Any of the various explanations of gravity as a quantum theory, including string theory and loop quantum gravity.
2
votes
1answer
222 views
ADM Hamiltonian formalism and Quantum gravity
is there a Hamiltonian reformultion of gravity ?=? if so if we use the usual Quantization scheme we can not we quantizy the gravity ??
in terms of a Gauge Theory with the potential $ A_{\mu}^{i} $ ...
0
votes
1answer
52 views
Can anything come out from the big bang?
If any configuration of matter can fall into a black hole and hit the singularity, and ditto for the big crunch, and there is time reversal CPT invariance, does it mean anything can pop out of the ...
7
votes
4answers
488 views
What exactly does the holographic principle say?
Does the holographic principle say given a spatially enclosing boundary satisfying the Bousso condition on expansion parameters, the log of the number of microstates in its interior is bounded by ...
1
vote
0answers
57 views
Thermal gravitational radiation and its detection
To my poor knowledge on the topic, the gravitational waves that are most likely to be detected by LIGO or other experiments do not have thermal spectrum. But I'm not certain.
I know that Hawking's ...
5
votes
1answer
168 views
How is the 'cluster decomposition principle' implemented in holographic theories?
Since holographic theories are non-local by definition, how is this principle implemented?
Naively, it seems to me it is not, at least, in some sense.
I would appreciate an explanation as simple ...
2
votes
2answers
178 views
Was Planck's constant $h$ the same when the Big Bang happened as it is today?
Was Planck's constant $h$ the same when the Big Bang happened as it is today?
Planck's constant :
$$h= 6.626068 × 10^{-34}\, m^2 kg / s,$$
$$E=n.h.\nu,$$
$$\epsilon=h.\nu$$
4
votes
2answers
181 views
What is the mechanism for fast scrambling of information by black holes?
Sekino and Susskind have argued that black holes scramble information faster than any quantum field theory in this paper. What is the mechanism for such scrambling?
4
votes
4answers
960 views
Is spacetime discrete or continuous?
Is the spacetime continuous or discrete?
Or better, is the 4-dimensional spacetime of general-relativity discrete or continuous? What if we consider additional dimensions like string theory ...
1
vote
1answer
160 views
Why are white holes the same thing as black holes in quantum gravity?
Why are white holes the same thing as black holes in quantum gravity? Their Penrose diagrams in semiclassical gravity are utterly different.
0
votes
2answers
124 views
Is the Chern-Simons integral of gauge fields over black hole singularities zero?
Suppose we have an evaporating black hole and a nonabelian Yang-Mills theory with a $\theta$ topological term. This counts the total number of instantons minus antiinstantons. Consider the total ...
1
vote
2answers
92 views
What is unification, unified interactions, or dualities between interactions?
Scientists succeeded in unifying EM with the weak force, then with the strong force to achieve the standard model. They then studied supersymmetry and GUTs that showed improved gauge coupling ...
0
votes
3answers
257 views
Laws of gravity for a universe that only consists of two objects?
So, we know that when two objects of normal matter get away from each other, the gravitational pull they feel from each other, decreases.
I wanted to see how that would work. And in my ...
5
votes
2answers
152 views
Are timelike diffeomorphisms really redundancies in description in quantum gravity?
Are timelike diffeomorphisms really redundancies in description in quantum gravity? Certainly Yang-Mills gauge transformations can be considered redundancies in description. Ditto for p-form ...
0
votes
0answers
19 views
In the Hartle-Hawking boundary condition, what should we set as the initial values for the scalar fields?
In the Hartle-Hawking boundary condition, what should we set as the initial values for the scalar fields? Fields like the Higgs assuming it's fundamental, and the inflaton fields.
1
vote
0answers
92 views
Euclidean quantum gravity and gravitational instantons
I have some questions concerning the calculations made by Hawking in the 80-90's using Euclidean (canonical) quantum gravity on gravitational instantons. Were those tunnelings only between identical ...
2
votes
3answers
124 views
Is BRST ghost number conserved in quantum gravity?
Quantum gravity needs Faddeev-Popov ghosts. Feynman showed that. Take a black hole. Hawking pair production of ghost-antighost pair. One ghost falls into the hole and hits the singularity. The ghost ...
