All Questions
5,503 questions
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Better equations for modeling and simulating a halo orbit?
I'm trying to model a halo orbit at low altitude (10m from surface). The satellite is using propulsion to trace the circular halo path. It looks like this "from the top" (the blue ball is ...
- 231
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0
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30
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Minimum energy to lift state of Utah one mile over 5-10 million years?
The following seems an elementary geophysics query, but I’m not sure
how to do a direct calculation. My geology text mentions that over the
past 5 to 10 million years, the state of Utah and some ...
- 159
2
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1
answer
60
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Does gravitational redshift conserve energy?
It is claimed that redshift due to cosmological expansion doesn't conserve energy. Does this exception also apply to gravitational redshift? Why or why not?
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13
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3
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580
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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♦
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42
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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-...
8
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3
answers
3k
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How does gravity overpower a vacuum?
While watching experiments with vacuum chambers, I had a thought.
If you put a sealed box at normal atmospheric pressure inside a vacuum chamber, pumped out the air and pierced the pressurized box I'd ...
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1
answer
67
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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 ...
- 441
4
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3
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591
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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 ...
- 4,833
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1
answer
757
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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=...
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4
answers
924
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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
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2
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118
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Why does a ball bounce back even when it is released and not thrown?
So according to my understanding when we thrown a ball towards ground we apply an additional amount of force which results in an acceleration greater than the one due to earth. When this ball reaches ...
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1
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47
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How does a mass that's dropped when strung by an inelastic, slack string continue its motion? [closed]
Say a mass is connected to a light, inextensible string of length $l$. The other end of the string is fixed to a point O. If initially, the mass is kept at a horizontal distance of $\frac {l}{2}$to ...
2
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1
answer
85
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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
100
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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 $2.2\times10^{-5}$ ...
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1
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44
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Dual of Newtonian gravitational field
In the static state, the laws of Newtonian gravity and Coulomb force have exactly same formulas,
$$F = K \frac{A_1A_2}{r^2}.$$
In the electrical case, moving materials produce a field, say a dual ...
- 1,419
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30
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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?
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1
answer
102
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Aren't there only 3 fundamental forces? [duplicate]
I have heard that there are 4 fundamental forces: Gravity, Electromagnetism, the Strong Nuclear Force and the Weak Nuclear Force. But I have also heard that gravity is not a force at all, it is simply ...
- 1,272
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2
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88
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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
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2
answers
49
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Stars that have fairly high gravitational redshift and calculation of their surface temperature by Planck emition spectra?
How high can the ratio between gravitational redshift and planck emition spectra be depending on the mass of the star so by how much this gravitational redshift could elongate the Planck spectra of ...
- 2,275
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4
answers
99
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Can a particle moving vertically upwards in space with constant velocity escape the earths orbit?
Let’s assume that a particle is moving vertically upwards towards space, said particle is having a constant velocity regardless of all forces acting. I repeat NO FORCES HAVE ANY KIND OF EFFECT ON ITS ...
4
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1
answer
182
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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 ...
- 482
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2
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73
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Gravitational Constant with ENM Units?
To give some context, there's a conspiracy 'theory' that I saw called Electric Universe that says that gravity is not a fundamental force and instead is a "incoherent dielectric acceleration"...
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1
answer
88
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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
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0
answers
71
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Gravitational time dilation near the Earth [closed]
I recently read the statement that near the Earth, in the Newtonian weak gravitational field, gravity is 99.9999% mainly due to "curvature of time" (ie gravitational time dilation), and only ...
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2
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1
answer
61
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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
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0
answers
54
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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 ...
- 202
3
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2
answers
94
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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
3
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0
answers
87
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Why does the wavelength of gravitational waves increase with larger energy?
Gravitational and electromagnetic waves are quite similar, as both are fundamental force waves that travel at the speed of light and have no limit to their range, but when it comes to electromagnetism,...
- 413
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1
answer
142
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Which of Kepler's laws would remain true if the force of gravity were proportional to the product of squares of each masses?
I was asked this question recently on which of the Kepler's Three law would remain if we changed the force of gravitation to be proportional to the product of squares of each masses instead of just ...
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0
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43
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Einstein's equation of gravitation field [duplicate]
I'm looking for the reason why there is the number eight $8$ at the r.h.s. of EI:
$$R_{\mu\nu}-\frac{1}{2}Rg_{\mu\nu}=\frac{8\pi G}{c^2}T_{\mu\nu}.$$
My attempt was to take the limit of this equation ...
- 255
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0
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78
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Variational description of modified Einstein equations
Let us suppose that we have an Einstein equation of the form
$$ R_{(\mu \nu)}-\frac{1}{2} g_{\mu \nu} R=8\pi T_{\mu \nu},$$
where $R$ is an affine connection, which differs from the Levi-Civita ...
