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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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})(\...
Damiano Scevola's user avatar
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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 ...
Krešimir Bradvica's user avatar
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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 ...
Astro's user avatar
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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 ...
vengaq's user avatar
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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-...
K. Pull's user avatar
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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 ...
Luis Berrios's user avatar
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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?
Manuel's user avatar
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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 ...
vengaq's user avatar
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2 votes
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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 ...
Zeesan's user avatar
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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: ...
zimia's user avatar
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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. ...
Rick's user avatar
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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 ...
WordP's user avatar
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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 ...
Lina Jane's user avatar
4 votes
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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 ...
LolloBoldo's user avatar
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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 ...
CoolFrog's user avatar
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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 ...
Physics Disciple's user avatar
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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 ...
Scibo's user avatar
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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 ...
ggg's user avatar
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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 ...
Ray Luxembourg's user avatar
6 votes
2 answers
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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 ...
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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 ...
Amber Lily's user avatar
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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 ...
Mantu Das's user avatar
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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 ...
Leo's user avatar
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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 ...
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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 ...
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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 ...
Alexander Kononovych's user avatar
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 ...
Manuel's user avatar
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1 answer
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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 ...
Motivix's user avatar
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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 $k_{\mu\nu}$?
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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 ...
maryam sadeghi's user avatar
1 vote
1 answer
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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 ...
Lory's user avatar
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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 ...
LolloBoldo's user avatar
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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 ...
Infinite Void's user avatar
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1 answer
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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 ...
Markoul11's user avatar
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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 ...
Infinite Void's user avatar
7 votes
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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 ...
Panopticon's user avatar
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1 answer
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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 ...
Justin Hodge's user avatar
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 ...
Angela's user avatar
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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: $ ...
Jason Jia's user avatar
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, ...
Lehs's user avatar
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3 votes
1 answer
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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 ...
aks.'s user avatar
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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 ...
Lorentz's user avatar
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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 ...
DimensionDestroyer's user avatar
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?...
Manuel's user avatar
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1 answer
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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}...
Astrolabe's user avatar
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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}{...
Dongba's user avatar
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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 $h_{\mu\nu}$...
yaron kedem's user avatar
2 votes
1 answer
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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 ...
Álvaro Rodrigo's user avatar
1 vote
1 answer
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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 ...
Lain's user avatar
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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 ...
Miles Gould's user avatar

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