All Questions
Tagged with gravity general-relativity
2,034 questions
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Can you directly feel the effect of gravity, or only opposing forces?
I was trying to think through the consequences of Einstein's observation that a freely falling body does not experience its own weight. And, as freely falling (or "inertial") bodies will ...
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Cosmological constant and gravitational lensing
The cosmological constant, $\Lambda$, leads to a uniform positive energy throughout the universe. It would lead to a more positive Ricci Scalar.
The question is whether it should lead to slowing down ...
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Should density be considered in the role of the strength of a gravitational field?
Ok, here me out. Black holes are usually formed from the compression of the mas released during the death of a supermassive star, however they have the same mass as the star (or even less). Yet when ...
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In GR, what is Gravity? A force or curvature of spacetime?
GR starts with a principle called the Equivalence Principle, which states that gravity is indistinguishable from acceleration locally.
Here, what do we mean by gravity?
Case 1: Do we mean gravity as a ...
user247935
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Stretching of space as distance to massive body like black hole decreases, true or false?
This question is about the stretching/elongation/expansion of a measuring rod in a gravitational field according to general relativity. It's not about the spagettification of something falling into ...
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Why can magnetic fields be blocked but gravitational fields can't? [duplicate]
Except for the stuff I know, I don't know anything about physics. I assume a magnetic field can be blocked, because they put Nixon's tapes in a lead box. And I know gravitational fields cannot be ...
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Portal Loop in a gravity well [duplicate]
Weirdly, I didn't find an answer or even close to one with a search through the forum. I probably missed it.
Portals! Speed thing go in, speedy thing go out. Lots of sci-fi fun.
I've got a hair-better ...
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Density changes of 3D projections of spacetime in relativity theory?
In relativity theory, the metric tensor $$g_{\mu\nu} $$ describes the gravitational field.
Can one treat the determinant of the metric tensor, normalized by the square of the Jacobian of the ...
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Query regarding the popular demonstration of spacetime curvature [duplicate]
These demonstrations usually begins with some guy taking an table covered with some fabric, put a heavy weight in the middle of it, calls them the "Sun", and makes some marbles ("...
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Variation of Chiral Plebanski Action for GR
In the Einstein-Cartan action for GR in the 'first-order' formalism, one has a solder-form $e^a$ together with a connection $A^{ab}$ assuming values in $\mathfrak{so}(1,3)$. The action
\begin{equation}...
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General relativity near an infinite plane
By Newtonian physics if we are above a infinite plane with acceleration of gravity $2$ then falling object reach to plane in $1$ second and this is independent of horizontal speed of falling body. Now ...
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Formulating the dynamics of mass distribution in Newton-Cartan gravity
I am a mathematician following the lectures of Schuller about General Relativity, in particular this lecture. My main motivation is to understand how do we use Einstein field equation to model a ...
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Why can’t acceleration in GR be defined the same way as in EM?
This question stems from page 36 of Misner, Thorne and Wheeler’s Gravitation, where they compare the Lorentz force equation with the equation of geodesic deviation. Specifically, see the second row in ...
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What is the cause for the acceleration of Earth?
I've read that spacetime curvature is present wherever initially parallel geodesics converge or diverge, and also that mass causes spacetime to curve. If there is indeed spacetime curvature due to the ...
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Problem understanding the bending of space-time curve as gravitational force! [duplicate]
I got the point of considering gravitational force as curvature on space-time fabric for bigger objects like stars, planets, blackholes. But my doubt is over the objects like us, what keeps us on this ...
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About Relativistic time in different planets and moons [duplicate]
Since the planets and moons in the Solar System has diferent gravitational fields, What happen to the relativistic course of time if we compare it to an absolute Newtonian time or in other words ...
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Why light have gravitational lensing? [closed]
I was reading about light in a book there it was there that light is not a physical quantity, light has no mass as photons are more and more smaller then atoms. Then how does gravity affect the path ...
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If the electrostatic force is so much stronger than the gravitation force, then how do black holes form?
