Questions tagged [gravity]
Gravity is an attractive force that affects and is affected by all mass and - in general relativity - energy, pressure, and stress. Prefer newtonian-gravity or general-relativity if sensible.
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Variation of the Einstein-Hilbert action in $D$ dimensions without the Gibbons-Hawking-York (GHY) term
Consider the standard Einstein-Hilbert action in $D \ne 2$ dimensions spacetimes :
\begin{equation}
S_{EH} = \frac{1}{2 \kappa} \int_{\Omega} R \; \sqrt{- g} \; d^D x,
\end{equation}
where $\Omega$ is ...
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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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Gauss-bonnet gravity constraints from string theory
Recently there have been advances in observational constraints of gravity theories that contain scalars coupled to the gauss-bonnet topological term:
http://arxiv.org/abs/0704.0175
http://arxiv.org/...
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Different features of Gravity and Yang-Mills
I am reading a famous paper by S.Hawking - "Quantum gravity and path integrals" https://doi.org/10.1103/PhysRevD.18.1747.
On the third page left column there is a statement, after the ...
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Free higher spin fields and gravity
There are soft theorems that suggest that any massless boson with spin higher than 2 should be a free field theory and cannot have interactions. Does this mean that one cannot embed such fields into a ...
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Entropy and equilibrium concepts at astronomic scales
I am always puzzled to read here and there discussions dealing with thermodynamic concepts applied to astronomic scales where gravity matters. To my opinion, there is a certain carelessness to go into ...
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Definition of gravity path integral?
In a non-abelian gauge theory there is a "fundamental" gauge field $A_\mu^a$ with gauge index $a$ often called connection. Although $ A_\mu^a$ is not gauge invariant, gauge invariant quantities can be ...
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Metric transformation, polygons and gravitons
I'm trying to understand the paper by Hitchin called: ''Polygons and gravitons". I'm stuck at page 471.
At this point, he does some computations and obtains a metric:
$$
\gamma dz d\bar{z}+\gamma^{...
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How do we decode the image formed by a gravitational lens?
Using our own sun as a gravitational lens, we can scrutinize planetary surfaces in distant solar systems with a good deal of accuracy. How do we translate this smeared out and curved image into a ...
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Propagator in Brans-Dicke Gravity
Consider an action of the form
$$
S = -\frac{2}{\kappa^2}\int d^4x\sqrt{-g}~\left(\phi R + \phi\mathcal{L}_{matter}\right).
$$
Expanding this to second order in $h_{\mu\nu}$ and including a harmonic ...
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Covariant derivative of the vielbein determinant
The vielbein postulate says that
$$\nabla_\mu e_v^{\,a}=\partial_{\mu}e_\nu^{\,a}+\omega_{\mu\,\, b}^{\,\,a}\,e^b_\nu-\Gamma^\sigma_{\mu\nu}\,e^{\,a}_\sigma=0.$$
$\nabla$ is the coordinate covariant ...
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Spin 3 vs spin 2 vs spin 1
I wanna to understand, why when one gonna to construct interacting theory of spin 3, one need also include infinite tower of spins 4, 5, 6 , ...
As I know, this statement correct even in classical ...
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What does black hole formation and evaporation actually look like as viewed from far away?
Many people on Physics SE (myself included) have been confused about what black hole formation and evaporation look like when viewed from far away. For example:
Does any particle ever reach any ...
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Situation after Saini & Stojkovic's paper on unitarity in gravitational collapse and non-formation of black holes?
In their paper, Anshul Saini and Dejan Stojkovic [1] claimed that by calculations it is possible to demonstrate that in a gravitational collapse of a disk, an event horizon is never made for a far ...
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Bargmann–Wigner equations in NP formalism
Bargmann-Wigner equations describe free particles of arbitrary spin $j$, namely
$$(-\gamma^{\mu}\partial_{\mu}+m)_{\alpha_r \alpha_{r’}}\Psi_{\alpha_1,..,\alpha_{r’},...,\alpha_{2j}}=0$$
where we have ...
