# Tagged Questions

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

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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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 ...
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### 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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### 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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### 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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### 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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### 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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### 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.
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 ... 3answers 497 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 ... 3answers 60 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. ... 3answers 271 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 ... 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 ... 2answers 139 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 ... 0answers 49 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 ... 1answer 192 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 ... 1answer 108 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, ... 1answer 119 views ### What does quantum gravity look like at or beyond the Planck scale? I am reading an article called The fundamental nature of space and time by Gerard 't Hooft. On page 3 he writes the following: Physically, however, the perturbative approach fails. The difficulty ... 0answers 17 views ### Is black hole complementarity compatible with canonical quantization? Suppose at time t, Alice and Bob are hovering just outside the event horizon of a black hole, sharing the same position, velocity and acceleration. Shortly afterward, in less than the Schwarzschild ... 0answers 32 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: ... 0answers 67 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 ... 1answer 371 views ### The cap on massive particles's speed is below the speed of light due to Planck length? This answer http://worldbuilding.stackexchange.com/a/8525/1991 claims that no particle can accelerate further after its de Broglie wavelength becomes Planck length. Given that speed at this point is ... 1answer 165 views ### Renormalization in Classical Field Theory 1) The statement that general relativity (GR) is not renormalizable - is it a statement only about the quantization of GR or is it non-renormalizable also as a classical field theory? 2) More ... 0answers 42 views ### Information loss from entanglement and cosmological horizons? Suppose we have a qubit P in an unknown quantum state. Unknown as in we didn't prepare it, and don't know how it was prepared either. Without measuring the qubit in any way, we encode it into two ... 0answers 150 views ### Are there any proposed alternatives to quantum mechanics as there are alternatives to general relativity? There are a lot of alternatives to general relativity and one of the motivations is attempting to formulate a working theory of quantum gravity. In some limit they reduce to general relativity. But ... 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 ... 1answer 154 views ### Can Einstein's 'Theory of General Relativity' exist in Harmony with 'Quantum Mechanics'? [closed] From the Book 'In Search of Schrodinger's Cat': Coordinates in space-time represent position; causality depends on knowing precisely where things are going, essentially on knowing their momentum. ... 0answers 24 views ### How would gravitons be detected? [duplicate] How would gravitons be detected indirectly or directly, in space or on earth? And what experiments are going on to find gravitons? 0answers 43 views ### No Hair systems and black hole entropy To my understanding, a black hole is a no hair system. So it can be described just by its mass, spin and charge. In other words it does not differentiate where its mass comes from, so it could be made ... 0answers 106 views ### What exactly is black hole complementarity and why is it necessary for solving black hole information paradox? What exactly is black hole complementarity and why is it necessary for solving black hole information paradox? The first question is about the story of vacuum fluctuation causing Hawking radiations. ... 1answer 82 views ### Why is the value of the action integral in general relativity the same on all regions that are homologous? In their famous paper Action integrals and partition functions in quantum gravity, Gibbons and Hawking argue that in order to avoid the singularity of a Schwarzschild black hole you can complexify ... 2answers 336 views ### Why isn't our universe symmetric? Why were random variations introduced in the spherically symmetric universe after Big Bang which made it non-symmetrical. Since the outcome of a coin toss depend on factors such as torque applied, air ... 0answers 62 views ### That the gravitational mass equals to inertial mass can imply that only Einstein-Hilbert action is satisfied I read Spacetime and Geometry by Sean Carroll. In p. 166 there is a comment that GR's action is nonlinear because if it is linear like the EM field, then graviton will not interact with each other, ... 0answers 60 views ### Regarding Randall Sundrum model In Randall Sundrum model 2, that is the one with non compact fifth dimension, there is only one brane, which is the Planck brane. The TeV brane is removed by taking the radius of the fifth dimension ... 2answers 7k views ### What are Stephen Hawking's main contributions to research-level physics? Without a doubt, Stephen Hawking is the most famous living scientist; indeed, his public visibility in all of history seems to be rivaled only by Einstein and easily eclipses giants of physics such as ... 0answers 34 views ### Should the answer to Einstein & Quantum gravity be found in conditions of our local observable universe? [duplicate] It seems like the discourse over a unified Einstein and Quantum Gravity disregards the phenomena encountered in our local observable universe, such as we find throughout the Intergalactic Medium. One ... 3answers 220 views ### Gravity - Force or Result? I am no Physicist, but I enjoy reading about Physics. However reading about leading theories such as M-Theory and others they speculate about the existence of the Graviton. In my past reading of ... 0answers 72 views ### Can anyone tell me what's this equation? Can anyone tell me what's this equation? 0answers 46 views ### CFT calculation of the Partitionfunction of 2+1 dimensional gravity I want to reproduce formula (4.29) in http://arxiv.org/abs/0804.1773v1 given by:$$ Z=Tr(q^{L_{0}}\bar q^{\bar L_{0}})=|q|^{-2k} \prod^{\infty}_{n=2}\frac{1}{|1-q^{n}|^{2}}  Where the trace is ...
The Standard Model of elementary particles is a gauge theory with gauge group $SU(3)\times SU(2)\times U(1)$, which is really a successful theory. We might be able to quantize gravity similarly. ...