Questions tagged [gravity]

Gravity is an attractive force that affects and is effected by all mass and - in general relativity - energy, pressure and stress. Prefer newtonian-gravity or general-relativity if sensible.

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How gravity would affect light "perceived" speed?

Here is the experience: A photon P travels from X to Y through tube drilled inside the earth from the North Pole to the South Pole. A photon Q travels from A to B, which is the exact same segment ...
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The behaviour of acceleration of a ball in motion

I have a few doubts on how the acceleration of a ball works when we throw it up in the air, In my mind, I could imagine it in some different "real world" cases, So I am listing them all down,...
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How can we be sure that gravity is not the reason for the perceived expansion of the universe? [closed]

The question has been partially discussed here but I believe the following formulation is a bit different. What experience could disprove the notion that the universe is not expanding despite the pull ...
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Black holes are not infinitely dense [closed]

If a black hole is infinitely dense, then therefore its terminal velocity in a determined field of gravity would be greater than the speed of light which is impossible, therefore a blackhole is not ...
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Do materials sort in order of density in space the way they do near Earth?

Obviously, if you drop materials into water near the surface of the Earth, they sort according to densities, with the least dense materials near the surface, and the most dense materials near the ...
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Proving relation between curvature of spin-connection (in 3 dimensions)

The curvature of the spin connection $\omega^{ij}:=\omega^{ij}_{\mu}\mathrm{d}x^{\mu}$ is defined by $$F^{ij}:=\mathrm{d}\omega^{ij}+{\omega^{i}}_{k}\wedge {\omega^{kj}}$$ As usual, in $3$-dimensions, ...
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Where does Okun's formula come from?

The formula is $(16)$ in Lev B. Okun's paper "The Concept of Mass". It gives the force acting on a light body by a heavy spherically symmetric mass according to general relativity. ...
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How we can visualise that gravitational potential, instead gravity is responsible for time dilation?

How we can visualise that gravitational potential, instead gravity is responsible for time dilation here on Earth? How possibly 'point A' can know that it should add some time dilation because at '...
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Speed due to gravity [duplicate]

Assuming a miraculous arrangement of a string of black holes for slingshot purpose, and a solid ball slinging by these black holes one after the other, gaining speed with each slingshot, is it ...
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Gravitational Potential Energy and mass [duplicate]

I understand when we lift an object (a mass), its potential energy increases. Also we know $E=mc^2$, does it mean when we lift an object in the gravitational field (like on earth), its mass increases?...
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If the Earth is constantly accelerating at 9.8m/s^2, according to General Relativity, won't Earth's speed be constantly increasing towards infinity? [duplicate]

As per Einstein's Theory of General Relativity, I understand that 'the Earth's ground is accelerating towards us at 9.8m/s^2. This is why we're falling towards the center of the Earth, as GR doesn't ...
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Does Newton's Theory with Retarded potentials give rise to the motion of perihelion of Planets

If we take into account the retarded potentials and the motion of the Sun(due to the planet), does Newton's Gravitational theory give rise to the motion of Perihelion of planets (qualitatively, not ...
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Is there a max terminal velocity much less than speed of light?

If you had an object falling into, say, a neutron star which had a very great gravity, is there some reason other than an atmosphere that would prevent the object from reaching any arbitrary speed up ...
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How does space change as the universe expands?

Consider a metre ruler. Despite the universe – and space itself – constantly expanding, the ruler maintains its size. If this ruler was alone in empty-ish space, other distant objects would appear to ...
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Are gravitational waves produced only when a mass accelerates? [duplicate]

Why? They are not a form of EMR, are they. Or are the rapidly changing ones the only ones we can detect?
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Why do gravitational waves move at the speed of light? [duplicate]

They are not a form of electromagnetic radiation, are they? So, why do gravitational waves move at the speed of light?
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Can the surface of a neutron star move faster than light? How length contraction manifests there?

