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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141 views
Confused about indices of the Ricci tensor
In an intro to GR book the Ricci tensor is given as:
$$R_{\mu\nu}=\partial_{\lambda}\Gamma_{\mu \nu}^{\lambda}-\Gamma_{\lambda \sigma}^{\lambda}\Gamma_{\mu \nu}^{\sigma}-[\partial_{\nu}\Gamma_{\mu ...
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1answer
114 views
Source term of the Einstein field equation
My copy of Feynman's "Six Not-So-Easy Pieces" has an interesting introduction by Roger Penrose. In that introduction (copyright 1997 according to the copyright page), Penrose complains that Feynman's ...
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3answers
222 views
Rocket launch from a mountain
If we were to build a high speed rail up the side of a mountain like in some ScFi movies, what is the velocity needed at the point of living the mountain excluding angular momentum from earth’s ...
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3answers
533 views
Paradoxical interaction between a massive charged sphere and a point charge
Suppose we have a sphere of radius $r$ and mass m and a negatively charged
test particle at distance d from its center, $d\gg r$. If the sphere is electrically neutral, the particle will fall toward ...
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1answer
354 views
Turbulence parameterization from gravity - fluid dynamics correspondence
I`m looking for a nice introductary reference that explains how the turbulence coefficient or any kind of turbulence parameterization (in view of applications to atmospheric turbulence for example) ...
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1answer
29 views
Gravitating sigma models
I am looking for a review or book on sigma models in (super)gravity theories, which arise from dimensional reduction.
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3answers
210 views
Simulate a physical impact of objects made of finite, small elements
I want to simulate an impact between two bodies according to gravity, and eventually considering other forces to stick matter together. I'd like to use python to do this, but I am open to ...
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1answer
118 views
What would gravity be like on a hemispherical planet?
On a hemispherical world half the size of Earth (ignoring that the planet should break apart and become a smaller sphere) what would the gravity be like? How would it change as you traveled from pole ...
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1answer
187 views
Are objects in a gravitational well shortened?
Bob is in a gravitational potential well, he moves a long vertical stick up and down a distance of 1 meters. Alice observes the upper end of the stick, at upper location.
There is the phenomenon of ...
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1answer
226 views
Determining Average Tidal Effects
Maximum tidal heights vary widely across the globe, from 16 m in the Bay of Fundy to mere centimeters elsewhere. These variations are due to coastline and shoreline differences. This makes it ...
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0answers
86 views
Equation of state of cosmic strings and branes
I'm sure these are basic ideas covered in string cosmology or advanced GR, but I've done very little string theory, so I hope you will forgive some elementary questions. I'm just trying to fit some ...
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0answers
91 views
Positivity of Total Gravitational Energy in GR
I read the following statement in the introduction to an article:
Over the last 30 years, one of the greatest achievements in classical
general relativity has certainly been the proof of the ...
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0answers
156 views
Penrose Conformal diagram for flat 2-dim Lorentz space-time
I have the following metric
$$ds^2 ~=~ Tdv^2 + 2dTdv,$$
defined for
$$(v,T)~\in~ S^1\times \mathbb{R},$$
e.g. $v$ is periodic.
This is the according Penrose diagram:
Question 1) Is the ...
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0answers
149 views
Why Brown York stress tensor == dual field theory's energy momentum tensor?
From the AdS/CFT dictionary, how to argue that the Brown York stress tensor for a gravity
system near the boundary is exactly the same as the energy momentum tensor of the dual field theory?
In the ...
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8answers
5k views
Is gravity a force and if so what is its opposite?
For every force there is an equal force in the opposite direction on another body, correct?
So when the Suns gravity acts on Earth where is the opposite and equal force? I also have the same ...
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5answers
1k views
Do we take gravity = 9.8 m/s² for all heights when solving problems? Why or why not?
Do we take gravity = 9.8 m/s² for all heights when solving problems?
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3answers
651 views
How is it possible for orbits to maintain stability?
