Any of the various explanations of gravity as a quantum theory, including string theory and loop quantum gravity.
7
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2answers
1k views
Is reality discrete at the quantum level? (…and what does it imply not only mathematically?)
On a quantum scale the smallest unit is the Planck scale, which is a discrete measure.
There several question that come to mind:
Does that mean that particles can only live in a discrete grid-like ...
1
vote
1answer
448 views
What is the smoking gun signature of string theory?
What is the smoking gun signature of string theory? Suppose we have a complete and consistent model of quantum gravity with a zero or negative cosmological constant, but all we are given is its ...
7
votes
1answer
219 views
Does the AdS/CFT correspondence for thermal states really imply time evolution for evaporating black holes is unitary?
You always hear theoreticians proudly proclaim the AdS/CFT correspondence implies time evolution for evaporating black holes is unitary. But if you examine the argument carefully, you find AdS black ...
7
votes
1answer
189 views
Scale set by cosmological constant
Following on Jim Graber's answer to: Can "big rip" rip apart an atomic nucleus?
If the cosmological constant is large enough, even the ground state of a hydrogen atom can be affected. So ...
9
votes
2answers
486 views
The Reeh-Schlieder theorem and quantum geometry
There have been some very nice discussions recently centered around the question of whether gravity and the geometry and topology of the classical world we see about us, could be phenomena which ...
2
votes
4answers
197 views
What are the conditions to be satisfied by a theory in order to be a quantum theory?
This is in continuation to my previous question. It is not a duplicate of the previous one. This question arises because of the answers and discussions in that question.
Can we call a theory, quantum ...
3
votes
1answer
564 views
Mathematical justification of Hartle-Hawking “no boundary” proposal
In Hartle-Hawking "no boundary" proposal it is proposed that Riemannian spacetimes rather than Lorentzian dominated the path integral near the big bang.
Moments after the big bang however spacetimes ...
-2
votes
1answer
538 views
What do gravitons interact with?
The other three forces' mediating particles (photons etc.) are absorbed by their appropriate charge-carrying particles, but I can't seem to find a clear answer that applies to the gravitational force ...
3
votes
2answers
567 views
What is energy in string theory?
Facts agreed on by most Physicists -
GR: One can't apply Noether's theorem to argue there is a conserved energy.
QFT: One can apply Noether's theorem to argue there is a conserved energy.
String ...
4
votes
1answer
197 views
Can T-duality resolve spacelike singularities?
Schwarzschild singularities are described by the Kantowski-Sachs metric with a contracting S2. Of course, T-duality doesn't apply to S2. But what about a Kasner-type singularity with two contracting ...
3
votes
1answer
354 views
Can PEPS explain the holographic principle in quantum gravity?
Condensed matter physicists have shown using quantum information that in many condensed matter systems, entanglement entropy only scales as the area of the boundary, and not the volume. This is the ...
7
votes
2answers
794 views
What happens when a black hole and an “anti-black-hole” collide?
Let's say we have one black hole that formed through the collapse of hydrogen gas and another that formed through the collapse of anti-hydrogen gas. What happens when they collide? Do they (1) ...
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0answers
256 views
Quantization of Gravitational Field: Quantization conditions
I'm begining to study Quantization of field with the second quantization formalism. I've studied phononic field, electromagnetic field in the vacuum and a generic relativistical scalar field.
I ...
1
vote
0answers
226 views
Are virtual photons affected by effective gravity in non-linear quantum electrodynamics?
Quantum electrodynamics based upon Euler-Heisenberg or Born-Infeld Lagrangians predict photons to move according to an effective metric, which is dependent on the background electromagnetic field in a ...
5
votes
0answers
119 views
When periodic solutions are combined with timelessness, do we get closed timelike curves?
In quantum gravity, ADM wavefunctional solutions have to satisfy the Wheeler-DeWitt equation. This leads to timelessness. What happens if we have a time periodic solution? In classical general ...
3
votes
4answers
448 views
Question regarding the validity of the big bounce
I have several questions regarding the "big bounce" theory. It appears to be popular among LQG researchers. My questions are as as follows.
1) How one reconciles it with the fact that it is now ...
