Questions tagged [gravity]
Gravity is an attractive force that affects and is affected by all mass and - in general relativity - energy, pressure, and stress. Prefer newtonian-gravity or general-relativity if sensible.
5,470 questions
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What does it mean that the Ricci scalar is zero? [duplicate]
I was watching this video on how to derive the Schwarzschild metric when at around 4:30, he mentions that because the Ricci scalar $R$ is zero, there is no immediate change in volume due to the ...
8 votes
1 answer
573 views
In a stationary, asymptotically flat spacetime, when does a Killing horizon coincide with the event horizon?
In general relativity, the black hole region (B) of an asymptotically flat spacetime $M,g$ is defined globally as $$ B := M \setminus J^-(\mathscr I^+), $$ and the (future) event horizon is its ...
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A Nuclear Quantum Gravity [closed]
I'm trying to publish this paper about a Quantum Gravity using the atomic nucleus, but by now the answer is it's controversial. What do you think? https://www.researchgate.net/publication/...
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Does dissipation have different gravitational entropy effects depending on which mass dissipates in a bound system?
Consider a bound two-body system ($M>>m$) such as Earth–Moon. I'm trying to understand whether there's a meaningful asymmetry in how identical dissipation processes affect gravitational entropy ...
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1 vote
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Can the electron beat the photon in this race?
I have two possible scenarios and I wonder whether the electron and arrive before the photon in either of them. Scenario 1: There is an emitter at A, and a reciever at B. Let the midpooint of AB be C. ...
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What do we know about mass equivalence principle for non-nucleons, mass in e.g. neutrino oscillations?
While special relativity says inertial mass is equivalent with energy, there are at least two more types of mass, for which equivalence seems not so certain - let me briefly summarize and ask for more ...
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1 vote
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What would someone surrounded by a collapsing shell observe?
Inside of a shell space time is flat, but you do get time dilation relative to distant observers. As the shell gets smaller, time dilation gets stronger. So does and observer inside of a collapsing ...
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2 votes
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Collapse of a charged dust cloud to a Reissner–Nordström black hole
The Oppenheimer–Snyder_model models the collapse of an object into a black hole. But what if the object is strongly charged, is there a model for the collapse of a spherical cloud of charged matter ...
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Why I am not expanding? [duplicate]
We know universe is expanding and space is also expanding. When why we dont feel the space around us and ourself expanding or stretching .even an big object Does it negligible?
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-3 votes
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Black hole evaporation [duplicate]
“If mass and energy are equivalent, and black holes evaporate by losing energy via Hawking radiation, does that mean singularities never actually form — that they’re replaced by ultra-dense, finite ...
3 votes
2 answers
162 views
Gravity at Initial Universe Creation Point $t_0$
Did gravity exist at our universes' $t_0$ (initial singularity)? Or, did gravity only exist after Mass was established with the creation of the Higgs Boson? What period of time was there between $t_0$ ...
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Induced metric on hypersurface of constant comoving radius in FLRW metric
I am currently looking at the Oppenheimer Snyder Collapse model analytically. The interior metric is described as the FLRW metric: $$ ds^2 = - d \tau^2 + a^2(\tau)(d \chi^2 + sin^2 \chi d\Omega^2) $$ ...
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Gravity, black holes and and the movie Interstellar [closed]
OKay, I know hollywood just be hollywooding science fiction movies. However, to my understanding the movie Interstellar had a physicist Kip Thorne who informed them on some aspects in astrophysics. ...
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Could the absence of local cosmological expansion be viewed as a balance of stresses in spacetime?
Cosmic expansion is described by the FLRW scale factor 𝑎(𝑡), while locally bound systems are well modelled by the Schwarzschild or Schwarzschild–de Sitter metric. The usual explanation is that ...
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1 vote
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When can discrete curvature on a weighted graph reproduce an Einstein–Hilbert–type action in the continuum limit? [closed]
In several discrete-geometry approaches — such as Regge calculus, causal sets, and graph-based Ricci curvature — local curvature is defined combinatorially rather than through differential geometry. ...
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