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 is the most accurate value of $G$ measured?

How do modern scientists update the measurement of $G$, the gravitational constant? Is CODATA the authority on this measurement and the experiment?
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Is the Moon freely falling towards the Earth?

The Moon is revolving around the Earth and its centripetal acceleration is towards the Earth. Does it mean that the Moon is freely falling towards Earth? What is the cause of the Moon orbiting around ...
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Is “scalar electromagnetics” real science?

This page claims that original Maxwell's equations, when formulated by Maxwell himself in quaternion form, had some special scalar part of electromagnetic field, which somehow appeared to describe ...
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How does the gravity well change as space expands? [duplicate]

How does the gravity well change as space expands? If we assume that the Earth's gravitational field curves flat space to create a gravity well then how does the gravity well change as space expands ...
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129 views

If gravitational radiation (or anything) cannot escape a black hole, how can it produce redshift or curve spacetime? [duplicate]

There is an apparent paradox in a Black hole. Keenan Pepper wrote: Electromagnetic radiation cannot escape a black hole, because it travels at the speed of light. Similarly, gravitational ...
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154 views

Is the concept of space-time curvature a recursive one? [duplicate]

A way some people explain (or try to explain) how gravity works is using space-time curvature: an object with high mass distorts the surrounding space-time plane like a bowling ball distorts a sheet ...
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Chemical effect on gravitation?

We know, that gravitation field of charged black hole is different than one of uncharged. I this true only for objects with singularity or is true for all objects? If true, then may we say, that ...
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Are gravitons bound by the event horizon?

I understand that photons, even when traveling at the speed of light, cannot escape the event horizon of a black hole. Are gravitons and other virtual particles traveling at the speed of light also ...
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Warped AdS geometry

I am having difficulty of finding more basic information on warped geometries. All the standard textbooks are not covering it. In the wiki article it's only said that warped geometry is the one which ...
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Classical electrodynamics formulated in terms of forces

The Newton's law of universal gravitation is described in terms of a force, which is produced by an action at a distance. It also can be described using the concept of a field, and that would be an ...
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162 views

How can photons exert gravity if they are wave-like?

As a reference, see this question: Does a photon exert a gravitational pull? It turns out the answer is "Yes" -- but this does not seem consistent with light being wave-like. I am imagining a ...
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219 views

Are laws of gravity time symmetric?

Time symmetry is often explained by the example of orbiting objects... What I can't find an explanation for is the moment when an object enters into orbit around another object. That clearly breaks ...
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If matter creates space, shouldn't there be experimentally detectable consequences?

Ernst Mach, a man to who influenced Albert Einstein significantly in his approach to relativity, did not quite seem to believe in space as a self-existing entity. I'm pretty sure it would be correct ...
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Can extremely charged objects simulate some black hole effects?

If we had a positive point charge of incredible quantity, does there exist an imaginary sphere about it, such that regardless of the initial speed and direction of any electron, that electron could ...
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508 views

Gravity of a disc

What proof is there that the center of gravity of a disc is at its center when the gravitational field is not uniform, such as objects close to the edge of the disc, or lying within the body of the ...
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246 views

Is there zero energy cost to generate a gravity field?

It seems that all objects with defineable mass emit a gravity field, which interacts with other objects within said field. My understanding on this may be very limited, but how is the field ...
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271 views

A loop quantum gravity toy inspired by an Aharonov-Bohm ring

Comparing my question to Give a description of Loop Quantum Gravity your grandmother could understand what I'm looking for here is a toy for a toddler ($\approx$ a pre-QFT graduate student). I seek ...
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metamaterials for gravitational waves

Is there such a thing as refractive index of a material for gravitational waves ? How would the index of a material depend on density - Say density of interstellar cloud vs planet vs star vs neutron ...
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Terminal speed of a body in a gravitational free fall? [closed]

What speed would a small body moving towards a large body reach under gravity alone? More importantly, is there a 'terminal speed' for the small body due to the increasing energy required to ...
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I think I am misunderstanding Einstein's equivalence principle and his elevator

I'm having difficulties understanding why a gravitational acceleration can be guaranteed to be locally equivalent to an accelerating frame. Doesn't it matter on how the force is being applied? If the ...
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88 views

Does my mass really affect objects on the other side of the universe?

I remember hearing back in school that each object is attracted to every other object in the universe. Of course this has to be taken with the understanding that most of those objects have very very ...
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Could the 4 Forces Split Off or “Decay” into Other Forces in the Distant Future? [duplicate]

From my basic understanding of popular-level physics articles and books and such, the 4 forces (Electromagnetism, Gravity, Strong and Weak Force) used to be 1 force in the early universe, then split ...
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2 different density liquids in space

Lets say you are in deep space with basically zero gravity. You have a ball of water floating in front of you. If you carefully injected that ball with a syringe filled with vegetable oil, what ...
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Deriving the Fierz-Pauli propagator

Fierz-Pauli action can be written as $$ \tag{1} S = \int d^Dx \; \frac{1}{2} h_{\mu \nu} \mathcal{O}^{\mu \nu, \alpha \beta}h_{\alpha \beta} $$ where the operator $$ \tag{2} \mathcal{O}^{\mu \nu} ...
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Does all matter emit gravitational waves all the time?

