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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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14 votes
3 answers
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Can perfectly stable orbits exist in GR?

Defining "stable orbit" between two bodies as one where, in the absence of other bodies or non-gravitational forces, the distance stays between some value pair $r_{min}>0$ and $r_{max}$. ...
SarcasticSully's user avatar
-3 votes
0 answers
44 views

What is the cause of gravity according to general relativity? [duplicate]

The question is based on general relativity and it can help big bang theory.
surya dev's user avatar
1 vote
1 answer
33 views

Cause of Coordinate Acceleration in Free Fall [duplicate]

So I understand that objects in free fall are in an inertial frame, at rest in terms of relativity. However, from a person on the surface of earth, a falling apple is accelerating constantly until it ...
Marco Chacon's user avatar
0 votes
0 answers
40 views

What is a simple example that contradicts the theory that light (photons) could have a super small mass? [duplicate]

The obvious example is that, to bring something that has mass to the speed $c$ requires infinite energy due to special relativity. But what if a kid asked me "What if light/photos had a super ...
chausies's user avatar
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Energy of the gravitational field within a sphere of radius $R$ in the Schwarzschild metric

The Landau-Lifshitz energy-momentum pseudotensor $t^{μν}$ is defined by $$16πt^{μν} = -2G^{μν} - g^{-1} \left[ -g \left( g^{μν}g^{αβ} - g^{μα}g^{νβ} \right) \right]_{,αβ}$$ where $g=\text{det}[g^{μν}]...
Khun Chang's user avatar
-2 votes
3 answers
100 views

What is the gravitational field of a hole in an infinite perfect crystal?

Or equivalently and more interestingly: In the early universe when there was uniform H/He gas everywhere, gravitational field was close to 0 everywhere. Every test particle was pulled from all sides ...
Alien from future's user avatar
3 votes
2 answers
628 views

Areas with anti-parallel gravity in classical physics

I cannot indicate an error in the following reasoning if it is done in the framework of classical physics. Let's make the imaginary setup with two wedges and the ball, when the gravity in the left ...
Artem's user avatar
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0 votes
2 answers
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Confused about Weinberg's result of gravitational time dilation

I am reading Weinberg's Gravitation and Cosmology. In section 3.5, the author got a result$$\frac{dt}{\Delta t}=(-g_{00})^{-1/2}\tag{3.5.2}.$$Here $dt$ is the time interval of a stationary observer in ...
rioiong's user avatar
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1 vote
0 answers
50 views

Does gravity accelerate you towards the geodesic of light between you and the mass?

If there's a planet far away, you will accelerate straight towards it due to gravity. If you place a Schwarzschild black hole right in the middle between you and the planet (the distance between the ...
Zach's user avatar
  • 171
3 votes
0 answers
52 views

Negative (absolute - not potential) energy of the gravitational field; how to generalize to GR?

Alan Guth gives a thought experiment to show that a gravitational field has negative energy. (See the picture below.) Consider a thin spherical shell of elastic, compressible matter, of radius $R_o$. ...
Khun Chang's user avatar
4 votes
0 answers
93 views

Lorentz force error in the present 2024 version of the gravitoelectromagnetism Wikipedia page? [closed]

I noticed that the Gravitoelectromagnetism (GEM) Wikipedia page has been edited recently. The factor of 4 in the GEM Lorentz force equation is now missing. But the GEM field equations are identical to ...
rdryne's user avatar
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2 votes
0 answers
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Gravitational halos made of neutrinos...?

I have been recently interested in how halos made of standard model particles could be formed and behave. After asking some questions in this site, I was told about how neutrinos could form such halos....
vengaq's user avatar
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2 votes
1 answer
76 views

How to properly combine kinetic and gravitational time dilation effect?

I developed a time dilation calculator that includes both kinetic (Lorentz Factor) and gravitational (Schwarzschild Metric Formula) factors to assess the time difference between Earth and satellites. ...
Eliot Mallamo's user avatar
8 votes
5 answers
1k views

Do you always experience the gravitational influence of other mass as you see them in your frame?