When most discuss black-holes, they refer to the 'singularity' at the centre. A point of theoretically infinite density at an infinitesimal point. However, if the electrostatic force repelling the ...
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Can gravitational attraction be thought of as a cascade of gravitational length contractions? [closed]
We know from the answer to previous questions that gravity contracts length and the length contraction only happens vertically, that is towards the centre of mass.
If we think of an object at a ...
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How to correctly find the "linearly extrapolated" retarded position of gravity
I'm unsure how to interpret the "linearly extrapolated" retarded position of gravity, or electromagnetic field as well. I've read up on as many explanations of this as possible and am still ...
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Why is a black hole black, according to theory (and observation, and ignoring QM & Hawking radiation)? [duplicate]
I had already edited this question in response to the criticisms of those who closed its first version, as described in my comment below following the first group of others' comments. Perhaps I ...
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Is there any way in which orbits emitting gravitational waves could avoid coalescence?
In the very far future, all orbits will eventually coalesce as they lose orbital energy through gravitational waves emission.
However, can there be 3-body or N-body interactions in which the members ...
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Would weightlessness (i.e. in thrill rides, planes, skydiving, etc.) be different on a Flat Earth?
I was listening to a Scientific American podcast, dated March 27th, 2020, about what Flat Earthers believe and why they believe it. As I listened, Michael Marshall mentioned that FEers believe that ...
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Non-metric gravity calculations
According to "Gravity and Strings" by T. Ortin (2015), the non-metricity tensor is calculated as
$$
Q_{\rho\mu\nu}\equiv\nabla_\rho g_{\mu\nu}=\partial_\rho g_{\mu\nu}-\Gamma^\beta_{\rho\mu}...
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Quantum mechanics does not fit well with gravity (GR) - does this mean one of these two theories is wrong or incomplete? [closed]
Quantum mechanics does not fit with gravity (GR) - does this mean one of these two theories is wrong or incomplete?
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Can we have negative Ricci scalar?
Recently I faced a negative Ricci scalar in some calculations and looking for a physical interpretation for it. Is there any physical Energy-Momentum tensor that could produce a negatively signed ...
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A study on the speed of gravity
I have just come across a paper which involves a study about the speed of gravity. This has rocked me a bit, since I thought it was well known gravitational fields travel at the speed of light, in ...
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How does string force us to include gravity & GR? [duplicate]
Some physicists (e.g. Witten) often claim that string theory does not only makes gravity possible, but actually force its existence.
I have studied an introduction to string theory (mainly from ...
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Basic understanding of Gravity Wells
In all the diagrams I have seen, a large astronomical body eg. a planet or star, shows its gravity well as one dimensional in spacetime. My basic question is: Is it truly one dimensional or multi-...
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Understanding Feynman: why gravity is not a spin-0 theory?
I'm struggling to understand a certain paragraph in Feynman's "lectures on gravitation". It's lecture 3, why gravity cannot be a spin-0 theory. Here's the text:
The rejection of spin-zero ...
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Is rotation invariant under gravitational time dilation?
Imagine a giant ferris wheel with a ten-mile radius on a planet with a strong gravitational field. As it spins, different parts of the wheel experience varying degrees of time dilation due to the ...
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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}$. ...
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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 ...
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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}$$
where $g=\text{det}[g^{μν}]...
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What is the gravitational field of a hole in an infinite perfect crystal?
Or equivalently and more interestingly: In the early universe when there was uniform H/He gas everywhere, gravitational field was close to 0 everywhere. Every test particle was pulled from all sides ...
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Confused about Weinberg's result of gravitational time dilation
I am reading Weinberg's Gravitation and Cosmology. In section 3.5, the author got a result$$\frac{dt}{\Delta t}=(-g_{00})^{-1/2}\tag{3.5.2}.$$Here $dt$ is the time interval of a stationary observer in ...
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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 ...
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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 $R_o$. ...
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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 ...
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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. ...
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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 ...
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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 ...
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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, ...
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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 ...
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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},...
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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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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-...
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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 ...
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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 ...
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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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