It is known that any Kerr black hole is surrounded by ergosphere, inside which everything rotates around the black hole faster than light. Since ergosphere is greater than event horizon, one can ...
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Mach's principle and a reference frame for acceleration

Although there are different statements of Mach's principle, one statement could be that acceleration is meaningless unless it can be defined relative to something. The distance stars provide a fixed ...
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Gravitational force acting on a small element on a sphere

I was given a formula that force acting on a small element on a sphere (made by connecting two hemispheres) to the center of the sphere equals: $$dF = \frac12 \cdot G\cdot m\cdot dm/r^2$$ I cannot ...
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Theory like general relativity for charges

According to GR, the distribution of mass curves spacetime and the curvature in spacetime then moves all masses in the spacetime in a specified fashion. Is there an analogous theory that says the ...
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What is the role of the dilaton in Jackiw-Teitelboim 2D gravity?

I read that the Einstein Hilbert action is topological in 2 dimensions. (What does that mean?). To write down a non-trivial action one introduces the dilaton field in JT gravity. Does this field have ...
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Will a massive particle be attracted to an infinitely long beam of light?

I was attended to Bonnor beams. A Bonnor beam is the general relativistic exact solution for the spacetime surrounding an infinitely long straight beam of light (it includes also the spacetime ...
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What would you see dropping a sufficiently strong chain with substantial length into a black hole?

Here's a visual representation of the scenario Here you can see we have a black hole on the left. The event horizon is the edge of the black hole. You are far away from the event horizon, and a chain ...
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If gravitational potential energy is negative, why does not it produce any repelling force? [closed]

The gravitational potential energy is usually considered negative. There is even zero-energy universe theory. If so, why it does not produce any repelling force? Or does it in form of cosmological ...
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What is the physical nature of a "force"?

This has application to E-M as well as gravity, but let me use the gravitational example as I think it's a bit more conceptually easy to grasp. Say we have two 1kg weights placed on the ground a metre ...
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The gravitational potential at the center of a solid ball (confusion)

I came across this question during my physics class. Suppose we have a solid, spherical planet with mass $M$ radius $R$ s.t. the density of this planet is uniform everywhere, then what is the ...
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What is the gravity at the event horizon of a black hole?

I've been reading and couldn't find any answer to this question. What is the minimum required gravity to prevent light from escaping?
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Conditions for when gravitational force acts on the centre of mass [duplicate]

In my physics material it says the gravitational force can be considered to be applied to the centre of mass of a body only in the following cases: 1.The effective gravitational field is homogeneous 2....
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Internal force and centre of mass

I have still not yet learn about the centre of mass of a system but I really do need to solve this question that keeps bugging me. I know that internal forces cannot accelerate the centre of mass of a ...
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Can matter be coaxed into becoming an anisotropic photon emitter?

Please allow me explain my line of reasoning: Consider an isotropic photon emitter at the origin of 3D space $(0, 0, 0)$. Also, consider a sphere-shaped photon absorber at position $(0, 100, 0)$, with ...
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Naked singularities in negative $M$ BTZ black hole geometry

Consider a BTZ black hole geometry, \begin{equation} ds^2=-N(r)^2dt^2+N(r)^{-2}dr^2+r^2(N^\phi dt+d\phi)^2, \end{equation} where $M>0$, $N(r)^2=-M+\frac{r^2}{l^2}+\frac{J}{4r^2}$ and $N^\phi(r)=-\...
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Does gravity besides frequency change the speed of beam of light? [duplicate]

Does gravity besides frequency change the speed of a beam of light?For me the easiest way to grasp this possibility is the so called frame dragging where the motion of the frame affects the overall ...
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Are the gravitational mass and inertial mass different qualities of a particle?

I can't understand how the same quality like mass can have different way to be defined. Is mass a quality that resist the movement of a particle in space (inertial mass) or the quality that moves ...
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Why does a system with zero resultant moment turn after temporarily force-tilted?

