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

learn more… | top users | synonyms

4
votes
2answers
898 views

Are all points in the universe connected?

Is it true that every point in the universe is connected or could be so theoretically? If so how is this mediated? Is it through the quantum nature of the fabric of space or is it through the ...
1
vote
1answer
76 views

measure higher frequencies then half of Planck-frequency?

The maximum frequency is defined by the Planck frequency $\omega_P$. Also there is the Shannon theorem which tells us that to lossless capture a signal, you have to sample it with minimum of the ...
3
votes
2answers
189 views

Phys.org Spectral geometry to unite relativity and quantum mechanics, restate in laymens terms?

Lingua Franca links relativity and quantum theories with spectral geometry Could someone give me a short synopsis of this article in laymens terms? What implications does this have in the physics ...
13
votes
3answers
1k views

What is the current state of research in quantum gravity?

I was browsing through this and was wondering what progress in quantum gravity research has taken place since the (preprint) publication. If anyone can provide some helpful feedback I would be ...
8
votes
0answers
157 views

Why is the Planck length the shortest measurable length? [duplicate]

I quote from the Wikipedia article on Planck length: According to the generalized uncertainty principle, the Planck length is in principle, within a factor of order unity, the shortest ...
7
votes
0answers
438 views

Can the laws of quantum mechanics be derived from a more fundamental theory? [closed]

String theory takes quantum mechanics and tries to make it compatible with gravity. If it turns out to be a theory of everything then would it explain why our world is described by the laws of quantum ...
4
votes
1answer
505 views

How do I quantize a classical field theory

I have not been able to find any information about this on the Internet. I am a middle-schooler, 14, who self-studies physics, and I know up to and including ODEs, and some of the calculus of ...
8
votes
0answers
171 views

Is it believed that all UV completions have “Maldacena duals”?

I have heard occasional rumors that effective field theories have gravity duals. For example, I've been told that UV momentum cutoffs in N=4 SYM become finite radii in AdS. I've heard speculations ...
2
votes
0answers
240 views

Status of Teleparallel Quantum Gravity

There are several approches to incorporate torsion into a theory of gravity. According to this article, requiring a consistent coupling to Dirac fields leaves us with the Einstein-Cartan approach or ...
0
votes
3answers
706 views

Why isn't the Bekenstein-Hawking Entropy considered the quantum gravitational unification?

Based on the Bekenstein-Hawking Equation for Entropy, hasn't the relationship between quantum mechanics and gravity already been established.
32
votes
2answers
2k views

Why is gravity such a unique force?

My knowledge on this particular field of physics is very sketchy, but I frequently hear of a theoretical "graviton", the quantum of the gravitational field. So I guess most physicists' assumption is ...
4
votes
2answers
643 views

The Uncertainty Principle and Black Holes

What are the consequences of applying the uncertainty principle to black holes? Does the uncertainty principle need to be modified in the context of a black hole and if so what are the implications ...
1
vote
4answers
973 views

Could all strings be one single string which weaves the fabric of the universe?

This question popped out of another discussion, about if the photon needs a receiver to exist. Can a photon get emitted without a receiver? A universe containing only one electron was hypothetically ...
13
votes
2answers
742 views

Why one-dimensional strings, but not higher-dimensional shells/membranes?

One way that I've seen to sort-of motivate string theory is to 'generalize' the relativistic point particle action, resulting in the Nambu-Goto action. However, once you see how to make this ...
14
votes
0answers
496 views

How is Matter Modelled in Loop Quantum Gravity?

I have a basic understanding of how gravitational degrees of freedom are modelled in loop quantum gravity, but as far as I know, the main machinery - spin network states and observables, does not ...
6
votes
1answer
326 views

Is the quantization of gravity necessary for a quantum theory of gravity? Part II

(At the suggestion of the user markovchain, I have decided to take a very large edit/addition to the original question, and ask it as a separate question altogether.) Here it is: I have since ...
21
votes
3answers
4k views

Why is gravity so hard to unify with the other 3 fundamental forces?