According to $a = v^2/R$, the circular velocity and radial distance between two attracting objects (such as planets), must remain in perfect proportion in order for orbital motion to take place. How ...
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2answers
551 views
Noticing that Newtonian gravity and electrostatics are equivalent, is there also a relationship between the general relativity and electrodynamics?
In classical mechanics, we had Newton's law of gravity $F \propto \frac{Mm}{r^2}$. Because of this, all laws of classical electrostatics applied to classical gravity if we assumed that all charges ...
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4answers
2k views
Why can't a rope be pulled completely straight?
I have found several discussions on how to calculate the sag of rope that is tied off at two points (like a tightrope), and I understand it to a certain extent. What I can't wrap my head around is how ...
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3answers
324 views
Why don't astronauts in orbit get stuck to the “ceiling”?
When a shuttle is in orbit, it is essentially rotating around the "centre" of the Earth at a great speed. So why does there seem to be no centrifugal force sticking them to the 'ceiling' of the ...
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2answers
908 views
How does Newtonian mechanics explain why orbiting objects do not fall to the object they are orbiting?
The force of gravity is constantly being applied to an orbiting object. And therefore the object is constantly accelerating. Why doesn't gravity eventually "win" over the object's momentum, like a ...
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3answers
431 views
Gravitational lensing or cloud refraction?
My current understanding of gravitational lensing follows
When a star or other massive body passes between us and another star, the phenomenon
generally labeled 'gravitational lensing' occurs. The ...
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4answers
311 views
What is the medium that allows magnetic fields *or any forcefield* to exist?
Magnetic fields are obvious distortions.. of.. something, but what exactly are they distortions of? Massive objects produce curvatures/gradients in space-time resulting in what we observe as ...
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2answers
190 views
In a spaceship, if a vessel suddently stops will an object inside the vessel keep going?
My question is a 2 part question.
First if a vessel in space is going very fast and suddently stops (maybe it is not possible but that is not the point) will things/humans inside the vessel keep ...
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3answers
370 views
Vacuum and repulsive gravity
How can one show from General Relativity that gravity is attractive force, and under which conditions it becomes repulsive, also why positive energy vacuum drives repulsive gravity?
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1answer
170 views
Can a hovering helicopter travel half the globe in 12 hours? [duplicate]
Suppose we have a helicopter that is able to stay stationary in flight for extended periods of time. If such a helicopter stayed at point A in the sky for 12 hours straight, would it reach the other ...
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2answers
186 views
How does one get the value of acceleration of gravitation on earth accurately by experiment without electronic device?
How does one get the value of acceleration of gravitation on earth accurately to 5 significant digits by experiment without electronic device?
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3answers
322 views
Has somebody ever tried to measure the gravity oscillations of nearby rotating masses?
I'd like to extend this question:
Are Newton's gravity waves detectable by a laser interferometer?
but with some changes.
Has somebody ever tried to measure the gravity oscillations nearby ...
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2answers
828 views
“Artificial gravity” on a rotating space stations
There have been a lot of sci-fi shows recently using the "rotating space station" explanation for gravity on space stations.
After watching these videos:
...
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3answers
294 views
The Sun as a gravitational lens
Since the Sun is a gravitational lens with as focal length of 550 AU for visible light, with an immense amplification factor, shouldn't it light up objects hanging out there?
We should get solar ...
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2answers
713 views
What is the escape velocity of a Black Hole?
The escape velocity of Earth is $v=\sqrt{\frac {GM}{R}}$, where $M$ is the mass of the Earth and $R$ it's radius (approximating it as a sphere), and is much less than light speed $c$.
What is the ...
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3answers
259 views
Do rotating bodies emit gravitational waves?
Suppose we have a cylinder of mass $m$, radius $R$ and height $h$ in rotation with speed $\omega$ around its symmetry axis with no friction (ideal situation).
I'd expect this cylinder to emit ...
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2answers
319 views
How much light is there in space and how heavy is it?
Our night sky is filled with stars. On a dark night a significant fraction of the sky is light. This light, we are told, has been in transit for many millions of years. There must therefore be quite a ...
