Questions tagged [linearized-theory]
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77
questions
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1answer
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Stuck Solving MTW Gravitation Problem 20.5
I am stuck on exercise 20.5 part a) from Misner, Thorne, and Wheeler's Gravitation chapter 20. The Einstein summation convention is used throughout this post.
Problem Statement
Calculate $t^{\...
0
votes
0answers
23 views
Why linear response theory is exact for quadratic Hamiltonian?
In linear response theory, we consider the Hamiltonian $H(t)=H_0 + \theta(t)H'(t)$, where $H'$ is a perturbation that is turned on $t=0$. A standard result is that for an observable $A$,
$$ \langle A(...
1
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0answers
61 views
Gravitons from linearized gravity
Linearizing gravity as follows:
$$
g_{\mu\nu}=\eta_{\mu\nu}+h_{\mu\nu}
$$
Up to this point, everything is a 4x4 matrice.
How does one eventually recover a spin-2 particle which, according to https:/...
0
votes
2answers
57 views
Contracting Riemann Tensor Troubles
It has been several years since I looked at General relativity, and I am trying to brush up on it because it was always interesting and I am in need of it for my research.
Specifically, I am looking ...
0
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1answer
42 views
Linearizing the Lugiato-Lefever Partial Differential Equation
Problem Statement:
Given the Lugiato-Lefever equation, linearize the equation and determine the dynamics near a stationary solution by looking for a stationary solution with a small perturbation.
...
0
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0answers
43 views
General Relativity Lorentz-like equation
In the literature, it says that, in the weak-field,
$$g_{µν} = η_{µν} +h_{µν},$$
slow-motion limit, the Geodesic equation reduces to the Lorentz-like equation.
Can anyone explain this?
3
votes
1answer
172 views
Perturbed Ricci tensor due to metric perturbation i.e. $R^{(2)}_{\mu\nu}[h]$ in Linearized theory of Einstein field equation
This is an equation (7.153) from Chapter-7 of Sean Carroll's An introduction to General Relativity: Spacetime and Geometry book. I think all of you who studied GR and went thorugh Carroll's book have ...
2
votes
1answer
85 views
Reference on quantization of linearized gravity
I often see claims saying that quantization of linearized gravity can be done. Is there some standard references for it? such as original papers/ review papers / textbooks etc.
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0answers
92 views
Perturbation to the flat space metric
from the geodesic equation for non-relativistic case where $$v_i\ll c$$
$$\frac{dx^i}{dt}\ll1,{\rm for }\ c =1$$
$$\frac{dx^i}{d\tau}\ll\frac{dt}{d\tau}$$using this the geodesic equation for proper ...
1
vote
3answers
123 views
How to get space component of weak field (linearized) metric?
For Minkowski space with a weak gravitational field the metric takes the form
$$
ds^2 = (1+2\phi)dt^2 -(1-2\phi)(dx^2+dy^2+dz^2),
$$
where $\phi$ is the Newtonian gravitational potential.
You can get ...
0
votes
3answers
121 views
Is Stokes equation a reduction of Navier-Stokes equations?
The following Stokes problem:
$$\begin{cases}-\nu\Delta u+\nabla p=f&,\textrm{in }\Omega\\ \nabla\cdot u=0&, \textrm{in } \Omega\end{cases}$$
is a reduction of the Navier--Stokes equations?
...
1
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1answer
82 views
Metric tensor in linearized gravity [duplicate]
I'm studying linearized gravity, for which the weakness of the gravitational field is expressed as ability to decompose the metric into the (flat) Minkowski metric plus a small perturbation, i.e. $$g_{...
3
votes
1answer
135 views
Constraints in general relativity
In this review on inflation, on Pg. 135, Baumann talks about the energy and the momentum constraints for gravity. Are these equations the $G_{00} = T_{00}$ and $G_{0i} = T_{0i}$ components of the ...
4
votes
2answers
175 views
How do you derive a quantum field theory from a spacetime metric?
What are the first steps in converting a metric into a quantum field theory? I know roughly what to do once I have a pair of non-commuting operators, but how do I get to that point?
Specifically, I'd ...
3
votes
1answer
349 views
Linearizing the Einstein-Hilbert action; shortcuts?
I am interested in linearizing actions that are constructed out of geometrical objects. By this I mean perturbing the metric (or vielbein) and keeping in the action terms which are quadratic in the ...
0
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1answer
37 views
What is a quasistationary approximation
I was reading an article which states :
The linear-stability analysis for this system can be
performed in complete generality; but it will be best
for purposes of this review to go directly to ...
0
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1answer
2k views
How can I Derive the Equation for Coefficient of Linear Thermal Expansion?
I know the relationship between change in temperature and change in length. When the ambient temperature around any substance is increased, its length increases. This is due to molecules gaining more ...
