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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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Does the observable part of universe become smaller, larger or the same near event horizon?

I am pondering a lot about the nature of event horizons, but this one is not obvious to me. When you are in a gravitiational well, does the boundary of observable universe become closer to you or ...
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Does the same object have less mass on Earth than in space?

Would it be true to say a person is less massive on the surface of the Earth than they are in space because they have less potential energy? Would the Earth also be reduced in mass slightly?
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Space time and its existence [on hold]

I was just thinking if we could analyze the ellipses in which all of the planets move around the sun and then see if they resemble a cone with the sun at the apex(because that what it looks in the ...
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If space-time can be warped or curved, Doesn't that mean that It need space to be curved? [on hold]

There is a doubt that i've always have and no one has given to me a satisfing or good response, i hope someone give me the response. Lets Talk about space only. If space can be curved or warped, that ...
659 views

Justification for excluding gravitational energy from the stress-energy tensor

I did general relativity years ago at Uni and was just revising with the aid of Dirac''s brilliant book; the beauty of this book is that it is so thin and concise. On reading this book I find that I ...
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Perturbed Ricci scalar in Modified Gravity

When getting the perturbed Ricci scalar in a Modified Gravity theory of the form $\mathcal{L}_{gr}=F\left(\phi,R\right)R$, $\phi$ being a scalar field, it is easy to arrive at an expression of it in ...
110 views

How does exactly time dilate in a gravitational field, and how is it judged by different inertial observers?

When I tried to answer the SE question, Clock on a pendulum, I faced an ambiguity regarding time dilation in the gravitational field. To make the argument clearer, I designed the simpler thought ...
21 views

Time for two masses to collide in space [duplicate]

Take two equal masses $m_1=m_2=m$, separated by a distance $r$ in space. The potential energy of the system would be $$U=-\frac{Gm^2}{r}$$ When they collide the total kinetic energy is $$K=mv^2$$ ...
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Geometry of Tegmark's Level I multiverse

Consider Tegmark's Level I spatially infinite multiverse: it has the usual gravity, and each local copy of some material configuration has the same gravity. But how does the geometry of the Level I ...
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If gravity can be thought of as masses leaving dents on a spacetime 'sheet', what is holding up that sheet? [duplicate]

If the force of gravity can be thought of as masses leaving dents on a sheet of spacetime, what is holding up that sheet?
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Gravity-Electromagnetic field coupling [on hold]

How does electromagnetic field couple with gravitational field?
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How much of general relativity does the equivalence principle actually imply, why is it different?

From EP we have that gravity is not a force but a pseudo force, i.e an inertial force due to a gravitational field an accelerating a body independantly of its mass, in other words, the trajectory of ...
137 views

We all accept/believe in $c$. But what happens when light travels away from the center of gravity of a heavy star? Does it slow down?

I'm basically trying to understand a mysterious characteristic of the universe. Why light has to travel at $c$? I understand and accept that from experiment. Not arguing that it does not. Just saying ...
37 views

Is the relativistic time dilation compatible with the gravitational field of a moving massive planet?

Assume that a satellite orbits a massive planet/star at a distance $r^\prime$ away from its center in a circular path in plane $x^\prime y^\prime$. Suppose that the gravitational field is $g^\prime$ ...
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Motivation for tensor theory of gravity

In class we were shown that $$\rho = \frac{dm}{dV}$$ has the transformation properties of the 00 component of a rank 2 tensor. So we'd like to turn the classical Poisson equation for gravity into a ...
27 views

Gravitational wave energy fluxes and bracket averaging notation

Typically the rate of energy loss due to gravitational waves is given in the form (e.g. MTW or Eq 33 of these notes), $$\dot{E} = - \frac{1}{5} \langle \dddot{I}_{jk} \dddot{I}_{jk} \rangle$$ where ...
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Could Breakthrough Starshot spacecraft detect hypothetical Planet Nine?

Since we just want to prove if Planet Nine exists in our solar system and know the approximate position of the planet if it exists, we can avoid most of the technical challenges for StarChip (camera ...
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Undertsanding gravitational time dilation in terms of four-vectors

I am struggling to understand how to calculate gravitational time dilation using four-vectors, based on my understanding of them. Specifically, I would appreciate if someone could point out the ...
83 views

If I surrounded a black hole with gravitometers and observed the gravitational field as mass passed the event horizon, what would I observe?

By surrounding a black hole with gravitometers I would be able to get a 3-D map of the gravitational field. If I observed this field as an object approaches the event horizon, what would I see? Would ...
34 views

Damped Oscillation and Period

In my school experiment, I wanted to measure the gravitational constant ($9.81 \ m/s^2$) by using a pendulum. If we take into account the damped oscillation (i.e. friction forces), does that affect ...
71 views

What is the effect of acceleration due to gravity on horizontal acceleration?

The question is the following: An object accelerates from rest to $100\,\mathrm{km}$ per hour in $4.0\,\text{seconds}$. What fraction of the acceleration due to gravity is the car's acceleration? ...
72 views

Is The Third Time Derivative of Position Relative, and if it is, Can it be represented in 6 Dimensions?

Einstein’s Theory of Special Relativity only applied to objects at constant velocities. This could be represented in a four dimensional Minkowski Space. From what I understand, Einstein compared ...
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Acceleration of a falling object as it hits the ground

I am pretty new to physics so forgive me if this seems like a really dumb question. My Physics I teacher is having us develop a solution to the question: You drop your laptop; what is its ...
44 views

Are high-frequency gravitational waves inherently stronger/higher energy? Like the way gamma photons have more energy than radio photons?

