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.
5,069
questions
0
votes
0
answers
24
views
How to derive the surface gravity in terms of redshift factor in static spacetime?
I am following Carroll's book "Spacetime and Geometry" p. 245-247. I have the following expression for the surface gravity of a Killing horizon:
$$\kappa^2=-\frac{1}{2}(\nabla_{\mu}K_{\nu})(\...
0
votes
0
answers
14
views
Can gravitational waves be compared with a sinking water phenomena?
Let say we have some water in the sink and open the closure.. The water starts to move towards it in a whirpool like manner.. If we have a table tennis ball and leave it near the hole of the sink it ...
- 2,223
-5
votes
1
answer
57
views
Have we misunderstood gravity? Instead of gravity being the result of dense mass, might mass (eg planets) be a result of dense pools of gravity? [closed]
Consider how “Jumbos” might form without the traditional premise of star and planet formation.
Consider how there isn’t sufficient known mass in the universe to “hold it together” in gravitational ...
- 7
2
votes
1
answer
37
views
Is there a distance from a gravitational source where the influence of gravity and dark energy are balanced out?
While gravity is a force that attracts
objects with mass, dark energy (or the accelerated expansion of the universe) is not.
However, I have found numerous articles, forums, questions in the stack ...
- 1,810
2
votes
2
answers
1k
views
What kind of coordinate change is needed to make gravity disappear?
I understand that the Christoffel symbols associated with the metric will vanish locally once you perform the appropiate change of coordinates. These new coordinates correspond to an observer in free-...
- 51
-5
votes
0
answers
38
views
Exploring Quantum Wiggles with AI: Gravitational Waves and the Double-Slit Mystery [closed]
I would like to preface this by stating that I am by no means a physicist, but I am an engineer who can't stop thinking about how similar the macrocosmic universe is to the quantum realm.
I asked ...
1
vote
0
answers
36
views
What is the difference between GPE and gravitational self energy in GR?
What is the difference between gravitational potential energy and gravitational self energy in General Relativity? Are they both the same in Newtonian gravity?
- 359
1
vote
1
answer
80
views
Dark energy contributing to, or modifying, mass estimates?
I have found some papers (like this one: https://www.aanda.org/articles/aa/full_html/2009/45/aa12762-09/aa12762-09.html) which say that dark energy increases the potential energy in a system of a ...
- 1,810
2
votes
2
answers
88
views
Can a body escape a black hole by being thrusted? [duplicate]
I am told many time that nothing can escape black-hole because black-holes escape velocity is more than speed of light. But we know object don't necessarily have to exceed speed of light to escape a ...
- 41
0
votes
0
answers
23
views
Calculate mass from time dilation
Hi not a physicist in any way so was wondering if I know the age (time passed) and mass of an one object and the age (time passed) of another object, can I calculate its mass?
So Object 1 is Earth:
...
- 101
-4
votes
0
answers
90
views
Why don't black holes expand forever? [closed]
I was watching a video by Leo Susskind https://www.youtube.com/watch?v=yLOHdW7dLug discussing the expansion of black hole internal structure. He originally felt this broke the laws of thermodynamics. ...
- 2,643
4
votes
2
answers
342
views
Are objects in an uniform field inertial?
It is currently understood that gravity is not actually a force, and a fact that is often used to show this is that an object in free fall doesn't "feel" that it is accelerating and is thus ...
- 366
3
votes
2
answers
159
views
The limit of GR with infinite speed of light $c$
Just answer what you can. I don't mean the zero curvature flat space time version. I know that the Einstein Field equations use $c$ as a constant, but what would the universe be like if gravity was ...
- 53
4
votes
2
answers
211
views
Allowed Topologies for General Relativity
Studying the ADM formulation of General Relativity the ADM splitting comes out from the assumption that the spacetime is globally hyperbolic.
From that assumption thanks to Geroch's theorem, it is ...
- 1,104
-3
votes
1
answer
43
views
Could black holes be a three dimensional object breaking through space time and falling 4th dimensionaly [closed]
I was thinking about general relativity and I was thinking about it in two dimensions where a heavy metal ball would be placed on a mesh fabric (this is just how I’m imagining it) and if the ball was ...
- 11
1
vote
1
answer
44
views
Why are my heavier objects sliding on a smaller incline than lighter objects? Coefficient of Static Friction
I gave my students a lab on the coefficient of friction in 2D. I use a wooden plank and sandpaper as my IV. I have done this experiment in a few different ways, but the normal consensus was still the ...
0
votes
1
answer
43
views
Calculating an arbitrary metric tensor (field) in vacuum: Make use of the constant speed of light?
The metric (=field of metric tensors) is the solution of Einstein's field equations when a special distribution of matter is given. It is among the unsolved problems of physics to calculate the metric ...