4
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1answer
95 views
Gravitational waves energy source in linearized theory
By linearizing the metric in the following way (approach in most textbooks):
$g_{\mu\nu}=\eta_{\mu\nu}+h_{\mu\nu}\text{ with } |h_{\mu\nu}|\ll 1$
and choosing the transverse-traceless gauge a wave ...
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1answer
109 views
Can anyone explain the harmonic oscillator (in context to quantum mechanics) 2.3 (Griffiths) using Taylor series?
At the end he concludes $V(x) = V''(x_0)(x-x_0)^2$. How does he get to know that the rest are $0$? How does he conclude $V''(x_0) = k$. Please try to explain in easy ways and tough vocabulary. I don't ...
0
votes
2answers
140 views
meaning of “sufficiently small” in approximations to behaviour of a Harmonic Oscillator
So in my classical mechanics book it states:
"For any sufficiently small displacement, any system of this kind behaves like a harmonic oscillator."
When discussing SHO. So I am curious what is ...
2
votes
1answer
181 views
Why do materials obey Hooke's law? [duplicate]
Why do materials extend proportionally to the force exerted on them (Hooke's law)? I thought that
when materials are compressed or extended under force, their atoms become closer or further apart;
...
3
votes
1answer
182 views
Factor of 4 (or 2) in the gravitoelectromagnetic (GEM) Lorentz-force law. Which is correct? Why is it there?
I realize that the Gravitoelectromagnetic equations (GEM) are derived from the Einstein field equation (EFE) in the degenerate case of reasonably flat spacetime, which is the case for the propagation ...
2
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1answer
190 views
How can I find the metric in weak field limit for specific theory?
What is the general approach to finding a modified version of Poisson equation by means of the weak field limit of a specific gravitational theory? What is the first step? Can you introduce the main ...
2
votes
1answer
107 views
In what physical situations is the weak-field limit invalid?
in the weak-field limit gravitation is described by a symmetric tensor field $h_{μν}(x)$ in flat spacetime. Linear theory suffices for nearly all experimental applications of general relativity ...
1
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0answers
56 views
Has anyone ever tried to derive gravitoelectromagnetic waves equation?
Has anyone ever tried to derive gravitoelectromagnetic waves equation? As we know, there is Maxwell-like equation in gravity. Has anyone here ever formulated gravito electromagnetic waves equation ...
2
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0answers
160 views
Effective stress-energy tensor for a gravitational wave, compared to static semi-Newtonian case
There is a calculation that I had been thinking for a long time of working out to my own satisfaction, both because of its intrinsic importance and because it seemed like it would be fun. This was to ...
4
votes
1answer
716 views
What are the Maxwell's equations for gravitational waves?
Maxwell's four equations can be used to describe the propagation of electromagnetic waves. What is the equivalent for gravitational waves - if that question makes sense?
3
votes
4answers
328 views
Why change in resistivity is proportional to the original resistivity?
When there is a temperature change $\Delta T$, the change of resistivity is
(1) proportional to $\Delta T$
(2) proportional to the original resistivity $\rho_0$
Hence we can define the temperature ...
4
votes
1answer
258 views
Why is the Ricci scalar non-zero in this case?
The Einstein equations can be written as (1):
$$R_{ab}-\frac{1}{2}Rg_{ab} = -8\pi GT_{ab}$$
or by contracting the above equation with the metric tensor and resubstituting: (2)
$$R_{ab}=8\pi G(\frac{1}...
5
votes
2answers
275 views
Trace-reversed EFE and linearized gravity
I have a question about the linearized Einstein Field Equations, and in particular about the Newtonian limit. It goes as follows. If one uses the trace-reversed form of the EFE for the 00-component ...
1
vote
2answers
136 views
How to use Ashtekar's variables in classical gravitational physics?
I have often heard of Ashtekar's variables in General Relativity, because of the naturalness with which they would allow a canonical formulation of gravity, useful for a hypothetical quantum gravity ...
0
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1answer
80 views
Understanding Boyle's Law and Charles's Law
Boyle's Law is defined as follows: $PV=k$
This implies that $P_{1}V_{1}=P_{2}V_{2}$ is true while temperature and mass of confined gas is constant.
This would mean that $P_{2}=P_{1}V_{1}/V_{2}$ ...
3
votes
0answers
355 views
Gauge transformation of trace-reversed metric perturbation
This question is in reference to Exercise 30.4.2 in Thomas Moore's A General Relativity Workbook, which asks you to show that a gauge transformation of the trace-reversed metric perturbation $H_{\mu\...
1
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0answers
71 views
A question about the metric in linearised gravity
In linearised gravity the usual approach is to perturb the metric around some fixed background (often taken to be Minkowksi). My question is, does one literally perturb the metric tensor itself, i.e. $...
2
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0answers
175 views
Rationale behind the linearised Navier-Stokes equation
Some applications of fluid dynamics consider the linearised Navier-stokes equation where the advection term $(\vec{u}\cdot\vec{\nabla})\vec{u}$ is dropped.
