All Questions
Tagged with gravity general-relativity
2,007
questions
11
votes
2
answers
513
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Why is nonzero net charge density incompatible with the cosmological principle?
In an answer to a question about the overall charge-neutrality of the universe, benrg writes,
A nonzero net charge density is incompatible with the cosmological
principle. Unlike the gravitational ...
rob♦
- 91.5k
0
votes
0
answers
40
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Topological illustration of spacetime dilation: which function should I use for isometric lines spacing?
Scientific popularization, when it comes to illustrating spacetime dilation around massive objects, often relies on the description of a two-dimensional square-grid, which can be regarded as a cross-...
0
votes
1
answer
57
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When is it appropriate to say Newtonian gravity is a force? When is it not appropiate? [closed]
Please help me understand the notion of force when it is applied to Newtonian gravity. From my understanding forces in physics involve interactions with at least 2 objects and can cause an ...
- 431
5
votes
3
answers
414
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Why is the universe charge-neutral?
The positive charges (such as from the protons) of the universe are almost neutralized by the negative charges (such as from the electrons).
Is there an explanation for this neutrality? Does it ...
- 4,452
7
votes
1
answer
747
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Can gravity radiate?
In electromagnetism, when a charge accelerates, it emits radiation. We know this because we can write the retarded potentials, apply $\vec E=- \nabla V-\frac{\partial \vec{A}}{\partial t}$ and $\vec B=...
- 1,616
4
votes
4
answers
911
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Thought experiment regarding gravity
Let there be a mass $M$ which is a spherical shell of radius $R$.
Now the Gaussian flux about a enclosing sphere of radius $r$ where $r>R$ will be proportional to $M$ or $GM$ (where $G$ is the ...
- 627
2
votes
1
answer
74
views
Definition of the gravitational constant in 1+1 gravity
In this paper, the author formulates a $(1+1)$-dimensional theory of gravity by taking the trace of the Einstein equations
$$\left(1 - \frac{D}{2}\right)R = 8\pi G_D T,\tag{2}$$
(where $G_D$ is the ...
0
votes
1
answer
86
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Is the Planck mass the "lower limit" for gravity?
The Planck units are often treated as being the "lower limits" to things: the Planck length for length, the Planck time for time, etc.
But the Planck mass, which is about $2.2\times10^{-5}$ ...
- 413
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0
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27
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Detailed derivation of ESCK gravity and Extended Friedmann Equations with Torsion
Do you know a textbook on the Einstein-Cartan-Sciama-Kibble theory of Gravitation and its application to derive Extended Friedmann Equations with Torsion, which shows the calculations in detail?
0
votes
2
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83
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How could I calculate the time it will take for light and mass to go towards a black hole and come back, to and from constant radial distances?
If you have a "perfect mirror" and a "perfect trampoline" at some constant distance outside a black hole's event horizon:
a) How would a shell observer at some distance farther ...
- 171
4
votes
1
answer
169
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How are objects inside a black hole affected by the gravity of objects outside the black hole?
There are many Q&As about whether something inside a black hole can escape the event horizon if another massive object gets close enough to pull it out. I realize the answer (I think universally ...
- 450
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votes
1
answer
85
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Questions about speed of gravity [duplicate]
If gravity "travels" at $c$, and the sun is travelling "forward", does it mean the planets are actually orbiting various points "behind" the center of the sun? Does it ...
2
votes
1
answer
60
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Acceleration at peak of a gravitational wave
The amplitude of the strongest gravitational wave signal detected by LIGO sofar can maybe be expressed as an acceleration? If so, what would the numerical value be (in m/s^2)? I would like to compare ...
- 243
1
vote
0
answers
48
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How can you use gravity while trying to model gravity? [duplicate]
So consider the usual pop-science spacetime model, a bowling ball on a trampoline. Apparently, the ball should sink into the trampoline, causing a dip in the fabric which causes nearby objects to fall ...
- 141
3
votes
2
answers
84
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How is it that energy of matter yields gravity if the amount of energy in a system is frame dependent while the force caused by gravity is not?
I've been told that the gravitational field arises due to the energy density terms in the stress-energy tensor of matter and therefore that all energy of matter exerts a gravitational field effect, ...
- 531