0
votes
2answers
136 views
How do we end up with a gravity-dominant macroscopic universe from a quantum world having weakest gravity?
At quantum scale, gravity is the weakest force. Its even negligible in front of weak force, electromagnetic force, strong force.
At macroscopic scale, we see gravity everywhere. Its actually ruling ...
3
votes
1answer
138 views
Hilbert of quantum gravity: bulk $\otimes$ horizon
I was reading a paper dealing with the Hilbert of quantum gravity (or more precisely what should it look like considering what we know from QM and GR) ref: http://arxiv.org/abs/1205.2675 and the ...
-4
votes
1answer
153 views
Why did Standard Model never sense a requirement to include gravitational quantum?
Standard Model is advanced version of Quantum physics. It tried to include everything which came in the way while understanding quantum world. It even didn't bother to include even Higgs Boson which ...
4
votes
2answers
159 views
Motivation for “discretized quantum state spaces”
I know that the title of the question is rather vague, so let me clarify what I mean.
For a quantum system, the set of states has infinitely (even continuously) many extreme points, i.e. there are ...
3
votes
1answer
197 views
Is the only diffeomorphism invariant anthropic principle the final anthropic principle?
Quantum gravity is a gauge theory with the gauge symmetry spacetime diffeomorphisms. Presumably the quantum state of our universe is invariant under spacetime diffeomorphisms, including timelike ...
2
votes
3answers
347 views
What happens to matter in extremely high gravity?
Though I am a software engineer, I have bit interest in sciences as well. I was reading about black holes and I thought if there is any existing research results on How matter gets affected because of ...
3
votes
1answer
176 views
What does AdS/CFT have to say about quantum gravity in our world?
The Ads side of the AdS/CFT correspondence is a model of quantum gravity in 5 dimensional antidesitter space. What can it say about quantum gravity in our 4-spacetime dimensions? Or is it just a toy ...
-2
votes
1answer
122 views
Is existence subjective or objective? [closed]
Is existence subjective or objective? There are two observers, Alice and Bob, in a de Sitter cosmology with a positive cosmological constant. Both coast along their geodesics with some initial spatial ...
2
votes
1answer
257 views
Does a complete theory of quantum gravity require anthropic post-selection?
Does a complete theory of quantum gravity require anthropic post-selection? Certainly the black hole complimentarity and causal patch conjectures highlights the essential role of observers, at least ...
3
votes
2answers
513 views
How does String Theory predict Gravity?
Firstly, General Relativity states that Spacetime is dynamic and is consonant with the distribution of matter/energy. How does String Theory predict gravity, when it is background dependent, that is ...
0
votes
1answer
89 views
Entropy, Mass and Brane gravity
Does string theory state as vibrational entropy increases, mass increases?
Related: What is a D-brane?
Reference: Cambridge Relativity
3
votes
1answer
255 views
Set theory, category theory, realism and the recent “reality of the wavefunction” papers
I will add a better phrased question here. Do we need to consider quantum foundations to form a quantum theory of gravity? The kind of foundational question I am thinking of is expressed in the ...
6
votes
1answer
147 views
If a fundamental theory exibits e.g. a mirror symmetry, in what sense it the underlying geometry real?
Are the more recently discovered symmetries in string theory such that the theories based on mirroring geometries are absolutely the same from an observable point of view?
I have mirror symmetry ...
1
vote
1answer
176 views
Quantum gravity and relevant/irrelevant operators
I am familiar with the casual dichotomy in QFT between coupling with positive dimensions in energy implies relevant operator on one side and negative dimension implies irrelevant operator on the other ...
-5
votes
1answer
215 views
What do scientists believe about existence in dimensions? [closed]
I couldn't really think of a suitable question title, I'm not sure if it's completely related or not.
But this is as far as I know (well, I thought it all up last night and it seemed extremely ...
9
votes
3answers
442 views
Is there any quantum-gravity theory that has flat space-time and gravitons?
Many quantum-gravity theories are strongly interacting. It is not clear
if they produce the gravity as we know it at low energies. So I wonder, is there
any quantum-gravity theory that
a) is a well ...