Say a beam, 3m long from end to end, has a pivot placed at its centre of mass, at half its length i.e. 1.5m. Objects with weight 200N each are placed on both ends of the beam. The system is in ...
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Force per unit mass exerted on an object at the surface of a planet

A planet has mass $M$, radius $r$ and orbital period $\tau$. Then the force per unit mass exerted on an object at rest at the equator on the surface of the planet is given by $$\frac{GM}{r^2} - \frac{...
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How are relativistic tidal forces different from their Newtonian counterparts?

Conceptually, the tidal effects of Newtonian gravity differ drastically from that of general relativity. In Newtonian gravity, the effect is due to the change in gravitational force across a certain ...
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Differences between observations of light versus gravity for a satellite traveling by the sun

A satellite travels in a geodesic by the sun with sufficient velocity to escape the sun's orbit. The distance of closest approach is 100 light seconds when the satellite's velocity is perpendicular ...
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Does an object with kinetic energy have increased mass? If so, how is this reconciled in relativity?

Let's imagine that you are standing in a field. You have a ball that weighs 10 grams. You throw this ball at 40 kph. If I understand things correctly, this ball now has kinetic energy relative to your ...
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Entropy seems to behave in peculiar way [duplicate]

While I was learning introductory statical physics/ thermal physics, I have following doubt below Suppose that we have a system which is very very big, consisting of objects which are very very small (...
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Projection on transverse traceless Gauge

I am reading this book about Gravitational waves. On page 10 and 11 Maggiore says that Given a plane wave solution $h_{\mu\nu}$ propagating in the direction $\hat{n}$, outside the sources, already in ...
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Gravity is not a force - how does "accelerating up" work for the entire earth? [duplicate]

So this question has been bothering me for several days now. I've seen many YouTube videos on how "Gravity is not a Force", but no one seems to offer a simple explanation on how does this &...
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Can we give a sense or put in context the fact that gravity isn't a force yet is one of the fundamental ones? [duplicate]

As per the title. Not that the title fact poses me big troubles, personally, but is at least confusing that the two assertions coexist within the same article, interview, and thread — or can be ...
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Ridiculously large velocity: free fall, Stokes

According to free fall by Stokes law the terminal velocity is given by: $$ v_T = g\cdot\dfrac{m}{6\cdot r\cdot\pi\cdot \eta}$$ If you're talking about a chutist and use for air as viscosity $\eta = ...
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Is there a valid solution to Einstein's equations for this cosmological model?

The cosmological model below has been developed in order to explain the flatness problem. At first it's from Newtonian considerations, then a solution of the Friedman equations is looked for $$\left(\...
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Gibbons-Hawking-York (GHY) boundary term

In the Hilbert action, apart from the bulk term, there is a boundary term called GHY $\int d^{3}x\sqrt{h}K$. Why do we choose to calculate this on the $r=$constant hypersurface instead of the $t=$...
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If a Spacecraft Was To Launch From An Asteroid Would it Be Easier To Minimize The Required Escape Velocity By Launching At A 45 degree angle? [duplicate]

If a Spacecraft Was To Launch From An Asteroid Would it Be Easier To Minimize The Required Escape Velocity By Launching At A 45 degree angle? Or Would It Be Better To Head Straight Up?
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Does anyone know where I can find values of gravity?

Does anyone know where I can find the values of gravity at different places across the globe and at different heights? I am doing an assessment on gravity and can't find any data for the values of ...
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Curvature vs Gravity [closed]

I understand how and why spacetime curvature is responsible for the formation of geodesics and how objects follow those curved spacetime paths. "Matter tells spacetime how to bend and spacetime ...
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Why doesn't a car moving on a curved path have a horizontal gravity force but it has a horizontal normal force?

So I was wondering.. When we solve questions of an object sliding on an incline, we consider the force of gravity to have two components, a horizontal and a vertical, and the normal force to have only ...
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What is Observed by a Planet in a Region of Spacetime in a Theoretical Universe that has a Mass Density Gradient?

Planet M exists in a theoretical Universe that is like our Universe in every way except it can contain Large regions of Spacetime that have mass density gradients. For a specific time, t, Planet M is ...

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