Tagged Questions
1
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1answer
45 views
How does gravity effects both time and light if they have no mass [duplicate]
I've been reading about how black holes can effect both time and light with gravity. So I was wondering, doesn't something have to have mass to be effected by gravity? And if so, does this mean both ...
0
votes
1answer
91 views
can be exist the negative mass? [duplicate]
I'm not sure about this but I guess there must be negative masses in the universe because of the symmetry. If the gravity is one of the main forces in nature it must has negatives mass to be able to ...
1
vote
1answer
29 views
Can we build a synthetic event horizon?
If we imagine ourselves to be a civilization capable of manipulating very heavy masses in arbitrary spatial and momentum configurations (because we have access to large amounts of motive force, for ...
13
votes
1answer
149 views
Is period of rotation relative?
My question is inspired by the following answer by voix to another problem:
"There is a real object with relativistic speed of surface - millisecond pulsar. The swiftest spinning pulsar currently ...
1
vote
2answers
105 views
Negative potential energy of gravity
Does the negative potential energy in the gravitational field have to be considered in calculating the total mass of the system in question (because of $E=mc^2$)?
If so it seems to me that the ...
1
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5answers
138 views
How universal gravitation falls short
As a non physicist I can understand how Newtonian mechanics falls short in cases of high velocity etc. and is properly generalized by the special theory of relativity.
What is not clear to me is how ...
2
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2answers
49 views
Can the effects of a person's mass upon the local gravitational field be detected and measured remotely?
As the title suggests, Can the effects of a person's mass upon the local gravitational field be detected and measured remotely?
I am aware any mass produces and effects gravity but couldn't find ...
10
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5answers
535 views
Why do we still need to think of gravity as a force?
Firstly I think shades of this question have appeared elsewhere (like here, or here). Hopefully mine is a slightly different take on it. If I'm just being thick please correct me.
We always hear ...
-1
votes
1answer
54 views
What is mathematical definition of a strong gravity?
Mathematical definition of a weak gravity is simple $g=\frac{GM}{r^2}$
but what is mathematical definition of a strong gravity? (blackhole-like or close to a blackhole-like object)
7
votes
1answer
149 views
Why is Einstein gravity not renormalizable at two loops or more?
(I found this related Phys.SE post: Why is GR renormalizable to one loop?)
I want to know explicitly how it comes that Einstein-Hilbert action in 3+1 dimensions is not renormalizable at two loops or ...
1
vote
3answers
147 views
What truly is mass, and is there a direct way to measure it?
We know a mass of an object of one kilogram as an object that weighs W = mg = 9.8 N and we reference it to that, (when it should as a fundamental parameter describe weight not the opposite). But if we ...
6
votes
1answer
103 views
Why is $R^2$ gravity not unitary?
I have often heard that $R^2$ gravity (as studied by Stelle) is renormalisable but not unitary. My question is: what is it that causes the theory to suffer from problems with unitarity?
My naive ...
0
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2answers
110 views
Einstein's theory tells us that gravity is a curve in space and time but how does that causes attraction in mass? [duplicate]
The sun is incredibly massive object and it causes the space around it to bend. This causes the planets to pulled to the sun or the planets move in an elliptical path around the sun. But I don't ...
1
vote
3answers
137 views
About gravity through space time curvature
Is it possible to produce virtual gravity? I mean gravity without the help of mass by curving spacetime with other effects like fast rotating objects?
2
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4answers
125 views
Does everything with mass or energy have a gravitational pull?
As small as it may be, does every 'thing' have a gravitational pull? That is, something with mass or energy. No matter how obsolete or negligible it may be, is it there? If so, how is it calculated? ...
0
votes
2answers
139 views
General Relativity & Kepler's law
According to Kepler's law of planetary motion, the earth revolves around the sun in an elliptical path with sun at one of its focus.
However,
according to general theory of relativity,
the earth ...
2
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5answers
251 views
Einstein gravity versus Newton's gravity
What's the basic difference between the gravity as seen by Einstein, and that by Newton?
1
vote
0answers
31 views
In which direction does space “bend”? [duplicate]
Gravitation is often depicted as a ball on a cloth that curves a hole into space. But in what direction does this hole form? Into the direction the object is moving?
