic principle? I read in a blog here about asymptotic safety that it’s incompatible with the holographic principle https://motls....
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Why can't QFT in curved spacetime be a valid hypothesis for quantum gravity? [duplicate]

Disclaimer: I've only learned basic QFT as part of an undergraduate course, if there's something I don't understand about this then please let me know. I've recently been learning about Dirac ...
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52 views

Fuzzy sphere of polarized D0 branes, non-abelian worldvolume action regime validity

I am studying Myer's paper "Dielectric Branes" (https://arxiv.org/abs/hep-th/9910053v2), where it is shown that a stack of D0 branes polarizes into a spherical D0-D2 bound state. The stack of $N$ D0 ...