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Time dilation in quantum gravity
In General Relativity, time moves slower near massive objects where spacetime is curved stronger. In quantum gravity, the gravitational force is represented by the quantum field that refers to the ...
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How to find the Hawking temperature for this metric?
I am reading this paper about "Hawking radiation of Kerr-Newman-de Sitter black hole", where the authors find Hawking temperature of this metric
The authors state that hawking temperature is given by
...
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Vafa-Witten theorem for gravitational $\theta$-term
"The Vafa–Witten theorem shows that vector-like global symmetries such as isospin and baryon number in vector-like gauge theories like QCD cannot be spontanteously broken as long as the theta angle is ...
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Gravitational potential energy and center of mass
I've been reading about the Nordtvedt effect, and how Gravitational Binding Energy (GBE) affects total mass. According to the WP article, experimental evidence rules out the existence of this effect.
...
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On-shell action in asymptotically AdS space
Consider a field theory coupled with gravity described by the action:
$S=\int d^Dx \sqrt{-g} \left( \mathcal{R}-\Lambda+\mathcal{L}_m[\phi] \right)$,
with the requirement that g must be ...
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Proportionality Constant in Einstein Field Equations
The Einstein Field Equations: $$G_{ab}~=~8\pi T_{ab}.$$ I am familiar with how to obtain the $8\pi$ proportionality factor through correspondence with Newtonian gravity, but am wondering if this ...
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Effective field theories in curved spacetime
Loosely speaking, in flat spacetime, one defines the effective Lagrangian by writing down all possible operators compatible with the symmetries and suppressed by some energy scale, and one usually ...
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Gravity's self-energy
Suppose we have a single massive point particle.
In the absence of "potentials", the content of the stress-energy tensor would be dictated uniquely by the particle's mass and trajectory (...
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Linearised diffeomorphisms on an arbitrary gravitational background Part 2
This question is a follow on from my recent post here, in the sense that I will use the notation introduced there. In that post, I considered infinitesimal diffeomorphisms of a metric $g_{\mu\nu}$ ...
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Linearised diffeomorphisms on arbitrary gravitational background Part 1
Consider some spacetime $\big(\mathcal{M},g_{\mu\nu}\big)$ parameterised by local coordinates $x^{\mu}$ ($\mathcal{M}$ is a smooth differentiable manifold equipped with a Lorentzian metric $g_{\mu\nu}$...
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Relation between bulk Hamiltonian in AdS and stress energy of CFT
Consider the following two situations:
One can define a stress energy for AdS which matches with the expectation value for the CFT stress tensor.
Consider bulk metric perturbations of the form:
$$g_{...
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Effect (if any) of strong(ish) gravity radiation on stars
Two black holes merge, and a good few percent of their total mass is converted into gravitational radiation.
Years or decades later, the resulting gravity wave passes through nearby stars. Does it ...
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If gravity turns out to be mediated by exchange particles, would that imply a problem with gravitational fields around a black hole?
In general relativity, gravity is a distortion of spacetime due to mass. Its effects travel (if that's the right word) at the speed of light. In the SM all 3 other known interactions are mediated by ...
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What is the relationship between spin network spacetime and tensor network (entanglement) spacetime?
In 1971, Sir Roger Penrose, suggested a combinatorial construction of spacetime using the angular momentum of particles. This work led to and introduced the idea of spin networks which are ...
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The (Newton-Laplace-Ivory-Arnold) shell theorem in general relativity
It is well-known that Birkhoff's theorem and the classification of LTB spacetimes proves one version of Newton's shell theorem in the context of GR. Another statement in Newtonian gravity, often ...
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Deriving a path-integral expression for a thermal density matrix with position-dependent temperature
I've been fiddling with deriving a path-integral expression for a thermal partition function with a position-dependent temperature but I'm not sure how to get started on this. Concretely, I'm trying ...
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Why gravity does not have any internal symmetry?
The QFT description of forces other than gravity assumes some internal symmetry such as the $SU(3)$ color symmetry for strong interactions. Gravity is based on spacetime symmetries. What forbids the ...
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Cosmological constant phase transition?