3
votes
1answer
85 views
How can photons exert gravity if they are wave-like?
As a reference, see this question: Does a photon exert a gravitational pull?
It turns out the answer is "Yes" -- but this does not seem consistent with light being wave-like.
I am imagining a ...
3
votes
2answers
91 views
Gravitational distortion of an object's diameter, at a distance,
Does the curvature of space-time cause objects to look smaller than they really are? What is the relationship between the optical distortion and the mass of the objects?
3
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1answer
153 views
Calculating Riemann Tensor Using Tetrad Formalism
I was trying to calculate the Riemann Tensor for a spherically symmetric metric:
$ds^2=e^{2a(r)}dt^2-[e^{2b(r)}dr^2+r^2d\Omega^2]$
I chose the to use the tetrad basis:
$u^t=e^{a(r)}dt;\, ...
3
votes
3answers
118 views
Relation between the determinants of metric tensors
Recently I have started to study the classical theory of gravity. In Landau, Classical Theory of Field, paragraph 84 ("Distances and time intervals") , it is written
We also state that the ...
4
votes
1answer
176 views
Is the quantization of gravity necessary for a quantum theory of gravity? Part II
(At the suggestion of the user markovchain, I have decided to take a very large edit/addition to the original question, and ask it as a separate question altogether.)
Here it is:
I have since ...
1
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3answers
260 views
Similarity between the Coulomb force and Newton's gravitational force
Coulomb force and gravitational force has the same governing equation. So they should be same in nature. A moving electric charge creates magnetic field, so a moving mass should create some force ...
5
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1answer
254 views
In what limit does string theory reproduce general relativity?
In quantum mechanical systems which have classical counterparts, we can typically recover classical mechanics by letting $\hbar \rightarrow 0$. Is recovering Einstein's field equations (conceptually) ...
5
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0answers
87 views
Equation of state of cosmic strings and branes
I'm sure these are basic ideas covered in string cosmology or advanced GR, but I've done very little string theory, so I hope you will forgive some elementary questions. I'm just trying to fit some ...
1
vote
1answer
192 views
In what way is the Riemann curvature tensor related to 'radius of curvature'?
In Misner, Thorne & Wheeler, they say, in their delightful 'word equations' that
$$\left(\frac{\mathrm{radius\,\, of \,\,curvature}}{\mathrm{of\,\, spacetime}}\right) = ...
1
vote
1answer
105 views
What does it mean that Einstein's equations are hyperbolic-elliptical?
I says on Wolfram MathWorld that Einstein's field equations are a set of "16 coupled hyperbolic-elliptic nonlinear partial differential equations".
What does it mean that the equations are ...
1
vote
1answer
139 views
Do residents of the Hudson Bay area have more time?
Apparently there is a gravity anomaly in the Hudson Bay Area in Canada: gravity is "missing" or it is slightly less than it is in the rest of the world.
Does that mean that things in the Hudson Bay ...
0
votes
2answers
479 views
Newtonian gravity vs. general relativity: exactly how wrong is Newton?
Is there a simple function I can use to describe the difference between simple Newtonian dynamics and the actual observed motion? Or maybe some ratios for common examples of, say, the motion of stars ...
3
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0answers
56 views
Gravitational effects and metric spaces
Could somebody please explain something regarding the Nordstrom metric?
In particular, I am referring to the last part of question 3 on this sheet -- about the freely falling massive bodies.
My ...
4
votes
1answer
165 views
Does conformal gravity explain the Bullet cluster lensing effects?
Conformal gravity is an "alternative" theory of gravity, where instead of using the Einstein-Hilbert action composed of the Ricci scalar, the square of the conformal Weyl tensor is used. It was ...
7
votes
3answers
464 views
What is the exact gravitational force between two masses including relativistic effects?
I was wondering if there is a closed-form formula for the force between two masses $m_1$ and $m_2$ if relativistic effects are included. My understanding is that the classic formula $G \frac{m_1 ...
4
votes
2answers
137 views
Is Brian Cox right to claim that Gravity is a strong force for large masses, is it wrong, or is it only a matter of interpretation?
I watched a program of his in which it was claimed that since mass bends space in accordance to General Relativity, then in the case of very large stars it becomes a strong force to the point of being ...