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4answers
221 views
Gravity from a singularity as distance approaches zero
If you had a singularity (that had mass but took up no space), what would happen to the acceleration of an object as it approached this singularity? I would assume that it would be infinite, since as ...
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1answer
187 views
If the Moon had gravity as good as the Earth and a magnetic field could it have supported life?
If the Moon had gravity as good as Earth and a magnetic field could it have supported life? Because if the Moon had gravity, it could have retained water more than is present today on the surface.
...
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3answers
181 views
Could we prove that neutrinos have mass by measuring their gravitational signature?
It is now said that neutrinos have mass. If an object has mass then it also emits a gravitational field. I appreciate the neutrinos mass is predicted to be small, but as there are so many produced ...
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2answers
163 views
A wonky gravitational potential and its critical points
I have tough problem I am not sure how to solve:
For this question, we are confined to a plane. Consider a gravitational field that is proportional to $\frac{1}{r^3}$ instead of $\frac{1}{r^2}$, and ...
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4answers
179 views
Are Newton's gravity waves detectable by a laser interferometer?
Newton's theory of gravity supports "gravity waves" in that moving objects cause changing gravitational fields. For example, two bodies rotating around their center of mass will have a stronger ...
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2answers
132 views
Place each foot on a scale: can you add the two to find your weight?
I frequent a blog from a British psychologist, and every Friday he likes to pose an interesting puzzle or riddle. The Monday after that he posts the answer. They're good fun, and IANAP but this week's ...
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2answers
209 views
What is the consequence of “infinite” gravitational force?
Introduction
I am a mathematically minded individual. I do not intuitively comprehend physics, and as a sophomore in high school who has only taken Intro to Physics in his freshman year, I may very ...
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1answer
89 views
Could this fountain, under the right conditions, technically be able to lift me up?
There is a big fountain in a lake in my city.
I was talking with a friend and we were wondering whether it would be able to lift me up. I sent a few emails and obtained information about the ...
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2answers
555 views
Lowest gravity on Earth's surface?
I am trying to determine which on Earth's surface has the lowest gravity. Googling is not finding anything concrete. My natural inclination would be to think of Mount Chimborazo in Ecuador, being on ...
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4answers
626 views
Increasing mass' effect on the balance between centripetal force and centrifugal force
Okay, this is nothing more than a thought experiment which popped into my head while driving home from work today.
Take the case of a single body orbiting another, larger body, as in a planet and a ...
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1answer
288 views
Can Black Holes be the Dark Matter?
Seems to fit the definition: interacts with gravity, doesn’t radiate energy (except Hawking Radiation) and could create gravity lensing without absorbing very much of the light.
Could 80% of the ...
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4answers
211 views
Maximal Gravity
I found this interesting problem in Introduction to Classical Mechanics with Problems and Solutions by David Morin:
Given a point $P$ in space, and given a piece of malleable material of
...
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2answers
98 views
What is the fastest a spacecraft can get using gravity-assist?
Assuming normal spacecraft and space objects (no neutron stars, black holes, etc). To what speed can a spacecraft accelerate using gravity-assist?
For example, if a spacecraft is moving at ...
4
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1answer
176 views
Is the quantization of gravity necessary for a quantum theory of gravity? Part II
(At the suggestion of the user markovchain, I have decided to take a very large edit/addition to the original question, and ask it as a separate question altogether.)
Here it is:
I have since ...
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1answer
100 views
Falling Electron
Suppose there are two objects in the universe. Earth, with a gravitational acceleration of g = 9.8m/s/s, and a typical electron.
The electron is dropped from a certain height, say 1000m above the ...
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2answers
179 views
why there is no accuracy of the measured value of $G$?
With the advancement of Modern Technology still there is no accuracy of the measured value of $G$ Gravitational Constant, why!?
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1answer
200 views
What is the current science on entropic gravity?
In the past couple of years there has been some buzz about 'entropic gravity', including here on stackexchange in Jan 2011. Many say that there is no way that gravity can be entropic, while others ...