Electricity and magnetism was unified in the 19th century, and unification of electromagnetism with the weak force followed suit, bringing into play the electroweak force. I've been told that ...
4
votes
1answer
310 views

Deriving the reduced Green's functions in Polchinski's volume 1

In equation 6.2.7, Polchinski defines his reduced Green's functions $G'$ on the 2-manifold to satisfy the equation, $$ \frac{-1}{2\pi \alpha '}\nabla ^2 G'(\sigma_1, \sigma_2) = ...
2
votes
1answer
413 views

Is Space-Time Quantisation necessary or even meaningful?

It is believed among people working on Quantum Gravity, that space-time must be quantised at the Planck scale. Although it is very hard to verify such proposition, it is interesting from a ...
8
votes
1answer
2k views

What happens to atoms inside a black hole?

Black holes have very high gravitational force that tends to crush everything. So as we know atoms in a molecule have inter-atomic spacing between them and further electrons also revolve at a certain ...
18
votes
1answer
620 views

Does local physics depend on global topology?

Motivating Example In standard treatments of AdS/CFT (MAGOO for example), one defines $\mathrm{AdS}_{p+2}$ as a particular embedded submanifold of $\mathbb R^{2,p+1}$ which gives it topology ...
3
votes
2answers
2k views

Higgs-Boson/Graviton [duplicate]

The Higgs boson gives particles mass. And the graviton is the theoretical force-carrier of gravity. Gravity depends on mass. So if the Higgs Boson gives things mass, it therefore gives them gravity. ...
4
votes
1answer
553 views

Is spacetime an illusion?

In consistent histories, for gauge theories, can the projection operators used in the chains be not gauge invariant? In quantum gravity, for a projection operator to be gauge invariant means it has ...
20
votes
6answers
2k views

Is the quantization of gravity necessary for a quantum theory of gravity?

The other day in my string theory class, I asked the professor why we wanted to quantize gravity, in the sense that we want to treat the metric on space-time as a quantum field, as opposed to, for ...
1
vote
1answer
154 views

Divergence in Supergravity

I'm not familiar with supergravity so here's my question: I've heard in talks that if one finds divergence for five-loop 4-graviton scattering amplitudes in five dimensions this translates to a ...
3
votes
1answer
595 views

Dirac Equation in General Relativity

Dirac equation for the massless fermions in curved spase time is $γ^ae^μ_aD_μΨ=0$, where $e^μ_a$ are the tetrads. I have to show that Dirac spinors obey the following equation: ...
6
votes
2answers
449 views

What is the “foamy space” hypothesis that has been debunked recently?

In "Space-Time Is Smooth, Not Foamy", a Space.com article, it is stated: In his general theory of relativity, Einstein described space-time as fundamentally smooth, warping only under the strain ...
1
vote
0answers
168 views

Approaches to Quantum gravity [closed]

I'm going to start my graduate studies in theoretical physics. My supervisor wants me to work on quantum gravity. He gave me the liberty to chose a particular approach to Quantum gravity (Excluding ...
4
votes
3answers
386 views

Question on inflation

I have two particular questions regarding the inflationary scenario. They are: 1.) What is the physical origin of the inflaton field? 2.) Why has the potential of the inflation field its particular ...
4
votes
1answer
122 views

Where and how is the entropy of a black hole stored?

Where and how is the entropy of a black hole stored? Is it around the horizon? Most of the entanglement entropy across the event horizon lies within Planck distances of it and are short lived. Is ...
1
vote
0answers
159 views

Black hole entropy from collapsed entangled pure light

Consider the following scenario, very similar to the one proposed in this question, but this time, the pure quantum radiation used for the black hole collapse, is now being split with down-converter ...
3
votes
1answer
281 views

Question on inflation as a phase transition

I have just finished watching the following video http://www.youtube.com/watch?v=beQ9fZ0jVdE where Laughlin, Gross and some students discuss e.g. about inflation. The following question is risen: Is ...
3
votes
1answer
126 views

Why are some extra dimension theories known as strongly coupled and others as weakly coupled?

I was looking at pdf file of the presentation of a conference talk. The speaker discusses two types of "mechanisms" for stabilizing the weak scale and calls them "weakly coupled" and "strongly ...
13
votes
3answers
573 views

Extremal black hole with no angular momentum and no electric charge

A black hole will have a temperature that is a function of the mass, the angular momentum and the electric charge. For a fixed mass, Angular momentum and electric charge are bounded by the extremality ...
6
votes
0answers
231 views

Finite quantum gravity?