9
votes
5answers
1k views
How to get Planck length
I know that what Planck length equals to.
The first question is, how do you get the formula
$$\ell_P~=~\sqrt\frac{\hbar G}{c^3}$$ that describes the Planck length?
The second question is, will any ...
7
votes
1answer
206 views
Area law for Entropy in Loop Quantum Gravity
In connection with the long saga of the (claimed) microscopic
calculations of the Hawking-Bekenstein entropy in (3+1) Loop Quantum Gravity (LQG) and related approaches I have the following question: ...
3
votes
1answer
163 views
Gravity and Planck scale
What is the connection between Planck's constant and gravity? Why is the Planck scale the natural scale for quantum gravity? I would have though the scale would be related to G, not h.
3
votes
2answers
494 views
Why does the force of gravity get weaker as it travels through the dimensions?
Some theories predict that the graviton exists in a dimension that we of course can't see, and that is why the force of gravity is so weak. Because by the time gravity has got from the dimension in ...
3
votes
3answers
585 views
How does gravity force get transmitted?
How does gravity force get transmitted?
It is not transmitted by particles I guess. Because if it was, then its propagation speed would be limited by the speed of light. If it is not transmitted by ...
1
vote
0answers
82 views
Canonical quantum gravity
I am looking for an introduction (textbook or paper, or if someone wouldn't mind writing a memo on it) to canonical quantum gravity, specifically the concept of minisuperspaces. I have been reading ...
1
vote
1answer
109 views
Classical black holes?
How big should the black hole be so we can consider it to be classical?
When they claim that we can not probe shorter distances than the Planck length, can it be true?
The argument says that, ...
1
vote
4answers
396 views
Why is gravity weak at the quantum level?
Why is gravity stronger than other forces at the macroscopic level, yet weaker than other forces at the quantum level? Is there an explanation?
4
votes
1answer
288 views
Interpretation of the Einstein-Hilbert action
Everyone knows the famous Einstein-Hilbert action $S_{EH} = \int d^4x \sqrt{-g} R$. I'd like to know if, after we first explicit the Ricci scalar in terms of the metric, it could be possible to ...
3
votes
3answers
192 views
Why must the excitation of closed strings in String Theory be spin-2?
In String Theory it is predicted that as a result of the closed strings we have spin-2 gravitons.
1) How do we know there must be an excitation of spin-2 particles?
2) Why does a spin-2 particle ...
7
votes
3answers
239 views
Does a UV completion of gravity necessarily need to be so drastic as String Theory or LQG?
First of all I, it is my understanding that the problems one encounters with the non-renormalizability of gravity are very similar (if not the same) as one encounters in any non-renormalizable theory. ...
2
votes
1answer
263 views
Why are there Gravitons among the modes of oscillation in String Theory?
Why are gravitons present among the modes of oscillation of the 'strings' in String Theory?
10
votes
1answer
187 views
What does the equivalence principle mean in quantum cases?
We know that electron trapped by nuclear, like the hydrogen system, is described by quantum state,and never fall to the nuclear.So is there any similar situation in the case of electron near the ...
2
votes
0answers
103 views
What results from particle collision would ensure the existence of the graviton?
I understand that particles are smashed together to try to enable us to detect some sort of graviton presence but we can't actually detect a graviton due to the fact that it 'exists' in some extra ...
5
votes
3answers
257 views
Information loss in a black hole
How does the Holographic Principle help to establish the fact that all the information is not lost in a black hole?
2
votes
1answer
24 views
Time Evolution of a Manifold Embedding
Given a smooth manifold $\mathcal{M}$ with a simplicial complex embedding $\mathsf{S}$, what specific tools or methods can be used to give an analysis of the time evolution of the manifold given some ...
4
votes
3answers
588 views
Why are there 4 Dimensions and 4 Fundamental Forces?
Is it a coincidence that there are four fundamental forces and four spacetime dimensions ? Does a universe with three spacetime dimension contain four fundamental forces? Can magnetism be realized in ...
5
votes
3answers
209 views
From the perspective of an observer inside a black hole's horizon, where does the energy for Hawking radiation come from?
Would energy be seen to "flow" to the outside of the black hole? Through what mechanism?