I am trying to build a convincing argument ...
3
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0answers
51 views
Linearized variation of $\sqrt{-g}(\nabla^\mu \xi^\nu - \nabla^\nu \xi^\mu)$
The Noether current associated with invariance of the Einstein-Hilbert action,
$$
S=\int d^dx \sqrt{-g}\,R[g_{\mu\nu}]\,,
$$
under infinitesimal diffeomorphisms $\delta_\xi g_{\mu\nu}=\nabla_\mu \xi_\...
0
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1answer
73 views
Why don't we write the symmetric term $\eta_{\mu\nu}\partial_\rho\xi^\rho$ in a change of coordinates in linearized gravity?
Reading through this lecture, section (5.3.1):
In linearized gravity, when we transform from one set of coordinates to another, the background metric remains Minkowski, but we may think of a metric ...
2
votes
1answer
61 views
Linearized Euler-Poisson equation
Normalized Euler-Poisson system have the form
$$n_t+(nu)_x=0$$
$$u_t+uu_x=\phi_x$$
$$\epsilon^2\phi_xx=n-e^\phi$$
where $u $: ion velocity, $n$: the ion density and $\phi$: electrical potential, The ...
1
vote
1answer
370 views
Propagating degrees of freedom of gravitational field
I'm following Carroll's Spacetime and Geometry, chapter on Linearized Gravity, pag. 282. He splits up the metric perturbation in scalar, vector and tensor components, writes the Einstein tensor with ...
0
votes
1answer
207 views
Linearized gravity: When do we let the metric be $\eta_{\mu \nu} + h_{\mu \nu}$ and when does it reduce to $\eta_{\mu \nu}$?
I am following a standard text on GR. In the chapter on linearized gravity, the metric $g_{\mu \nu} = \eta_{\mu \nu} + h_{\mu \nu}$ reduces to $\eta_{\mu \nu}$ when the metric act on tensor components ...
0
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1answer
33 views
General relativity, Linealized gravity — which transformations are compatible with the metric?
I am following lecture notes on linearised gravity. The author states that Lorentz transformations are compatible with the metric and begins to Lorentz transform (1) the metric components (2) all ...
2
votes
1answer
203 views
Tetrad formalism in linearized gravity
I have not seen so far any application of linearized gravity treated using the local orthonormal frame approach. Stuff, such as newtonian limit, gravitational waves, etc.
I can totally see the tetrad ...
0
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1answer
45 views
Reference for the integral form of the metric in linearised gravity
Reference wanted: I was looking for a source for the integral form for the weak field metric in terms of the stress-energy tensor, as given in the wikipedia article on Linearised Gravity. The problem ...
0
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1answer
165 views
About linearisation of equations of motion rigid body
I am modeling a system of solid bodies. Consider $\theta \approx 0$ and $\chi \approx 0$. At a certain moment I get the following formula for the angular velocity:
$$ \omega = \begin{bmatrix} 0 &-...
1
vote
2answers
81 views
Why is stress linearly dependent on strain in elastic materials?
The displacement vector $\mathbf{u}(\mathbf{x}, t) = \mathbf{r}(\mathbf{x}) - \mathbf{x}$ is used to keep track of the motion of the material points in a material. Firstly, we take the gradient of the ...
0
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1answer
402 views
How use Lorentz Gauge in Einstein tensor?
i have the Einstein tensor in terms h of first order
$$
\begin{eqnarray} \label {eq: wf5o}
G_{\beta \mu}& = & -\frac{1}{2}\left[ \bar h_{\beta \mu},_{\lambda}^{\;\;,\lambda}+\eta_{\beta\mu}\...
0
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1answer
93 views
How to find a linear approximation at a critical point for a potential of a conservative field?
I am trying to understand how Arnold derives his tangent equation for branches of a critical point in phase space.
Why is the slope at critical point in the phase space- $\sqrt{-U^{''}(\xi)}$ :
2
votes
2answers
636 views
Inverse metric in linearised gravity
From what I've read, in the framework of linearised gravity, one perturbs the metric around a Minkowski background, $\eta_{\mu\nu}$, such that
$$g_{\mu\nu}(x)=\eta_{\mu\nu}+h_{\mu\nu}(x)\tag{1}$$
...
5
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1answer
314 views
Why is there no Green's function when no gauge is chosen (Linearized Gravity)
I'm working on linearized gravity, and have come to the point where you need to choose a gauge to simplify the Einstein equation for the perturbative field.
Reading a paper, I came across the claim "...
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1answer
281 views
Gravitational waves (linearized gravity) [closed]
Even in the Schwarzschild metric we can write $g_{uv}=\eta_{uv}+h_{uv}$ where $h_{uv}$ is very small. So after some coordinate transformation (using gauge freedom) we can simplify the Einstein ...