A simple yes or no would be helpful, but a brief explanation would be nice too..... What about sound waves?
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Gravity in the multiverse

Consider Tegmark's multiverse theory, and single out a level. My question is: is each of the sub-universes endowed with gravity ? What about gravity between the sub-universes: does the multiverse ...
56 views

How does the fabric of space-time bends? [duplicate]

So I get the analogy of the fabric of space-time bending as a sheet would bend with the weight of a ball on it, but that explains the "outside" how would the fabric warp inside the earth? Like the "...
31 views

Constraints on models with spontaneous symmetry breaking

As I understand it, one of the loopholes of the Coleman-Mandula theorem is that models with spontaneous symmetry breaking (SSB) escape its scope. Is it therefore right to say that a quantum field ...
54 views

Unified theory of gravity [duplicate]

I have heard that a lot of times, but I have never completely understood what is a force theory unified with quantum mechanics, and also, why do we need this kind of gravitational theory to fully ...
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If objects slow down near a black hole, doesn't that mean the black holes won't absorb it until later? [duplicate]

First off, black holes grow (and also not die) by consuming matter, right? And if people (or any object) were to get close to a black hole, time would slow down for them which means they would ...
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Why do accelerations due to gravity and external forces get added during upward motion? [duplicate]

Suppose there is guy in an elevator, who has a mass of 100kg. The elevator is ascending with an acceleration of $10m/s^2$. What would be the apparent weight of the guy? Now, my resources tell me ...
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Overall pressure and movement in Earth making heat because of gravity

There are three sources of heat in the Earth's core, impacts of the planet when it was created (impacts made heat), radioactive decay of elements, and the extreme amounts of pressure and friction ...
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What do we mean by straight line in Inertia Law, autoparallel curve or extremal line?

As usually stated: Newton’s first law states that, if a body is at rest or moving at a constant speed in a straight line, it will remain at rest or keep moving in a straight line at constant speed ...
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Gravity in a spacetime with 2 indistinguishable dimensions, with all spacetime directions equivalent

A spacetime with 2 indistinguishable dimensions and all spacetime directions equivalent would have the signature (++) meaning that there would be no difference between spacelike and timelike ...
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Gravity as part of the metric but not of a 4-vector potential

In class, I've been told that the scalar potential $V(x)$ (except the one associated to gravity) has to be introduced in the Lagrangian as the time-component of a 4-vector potential $A_\mu$, so what ...
69 views

Geodesic equation in Einstein-Cartan manifold

In GR we use the Riemannian manifold without any torsion to describe the theory. Hence, the geodesic equation can be interpreted as "a trajectory of a free falling particle" or "equation of motion". ...
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Is there a significant difference between time at sea level vs top of mount Everest?

I heard about time dilation is somewhat proportional to gravity, and that gravity force may vary by latitude and elevation. So I am wondering, is there a significant time "speed" variation between if ...
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Does the curvature of light around a “centre star” in gravitational lensing affect the age at which we view the object being “lensed”

It occurred to me that during gravitational lensing, the light curves around a given object, creating a curved path. Because the path of light is curved, and thus, longer than a straight path, would ...
43 views

Will gravitational field of a body increase if the body is accelerated? [duplicate]

Suppose a body is at rest of mass m. If the same body is accelerated at velocity v, the mass increases to some definite amount. Since the mass of the body has increased, will the gravitational field ...
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Gravitational and Electric Forces in the Movements of Planets [duplicate]

Now that we know about the electric nature of the cosmos, the question arises: "How much are planets affected by electric forces while moving through the universe?" We have been taught that the ...
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Can photon Create gravitational wave when it travel through Space-time? [duplicate]

When big Explosion occurs in space it create gravitational wave. but it is possible that when photon (a packet of energy) make displacement in space-time it also create gravitational wave after all it'...
283 views

Is it theoretically possible to escape a classical black hole with force?

A stone being thrown from the surface must have a velocity of $11.2$ km/sec if it wants to escape Earth's gravity. However, a stone that has a constant force being applied on it need not have an ...
106 views

Equivalence principle doubt

There is something about Einstein Equivalence Principle that I don't quite get. This is my reasoning: Equivalence principle $\rightarrow$ locally, acceleration is equivalent to a gravitational field ...
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${}$Can the gravitational constant $G$ be calculated theoretically? [duplicate]

In an answer to a previous post of the same question, it was stated that the gravitational constant can not be determined theoretically yet! From my perspective, the reason this is not yet possible ...
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What is the experimental evidence for gravitational time dilation?

There are many experiments done to prove time can be changed. I don't know lots of them, but I think the most popular (most cited) is to put a clock on a fast moving object (airplane), fly around and ...
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Falling Speed of earth towards an object? [duplicate]

So I usually read, that the falling speed of an object towards the earth is independant of the mass of the object. Is this actually true? Shouldn`t the earth fall towards the object as well, therefore ...
56 views

Gravity, particle properties and magnetic monopoles

Generally, magnetic monopoles searches assume it has a mass. And that introduces gravity into the game. But, my question rough is: Why should a magnetic monopole have a mass or any other ...
95 views

Does general relativity account for the other 3 forces?

There are suppose to be 4 fundemental forces of nature, einstein described gravity not as a force but, put simply, the result of objects following curves in spacetime. However does it account for ...
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What do the terms attractive and repulsive gravity mean in Einstein's GR?

What does the jargon repulsive gravity mean in Einstein's GR or even attractive gravity in the same framework? The notions of attractive and/or repulsive only makes sense, or at least we are ...