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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Gravity = Yang-Mills squared? What is the Lagrangian?

In the following comment and references, it is mentioned that gravity can be understood as yang mills squared. What is the Lagrangian of a Yang Mills squared theory? Can anyone provide a quick primer? ...
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Required cushion/foam width to meet reduced force level in event of impact

Suppose a biker is riding a bike and bumps into roadside. If he/she is wearing good cushioned helmet, will not feel high 'g' force, but with an average helmet, impact can be high. So how does width or ...
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A new video about the "Real Cause of Gravity" that claims all other videos are wrong

This video on The True Cause of Gravity in General Relativity states that all other videos claiming that time dilation is the cause of gravity are "100% PERCENT UNEQUIVOCALLY COMPLETELY WRONG&...
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Is there any possibility that our gravity is inverted? [closed]

Imagine a world where earth and water fly on the sky, the earth nucleus is exposed and works as a son. Everything is upside down, the world is pointing towards the said core and not the sun. I hope I ...
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What happens when a neutron star becomes a black hole?

Suppose you have a neutron star that's as close as can be imagined to the required mass to become a black hole, perhaps just one proton mass away from this limit, when it collides with a dust grain ...
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When does a collapsing star become a black hole?

It’s said that from an outside observer’s reference frame, a black hole (as well as the event horizon) takes forever to form because of runaway time dilation, which causes falling matters to slow down ...
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Could a Black Hole Contain the Entirety of Space Time? [duplicate]

I was lying awake last night struggling to sleep as one does, thinking about time and Enstines concepts of relativity and having some fun with thought experiments similar to the one Enstine described ...
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Can gravity exist without time?

I have read this question: I think it's the opposite - when there is no gravity, the difference in time observers measure only depends on their relative velocity. It's when gravity gets very strong ...
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Measuring the speed of light with a black hole

is it possible to use the grabity a black hole has to bend the light back to the observer and measure it that way? also saying i made the light go from the right and bend back coming at my left and ...
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Emergent Gravity

One thing that I did not understood yet is how some physicists think of gravity, fundamentally, as a emergence phenomena? I see much of this "belief" through fluid dynamics approach to study ...
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Initial velocity of a body when the distance travelled by it in the last second before reaching its maximum height is $5\ \textrm{m}$ [closed]

I stumbled upon this problem recently, and it has been haunting me for more than a day at this point. The question is as follows:- When a ball is thrown up, it reaches a maximum height of $h$, ...
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Doubt regarding simplification of Lense-Thirring precession frequency formula

The standard result for Lense-Thirring precession frequency in the weak-field approximation is given by $$\vec{\Omega}_{LT}=\frac{1}{r^3}[3(\vec{J}\cdot\hat{r})\hat{r}-\vec{J}].$$ I am trying to ...
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Schwarzschild metric in terms of a falling observer's coordinates

The Schwarzschild metric tensor $\textbf{g}(r)$ in terms of a distant observer's coordinates $(t,r,\Omega)$, can be written as: $$ds^2=-\left(1-\frac{r_s}{r}\right)dt^2 + \left(1-\frac{r_s}{r}\right)^{...
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What exactly does Einstein's gravitational constant $8\pi G/c^4$ mean?

I understand that Einstein's gravitational constant is a sum of multiple constants and has a value of $8\pi G/c^4$, but what exactly does it represent?
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How can gravity change the wavelength of a photon without the two peaks or troughs to accelerate one from the other?

How can gravity change the wavelength of a photon without the two peaks or troughs to accelerate one from the other? So is it possible that gravity accelerates light? Can the a certain wavelength ...
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Do black holes exert infinite forces?