I'm working through an article that has some questionable assertions. The article is by Frank Tipler, "The structure of the world from pure numbers". (I'm going to ignore the fact that some of ...
1
vote
1answer
139 views

What are all the approaches that have been tried for a theory of quantum gravity? [closed]

I am aware that that the most researched approach is that of string theory. I have also heard about quantum loop gravity. What other approaches are there to unify gravity and QFT? Also, please include ...
1
vote
3answers
482 views

The Role of Gravity among the Fundamental Forces of Nature

If we look at the standard model, we have 4 fundamental forces which include Gravity, Electromagnetism, Nuclear weak force, Nuclear strong force. I would like to look at Gravity for a minute. ...
5
votes
1answer
416 views

Do all the forces become one?

Were the forces of nature combined in one unifying force at the time of the Big Bang? By which symmetry is this unification governed? Are there any evidence for such unification of forces? Has ever ...
4
votes
2answers
639 views

What is the motivation for assuming “Page” scrambling for Hawking radiation?

What is the motivation for assuming "Page" scrambling for Hawking radiation? Obviously, at the semiclassical level, we want the outgoing Hawking radiation to look thermal and mixed. However, surely ...
1
vote
3answers
685 views

Why can't light escape from inside event horizon of Black Holes?

The simple answer: Its because Gravity of Black Hole there doesn't allow it. See also this and this Phys.SE posts. Isn't it a classical answer? When we're unable to connect Gravity with Quantum ...
3
votes
0answers
49 views

Attractiveness of spin 2 gauge theories [duplicate]

Possible Duplicate: Why is gravitation force always attractive? I have heard that the attractiveness of gravitation is due to the fact that it is a spin 2 gauge theory. Why is this so? I ...
7
votes
1answer
136 views

Hawking radiation for closely orbiting black holes

Suppose we have two black holes of radius $R_b$ orbiting at a distance $R_r$. I believe semi-classical approximations describe correctly the case where $R_r$ is much larger than the average black body ...
4
votes
2answers
277 views

Assuming that extra dimensions will not be visible at LHC, what motivation will still remain to study them?

Many physicists believe that there is little possibility of observing extra dimensions at LHC so that some extra dimension models originally designed to solve hierarchy problem (ADD/Randall-Sundrum) ...
5
votes
2answers
968 views

What is background independence and how important is it?

What is background independence and how important is it? In order to be a theory of everything, will the final string-theory/m-theory have to be background independent? Does the current lack of ...
4
votes
1answer
142 views

Will Randall-Sundrum extra dimension scenario become defunct if not supported by LHC?

The Randall-Sundrum extra dimension scenario had been one of the most extensively studied class of theories. This offered a solution to the hierarchy problem. However, if this picture is not supported ...
3
votes
1answer
161 views

Why doesn't the firewall argument also apply to far away ingoing modes?

Gidom Mera's answer at http://physics.stackexchange.com/a/45511 is illuminating, but on closer analysis, it brings up further puzzles. Backscattering works in both directions. Let's see what we get ...
12
votes
1answer
347 views

Quantum uncertainty of particle falling in black hole

A stationary observer at infinity sees a particle of mass m falling in a supermassive Schwarzschild black hole. He observes an increasing redshift and sees the particle ceasing to progress when it ...
6
votes
1answer
131 views

Is there a black hole interior in black hole complementarity?

According to black hole complementarity, for an external observer, the interior of the black hole is replaced with a stretched horizon at a Planck distance above where the horizon ought to be. Is this ...
2
votes
0answers
72 views

Wick rotation for FRW in quantum gravity

There is no timelike Killing vector for FRW cosmologies. In the path integral formalism, is it possible to Wick rotate for quantum cosmology in quantum gravity? If yes, how? If no, how does one work ...
3
votes
1answer
969 views

Why does the universe exhibit three large-scale spatial dimensions? [duplicate]

Possible Duplicate: Is 3+1 spacetime as privileged as is claimed? Regardless of your favorite theory of how many dimensions the universe has in total, the universe seems to have a deep ...