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

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How much do we know about Quantum Gravity at energies larger than Planck energy?

What kind of predictions do current approaches to quantum gravity (superstrings/M-theory, loop quantum gravity,...) make at energies LARGER than Planck scale? Or is that a meaningless question? Does ...
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3answers
168 views

Why scale relativity is not mainstream physics? [on hold]

The astrophysicist Laurent Nottale develops since 1984 the scale relativity, which aims to unify quantum physics and relativity theory, using a fractal space-time. I would like to understand why ...
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1answer
111 views

Could the singularity that gave rise to our universe be past-eternal?

Are there any compelling models that include a past eternal singularity that ultimately gave rise to our universe? Does the "no-boundary" hypothesis that utilizes imaginary time have past eternal ...
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29 views

Gravity theory help? [on hold]

I'm looking for someone to help with math on a theory I have involving gravity on a quantum level. Anyone interested?
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3answers
477 views

Are length contractions limited by Planck length?

While we are getting closer to speed of light our length in the direction of the movement is according to Lorentz transformation getting shorter. But we can not (even theoretically) consider length ...
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4answers
3k views

Why do physicists trust black hole physics?

Based on popular accounts of modern physics and black holes (articles, video lectures), I have come to understand the following: Black holes are predicted by General Relativity, a classical theory ...
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39 views

Can we create graviton particle or gravitational wave? [duplicate]

Can we create graviton particle or gravitational wave? I want to know if we able to generate graviton particle out of some energy. Is it theoretically possible? And could we generate gravitational ...
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2answers
55 views

Do the stellar remnant in a black hole have spatial extent? [duplicate]

I am not talking about event horizons. I am talking about the actual remnant of stellar collapse. Is it just a point, hence the problem with the singularity? Or does it have a finite volume that we ...
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29 views

Palatini type gravities: Where could I find them?

I read the article Quantum gravity with torsion and non-metricity. Although I found interesting the analysis in the paper, I found quite interesting an statement in the abstract, The class of ...
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130 views

What experiment would disprove loop quantum gravity?

Loop Quantum Gravity (LQG) is a theory of quantum space-time that attempts to describe the interconnection between general relativity and quantum mechanics. It's main postulate is the granularity of ...
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2answers
370 views

Black hole entropy

Bekenstein and Hawking derived the expression for black hole entropy as, $$ S_{BH}={c^3 A\over 4 G \hbar}. $$ We know from the hindsight that entropy has statistical interpretation. It is a measure ...
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2answers
137 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 ...
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121 views

Why should the modes of the linearized metric perturbation be “wavefunctions” of gravitons (in the Randall-Sundrum model)?

In "An Alternative to Compactification" by Randall and Sundrum, they discuss the localization of "graviton modes" around the Planck brane in the Randall-Sundrum model where we have a compact fifth ...
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79 views

Hawking Temperature of the BTZ Black Hole

The metric of the BTZ Black Hole is given by $$ ds^2 = - N^2 dt^2 + N^{-2} dr^2 +r^2(d\phi + N^\phi dt)^2 $$ with $$ N^2 = -M+ \frac{r^2}{l^2} + \frac{J^2}{4 r^2}, \ \ \ \ \ \ N^\phi = ...
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4k 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 ...
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40 views

Absolute time as a common basis of quantum gravity?

Quantum mechanics has an absolute time concept, and general relativity has the dynamic time concept of spacetime. The incompatibility of both concepts is called the problem of time in quantum gravity. ...
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1answer
71 views

Can we observe a star collapse into a black hole? [duplicate]

Let's say that I happen by a star that has just burned out and is collasping into a black hole. I seat at infinity to watch it as a Schwarzschild observer. Will I ever see the black hole collapse into ...
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2answers
726 views

Special relativity paradox and gravitation/acceleration equivalence

One of the features of the black hole complementarity is the following : According to an external observer, the infinite time dilation at the horizon itself makes it appear as if it takes an ...
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3answers
5k 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 ...
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1answer
326 views

Graviton and principle of equivalence

Is it possible that in a theory of gravitons, i.e., a quantum field theory of gravitation, general relativity's principle of the equivalence of gravitational mass and inertial mass, no longer holds?
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20 views

How is the first law of black hole thermodynamics formulated for a non-vacuum solution with event horizon?

How is the first law of black hole thermodynamics formulated for a non-vacuum solution with event horizon?
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4answers
4k views

Is there a maximum possible acceleration?

I'm thinking equivalence principle, possibilities of unbounded space-time curvature, quantum gravity ...
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4answers
1k 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 ...
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5answers
4k 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 ...
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1answer
104 views

Can I teach myself physics (LQG specifically)? [closed]

OK I'm a second year undergrad student...have taken the introductory Math, Phys and Computer Science classes. I LOVE physics, and always have since I was a kid and wanted to major in that initially, ...
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2answers
744 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 ...
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0answers
42 views

Does the uncertainty principle affect event horizons? [duplicate]

I was thinking about black holes. For a simple black hole the event horizon is given by a distance of 2 times mass (energy) of the black hole. (2m). But according to quantum mechanics, if you try to ...
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3answers
6k views

Is the Planck length the smallest length that exists in the universe or is it the smallest length that can be observed?