- 81
-1
votes
1
answer
85
views
Gravity cause wavefunction collapse? (Roger penrose) [closed]
Roger Penrose suggested that gravity might play a role in the collapse of the wave function (which describes a system as a superposition of multiple values of Position, momentum etc.). According to ...
- 9
-1
votes
0
answers
32
views
Can energy redistribution Influence Spacetime Warping According to General Relativity? [duplicate]
I have a question regarding the interplay between modern technology and Einstein's general theory of relativity. According to Einstein's theory, the warping of spacetime is directly related to the ...
6
votes
2
answers
222
views
Why does rotation make black holes smaller?
A non-rotating black hole has a Schwarzchild radius of $2GM/c^2$. A rotating black hole of the same mass has a smaller outer horizon radius, down to a limit of $GM/c^2$ at the fastest possible ...
- 167
0
votes
1
answer
37
views
When calculating the amount of missing mass in a galaxy due to dark matter, do cosmologists take into account local effects of gravity on time? [duplicate]
If I understand correctly, massive objects cause time dilation, and so time seems to pass more slowly for observers closer to a massive object than those who are farther away. Do cosmologists take the ...
- 33
-1
votes
0
answers
32
views
My question learning gravity [duplicate]
so einstein told us two things: number one matter curves spacetime so if you put the earth on you assume so einstein said assume that there is a curved sheet or a curved rubber sheet and if you put ...
- 85
-4
votes
1
answer
56
views
Is gravity just a repulsion coming from all directions? [closed]
Why not explain the apparent attraction of masses by a repulsion coming from all directions in space (perhaps the dark force)? I.e. there is no gravitational force, just a repulsive force. A point in ...
- 1
2
votes
0
answers
57
views
Newton-Cartan from GR
How does EFE reduce to Newton-Cartan Field Equation $R_{tt}=4\pi G \rho$ in Newtonian Limit? I understand its direct derivation from geodesics in weak field, what I am curious about is how EFE reduces ...
- 773
1
vote
0
answers
47
views
Zel'dovich pancake derivation
I am looking for a derivation of the Zel'dovich pancake. Does anyone have a reference to the derivation or a link to the original paper?
Y. Zel'dovich, Gravitational instability: An Approximate theory ...
0
votes
0
answers
28
views
Mass Conversion from Potential Energy in Merging Black Holes
I am considering a hypothetical system of two black holes, each with a mass of 10 solar masses, uncharged and without spin (for the sake of simplification). Initially, they are separated by a distance ...
13
votes
2
answers
824
views
Is gravitational binding energy or gravitational self-energy a source of gravity?
The gravitational binding energy or self-energy of a system is the minimum energy which must be added to it in order for the system to cease being in a gravitationally bound state. Equivalently, the ...
- 359
0
votes
1
answer
27
views
How a force that is always perpendicular to velocity can change the kinetic energy when another force is acting oppposite to velocity? [closed]
When a satellite is revolving around a planet with an orbital velocity in presence of air resistance, the satellite's Kinetic Energy keeps on increasing as it falls down. The resistive force is trying ...
- 101
3
votes
2
answers
413
views
Reformulate Einstein equations to make them linear
Is it possible to reformulate the Einstein equation in terms of a new variable, say $k_{\mu\nu}$ in terms of the metric $g_{\mu\nu}$, in order to make the Einstein equations linear in $k_{\mu\nu}$?
- 726
1
vote
0
answers
79
views
Writing a gravity equation
I need a maple cod to variation this action with respect to tensor metric $g_{\mu\nu}$.
This called the Einstein equation. To obtain the Einstein equation, we vary the action with respect to the ...
1
vote
1
answer
61
views
Inertial Mass = Gravitational Mass. Why? [duplicate]
Okay, so the inertial mass of an object is always equal to the gravitational mass of the object. Conceptually, however, they seem different. Then what makes them identical? Is it because they are ...
- 995
1
vote
1
answer
128
views
Star Radius in the Oppenheimer-Snyder metric using ADM formalism
I'm working with gravitational collapse models, in particular with the Oppenheimer-Snyder model.
Short list of the assumptions for those unfamiliar with the model, you have a spherical symmetric ...
- 1,104
1
vote
0
answers
68
views
If positive mass distorts spacetime (spacetime gets curved toward the object) then would a theoretical negative mass make spacetime bulge out?
So lets assume for this question that negative mass exists then will a theoretical planet made of negative mass create an outward bulge in spacetime to create antigravity (repulsive). Also for this ...
-7
votes
1
answer
70
views
Contradiction of propagation speed of gravitational waves with non-locality of gravity?
Although LIGO at 2016 verified the speed of a traverse gravitational wave being the speed of light in a vacuum $c$ there is also the fact that gravity is a non-local phenomenon, meaning that it is not ...
