# 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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### Is mass the source of space-time?

The Einstein field equations (EFE) characterize how mass curves spacetime. $$R_{\mu\nu}-\frac{1}{2}g_{\mu\nu}R=\kappa T_{\mu\nu}$$ I try to understand the curvature of spacetime. In the EFE, there ...
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### Secondary vs. primary gravitation

Modified Newton dynamics (MOND) works surprisingly well to fit the rotation curves of galaxies. On the other hand, in the solar system, Newtonian gravitational law (which can be derived from general ...
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### About the gravity field around a lightbeam [duplicate]

Imagine there is an infinitely long beam of light. It has impulse and energy. How it affects the spacetime surrounding it? Will masses be pulled towards it perpendiculary and pulled in the parallel ...
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### What's the difference between a post-Minkowskian approximation and a post-Newtonian one?

I'm studying the book Gravity by Poisson & Will. Specifically, I'm interested in the post-Newtonian and post-Minkowskian approximations showed in chapters 6-10. The problem I'm having is ...
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### Do electric fields in a capacitor add to its weight?

So this article got me thinking: https://www.scientificamerican.com/article/do-electric-charges-and-m/ It states that according to GR, the energy in an electric field should curve spacetime and ...
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### Is it possible to make a magnetic box that will compensate Earth gravity allowing magnetic objects inside to fly on orbits like in space? [closed]

Is it possible to simulate solar system using magnetic spheres and some kind of box that will compensate the Earth gravity by magnetic force?
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### Question about time dilation and what an observer sees

This is a soft question I was discussing with a friend. Suppose a portal opens up (I know this is fiction) but for the sake of argument, let's suppose something like that happens. And the observer can ...
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### How does a static electric field behave in curved spacetime?

Imagine a simple capacitor of two parallel plates with a static electric field between them. This capacitor is oriented in a gravitational field such that the space between the plates is parallel to ...
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### Near horizon limit of near-extreamal black brane

It is known that the near horizon limit of a ($d+1$) dimensional extremal charged black hole (BH) is $AdS_2\times S^{d-1}$. I was looking at this paper by Faulkner et al. They consider a ($d+1$) ...
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### A satellite moves in parabolic or straight line path when it escapes and when its total mechanical energy is zero.How can ang. momentum cons. be used?

When a satellite escapes from the orbit of a planet it can have Mechanical energy equal to or greater than 0 but when it is equal to zero it can have a parabolic or straight-line path. When velocity ...
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### Would I be able to tell if I was inside a piece of distorted space? Are objects distinct from spacetime?

If one could hypothetically stretch / squash / in some way distort a piece of space, an outside observer could tell by looking at the distorted object/space that it had changed because the difference ...
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### How can an accelerated frame and a gravitational field be equal?

How can two things that are different be equal? In a gravitational field there is actually gravity that push us down, but in the accelerated frame there is nothing that push us down.
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### Increasing the speed of satellite to raise an orbit

Suppose we have a satellite orbiting the Earth in an elliptical-closed orbit. When the satellite reaches the perigee, its velocity is increased in the cross-radial direction. According to all the ...
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### A silly coincidence: $\frac{c}{\rm year}\approx 9.81 {\rm m/s^2}$ [closed]

Long ago, I sat and wondered about the equivalence of the gravitational pull on my body and upward acceleration. If I am not wrong, there is no way to distinguish between sitting on my chair here on ...
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### Conceptual Question: What would happen to satellites orbiting the earth if mass of earth suddenly increases? [closed]

Would the satellite crash onto the earth? If so, why, and in what kind of motion? A spiral motion sounds intuitive, but why? Or there is another possibility: will the satellite continue to execute ...
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### Statistical mechanical perspective of entropy in this computer simulation

I've scoured the internet for a clear answer on entropy and I see contradictory answers everywhere so I'm going to refine my question here. Please read carefully as the question is not as it seems at ...