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

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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 ...
13
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3answers
571 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 ...
22
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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 ...
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1answer
61 views

General relativity and quantum fields evolution in curved space [on hold]

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 ...
2
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1answer
97 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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37 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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0answers
62 views

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
157 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 ...
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1answer
224 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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1answer
64 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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65 views

How could general relativity and quantum mechanics ever be combined? [duplicate]

Maybe I'm mistaken but here it's what I understand: Einstein's theories have to do with space time curvature. Whilst quantum mechanics has to do with probabilities. The solutions to the Shrodinger ...
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2answers
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
98 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 ...
3
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2answers
714 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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1answer
73 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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0answers
724 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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1answer
56 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 ...
3
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1answer
93 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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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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2answers
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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4answers
808 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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1answer
72 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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1answer
89 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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3answers
117 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 ...
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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 ...
3
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1answer
60 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
88 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
44 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 ...
54
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7answers
10k views

Does gravity exist in a vacuum?

My understanding has always been that it does from conventional science courses, but really thinking about it, I was wondering if this is really the case. To my limited understanding there is a ...
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0answers
231 views

What are Galileons good for?

Lately I've seen many papers (for example "The galileon as a local modification of gravity"; 292 total hits on the arXiv) on types of field theories known as Galileons, and I'm wondering what the ...
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23 views

Planck Temperature

I have heard of two different things occurring at Planck Temperature: 1. A black hole forms 2. The quantum gravity takes over and the wavelength of light emitted from the object is Planck length ...
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0answers
27 views

Poincare patch of $AdS_2$

I know that a metric for global $AdS_{2}$ can be witten as $ds^2=\frac{1}{\cos^{2}\theta}\left(-dt^2+d\theta ^2\right)$, where $\theta \in [-\pi/2; \pi/2]$. So one can draw Penrose diagram for it as ...
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0answers
37 views

Force of gravity at quantum level is indetermined?

Newton's law of Universal Gravitation states that any two bodies in the universe attract each other with a force that is directly proportional to the product of their masses and inversely proportional ...
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0answers
37 views

Why should we want to quantize Gravity? [duplicate]

I understand that it will be "nice" to have a quantum description of Gravity as well, just like the other 3 forces. I would like to find out what problems arise in existing theory (not counting String ...
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1answer
144 views

We Don't NEED Quantum Gravity because Gravity isn't Even A Force! [duplicate]

Now, I understand the motivation for quantum gravity. I honestly want to work on a theory myself. However, gravity, according to General Relativity, is not a fundamental force of nature. To me, it's ...
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653 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 ...
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0answers
41 views

A question on intermediate step in deriving gravitational anomaly by Fujikawa's method

In Fujikawa's 'Path integrals and Quantum Anomalies', Eq.(10.26) in the derivation of gravitational anomaly in Chapter 10.1 is puzzling for me. ...
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0answers
52 views

Black Hole singularity in Loop Quantum Gravity

How is the singularity of a Black Hole treated in Loop Quantum gravity ? Does it go away ? And if it does, what's after the event horizon ?
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0answers
92 views

Quantum Gravity Singularity Question?

I am not a expert in QG (Quantum Gravity) or GR (General Relativity) for that matter so please forgive if I make any small mistakes, its just a curious question but I know that a singularity is a ...
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1answer
33 views

Value of Planck scale

I read several times that the Planck scale is the energy scale at which the effects of gravity are comparable to the effects of the other fundamental forces. How can I show that this actually happens ...
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2answers
826 views

Does anyone take the Wightman axioms seriously?

Does anyone take the Wightman axioms seriously? Mainly with respect to quantum gravity or gauge theores, abelian or non-abelian? Anyone doing any research on axiomatization of QFTs in some way?
102
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3answers
24k views

Why does Stephen Hawking say black holes don't exist?

Recently, I read in the journal Nature that Stephen Hawking wrote a paper claiming that black holes do not exist. How is this possible? Please explain it to me because I didn't understand what he ...
5
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2answers
90 views

Would the presence of B-modes in the CMB provide evidence for quantum gravity?

Finding B-modes in the CMB (which aren't due to foreground contamination) would be evidence for gravitational waves, because they cannot be produced by density perturbations (to first order, is my ...
4
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1answer
76 views

Is there a 2D manifold on which the Dirac equation has a zero mode?

The two-dimensional (2D) Dirac equation $(\sigma_1iD_1+\sigma_2 iD_2)\psi=E\psi$ admits zero mode ($E=0$) solutions on a non-trivial gauge background, such as the zero mode at the core of a U(1) gauge ...
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1answer
97 views

Why is the universe 3D if the holographic principle says it's encoded in a its surface?

The way I understand the holographic principle is that everything in a 3D space can be thought of as living on the 2D boundary of that space. If that is the case, why does everything in the universe ...
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5answers
2k views

Why don't we have a theory of everything?

What is currently stopping us from having a theory of everything? i.e. what mathematical barriers, or others, are stopping us from unifying GR and QM? I have read that string theory is a means to ...
3
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1answer
70 views

What is meant by the following divergent formula?

I have encountered the following formula a couple of times (in different physics contexts which I do not have a good understanding of) $$\int_{0}^\infty \frac{dt}{t}e^{-tx}=-\log x$$ Formally one ...
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2answers
26 views

Gravitational wave contribution of the Hawking radiation from a black hole

Black holes are expected to radiate like a perfect black radiator at the Hawking temperature, which means that they'll emit all particles according to the relevant formulas one can derive using ...
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2answers
294 views

How do gravitons impact on general relativity?

As I'm reading about GR a lot lately, I was wondering: how do gravitons (if they exist ofc) impact the general relativity? Since in GR, when we look at particles moving in space-time, we are only ...
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2answers
106 views

How can gravitions exist without violating GR? [duplicate]

How can gravitions exist without violating GR, since GR says that gravity is curvature in space-time.