# 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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### 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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### What is the cause of gravity according to general relativity? [duplicate]

The question is based on general relativity and it can help big bang theory.
1 vote
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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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### What is a simple example that contradicts the theory that light (photons) could have a super small mass? [duplicate]

The obvious example is that, to bring something that has mass to the speed $c$ requires infinite energy due to special relativity. But what if a kid asked me "What if light/photos had a super ...
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### Thought experiment regarding gravity

Let there be a mass $M$ which is a spherical shell of radius $R$. Now the Gaussian flux about a enclosing sphere of radius $r$ where $r>R$ will be proportional to $M$ or $GM$ (where $G$ is the ...
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### Why does a ball bounce back even when it is released and not thrown?

So according to my understanding when we thrown a ball towards ground we apply an additional amount of force which results in an acceleration greater than the one due to earth. When this ball reaches ...
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### How does a mass that's dropped when strung by an inelastic, slack string continue its motion? [closed]

Say a mass is connected to a light, inextensible string of length $l$. The other end of the string is fixed to a point O. If initially, the mass is kept at a horizontal distance of $\frac {l}{2}$to ...
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### Definition of the gravitational constant in 1+1 gravity

In this paper, the author formulates a $(1+1)$-dimensional theory of gravity by taking the trace of the Einstein equations $$\left(1 - \frac{D}{2}\right)R = 8\pi G_D T,\tag{2}$$ (where $G_D$ is the ...
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### Is the Planck mass the "lower limit" for gravity?

The Planck units are often treated as being the "lower limits" to things: the Planck length for length, the Planck time for time, etc. But the Planck mass, which is about $2.2\times10^{-5}$ ...
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### Dual of Newtonian gravitational field

In the static state, the laws of Newtonian gravity and Coulomb force have exactly same formulas, $$F = K \frac{A_1A_2}{r^2}.$$ In the electrical case, moving materials produce a field, say a dual ...
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### Detailed derivation of ESCK gravity and Extended Friedmann Equations with Torsion

Do you know a textbook on the Einstein-Cartan-Sciama-Kibble theory of Gravitation and its application to derive Extended Friedmann Equations with Torsion, which shows the calculations in detail?
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### Aren't there only 3 fundamental forces? [duplicate]

I have heard that there are 4 fundamental forces: Gravity, Electromagnetism, the Strong Nuclear Force and the Weak Nuclear Force. But I have also heard that gravity is not a force at all, it is simply ...
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### How could I calculate the time it will take for light and mass to go towards a black hole and come back, to and from constant radial distances?

If you have a "perfect mirror" and a "perfect trampoline" at some constant distance outside a black hole's event horizon: a) How would a shell observer at some distance farther ...
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### Stars that have fairly high gravitational redshift and calculation of their surface temperature by Planck emition spectra?

How high can the ratio between gravitational redshift and planck emition spectra be depending on the mass of the star so by how much this gravitational redshift could elongate the Planck spectra of ...
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### Can a particle moving vertically upwards in space with constant velocity escape the earths orbit?

Let’s assume that a particle is moving vertically upwards towards space, said particle is having a constant velocity regardless of all forces acting. I repeat NO FORCES HAVE ANY KIND OF EFFECT ON ITS ...
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### How are objects inside a black hole affected by the gravity of objects outside the black hole?

There are many Q&As about whether something inside a black hole can escape the event horizon if another massive object gets close enough to pull it out. I realize the answer (I think universally ...
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### Gravitational Constant with ENM Units?

To give some context, there's a conspiracy 'theory' that I saw called Electric Universe that says that gravity is not a fundamental force and instead is a "incoherent dielectric acceleration"...
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### Questions about speed of gravity [duplicate]

If gravity "travels" at $c$, and the sun is travelling "forward", does it mean the planets are actually orbiting various points "behind" the center of the sun? Does it ...
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### Gravitational time dilation near the Earth [closed]

I recently read the statement that near the Earth, in the Newtonian weak gravitational field, gravity is 99.9999% mainly due to "curvature of time" (ie gravitational time dilation), and only ...
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### Acceleration at peak of a gravitational wave

The amplitude of the strongest gravitational wave signal detected by LIGO sofar can maybe be expressed as an acceleration? If so, what would the numerical value be (in m/s^2)? I would like to compare ...
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### How can you use gravity while trying to model gravity? [duplicate]

So consider the usual pop-science spacetime model, a bowling ball on a trampoline. Apparently, the ball should sink into the trampoline, causing a dip in the fabric which causes nearby objects to fall ...