Black holes have infinite mass so according to Newton's law of universal gravitation should exert and infinite amount of gravity force.
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Apparent speed of an object in a circular orbit in Schwarzschild geometry

Does an object in a circular orbit around a massive body in Schwarzschild geometry seem to move faster for a stationary observer located close to the massive body, compared to how the same object ...
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Would a person split in half if entering a portal to a "mirrored world"? [closed]

I tried making the title worded as good as possible, but let me explain some more: SPOILER FOR STRANGER THINGS SEASON 4 In episode seven they climb a rope up into the portal and when they enter the ...
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Is there an upper boundary to magnetism?

This is gonna take some explaining, and full disclosure: I'm still undergrad, so please, forgive my ignorance here. Though please also hear me out: magnetism, like gravity, falls off with distance, ...
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How exactly did Einstein check if his equation on general relativity was correct? [duplicate]

I saw the equation on general relativity and then I wondered: how did Einstein check if his equation was right or wrong?
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Would gravity pulling person toward Earth change if velocity of Earth changes? [duplicate]

So I was reading the Albert Einstein's theory of how gravity works. From my understanding, the more mass an object has, the more space-time around it it bends. All objects travels in completely ...
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Do Two IDENTICAL Bodies Cancel-out Each-other's Gravity?

If we were to imagine two objects suspended in space* that were identical, with the exact same number of atoms, would the gravitational forces produced by each cancel each-other out? (maintain their ...
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Notion of mass of a quantum field theory in AdS space-time

Consider a scalar field theory in $AdS_{d+1}$ spacetime \begin{equation} S= -\frac{1}{2}\int d^{d+1} x \sqrt{-g} (\partial_\mu \phi \partial_\nu \phi g^{\nu \mu} + m^2 \phi^2 ) \end{equation} On ...
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In two particle system why can't we calculate the gravitational potential energy of each mass separately and find the net potential energy?

In a two particle system, suppose both are moving by their internal gravitational force, and hence the potential energies of the system is changing. Since both are acted upon by the conservative ...
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Direct calculation of the gravitational potential inside a hollow sphere

I calculated the gravitational potential inside a massive sphere with constant density and got the result: $$\Phi = -2\pi G\rho R^2 + \frac{2}{3}\pi G\rho R_p^2$$ Where $R$ is the radius of the sphere ...
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Can empty space 'press' galaxies?

If gravity slows the effects of time, then empty space will see greater expansion than space inhabited by massive objects. So the space within a galaxy will be expanding more slowly than the space ...
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Would the amount of balls in a newton's cradle make a difference if there was no friction?

If there was no friction or air resistance, would a cradle with 2 balls and 200 balls (or more) both keep moving forever? Thank you
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Is force exerted by a falling body greater if left from greater heights? [duplicate]

Consider a hammer undergoing a free fall from the second floor. It shatters a sheet of glass beneath (on the ground). However, if the same hammer is dropped on the sheet of glass from a distance just ...
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Is there an equation for the acceleration of gravity in a disk of uniform density?

I tried to solve this problem by entering this monster of an equation into Wolfram: $$G\rho\left(\int_{-R}^{2r-1}2\int_{0}^{\sqrt{R^{2}-x^{2}}}\frac{x-r}{\sqrt{\left(x-r\right)^{2}+y^{2}}^{3}}dydx+\...
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Does energy get attracted by gravity? [duplicate]

I will describe my question using a scenario since I don't know exactly what to ask that will convey my question properly. Let's say two identical faces were to get punched by a fist that generates ...
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Would the force transferred from a punch get affected by gravity? [closed]

I will describe my question using a scenario since I don't know exactly what to ask that will convey my question properly. Let's say two identical faces were to get punched by a fist that generates ...
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Lagrangian of a graviton

Recently in an interview for a phd program I was asked how would you write the lagrangian of a graviton. I answered that since graviton is a massless particle it's lagrangian should be similar to the ...
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Can you see on Miller's planet?

I have read this question: Miller's world would be fried by a strong flux of extreme ultraviolet (EUV) radiation. The cosmic microwave background (CMB) would be blueshifted by gravitational time ...
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3 answers
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How does string theory relate superposition and general relativity?