You see a galaxy far away. That galaxy is attracting you with a certain amount of gravity. I'm wondering if the gravity influence of the galaxy on you, as measured by you, always ends up being what ...
Zach's user avatar
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1 vote
3 answers
75 views

How much time does it take for an object to fall from space? [closed]

Let's say there's an object of mass $m$ in space, $h$ meters away from the surface of the Earth. $h$ is large enough that $g$ cannot be assumed to be constant. The acceleration varies according to ...
jazzblaster's user avatar
3 votes
2 answers
86 views

Tug of war between observers in frames with different rate of time

You have a very dense hollow sphere of matter. Observer A is inside the sphere inside a rocket. Observer B is in an identical rocket outside the sphere where the ring's gravity is negligible. They are ...
Zach's user avatar
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1 vote
3 answers
103 views

Do clocks tick faster when gravitational forces are weaker?

A professor last year taught us that "gravity slows clocks," when teaching about the relationship between gravity and time. This led me to think about places, such as intergalactic space, ...
William Solomon's user avatar
-3 votes
3 answers
76 views

Does Matter Cause Curvature or Vice-Versa [closed]

From the way explanations about gravity-acceleration-curvature equivalence are usually phrased here or elsewhere, it would appear many or most think that matter causes space-time curvature. I cannot ...
Prototypist's user avatar
4 votes
1 answer
197 views

Binary black hole merging condition

Assuming two black holes with the same rest mass $m$ collid coming from infinity with velocity $v$ and impact parameter $b$. Lets ignore spin at first. For which values of $v$ and $b$ would these ...
Okarin's user avatar
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0 answers
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How significant is the parabolic arch effected with an incline versus a decline in angle versus a decline in angle . For example throwing a dart

A dart players height can vary as can the height in which they raise their arm to aim. Is there less parabolic arch when throwing at the same strength from above the target compared to below, due to ...
Marc Kerr's user avatar
5 votes
1 answer
457 views

Bound states between neutrinos using Schrödinger's equation?

I would like to see if it's possible that neutrinos (with sufficiently slow velocities) could form bound states in a universe with matter (such as ours) There is a cosmic neutrino background in the ...
vengaq's user avatar
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2 votes
0 answers
33 views

Second-order equations of motion for higher derivative gravity?

We know that Lovelock gravity is the most general theory of gravity possible for Lagrangians which depend only on the metric tensor and the Riemann tensor \begin{equation*} L = L \left(g_{\alpha\beta},...
Ishan Deo's user avatar
  • 1,588
0 votes
0 answers
29 views

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 ...
Nico Brenner's user avatar
0 votes
0 answers
30 views

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 ...
user86742's user avatar
  • 149
2 votes
1 answer
48 views

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?
Derek Seabrooke's user avatar
11 votes
2 answers
513 views

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's user avatar
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0 votes
0 answers
40 views

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-...
olivierlambert's user avatar
8 votes
3 answers
2k views

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 ...
Walt Spring's user avatar
0 votes
1 answer
57 views

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 ...
Qubit's user avatar
  • 431
5 votes
3 answers
414 views

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 ...
MadMax's user avatar
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7 votes
1 answer
745 views

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=...
Lagrangiano's user avatar
  • 1,616
4 votes
4 answers
911 views

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 ...
Ajay's user avatar
  • 627
0 votes
2 answers
81 views

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 ...
Madly_Maths's user avatar
0 votes
1 answer
46 views

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 ...
Shakthi Weerawansa's user avatar
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 ...
James Warning's user avatar
0 votes
1 answer
86 views

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 $2.2\times10^{-5}$ ...
Quantum Wonder's user avatar
0 votes
1 answer
41 views

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 ...
moshtaba's user avatar
  • 1,409
0 votes
0 answers
27 views

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?
Alexandre Masson Vicente's user avatar
0 votes
1 answer
92 views

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 ...
user107952's user avatar
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0 votes
2 answers
83 views

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 ...
Zach's user avatar
  • 171
0 votes
2 answers
42 views

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 ...
Krešimir Bradvica's user avatar
0 votes
5 answers
89 views

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 ...
Aadarsh Bagul's user avatar
4 votes
1 answer
169 views

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 ...
Peter Moore's user avatar
0 votes
2 answers
56 views

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"...
null_set's user avatar
0 votes
1 answer
85 views

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 ...
Curious Steve's user avatar
2 votes
0 answers
69 views

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 ...
Rene Kail's user avatar
  • 928
2 votes
1 answer
58 views

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 ...
Wouter M.'s user avatar
  • 243
1 vote
0 answers
48 views

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 ...
stickynotememo's user avatar
3 votes
2 answers
84 views

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, ...
Hadi Khan's user avatar
  • 531
3 votes
0 answers
80 views

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,...
Quantum Wonder's user avatar

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