I recently watched at a talk by Cumrum Vafa in which he stated that the cosmological constant allows us to define a time-scale $T_\Lambda=1/\sqrt{E_\Lambda}$. The time scale of this time is about 10¹¹ ...
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How does the Newton's constant get renormalized in Quantum Gravity with matter?
In quantum gravity with matter, say e.g. $R$ + $\partial\phi_{\mu} \partial\phi^{\mu}$, even the one loop correction is non-renormalizable. I am sure many smart people have already worked on this, so ...
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Closed trajectories for Kepler problem with classical spin-orbit corrections?
Kepler problem explains closed elliptic trajectories for planetary systems or in Bohr's classical atomic model - let say two approximately point objects, the central one has practically fixed position,...
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Gravitational force for a binary of point particles with GR term
i'm trying to simulate the two body problem with 2 equal masses and I want to account for general relativistic effects. I know that the difference in the gravitational force would be an additional ...
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Non-locality of gravitational energy
Gravitational energy is non-local which is essentially because of the equivalence principle. The equivalence principle says that you can always transform your frame so that you feel like in a ...
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Will the photon wavelength fluctuate in the presence of a gravitational wave?
The microwave background is due to the expansion of the Universe where the wavelengths of radiations are stretched by spacetime.
As in the LIGO experiment, in the presence of gravitational wave, ...
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How does gravity affect entanglement?
Hypothetically an entangled pair of atoms are placed inside two similar satellites orbiting at the same altitude, one of the satellite will then slow its speed and descend towards the surface of Earth....
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Generalized gravitational entropy and entanglement entropy
What are the differences (if any) between Generalized gravitational entropy (Lewkowycz-Maldacena) and holographic entanglement entropy (Ryu-Takayanagi)?
More specifically, I was wondering following ...
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Inertial frames and Sagnac interferometers
Let's posit:
I am in orbit around a large body, like a planet, and
I am close enough to be tidally locked to the large body.
Am I in an inertial frame? Even without looking at the stars, couldn't I ...
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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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Would a very long massive rod exhibit a large deviation from Newtonian gravity (specifically a deficit angle rather than 1/r force)?
In General Relativity the metric corresponding to an infinitely long massive rod is flat but with a deficit angle. It exhibits a very large deviation from Newtonian gravity in all regions of space in ...
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$\langle TT\rangle$ correlator of the boundary CFT from metric fluctuations in the bulk gravity
Is there a reference which explains how the $\langle TT\rangle $ correlation of the boundary conformal field theory (CFT) can be holographically calculated from the bulk gravity? (..I am often getting ...
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How to calculate gravity path integrals about an AdS background?
Suppose I have some Lagrangian of some higher derivative gravity coupled to a may be matter fields. Now I want to fluctuate it to quadratic order about an AdS background and calculate the 1-loop ...
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Squashed 3-sphere?
What is a squashed 3-sphere? In context of quantum gravity. I stumbled upon a term 'squashed 7 sphere' but that's concerning supersymmetry.
Is it just normal 3-sphere metric, that is just 'squashed' ...
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Calculating dynamical friction of a black hole passing through solid matter
Following up this question I am trying to calculate the dynamical friction of a black hole passing through solid matter
$$\frac{d\mathbf{v}_M}{dt} = -16 \pi^2 (\ln \Lambda) G^2 m (M+m) \frac{1}{v_M^3}...
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Is Brans-Dicke theory really ruled out by solar system tests?
Brans-Dicke theory with small values of parameter $w$ are said to be ruled out by solar system general relativity tests like the Shapiro time delay test and the deflection of starlight by the sun.
But ...
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Would pair production be affected by general relativity?
Given that during pair production a very small amount of energy from the photon becomes gravitational potential energy in the particles, I was curious how this would be affected by general relativity? ...
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Can a CTC contaning spacetime be purely electric?
Take a time-oriented Lorentzian manifold $(M, g)$ where $M$ is a topological 4-manifold and $g$ a Lorenzian metric.
Assume such a spacetime contains a CTC.
Since the manifold is time-oriented, one can ...