So I know that in general relativity, superposition cannot be true. However, since string theory supposedly connects quantum mechanics and gravity, how does it relate superposition and gravity?
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Calculation of interaction of gravitational waves with a black hole

We have a well understood interacting electromagnetic system, an electromagnetic wave interacting with an atom. We use perturbation theory to calculate what happens in such a system. The result is of ...
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If gravity was lower but air density was the same, how would evaporation be affected?

I'm confused about the relationship between air density, air pressure and gravity. Does water evaporate faster under low air pressure simply due to the air density being lower? If the air pressure was ...
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Energy conservation in the Sachs-Wolfe effect

When energetic photons climb out of a potential well (eg Sachs-Wolfe effect) they lose energy. According to conservation of energy, where does this energy go? Into the gravitational potential making ...
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3 votes
5 answers
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Why doesn't light 'orbit' Earth? (why doesn't light follow a geodesic?) [duplicate]

I learned that things like the international space station or the moon are not "pulled" by the gravity of the earth but instead are following a geodesic (a straight line in curved spacetime)....
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Gravity Differential Equations

I was just messing around with Newton's Law of gravitation, when I had the idea of converting Newton's Law into differential form (more or less like Maxwell's equations). I did the following: #1 ...
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Is the volume in general relativity independent or dependent on the coordinates?

The volume in curved space is calculated as: $$V=4 \pi\int_{\Omega}r^2\sqrt{g_{rr}} d\Omega$$ Is this volume dependent or independent from the chosen coordinates? As I understand it should be ...
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State of the art on Modified gravity : going beyond the 2nd order differential equations, diffeomorphism invariance breaking, extra degrees of freedom

I am going to do a state of the art on Modified gravity models. I have found a talk that presents the problematic. In particular, it is said the following things : Modifying General Relativity How to ...
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Equation of motion in quadratic gravity

I am going through the paper https://arxiv.org/abs/1502.01028 which considers the quadratic gravity with the action \begin{align} S = \int d^4x \sqrt{-g} (R - \alpha C_{\mu\nu\rho\sigma}C^{\mu\nu\rho\...
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4 answers
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Does an observer experience stronger gravity on the way towards the singularity?

I have read this question: Clearly as you approach the singularity the tidal force on a body increases. Are the fundamental forces unified in a black hole? Based on the questions on this site, ...
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2 answers
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What is the real world interpretation of $E = mc^2$? [closed]

Why I am asking this question: There appears to be a logical contradiction in the notion that gravity is "one thing" mass moves in gravity much differently than light (a massless particle) ...
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Why would black hole rip me apart?

According to my understanding of General Relativity, gravity is not a force and an observer which is falling freely under the influence of gravity should be considered inertial. Now, I have come ...
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What is the component of gravity other than curvature?

This question started long, but it summarizes as follows: there is light there is mass These are 'many' things for the purpose of a single force, gravity. Yet there are 'many' components of gravity: ...
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How compact can a thin shell be without collapsing?

I've heard of https://en.wikipedia.org/wiki/Buchdahl%27s_theorem. It states that as the ratio between a spheres radius a it's Schwarzschild radius approaches $9/8$. the pressure at the center tends to ...
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How velocity affects different orbits?

Assume a sun with mass $M$ and a planet with mass $m$. Assume at $t_0$ the planet is $r_0$ (distance) away from sun and has an initial velocity of $v_0$. Also, let’s assume the angle between the $r_0$ ...
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-6 votes
1 answer
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The speed of time [closed]

If the speed of light is 14,545.075 x the speed of sound, could the speed of time (and/or gravity) be 'something' like that times the speed of light, say 2,705,384,023.877 m.p.s? (Sonic boom > ...
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2 votes
1 answer
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Does convection occur in microgravity due to pressure gradient?

Context: On Earth, with gravity, when heating a fluid, the particles gain thermal energy and decrease in density, and a difference in density throughout the fluid causes thermal energy transfered by ...
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