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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Friedmann equation for a universe contains both pressureless matter and radiation in FRW model

I need to write a code in Fortran to solve the Friedmann equation to get a(t) in FRW model, and I need to assume that the universe contains pressureless matter and radiation. For radiation w=1/3 and ...
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Mechanical energy conservation for different chosen systems

I studied work-energy theorem and the equation for mechanical energy conservation yesterday. I’m not able to understand how to identify forces when choosing a system (which force is which). This is ...
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Plane and gravity force [closed]

If a plane, once in flight, continued to accelerate on a straight trajectory, but without exceeding the escape velocity, and without considering stall or lift effects, would it leave Earth despite the ...
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In general relativity, what makes an object released in a gravitational field START to fall? [duplicate]

I understand why a released object moves as it does once it has started moving, and why all objects fall at the same speed, removing other effects. But I don't see why the curvature of space would ...
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Definition of orbits of a Killing vector field

Given a Killing Horizon $\mathcal{N}$ of Killing vector field $\xi$, one can prove that the surface gravity $\kappa$ is constant on orbits of $\xi$, i.e $$\xi\cdot\partial\kappa^2 = 0.$$ What is the ...
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How far the gravitational waves can travel?

How far the gravitational waves can travel? Does it depend on the mass which is converted to energy in a black hole/neutron star merger? If there is a finite distance, were we lucky to be in the range ...
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How to visualize spacetime curvature? [duplicate]

I find it very difficult to visualize spacetime curvature in my mind. When I look at the depiction like the one below it doesn't make much sense to me, instead, it is making it more confusing for me ...
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Why are we still considering gravity as one of four fundamental forces? [duplicate]

if gravity is just bending of spacetime according to general relativity why are we still considering it as one of the 4 fundamental forces. https://astronomy.stackexchange.com/q/12408/ this link ...
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Finding parallel distance between a spherical body and a mass attracted from infinity [closed]

Body B with mass M is attracted by the spherical body A off radius R and mass M (M>>m). Body B starts at infinity with initial velocity $ v_{\inf} $. It travels parallely in the beginning with ...
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Free fallen object: convergence Schwarzschild with classic mechanics

I'm trying to verify that, using Schwarzschild metric and using Newton mechanics, the same velocity over time for an object that is fallen to a planet in straight line, until some amount of ...
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Silly question about a realization given by gravitomagnetism: could there exist a “apparent gravitational magnet”?

First of all, I will use a conclusion given by gravitomagnetism, but the example will concern Kerr black holes, and therefore lies in the realm of strong gravitational fields. Well, gravitomagnetism ...
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Do bodies moving at (near) light speed produce gravity? [duplicate]

If a massive body is moving at near light speed (or, hypothetically at light speed) and on course to hit Earth, will we feel its gravitational pull before it collides with us? I think the answer is ...
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Will a pendulum oscillate on the ISS?

A question I encountered is whether a pendulum undergoing SHM on earth will oscillate on the ISS. Apparently the answer is it won't due to no effective gravity. The argument is there will be no ...
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How does the philosophy of 'Emergent Gravity' differ from that of 'Quantum Gravity'?

How are the two approaches of understanding gravity, the so-called 'Emergent/entropic gravity' and 'Quantum gravity' (String theory, LQG etc) differ from each other? I am very curious and will ...
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How much would increased gravity affect the particle count in air?

If everything else on Earth stayed the same (as much as possible anyway) but the mass was scaled up, how much would that affect the number of non-gas particles in the air such as dust, hairs, viruses, ...
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What exactly is the cosmological constant? [duplicate]

I know that the cosmological constant was developed as an addition to the Einstein Field Equation as an anti-gravity force: $$R_{\mu \nu} - \frac{1}{2} R g_{\mu \nu} + \Lambda g_{\mu \nu} = \frac{8 \...
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How is the orientation of quantum spin testing of entangled particles made the same between two distant tests?

