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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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

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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How much centrifugal force would it take to create gravity?

If say your car was stuck in the mud. You press the gas the wheel spins and the centrifugal force shoots mud out the back, how fast would the tire need to go to keep the mud perpetuating around it ...
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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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Is Curve in spacetime fabric really possible? [duplicate]

According to The theory of special relativity gravity is in the form of curve in space - time fabric similar to the curve which is made by any massive body when kept on a trampoline. But in case of ...
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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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If you could somehow nullify gravity in a volume, what would happen to a planet? [closed]

The Gravitonians of Zarquon Beta have shown up, shone a giant ray on the planet Earth that nullifies gravity in a region of about a million miles across and left unceremoniously. The ray works by ...
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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

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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How gravity works? i found found a paper [closed]

I was looking for new ideas on gravity in internet and i found this. this paper is strange, contains different diagrams and modals of gravitational force. it looked interesting to me but couldn't get ...
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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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If an astrophysical jet contains gamma rays does it mean that at the source poles there is small gravitational redshift?

If an astrophysical jet contains gamma rays does it mean that even though the source has incredibly strong gravitational pull, at the source poles there is small gravitational redshift? Maybe there is ...
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Do quasiparticles (phonons) bend spacetime or not?

I have read this: Recent research has shown that phonons and rotons may have a non-negligible mass and be affected by gravity just as standard particles are.[19] In particular, phonons are predicted ...
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Attempting to model constant acceleration with air resistance

I am working on a model of a reflective sail being accelerated by a laser beam. The primary equation is pulled from a physical description of the Breakthrough Starshot, using initial velocity as a ...
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The Sun bends planets' trajectory and speeds up it towards the perihelion; does it both actions with light? [duplicate]

The Sun bends planet's trajectory and speeds up it towards the perihelion; does it both actions with light? If it speeds up light too, then does it mean that gravity is a consequence of a vortex that ...
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Can gravity from a massive object outside our cosmological event horizon have any effect on Earth?

The gravity of a very massive luminous celestial object traveling at the speed of light away from us (at the Hubble Sphere) would have an Extremely small, but non-zero gravitation effect on Earth (...
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Atmospheres and oceans on lower gravity planets

Using Earth as an example, how much less gravity could the planet exert and still retain its atmosphere and oceans? Or is it a matter of size, mass, magnetosphere, place in the solar system, all of ...
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Do freefalling clocks within a small region of spacetime undergo gravitational time dilation?

I have several times seen explanations of gravitational free fall (eg, of a small object toward earth with no air resistance) that begin with the following claim about the particles of the free-...
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Black hole and white hole symmetry

NB: This question comes from an idea I had while watching this video about the video game portal and its sequel (link: https://www.youtube.com/watch?v=aj8NDOA0Sgw at 15:30 the idea of a black hole was ...
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Understanding the expansion of the universe

The universe is not only thought to be expanding but the rate at which it is expanding is said to be accelerating. Which of these models (if any) is a correct way of looking at this? Model A: Let’s ...
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Why is it incorrect to to add accelerations when something is accelerating downwards and gravity is downwards? [closed]

I know it is related to apparent weight and how one feels lighter when accelerating downwards in an elevator. But I have no idea to explain it.
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Black holes bending light [duplicate]

Ok so I've been watching some videos about black holes but there is one thing I don't get: how can light be bent by black holes since photons don't have any mass?
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Where do gravitons reside - in atoms?

As a non-physicist, I imagine that if we have an atom of lead and an atom of copper floating near each other in deep space with no initial relative velocities, they will exert a gravitational pull on ...
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Einstein-Hilbert action on lattice

I am wondering how the Einstein-Hilbert action is written on lattice in Euclidean spacetime, and if the metric will still be diagonal if it is possible to write Einstein-Hilbert action on lattice.
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Bell's paradox but with acceleration caused by a uniform gravitational field rather than rocket engines

Bell's paradox has in the past been the topic of quite heated discussions. It is posed in the context of a silk thread connecting two identical rockets whose engines are ignited at the same instant. ...
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Gravitational Force & Normal Force

Imagine a moving ice-block on an extremely smooth (i.e. friction is zero) and circular, rogue planet devoid of an atmosphere. Assuming that the planet in question is far away from any (massive) object ...
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Is nature (effect) of Dark Energy opposite that of Gravity?

Does the left side of Einstein's field equations as displayed above imply that the cosmological constant expands space in precisely the opposite way gravity collapses or curves space? I.e. could there ...
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Thought experiment: Gravitation effect of mass converted to energy

Two masses M1 and M2 are separated by some distance. M1 accelerates towards M2 and reciprocally. Then because e=mc2, M1 converts totally into energy (which I believe would be photons) and vanishes. ...
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State space of free falling object is time variant?

In the case of a free fall object, if $h$ denotes distance from floor and $v$ the speed, then, given an input $U = g$, and $X=[h \quad v]^T$ and $\dot{X}=[\dot{h} \quad \dot{v}]^T$, I found that the ...
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Can we detect gravitons this way?

First, we shoot a collection of particles (let's exclude gravitons from these, just for the sake of clarity) through a large region of spacetime in which the stress-energy tensor is zero and make them ...
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Linearised diffeomorphisms on arbitrary gravitational background Part 2

This question is a follow on from my recent post here, in the sense that I will use the notation introduced there. In that post, I considered infinitesimal diffeomorphisms of a metric $g_{\mu\nu}$ ...
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Linearised diffeomorphisms on arbitrary gravitational background Part 1

Consider some spacetime $\big(\mathcal{M},g_{\mu\nu}\big)$ parameterised by local coordinates $x^{\mu}$ ($\mathcal{M}$ is a smooth differentiable manifold equipped with a Lorentzian metric $g_{\mu\nu}$...
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Casimir Vacuum gravity propulsion?

This question is quite similar to Poor man's Alcubierre drive? but definitely not a duplicate since the direction of movement, in my proposal, is the opposite. This is also similar but different. (...

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