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 Work Done by Gravity Equal to Zero in General Relativity?

I've heard it said that, according to Einstein's equations, gravity isn't actually a force but is instead an effect of the curvature of spacetime. In other words, a body in a gravitational field ...
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Can we construct a logically self-consistent relativistic theory of gravity just by tweaking EM?

This question considers a modification of classical E&M where we simply reverse the relative sign in Maxwell's equations and change the "$q$" in the Lorentz force law to an "$m$&...
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What is the magnitude of acceleration of both blocks?

A solid disk of mass M and a radius 3R is attached to a thin hoop of mass M and radius R such that the two objects would rotate together. A block of mass M is attached to the large solid disk via a ...
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Gravitons and general relativity

First I want to say that I am a layperson, so I want intuitive answers. So all the 3 fundamental forces in nature has a carrier particle except gravity. So we have hypothesized the existence of ...
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How to generate longitudinal acoustic waves in a granite disk. Starting from the center going outwards like dropping a rock in a pond

Diameter of disk: 25cm Thickness: 1cm composition: pink granite Follow-up on my earlier question about injecting 200 watts of power in granite. Among the answers already provided, was the idea of ...
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If a system is placed in a gravitational field, can you define internal energy for it?

My textbook says that internal energy for a system can only be defined if the system is isolated i.e. it has no external $K.E.$ and is not present in any external force field. But why? Why is it ...
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Will Earth's rotation be altered if a circularly polarized gravitational wave passes through it?

I have read this question: If you rotate a polarized classical electromagnetic wave by 180° about its axis of propagation, you don't get the same wave back; you get the same wave phase-shifted by ...
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Experimental Test for the cyclic $G_{earth}$ prediction of a Cosmological Model

Can anyone suggest a way to measure or rule out a tiny cyclic variation in the earth’s gravitational constant $G_{earth}$, predicted by an alternative cosmological model? It’s an annual cyclic ...
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Why do we need to equation of state (EOS) equation?

I cannot understand why we need the equation of state of star for the numerical solution of TOV equation. In fact, I do not know how EOS equation helps with solving TOV equation. How is EOS equation ...
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Randall–Sundrum graviton and experimental bounds

What is the physical property and math equation description of Randall–Sundrum graviton? How is this Randall–Sundrum (RS) graviton differed from the usual spin-2 hypothetical massless graviton? Is ...
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What motivated Einstein to formulate general relativity?

I never really fully understood what motivated general relativity or why the Newtonian concept of gravity was considered problematic. One thing I always hear is that it is because it doesn't address ...
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Why do the sensitivities of magnetometers and gravimiters have such strange units?

It looks like the sensitivities of magnetometers and gravimeters are usually reported with the units of $\text{Tesla}/\sqrt{\text{Hz}}$ and $\text{Gal}/\sqrt{\text{Hz}}$, respectively (where "Gal&...
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Why did we expect gravitational mass and inertial mass to be different?

I've read many times that the fact that gravitational mass is equal to inertial mass (as far as we can tell) used to be a puzzle. I believe that Einstein explained this by showing that gravity is ...
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Hawking radiation and Einstein-Cartan theory

Einstein-Cartan theory is an alternative to GR which allows for spin to affect torsion. The result is that a black hole doesn't collapse to a point and instead bounces back into a white hole and has ...
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A satellite revolving around an spacecraft around the earth? [closed]

So, I was solving some questions from gravitation and came across this question a spacecraft A of mass $M$ is orbiting round the earth with the constant angular velocity $\omega$ in a circular orbit ...
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Ambiguities regarding dimensions of Planck length

There is a couple of posts circulating around about how to express higher dimensional (Newtons) gravitational constants in terms of each other. In these discussions, summarized below, there is one ...
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Can you have quantum gravity without gravitons? [duplicate]

I was thinking about whether quantum gravity needs gravitons. One can do a sum over histories of curved spaces perhaps without having to have gravitons. But the term "quantization" implies ...
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Barometric formula with variable gravity

Recently, I have wondered about what would be the atmospheric pressure as a function of altitude in a planet that only consists of gas. The equation that does just that is the barometric formula (see ...
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Gravitational Time Dilation and Equivalence Principle

My question relates to page 118 of these notes, which is reproduced below for your convenience. Specifically, I have difficulty understanding the line "This shows that time slows in a region of ...
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Force of gravity at a level of fixed escape velocity [duplicate]

A brick (1 Kg) is placed on a surface that is stationary, relatively to a non-spinning black hole. The escape velocity at this level is 1/2 of the speed of light. Can we compute the weight of the ...
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Why and where Poincaré gauge theory fails?

I would like to post this question because I have seen no one post it in this explicit way. From what I have seen, Poincaré gauge theory uses connections (gauge fields) like the spin connection and ...
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Gravitational Time Dilation at height H Formula

The Wikipedia lists this formula for finding the time dilation in a gravitational field from one point to another point at "height" h, by integrating over the gravitation. However there is ...
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Frame dragging: is the velocity of the points of the 'frame' around a star inversely proportional to the square root of distance from the star?

Is the velocity of the frame around a star inversely proportional to the square root of distance from the star? In other words does it move simmilarly as astrophysical objects like planets or comets ...
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Going from 4d Planck mass to 3d Planck mass

I have an expression where I have converted all units such that the answer is expressed in 4d Planck mass, $m_p^{(4d)}$. In David Tong's lecture notes, Newtons gravitational constant in 4d is given by ...
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How work done by a force on a body is negative?

Work done is the product of force acting on body and the displacement of body in the direction of force. Gravitational potential energy=Mgh, which is negative of work done by gravitational force.But ...
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According to general relativity planets and Sun bend the spacetime (explaining gravity), but does this hold true for smaller objects?