3
votes
2answers
255 views
Gravitational and gauge-gravitational anomalies in N=1 D=4 supergravity coupled to a SUSY gauge theory with chiral matter
When people talk about the first superstring revolution they often mention the miraculous cancellation of anomalies via the Green Schwarz mechanism. My question is whether such a string-theoretic ...
4
votes
3answers
726 views
Give a description of M-theory your grandmother can understand
Inspired by this question, let me ask a similar question. Is it possible to do the same (give a description of M-theory your grandmother could understand)for M theory? While I know even experts don't ...
6
votes
3answers
476 views
Does Wick rotation work for quantum gravity?
Does Wick rotation work for quantum gravity? The Euclidean Einstein-Hilbert action isn't bounded from below.
4
votes
2answers
406 views
Global symmetries in quantum gravity
In several papers (including a recent one by Banks and Seiberg) people mention a "folk-theorem" about the impossibility to have global symmetries in a consistent theory of quantum gravity. I remember ...
5
votes
1answer
255 views
Why can't fermions be affected by effective gravity in non-linear quantum electrodynamics?
Quantum electrodynamics based upon Euler-Heisenberg or Born-Infeld Lagrangians predict photons
to move according to an effective metric which is dependent on the background electromagnetic
field. In ...
8
votes
5answers
454 views
How important is the cosmic censorship conjecture?
I would like to know how important the cosmic censorship conjecture is? Should a quantum theory of gravity must obey this? It was never rigorously proved in classical GR too. What would be the ...
6
votes
2answers
438 views
Does perturbation theory break down for quantum gravity?
Perturbation theory presumes we have a valid family of models over some continuous (infinitely differentiable, in fact) range for some parameters, i.e. coupling constants. We have some special values ...
7
votes
2answers
630 views
Do traversable wormholes exist as solutions to string theory?
There has been some heated debate as to whether the laws of physics allow for traversable wormholes. Some physicists claim we require exotic matter to construct wormholes, but then others counter the ...
3
votes
3answers
522 views
How could spacetime become discretised at the Planck scale?
I didn't have much luck getting a response to this question before so I have tried to reword and expand it a little:
In early 2010 I attended this inaugural lecture by string theorist- Prof. ...
9
votes
1answer
723 views
Diff(M) as a gauge group and local observables in theories with gravity
In a gauge theory like QED a gauge transformation transforms one mathematical representation of a physical system to another mathematical representation of the same system, where the two mathematical ...
5
votes
2answers
486 views
Other possible theories (other than string theory) which are generalizations of the standard model with incorporation of gravity
The only finite mathematical framework that incorporates both the standard model of particle physics and gravity under one umbrella that I am aware of is string theory. I would like to know whether ...
4
votes
1answer
236 views
What are the current (popular(ish)) approaches to modelling the quantum nature of spacetime at the Planck scale?
My guess at a list of them would be: spin foams, casual sets, non-commutative geometry, Machian theories, twistor theory or strings and membranes existing in some higher-dimensional geometry...
...
11
votes
6answers
1k views
Is the Planck length Lorentz invariant?
The planck length is defined as $l_P = \sqrt{\frac{\hbar G}{c^3}}$. So it is a combination of the constants $c, h, G$ which I believe are all Lorentz invariants. So I think the Planck length should ...
6
votes
1answer
304 views
Are there any versions of LQG that claim to not violate Lorentz symmetry?
LQG formulations have a minimum length/area. Since say, a Planck area can always
be boosted, any minimum area in space can be shrunk. Do LQG proponents worry about local Lorentz invariance violation, ...
7
votes
4answers
1k views
Can Loop Quantum Gravity connect in any way with string theory?
The one difficulty I see with LQG is that it requires an enormous number of degrees of freedom, e.g. these spin variables in the net. This is in contrast to stringy holographic theory where the ...
12
votes
3answers
780 views
Has the black hole information loss paradox been settled?
This question was triggered by a comment of Peter Shor's (he is a skeptic, it seems.) I thought that the holographic principle and AdS/CFT dealt with that, and was enough for Hawking to give John ...
-4
votes
2answers
2k views
What are the main differences between these quantum theories?
What are the main differences between these quantum theories?