Quantum entanglement says that two spatially-separated particles tested for quantum spin on the same orientation will be guaranteed to produce opposite results, but testing in a different orientation ...
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de-Sitter space as near-horizon limit of Black Holes

We know that we can obtain $AdS_{2}$ x $S_{2}$ spacetime as the near-horizon geometry of an extremal/ near extremal RN (or Kerr) Black Hole in asymptotically flat spacetime. It is also known that we ...
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Is three dimensional supergravity dynamical?

So it is well known that standard $D = 3$ Einstein gravity is non-dynamical in the sense that the graviton has no on-shell degrees of freedom (d.o.f $= D(D-3)/2$ and the theory is topological). ...
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Boundary terms for stringy correction to GR

We know that there can be possible higher derivative corrections (stringy corrections) to the Einstein-Hilbert action. In GR, to ensure that we get the Einstein Field equations from varying the E-H ...
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Relationship between gravitational field strength, mass, density and altitude [closed]

Explain why both mass and density affect the gravitational field strength at the surface of any celestial body. Explain why the gravitational field strength does not depend on the density of the ...
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Planck length compression of gravitational waves due to gravitational time dilation

I've made an error in my thinking somewhere concerning gravitational waves in the following thought experiment, and would appreciate knowing where I went wrong: Consider two overlapping gravitational ...
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Why do I feel a force from a non-accelerating punch? [duplicate]

If I'm getting punched by a fist that is moving at a constant velocity (let's say $20 \mathrm{m/s}$), I still feel a force that is being applied to my body. However, according to $F=ma$, in order to ...
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Conserved charges without conserved currents?

I recently read (Section 9.3 of J.Kaplan AdS/CFT notes) that we can have QFT's s.t. we have conserved charges but NO associated conserved currents. How is this related to the gauging of global ...
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Thought Experiment - Gravitation, gravitational lensing, and negative mass matter

Suppose you had a giant mass of some exotic form of matter that has a negative mass. How would that mass behave with respect to gravity? What would a sufficiently large clump of this matter do to ...
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Are there any experiments going related to gravitational shielding? [duplicate]

Is it possible somehow to achieve gravitational shielding? Are there any experiments presently going on or future projects?
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What is the physical interpretation of the equation for the invariant interval in general relativity?

In my GR lecture I was given the following equation for the spacetime interval (signature $(+,-,-,-)$): $$ ds^2=(1+\frac{2\phi}{c^2})c^2 \, dt^2-(1-\frac{2\phi}{c^2})\delta_{ij}\, dx^{i}dx^{j} \tag{1}...
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Would naked singularities have the same gravitational properties as black holes of the same mass? [duplicate]

This is just a thought experiment before someone mentions the cosmic censorship conjecture. If physicists are discussing it in papers and scholarship, I wanna see some of those theoretical angles too ...
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Gravitational waves of spherical body with mass $M$ which collapses to a black hole with mass $m<M$

Consider a body with spherical symmetric mass distribution in asymptotically flat space with a total mass $M$. Assume it collapses to a black hole with mass $m<M$. Does this body emit ...
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Why is Principle of superposition not oftenly used for gravitational potential?

We talk a lot about this principle for electric potential at point by many charges but not for gravitational potential by other masses around it, why?
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Can we increase the potential of a charge/mass by keeping many charges/masses next to it? [duplicate]

The "principle of superposition" says that a charge might have higher or lower potential energy depending on the number of charges around it because the potential, according to formula, is the sum of ...
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Hamiltonian of a quantum field that is minimally coupled to gravity

The action for the gravitational field is known as the Einstein-Hilbert action: $$\begin{equation} S_{G}=\int d^4 x \sqrt{|g|} R \end{equation}$$ where $R$ is the Ricci scalar. The ...
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Do theoretical naked singularities have the same gravitational properties as black holes of the same mass? [duplicate]

Just a thought experiment before someone mentions the cosmic censorship conjecture. If you take a black hole and turn it into a naked singularity, does the naked singularity have the same ...
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What is the analog of the Aharonov-Bohm effect for general gauge fields and for gravity?

