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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Riemann tensor of $R\times \mathbb{S}_{d-1}$ metric

I would like to give a general expression to the Riemann tensor of a metric \begin{equation}\text{d}s^2=r^2(-\text{d}t^2+\text{d}\Omega^2_{d-1}).\end{equation} This metric is actually decomposed in $R\...
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Inertial vs gravitational mass at different temperatures

So, I know more energetic objects have more mass according to Einstein. I'm aware that this has been verified by weighing an object at different temperatures where the object weighed more at higher ...
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What is the physical interpretation of negative Unruh temperature?

Actually the Unruh temperature is vectorial since it is proportional to acceleration but derived for only one space dimension so at least positive and negative. What can negative Unruh temperature ...
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Is the Newtonian gravitational potential $-\frac{GMm}{R}$ just an approximation?

Is $-\frac{GMm}{R}$ just an approximation? I believe that it is since we assume that one of the mass is at rest when deriving it.
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Can you calculate the sedimentation rate of a coarse chemical suspension under gravity using a centrifuge?

If the sedimentation rate can be determined for a coarse chemical suspension or a suspension containing large particles (i.e., particles with radii between $100$ to $200 \mu m$) in a medium like ...
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Surface gravity and mass of a black hole

The surface gravity of a Schwarzschild black hole is said to be inversely proportional to the mass of the black hole. But if the event horizon represents the "point of no return" even for light, then ...
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Isn't the Equivalence Principle technically wrong?

Isn't the Equivalence principle not quite true, technically speaking? A gravitational acceleration and an inertial acceleration are communicated very differently. A gravitational acceleration is ...
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How is spacetime curvature effective for objects on the surface of the earth? [closed]

Spacetime curvature is explainable when the earth exerts its so called gravity on objects out of the earth's atmosphere but what of objects which are on the earth's surface and move away from the ...
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Topologically, is a curvature singularity just a hole?

Topologically speaking, a hole can be introduced into a manifold and it will still be a manifold, e.g. remove points within a 2-sphere of some radius from the cartesian plane and you'll still have a ...
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Can anyone explain how does an object move upward from the surface of the earth defying the spacetime curvature?

I am trying to understand the while concept of spacetime curvature. Space is a fabric which is bent by heavy masses. But I don't understand that why objects follow the geodesics and get attracted to ...
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Which location on Earth has the strongest deviation from the vertical due to gravity?

So, I've recently learned about the Schiehallion experiment, performed in 1774, where scientists detected the deviation from the vertical of a plumb due to the gravitational attraction of a single ...
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The spin of a field

I have searched for an explanation for the math behind the spin of fields, such as the electromagnetic field has a spin of 1 and the gravitational field has a spin of 2. The internet did not provide a ...
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Speed of light measurement using a star mass, just like a satellite orbits the earth

This is just an idea than came to my mind last night, would it be possible to, launch a satellite, close enough to the sun (To be safe for it) to light a laser, somewhere close enough to the sun, ...
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Does Newton's Universal Gravitational Law get replaced or enhanced by Einstein's General Relativity Theory? [duplicate]

Regarding that both theories work on equivalent gravitational fields, would GR substitute or upgrade Newton's Gravitational Law?
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Better explanation of the common general relativity illustration (stretched sheet of fabric)

I've seen many science popularisation documentaries and read few books (obviously not being scientist myself). I am able to process and understand basic ideas behind most of these. However for general ...
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Jumping out of upside down helicopter, will turbine wind cancel me out like a bloey joey?

How long would it take to cancel the speed you're going when you leap out of your helicopter, after it does do an aerial flip and is flying straight downward? I used to say, "college felt like ...
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Basic Gravity/Projectile Question

I have a really basic question before I start some code. Can a projectile, launched into space, hit the opposite side of a planet with just its initial trajectory and force (assume an ideal, perfect ...
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What will be the tension force in this case?

Suppose I have a block of mass $2\, {\rm kg}$ sitting on my table. Then I connect a string to it and pull it vertically upwards. The resultant acceleration is vertically upwards: $2.2\, {\rm ms}^{-2}$....
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What is the current status of the convergence of the post-Newtonian approximation?