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2
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1
answer
83
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On the existence of Gravitational energy in GR [duplicate]
I was reading this paper that puts forward the argument that Gravitational energy in GR is unnecessary and doesn't exist and that got me wondering if this is a fringe theory or what exactly is the ...
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1
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146
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Do we really know the universal gravitational constant?
We've all heard $$F_g=\frac{gm_1m_2}{r^2}.$$ However, since I took physics, we've discovered "dark energy," which if I have any concept of the current thinking is caused by space being ...
1
vote
1
answer
114
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Does dark matter have mass?
When trying to understand what dark matter is, it is helpful to know that some properties of it can already be derived from various observations, such as, it only interacting via gravity and no other ...
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1
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Why sun revolve around the sun ? Why cant it just rotate ? ( gravitas attraction force makes it revolve , how?) [closed]
Why the revolution ?
How General relativity theory explains it
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0
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79
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Polarization tensor of graviton in $d$ dimensions
Take the following tensor, that is the sum over the two polarizations of a gravitational wave in 3 spatial dimensions:
$$E_{ijkl}(\vec{k})\equiv\sum_{\lambda = +,\times} \epsilon^\lambda_{ij}(\vec{k})\...
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70
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Is it possible to know if you're moving or standing still due to the definition of Einsteins equivalence principle?
I have a question regarding Einstein's theory of relativity.
Einstein's equivalence principle states that locally it's not possible to tell if you're accelerating or being stationary in a ...
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5
answers
311
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How is Gravity, assuming only General Relativity, *not* like Centrifugal Force?
It is common to state that "Gravity is not a force" due to its interpretation as a curvature effect in general relativity. By this, is it right to say that gravity is a fictitious force due ...
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0
answers
55
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Can there be structures made from neutrinos that can have angular momentum?
Would it be possible to eventually have structures made from neutrinos somewhere in the universe, as it is indicated in this question (Are neutrino stars theoretically possible?), like halos of ...
- 2,878
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1
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101
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Does dark energy work on the principle of anti-gravity, i.e. repulsive gravity?
Our universe is made up of 95% dark energy+ dark matter (of which most is dark energy), and this dark energy is considered to be the main reason for the expansion of our universe. But, anything that ...
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2
answers
76
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Gravitational potential due to arbitrary shape
Outside the uniform sphere, the potential is expressed as if all the masses are concentrated in the center of the sphere.
Is it also true for arbitrary shape?
That is, outside any body, is potential ...
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0
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37
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Contribution of gravitational energy to mass of spherical shell
Suppose we have a spherical shell of mass M. Suppose we are able to variate it's radius while maintaining the same mass.
When two masses are subjected to attraction and are approached we may extract ...
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3
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114
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Suppose a shell of a sphere formed by matter and with large radius, start afterwards to diminish the radius, give $E=mc^2$ the system stops
Thinking in classical mechanics terms but with the knowledge that $E=mc^2$ let's make the below thought:
Suppose you have a shell of a sphere formed by a mass uniformly distributed over the surface of ...
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5
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2
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532
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Do two bodies with the same mass but different volumes, exert the same gravitational pull?
Asking this strictly from GR, not Newtonian gravity. I am not well versed with GR, so let me know if my question fundamentally misunderstood the concepts of GR, but if there are two independent bodies,...
-7
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1
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146
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So just because gravity "merely" bends space and isn't "really" a force at a distance - isn't it still a thing at a distance? [closed]
As a preamble, just for clarity as far as I can remember (I was awfully drunk) I have a degree in physics, math and comp sci: my point is "here's a probably stupid question at the level of person ...
- 1,004
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6
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9k
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If gravity is not a force, what makes massive objects spheroid?
For most of my life, the explanation given for why celestial bodies like stars, planets, etc. are round is due to gravitational force. Simply put, if an object has enough mass, it will, in turn, have ...
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3
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1
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321
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Ideal Gas law and Universal gravitational constant
In a hypothetical universe with a different Universal gravitational constant $G$, will the nature of ideal gas change by any means?
i.e, will $PV = nRT$ be no longer applicable in that case?
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2
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79
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Energy created by gravity
So just a thought experiment: I take my rocket and fly through space. Meanwhile I pick up some piece of debris that experiences no (big) gravitational pull. I attach it with an infinitely long rope to ...
1
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0
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25
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Is it possible to change the amount of gravity applied to an object? [duplicate]
The question might be silly. I'm not a physics student either my question might be asked incorrectly. I would love to learn from you.
Is it possible to change the gravitational force applied to an ...
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1
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91
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How does gravity act and propagate in a 2+1D universe?—Newtonian versus general relativity
In a hypothetical 2+1D universe:
if we apply the Newtonian concept of gravity, we might expect that the gravitational force between two mass points with a distance of $r$ would diminish linearly with ...
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