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

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277 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 ...
14
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3answers
650 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
73 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 ...
2
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1answer
127 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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0answers
74 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 ...
2
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0answers
72 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 ...
7
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1answer
170 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 ...
9
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1answer
414 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
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 ...
2
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1answer
420 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 ...
0
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1answer
187 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 ...
1
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0answers
907 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 ...
0
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1answer
125 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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4answers
1k 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 ...
-4
votes
1answer
107 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 ...
3
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1answer
99 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?
4
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3answers
178 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
votes
1answer
78 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?
6
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2answers
105 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 ...
2
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1answer
52 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 ...
56
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7answers
15k 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 ...
6
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0answers
267 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 ...
0
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0answers
36 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 ...
1
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0answers
28 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
55 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
39 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 ...
0
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1answer
373 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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0answers
61 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
107 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 ...
2
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1answer
42 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 ...
119
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3answers
26k 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
votes
2answers
107 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 ...
0
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1answer
179 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 ...
19
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5answers
3k 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
81 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 ...
0
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2answers
45 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 ...
0
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2answers
500 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
154 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.
2
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1answer
60 views

Decomposition of the gravitino into helicity $\pm \frac{3}{2}$ and $\pm \frac{1}{2}$ components

I'm reading this book on string theory. When they decompose two dimensional gravitino (formula 7.16) $$ \chi_\alpha = \frac{1}{2}\rho^\beta \rho_\alpha \chi_\beta + \frac{1}{2}\rho_\alpha \rho^\gamma ...
4
votes
3answers
321 views

Could future experiments on “Gravitational Casimir Effect” confirm the existence of gravitons?

From Casimir effect, we know that when two plates are placed very close to each other in vacuum, they attract each other because the quantum fluctuations that press on the two plates' outer surfaces ...
2
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3answers
62 views

How does one experience single gravitons and/or Planck-force?

Moving sufficiently far away from a light source one would not be able to measure a steady stream of light, but only single photons every now an then. The experience would be a very faint blinking. ...
17
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5answers
2k views

Why doesn't matter clump together such that it can't be taken apart again?

Given the inverse square law force of gravity shouldn't two particles that are infinitely close to each other be infinitely attracted to one another? For example, suppose the hands of some super deity ...
2
votes
2answers
201 views

Gravity as a particle-based force vs space-time warp

I'm interested to understand the interpretation of gravity as a result of exchange of force particles (ie gravitons) vs General Relativity-based warping of space-time. Related to this is while a ...
0
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0answers
57 views

Would a directional “graviton” emitter violate any known laws of physics?

Setting aside that we don't known what the mediating partial in quantum gravity looks like and have no way to manipulate it, what would the implications be of a directional graviton source be? Would ...
2
votes
1answer
272 views

How does this new theory of a possible infinitely old universe not violate the second law of thermodynamics

I read the following article: http://phys.org/news/2015-02-big-quantum-equation-universe.html And followed it back to this journal reference : http://arxiv.org/abs/1404.3093 It appears to be ...
4
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1answer
127 views

Theories that predict the number of space-time dimensions

My impression in that most theories assume three spatial dimensions and one temporal dimension, though could in principle be formulated in others numbers of dimensions without inconsistencies. I know, ...
38
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2answers
4k 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 ...
2
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0answers
43 views

Ground state energy in a simple quantum gravity situation

I found this problem in an MIT undergrad QM problem set; it is problem number 2, part a, number iv. I'll summarize everything below, but here's the link: ...
8
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2answers
478 views

Is quantum gravity, ignoring geometry, the theory of a fictitious force?

This question is motivated by this question and this one, but I will try to write it in such a way that it is not duplicate. In short, I don't understand the motivation for a "quantum theory of ...
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0answers
79 views

Gravitational singularity

Is it possible that the gravitational singularity actually turns out to be a genuine singularity once we have a true theory of quantum gravity in place. There is a lot of talk about singularity but ...