In the very well written article by C. Will, On the Unreasonable Effectiveness of the post-Newtonian Approximation in Gravitational Physics, he states: The one question that remains open is the ...
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Does the clock tick faster when falling? [duplicate]

I just read this answer that explains that the time dilation due to the gravity and the time dilation due to velocity are the same thing. Does that mean that, if I fall from the top of a skyscraper on ...
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Does gravity affect dark matter?

On this photo the red color is ionized gas and a blue color is a fark matter. We see the shockwave on gas but the clouds of dark matter move through each other without any interactions. The dark ...
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Can buoyancy and electrical charge explain atmospheric pressure aside from gravity? [closed]

It seems that buoyancy can explain most of the effects that are attributed to gravity. Flat Earth suggests that it is the source of what is called gravity, however they are omitting a very important ...
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If frequency shift is positive then how to interpret it?

Let say lightning is happening at $r=r_{A}$ from center of gravity and our observer at $r=r_{B}$ with $r_{A}>r_{B}$ then if we get frequency shift is positive (by considering let say schwarzschild ...
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How does spacetime curvature "CREATE" gravity? [closed]

What exactly causes an object to follow geodesics? If we take an analogy of the drawing lines on a sphere and taking two objects moving towards the poles then how do objects on earth move "above&...
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Expanding an expression involving covariant derivatives

Given $$ \sqrt{(-g)}g^{\mu\nu}\nabla_\mu \nabla_\nu \phi, $$ where $\nabla$ is the covariant derivative and $\phi$ a scalar field. Furthermore we define $\sqrt{-g}g^{\mu\nu}=\eta^{\mu\nu}+h^{\mu\nu}$,...
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Detection of gravitons in gravitatonal wave detectors: what is the expected value of the squeezing parameter for astrophysical sources?

The recent progress regarding the detectability of gravitons is quite interesting, and has been theoretically controversial. A series of papers involving F. Wilczek (the full-glory paper is titled ...
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Combination of time dilation due to motion and gravitational time 'contraction' due to negative mass

Consider a system with a moving spaceship of mass $m$ and a huge, fixed negative mass $M$ (with $- M \gg m$). The negative mass will accelerate the spaceship. The astronauts will experience time ...
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If kinetic and potential energy are relative, how do they exert a gravitational force? [duplicate]

I am imagining two bodies flying through empty space near the speed of light relative to their (distant) surroundings. Let’s say they are a bowling ball and a tennis ball. They are not moving with ...
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How to calculate orbit of a sphere around a bigger one in microgravity?

I don't have any knowledge in physics, but in a world building related question, I need to know how can I calculate the orbit or the distance in each point of the orbit around a spherical object in ...
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How can an observer establish a coordinate system in curved spacetime only with GR?

Consider the theory only posseses GR. If an observer lives in a curved spacetime, how can he measure the metric of the spacetime without knowing the matter distribution? Suppose he can establish a ...
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What are $δ$ and $ε$ in the list of Dirac's five 'fundamental constants', concerning his 'Large number hypothesis'?

From Jean-Philippe Uzan's Varying Constants, Gravitation and Cosmology: Dirac formed five dimensionless ratios among which1 δ ≡ H0ħ/mpc2 ∼ 2h × 10−42 and equation M1 and asked the question of which of ...
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Kruskal coordinates from GR field equations

The "usual" way (as I've seen in most text books) of dealing with Schwarzschild spacetime is to start with Schwarzschild coordinates, then explore the nature of coordinate singularity at $r=...
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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 ...
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There is a way of making a Balloon lighter than Helium/Hydrogen (or vacuum)? [duplicate]

I don't know much about physics, but I wonder if there is a way of making something like a Balloon to float more than Hydrogen, Helium or Balloon with Vacuum. I say "Vacuum Balloon" because, ...
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Is the non-technical definition of weight same as mass or is it also affected by gravity?

Technically, weight is the force which an object applies in downward direction, with Newton as its SI unit. However in non-technical usage, weight is a measure of heaviness or lightness of an object, ...

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