According to general relativity planets and the sun bend spacetime, and that is the explanation of gravity. However, does this hold true for smaller objects, like toys, pens, etc.? Do they also bend ...
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What would happen to a long ruler in a strong gravitational field?

So, let's say that we have an incredibly long, virtually indestructible ruler. We have advanced enough to move it wherever we want. Let's also say that we have another, identical ruler of the same ...
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At what radius from the center of a black hole is space infinitely warped?

I read that at the singularity point at the center of a black hole, space is infinitely warped. However, how is space not infinitely warped already at the Schwartzchild radius, since at that radius, ...
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How large would a black hole have to be do devour a rocky planet such as Earth?

I had the idea of using a black hole as a weapon to engulf inhabited planets. I wanted to know the minimum mass required to absorb more than its hawking radiation worth of energy each second, which ...
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Can relativistic mass actually change gravitational pull?

I've heard that relativistic mass can influence gravity, but this seems to create a paradox, unless I am missing something. It seems to me that if there were two celestial bodies that are observed to ...
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Why is the group of gauge transformations on the frame bundle isomorphic to $\text{Diff}(M)$?

Consider the frame bundle $LM \to M$ for given Lorentzian manifold $M$. The group $\mathcal{G}$ of gauge transformations of the second kind are automorphisms $\phi:LM \to LM$ covering the identity $\...
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Does glass flow? [duplicate]

In old heritage buildings(50-100yr old). The glass windows (generally coloured) if we tap on the base of the glass and the top of the glass its a completely diff sound. You can clearly make out that ...
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Why does time 'change' when gravity increases?

I was doing some research on gravitational time-dilation, my topic for the Breakthrough Junior Challenge (a science video contest where contestants have to explain a hard science/math topic in 3 min ...
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Lasting gravity-free vacuum on the Earth [closed]

As the title says, Is it possible to create a lasting gravity-free vacuum here on Earth? sorry for my bad English
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If gravity is spacetime curvature, and spacetime doesn't show any discreteness, then gravity doesn't show any discreteness and can't be quantized?

I am not asking about why or how gravity should be quantized, or what the problem with renormalization is, or what the discrepancy is between QM and GR is. Those are beautifully described in other ...
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Event horizon visibility

I am attempting to correlate if this could be related in part to flow on negative Gauss curvature surfaces.. for an elementary explanation if possible. What dynamic mechanical or electromagnetic ...
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Why can't we do a Fourier transform of Einstein-Hilbert action?

In QFT, going to the momentum space to extract the propagators and the vertices is a standard procedure. However, the same is not true for the Einstein-Hilbert action. I want to know the precise ...
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Is gravity a force? Can there be a new BETTER theory for it? [closed]

what I'm asking is simple in look, but actually, it goes into depth and is complicated. Can there be a new better theory for gravity? and how and why is it not a force? it seems like the theory's the ...
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Does gravity reverse entropy? [duplicate]

How I got interested:- A few days ago I was watching a few YouTube videos about reversing entropy and how it was impossible. But while thinking about it, it suddenly seemed like gravity reverses ...
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Are there consistent theories of GR where our Universe is embedded in higher dimensional space?

In GR we have a metric space given by the metric $g^{\mu\nu}(x)$. This allows us to deetermine the curvature $R^{\mu\nu}$ at each point in space-time. This curved 4D manifold is not required to be ...
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Why did Dirac say that atomic time is different from relativistic time, and that gravity is becoming weaker? What is the relation between the two?

In this gem of an interview in 1982 with Friedrich Hund, Dirac says at 09:17 that there is some theoretical basis and observational evidence that atomic time and distances are different from ...
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Difference in the precession regarding the gravitational gradient

Let say a less massive star acts on a planet causing it to precess.Does a more massive star cause less precession of this planet if in both cases the gravitational force applied to the planet center ...
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Is there a good reason not to think of gravity as the signaling of particles inward motion?

This is not about inventing new theories, let alone rejecting canonical ones. I am just wondering if general relativity (GR) could be interpreted like the following, or if there is something in GR ...
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If incoming photons increase the mass and gravity of a black hole are they then some kind of source of gravity? [duplicate]

If incoming photons increase the mass and gravity of a black hole are they then some kind of source of gravity?I can not understand why equivalence of mass and energy lead as to say mass and energy ...
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Does General Relativity allow a reduction in the strength of gravity?

Observations of galaxies commonly show ‘jets’ and other ejection phenomenon like these ‘Fermi bubbles’ The Fermi Bubbles are two enormous orbs of gas and cosmic rays that tower over the Milky Way, ...
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Particular behaviour of 'black hole' horizon in modified gravity

When working in a particular theory of modified gravity, one can see that a solution for a spherically symmetric and static puntual mass is given by \begin{equation} ds^2=-B(r)dt^2+A(r)dr^2+r^2d\theta^...
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What balances out the Normal force in this scenario of uniform circular motion?

In my Physics course we are dealing with uniform circular motion. In this scenario there is an object in horizontal uniform circular motion. In the FBD provided in the problem the gravitational force ...
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By how much does the gravitational redshift change a neutron star emission spectrum, thus disturbing the measurement of its surface temperature?

How much does the gravitational redshift change a neutron star emission spectra disturbing so the measurement of its surface temperature? I remember some tv-shows talking about Italian pasta when ...
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If I throw an object upward will its acceleration change or it will stay constant at $-g$?

I mean if, for example, I throw a rock upward, its acceleration will always be $-g$ or it will be $-g+a$ because I apply a force on the object when I throw it? (without considering friction)
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Why Newtonian mechanics works in cosmology? [closed]

Could anyone please provide an intuitive explanation of why Newtonian mechanics can be used to describe our universe at extremely large scales? A typical explanation goes "since speeds and energy ...

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