I have heard both that Planck length is the smallest length that there is in the universe (whatever this means) and that it is the smallest thing that can be observed because if we wanted to observe ...
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35 views

Need some intuition behind spontaneous dimensional reduction

In quantum gravity there is the notion of spontaneous dimensional reduction. Namely, at small scales, the dimension of spacetime is less than the ostensible four. In the causal dynamical ...
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33 views

In discrete models of spacetime, what are some implications of the Einstein equation

We have several models of discrete spacetime. Sorkin has a causal growth dynamics, there's spin foams, Panangaden showed a correspondence between interval domains and spacetimes. I am looking for ...
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2answers
115 views

Why can't gravitons be created in traditional particle accelerators?

I've heard that it is incredibly difficult to detect a graviton, but I don't quite understand why. With all of the knowledge I have at the present time it seems like it should be possible to create a ...
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3answers
579 views

Why does nonlinearity in quantum mechanics lead to superluminal signaling?

I recently came across two nice papers on the foundations of quantum mechancis, Aaronson 2004 and Hardy 2001. Aaronson makes the statement, which was new to me, that nonlinearity in QM leads to ...
2
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1answer
68 views

General relativity and quantum fields evolution in curved space [closed]

There are many cases when we have to discuss the problem of evolution of quantum fields on GR background (inflaton evolution during inflation, axion field evolution etc). But GR isn't quantized as ...
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1answer
117 views

Quantum mechanical explanation of the expansion of the universe?

All the explanations for the expansion of the universe that I have heard about are usually based off Einstein's General Relativity, and many quote the proposed all permeating Dark Energy as the cause ...
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44 views

About spin 2, graviton, in the context of attractive force.

excerpt from Introduction to Black hole physics, Valeri P. Frolov and Andrei Zelnikov In this textbook, there is comment about spin of gravitons as follows For gravitating bodies the gravitational ...
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Where are the gravitons in the Wheeler-de-Witt formalism?

As I understand it, in the Wheeler-de-Witt formalism you sum over the spaces bounded by a start space and and end space. What I want to know is where do gravitons fit into this picture? I ...
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1answer
158 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 ...
8
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1answer
250 views

Is John Nash's “Interesting Equation” really interesting?

As recently mentioned in the news, before his passing, John Nash worked on general relativity. According to the linked article John Nash's work is available online from his webpage. His work is ...
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1k views

Is Gravity a Problem?

I was watching the movie "Interstellar" recently and in that a character "Romly" mentions that if he could peep into the black hole "Gargantua" he could solve gravity. I have a questions, is that a ...
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1answer
142 views

What does the universe look like at the Planck length (in a drawing)?

What is an informative drawing of the universe at the Planck length, to get a deeper sense of the meaning of it? For example, you see stuff like this: But that confuses you because there is no ...
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1answer
89 views

What happens as you approach/cross the Planck temperature?

According to IFLScience, above the Planck Temperature (absolute hot) conventional physics break down. My question is what happens as you approach this temperature, and, if it is possible, what ...
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772 views

Apparently John Nash found a way to replace Einstein's theory of relativity days before crash [closed]

True story, John Nash told his friend that he revised Einstein's theory of relativity to account for quantum gravity days before dying in the car crash. This guy worked on manifold theory big time (in ...
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70 views

What is the significance of Planck charge?

It seems for me that Planck units are somehow connected to limits where our current knowledge breaks down because of (quantum) gravitational effects. Please correct me if I'm wrong. For example ...
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832 views

Why should one expect closed timelike curves to be impossible in quantum gravity?

From the Wikipedia article, it seems that physicists tend to view closed timelike curves as an undesirable attribute of a solution to the Einstein Field Equations. Hawking formulated the Chronology ...
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81 views

What areas of research question the existence of the Planck length?

As a follow up to What are the structures that exist at roughly the scale of the Planck length?, what areas of research question the idea of a Planck length (or Planck scale)? Or if there are no areas ...
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91 views

What are the structures that exist at roughly the scale of the Planck length? [closed]

What are the mathematical/geometric structures that exist at the level of the Planck length?
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124 views

What do gravitons do?

Yes, I know gravitons are 'just a theory', but I'm wondering how they theoretically act. Are they raining down on everything with mass to "push" other things towards it? Or do the gravitons form a ...
3
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1answer
65 views

How are gravitons supposed to work?

If gravitons are particles, how is it that they can hold us to the ground, or create gravity between two stars millions of miles away from each other?
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2answers
90 views

Speed of gravity within a mass

We all consider that gravity travels at the speed of light. Light travels at the speed of light except when it is in a medium ,say glass, where it travels slower. What happens when gravity passes ...
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1answer
48 views

Can anyone explain to a novice physicist whether there is a gravitational-electromagnetic symmetry?

I am trying to understand how the four fundamental forces relate to one another and to a theory of everything. As I understand it the unified force that is thought to exist at very high energies gets ...