The Aharonov-Bohm effect concerns the electromagnetic fields, where a quantum particle can feel the effect of an electromagnetic field in regions where it vanishes, through the electromagnetic ...
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Will Earth be devastated when the orbit shifts 1 metre towards the Sun? [closed]

There are some arguments in my country from a population that they are "religious". Then, their argument is that there is a perfect balance in universe and solar system. Hence they say that Earth ...
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Why is an event horizon dependent on density?

It’s frequently stated in Astronomy documentaries that you could replace the sun with a black hole of equivalent mass and the orbital mechanics will continue as normal. This works fine in dispelling ...
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How is time defined in time dilation?

It appears that there are two different concepts of time dilation (in my amateur point of view): Time delation due to relative velocity. Let‘s say a second is measured by the time it takes for light ...
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Speed of propagation of gravity in full non-linear theory

It is often stated that gravity propagates at the speed of light. Justifications for that statement usually follow from the linearized theory, for which it is more or less obvious. However, I was ...
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How do gravitons and photons interact?

First of all, I am a noob in physics (I‘m a computer scientist) and started reading Hawking‘s „A brief history of time“. In Chapter 6 he says that “electromagnetic force [...] interacts with ...
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Prove that we can always obtain De-Donder gauge

In linearized gravity $\bar{g}_{\mu \nu}$ is the metric of the background and $h_{\mu \nu}$ is a small perturbation we have the metric $$g_{\mu \nu}=\bar{g}_{\mu \nu}+h_{\mu \nu}.$$ I often read in ...
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If you were to hang a wrinkled shirt, would it ever become unwrinkled over time via gravity? [closed]

If you were to hang a wrinkled shirt, would it ever become unwrinkled over time via gravity?
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Gravity at light speed

my question in regards to light. Since gravity warps space time, and we have discovered gravitational waves. Would the light at the front of the wave, be traveling faster than lightspeed? Essentially ...
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Does gravity induce current?

Suppose I have a long metal copper wire (for example $100\ \mathrm{m}$ or $1\ \mathrm{km}$) and put it vertically on the earth's surface, then I put direct current (DC) of 1 ampere into it. I make it ...
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How to measure the gravity component in all the three dimensions using three-axis accelerometer?

I am reading this research paper and trying to understand how the three-axis accelerometer MMA8452Q use in this study to estimate the gravity component for three-axis i.e. $a_x$, $a_y$ and $a_z$. My ...
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Why is there not a General Relativity for forces other than gravity?

I think what I'm looking for here is some sort of a bridge between the very material terms and mental images that I have access to and more of a pure math understanding. My deepest exposure to ...
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Can I cancel out the gravitational force when calculating the gravitational constant? [closed]

Basically, this is what i’m doing: $$F = m_1a$$ $$F = \frac{Gm_1m_2}{r^2}$$ where: $F$: gravitational force $m_1$: mass of the earth $m_2$: mass of object dropped $a$: gravitational ...
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Never underestimate thermodynamics (especially when it relates to gravity)

In the review paper by Subir Sachdev, the low-temperature effective action of charged SYK models has been derived using holography. In section 4, arguments of thermodynamics are used to derive the ...
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Einstein equations in 2 dimensional dilaton-gravity theories

In Ref. as Jensen, the model Eq.(12) does not contain any kinetic term for the field $\varphi$: \begin{equation} S = \int d^2 x \sqrt{-g} \left( \varphi R + U[\varphi] \right) \end{equation} The ...
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Is one dimension the same as one space-time?

In this Cern article, https://home.cern/science/physics/extra-dimensions-gravitons-and-tiny-black-holes, it states: "In our everyday lives, we experience three spatial dimensions, and a fourth ...
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Why is the graviton spin supposed to be 2, and how can it interact with matter? [duplicate]

As far as I know, gravitons are predicted to be spin-2 particles because some extrapolation from GR about non-linearity and antisymmetry. I think I want some clarification on that. What are some ...

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