- 3,728
1
vote
2
answers
45
views
Behaviour of weightless objects (objects in a state of zero gravity) in a vacuum
SCENARIO 1: So lets say there is a room with no air in it, meaning the room is a vacuum. Now lets say there is a single metal cube (the metal cube is 5kg) in the room, of course gravity still exists ...
7
votes
1
answer
258
views
What evidence do we have for GR in the nonlinear regime?
The classical equations for Einstein's GR (modulo the cosmological constant) read
$$R_{\mu\nu} - \frac{1}{2} R g_{\mu\nu} = \kappa T_{\mu\nu}.$$ These equations have a complicated linearization that ...
- 557
-2
votes
1
answer
58
views
A friend of mine seems to believe that E=mgc. He believes that gravity and light are the same. That gravity is just photons not moving. Silly I’m sure [closed]
I don’t know how to frame the question because I lack understanding of physics. But it doesn’t make sense to me when he says E=mc2 is wrong or incomplete and that his formula of E=mgc is the ...
- 25
4
votes
4
answers
485
views
Interpretation of gravitational waves
A wave has peaks and valleys. I can think of a sine wave as a wave with peaks and valleys.
Now, if gravity is a wave, can we say that gravity would have peaks and valleys, with the valley becoming ...
- 963
0
votes
0
answers
45
views
How to calculate the energy in a hyperbolic orbit?
I'm recently reading a book about rocket science which involved orbital mechanics. I know that in an elliptical orbit, the energy $ E=-\frac{GMm}{2a}$, and therefore can get the vis-viva equation: $ ...
1
vote
1
answer
103
views
Dark matter, MOND or flattened gravitational fields? [closed]
Could there not be a third variant to explain why e.g. long-distance multistar systems rotate faster than Newton's law of gravity suggests?
In addition to the Dark matter hypothesis and MOND then, ...
- 521
3
votes
1
answer
44
views
Why can this magnet hold more weight vertically?
I was browsing Amazon for magnetic hooks for my fridge and saw this diagram which surprised me because it is the opposite of what I would expect. Wouldn't the "Vertical" magnet be the ...
- 133
0
votes
2
answers
49
views
Accounting for air resistance in a free fall experiment
Suppose we are working with masses of equal density. The effect of air resistance is bigger on objects which have a larger surface area $A$ (perpendicular to $g$). In a free fall experiment, this ...
- 43
2
votes
0
answers
35
views
What are the current theoretical limits on the size of a wormhole?
What are the current theoretical limits on the size of a wormhole?
Some physicists believe that wormholes could be as small as the Planck length, which is about 10^-35 meters. Others believe that ...
1
vote
2
answers
116
views
What is the gravitational field intensity of a uniformly distributed mass content in Newtonian gravity?
In an infinite universe composed of single point masses which can be simplified as a uniformly distributed mass density, what is the equation for the gravitational field intensity in Newtonian gravity?...
- 359
0
votes
1
answer
39
views
Second mass moment for a specific rotating object
The polarization amplitude for gravitational waves can be written as a function of second mass moment. For example for a propagating wave along the $z$-direction, we have
$$ h_+ = \dfrac{1}{r}\dfrac{G}...
- 159
0
votes
0
answers
76
views
A question about de Sitter-invariant general relativity
In the case of standard general relativity, all solutions to the gravitational field equations are spacetimes that reduce locally to Minkowski. Einstein’s equation is written as
$R^{\mu \nu}-\frac{1}{...
- 73
1
vote
0
answers
57
views
What is the regime of validity for linearized gravity?
A common formulation of the approximation is to assume the metric is give by
$$g_{\mu\nu}= \eta_{\mu\nu} + h_{\mu\nu},$$ where $\eta_{\mu\nu}$ is the Minkowski metric and the components of $h_{\mu\nu}$...
- 67
2
votes
1
answer
220
views
How to calculate the period of non-circular orbits?
How to calculate the period of non-circular orbits?
By conservation of mechanical energy:
$$
E = -\frac{GMm}{r} + \frac{1}{2}\mu \left ( \dot{r}^2 + r^2 \dot{\theta}^2 \right )
$$
By the conservation ...
- 1,213
1
vote
1
answer
93
views
Gauge invariance of linearized gravity with an arbitrary background spacetime
Consider here a background metric $g_{\mu\nu}$, we impose a perturbation $g_{\mu\nu}+\epsilon h_{\mu\nu}$ with $\epsilon\ll1$. Then we can write down the modified Einstein-Hilbert action with zero ...
- 330
0
votes
0
answers
34
views
What is the evidence against a variable gravitational constant? [duplicate]
I understand that our main supporting evidence for dark matter is the anomalous speed of objects orbiting around the edges of distant galaxies. Is there a reason why dark matter solves this problem ...
- 101