2
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2answers
149 views
Gravitation force- Attraction and repulsion
Gravitation force is always attractive.
Now assume(not a practical one): I took our Earth in my hand and started shaking up and down. This will create a disturbance in space-time warp and it moves ...
6
votes
2answers
249 views
What is the weight equation through general relativity?
The gravitational force on your body, called your weight, pushes you down onto the floor.
$$W=mg$$
So, what is the weight equation through general relativity?
4
votes
1answer
124 views
Is this closed time-like curve in a Godel universe a “circle”?
Would the observer moving along the circle in this Godel space-time diagram feel fictitious forces as though he is accelerating along a circular path or would he simply arrive at an earlier point in ...
3
votes
0answers
80 views
Materials with different gravitomagnetic permeability?
If you start with general relativity, and assume small perturbations around a nearly flat metric, it is possible to obtain linearized equations of gravity that look a lot like Maxwell's equations, ...
8
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2answers
256 views
Wavefunction collapse and gravity
If gravity can be thought of as both a wave (the gravitational wave, as predicted to exist by Albert Einstein and certain calculations) and a particle (the graviton), would it make sense to apply ...
4
votes
2answers
209 views
What is the consequence of “infinite” gravitational force?
Introduction
I am a mathematically minded individual. I do not intuitively comprehend physics, and as a sophomore in high school who has only taken Intro to Physics in his freshman year, I may very ...
5
votes
0answers
91 views
Positivity of Total Gravitational Energy in GR
I read the following statement in the introduction to an article:
Over the last 30 years, one of the greatest achievements in classical
general relativity has certainly been the proof of the ...
3
votes
3answers
311 views
Gravity stronger than electromagnetic force in a black hole?
Well, the question has somewhat been answered before, but there's one part missing, which - I'd think - is in conflict with the physical laws.
The earlier reply says that the gravitational pull even ...
2
votes
2answers
198 views
Is the Schwarzschild black hole unphysical?
To obtain the Schwarzschild metric from Einstein equations of general relativity, we suppose that the energy density is a distribution :
$$ \rho (\vec{r}) = M \delta(\vec{r})$$
The Schwarzschild ...
5
votes
0answers
156 views
Penrose Conformal diagram for flat 2-dim Lorentz space-time
I have the following metric
$$ds^2 ~=~ Tdv^2 + 2dTdv,$$
defined for
$$(v,T)~\in~ S^1\times \mathbb{R},$$
e.g. $v$ is periodic.
This is the according Penrose diagram:
Question 1) Is the ...
6
votes
2answers
277 views
Why does no physical energy-momentum tensor exist for the gravitational field?
Starting with the Einstein-Hilbert Lagrangian
$$ L_{EH} = -\frac{1}{2}(R + 2\Lambda)$$
one can formally calculate a gravitational energy-momentum tensor
$$ T_{EH}^{\mu\nu} = -2 \frac{\delta ...
-1
votes
3answers
182 views
How many pieces of toast would you need to make a black hole?
I am trying to find the following:
How many pieces of toast would you need to make a black hole?
From what I've learnt so far I need to find an equation for the compression force the massive amount ...
-9
votes
1answer
202 views
Why are we talking about space curvature as if we know what space is? [closed]
1) Why are we talking about space curvature as if we know what space is?
Every question about gravity seems to evoke an answer involving "space curvature" which seems like an undefined placeholder ...
1
vote
2answers
120 views
Most suitable metric for the Solar system?
If I wanted to solve the Einstein equations for the solar system, which choice of $g_{\mu\nu}$ and $T_{\mu\nu}$ is more suitable?
I thought about using a Schwarzschild metric near each planet, but ...
1
vote
1answer
222 views
Relationship between Alcubierre drive space-time evolution and speed of gravity
The top rated answer to this question about the Alcubierre drive asserts, "spacetime can dynamically evolve in a way which apparently violates special relativity," but according to the Wikipedia ...
2
votes
1answer
90 views
Are all static solutions of Einstein's equations spherically symmetric?
Is it true that all static solutions in GR are also spherically symmetric? Is there a proof of this?
Similarly, are all stationary solutions axisymmetric?