I understand that gravitational force is a result of the curvature of spacetime and that the analogy with electromagnetic waves falls apart at a certain point. However, I have been confused when ...
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Gravity and its energy

Where does gravity get its energy from? So, I'm taking a high school environmental science class and we just started a unit on energy. We were talking about different types of energy and one was ...
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Gibbons-Hawking-York (GHY) boundary term for Schwarzschild metric

What is the simplest way to calculate Gibbons-Hawking-York boundary term for Schwarzschild metric? \begin{align} \int ...
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Conservation of angular momentum in Earth-Moon system

We all know about the fact that tidal friction is slowly slowing down the Earth's rotation about its axis, and that subsequently the Moon is slowly drifting away, in order for the angular momentum of ...
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What is the physics of tilt compensation of an electronic compass

This question concerns the physics behind the implementation of electronic compasses to find the orientation of a device. In the robotics community, 3-axis magnetometers are often used for this ...
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299 views

Jump from a falling object [duplicate]

There have been quite a few plane crashes the past week and this question popped into my head of which I need a good explanation in science about the possibility of survival or not and why. Assuming ...
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442 views

Black hole thermodynamics in a time dependent metric

For a time dependent space time metric, to get the thermodynamics, does the standard procedure of Wick rotating the time, and then calculating the free energy, work ?
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81 views

Modified gravity and dark energy

Is there a model of modified gravity that either was close to or have succeeded in modelling the problem of dark energy, and suffers from no instabilities and explains the observational data. Also has ...
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267 views

Does decreasing the length of the line between a harness and zip wire increase the velocity at which you travel?

My friend is an instructor at an outdoor adventure centre which has lots of zip wires. The instructor's harness has a shorter line that attaches to the zip wire than the normal harness does and she ...
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213 views

How can you calculate the tidal gradient for an orbit?

In the movie Gravity, two characters are dangling from the international space station by a long tether. I've previously wondered exactly how you could calculate the tidal forces that act on an ...
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208 views

Divergence of One and Two Graviton Exchanges

At the bottom of pg. 3, Kiritsis states the following To appreciate the difficulties with the quantization of Einstein gravity, we look at a single-graviton exchange between two particles (Fig. ...
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What is a “gravitational cell”?

I am not a physicist, and I don't understand the details of electromagnetism. Anyhow, I was looking for how the batteries work in Google. So, I came across this article: "How batteries work: A ...
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Transformation law for fermionic measure in functional integral

I am reading the paper "Bosonization in a Two-Dimensional Riemann-Cartan Geometry", Il Nuovo Cimento B Series 11 11 Marzo 1987, Volume 98, Issue 1, pp 25-36, ...
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414 views

Wave pulse on a freely falling rope

Consider a rope hanging from the ceiling (massive / massless irrelevant, I suppose). A wave pulse is set up on the rope. Just as the wave pulse starts propagating on this rope, the top of the rope is ...
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How to measure force of impact inside container?

I am in 7th grade and for my science fair project, I need a way to measure the force on a dropped object when it hits the ground. What I am trying to determine is which packing materials provide the ...
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The trajectory of a projectile launched from a hilltop

Here is the problem: A boy stands at the peak of a hill which slopes downward uniformly at angle $\phi$. At what angle $\theta$ from the horizontal should he throw a rock so that it has the greatest ...
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Computing period, semi-major axis of binary

I have mass, $g$, and luminosity of each of the stars in a binary system, extracted from a model. I calculated the individual radii from $g$ and the mass. I am trying to compute $a$, but I seem to be ...
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Clarification on Wald's book

I have a question concerning the Wald's book: General Relativity. In the appendix E, he derived the Einstein equation by considering the surface term (GHY). I do not understand what he said after the ...
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Action principle for the Electromagnetism and Gravity

Here is the formula for the stress energy tensor: $$ T_{\mu\nu} = - {2\over\sqrt{ |\det g| }}{\delta S_{EM}\over \delta g^{\mu\nu}} $$ (This follows from varying the total action $S ...
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Black hole complementarity

Black hole complementarity states that two observers, one falling into a black hole, and one observing outside, experience two different histories but since they can not communicate there is no ...
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If it takes infinitely long for someone to fall in a black hole, wouldn't it evaporate first? [duplicate]

Let's say I decide to jump into a large black hole. A distant observer never sees me enter the black hole, but he does see the black hole evaporate. According to this reasoning, I would then keep ...
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53 views

Can stable non-circular orbits exist in 2 or 3 dimensional hyperbolic space

I know that stable non-circular orbits in euclidean space exist only in 3 spatial dimensions but what about if the spatial dimensions are hyperbolic instead? Are there any stable non-circular orbits ...
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Palatini type gravities: Where could I find them?

I read the article Quantum gravity with torsion and non-metricity. Although I found interesting the analysis in the paper, I found quite interesting an statement in the abstract, The class of ...
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Is there a relation between $G$ and the age of the universe?

Here is a recording of Paul Dirac, talking about dimensionless constants and their significance. He gives some examples of such constants(ratio of the masses of an electron to a proton, the ...
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76 views

Are moving objects producing stronger gravity fields? [duplicate]

If the strength of gravitational influence exerted by a body is derived from its mass and energy then is it true that a moving object which has some kinetic energy should also produce stronger ...
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How much work can a single grain of rice do? [closed]

I found a website saying a grain of rice contains 1/10 kcal. I'm not a physicist and haven't done maths for a long time. But here's what I came up with: E = m * g * h One small calorie equals ...