Quantum Mechanics
Quantum Field theory
Quantum Gravity
EDIT:
I ask this question because when I asked a question before people talk ...
11
votes
4answers
1k views
Is there a maximum possible acceleration?
I'm thinking equivalence principle, possibilities of unbounded space-time curvature, quantum gravity ...
2
votes
5answers
434 views
Black Hole Singularities
If two black holes collide and then evaporate, do they leave behind two naked sigularities ore? If there are two, can we know how they interact?
8
votes
1answer
295 views
Summing over topologies
Sometimes in quantum cosmology, when we are thinking about 'wave functions of the universe' we have in mind some sort of formal path integral, where we include not just the variations in the dynamical ...
6
votes
1answer
632 views
Where does the $10^{500}$ estimate for the number of stringy vacua come from?
This number is thrown around a lot so I'd like to understand its origin. I know that it counts compactifications of string theories on (not only) Calabi-Yau manifolds and probably also some other ...
7
votes
2answers
314 views
Is taking the “square root” of the densitized inverse triad irregular in loop quantum gravity?
In loop quantum gravity, the canonical (Ashtekar) variables are chosen to be the densitized inverse triad $\mathbf{E}$ and some rotation connection field $\mathbf{A}$. To get the ordinary triad from ...
9
votes
4answers
870 views
Discussion of the Rovelli's paper on the black hole entropy in Loop Quantum Gravity
In a recent discussion about black holes, space_cadet provided me with the following paper of Rovelli: Black Hole Entropy from Loop Quantum Gravity which claims to derive the Bekenstein-Hawking ...
5
votes
4answers
515 views
Does string theory tells us anything concrete about black hole decay?
EDIT: I edited the question to reflect Moshe's objections. Please, look at it again.
It's apparently a black hole time around here so I decided to ask a question of my own.
After a few generic ...
8
votes
6answers
1k views
The final death of a black hole
What are the different death scenarios for a black hole? I know they can evaporate through Hawking radiation - but is there any other way? What if you just kept shoveling more and more mass and ...
9
votes
5answers
606 views
What is the role spin-networks play in Loop Quantum Gravity?
This is a second question (in what will probably become a series) in my attempt to understand LQG a little. In the previous question I asked about the general concepts behind LQG to which space_cadet ...
14
votes
4answers
2k views
Give a description of Loop Quantum Gravity your grandmother could understand
Of course, assuming your grandmother is not a theoretical physicist.
I'd like to hear the basics concepts that make LQG tick and the way it relates to the GR. I heard about spin-networks where one ...
4
votes
1answer
326 views
Quantum Gravity and Calculations of Mercury's Perihelion
In an astronomy forum that I frequent, I have been having a discussion where the state of quantum gravity research came up. I claimed that Loop Quantum Gravity theories couldn't prove GR in the ...
6
votes
1answer
1k views
What specifically does the phrase “continuum limit” mean?
I'm interested in the meaning of the phrase "continuum limit" specifically as it is often used in expressions relating to the ability of a quantum gravity theory to recover GR in the continuum limit.
...
10
votes
3answers
434 views
Hawking radiation and quark confinement
The simple picture of Hawking radiation is that a pair-antiparticle pair is produced near the event horizon, then one falls into the black hole while the other escapes. Suppose the particles are ...
2
votes
1answer
399 views
Introductions to discrete space-time
It's comparatively easy (cum grano salis) to grasp the following concepts:
Euclidean space-time (continous space and continuous time)
classical mechanics (discretely distributed matter in continous ...
2
votes
1answer
311 views
Possibly naive question about quantized space-time
I beg your pardon in advance if this question is naive.
In Quantum Mechanics, discrete values of measurements occur only in relation to bound states. This is because of the general solution for the ...
2
votes
3answers
624 views
What are some approaches to discrete space-time used in modern physics?
This thought gave rise to some new questions in my mind.
What are the consequences for:
How would it affect duality i.e. particle, wave property of photons?
How does this statement affect the ...
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votes
7answers
2k views
Does Quantum Physics really suggests this universe as a computer simulation? [closed]
I was reading about interesting article here which suggests that our universe is a big computer simulation and the proof of it is a Quantum Physics.
I know quantum physics tries to provide some ...
