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.
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What are the ways to solve the problem of the negative heat capacity of black holes since the canonic approach gives a positive heat capacity
From one hand, it is known that the heat capacity of a black hole is negative: \begin{align} M=\frac{1}{8\pi T} \quad \to \quad C_v=\frac{\partial Q}{\partial T}=\frac{\partial M}{\partial T}=-\frac{1}...
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The flatness in the teleparallel gravity
Curvature is an almost directly measurable quantity: I measure the surface area $S$ of a body (massive ball, e.g. Earth, how much color I need to paint it). I measure its radius $r$ (by digging a ...
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How does Gravity work with White Holes? [duplicate]
Black holes are so dense that they don't even let light escape them. So what about white holes? They are opposites of black holes and reflects (i don't have a better term for this, sorry) light. So ...
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What happens in a free fall if Earth's gravity changes? [closed]
I am trying to imagine a thought experiment in which a physics lab is in free fall in relation to Earth. Suddenly, Earth changes its mass---it loses mass and becomes equal to the Moon (I'm ignoring ...
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Gravitational instantons and normalization
The normalization factor for the gravitational instanton number is commonly stated as $1/384\pi^2$ (see for example Equation (2.27) of Dumitrescu) $$ \frac{1}{384\pi^2}\int\text{tr}(R\wedge R)\in\...
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Spacetime as condensed matter or fluid
Nowadays it seems that the thought of "Spacetime as condensed matter or fluid" is called analogue gravity which I can find a good review article recording all the PhD thesis. We have the ...
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Why do we need the metric in GR?
The equivalence principle only needs the connection coefficients to describe gravity so why isn't the metric just the Minkowski metric and we work with the connection coefficients? Is there a ...
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Perhaps long-range Brans-Dicke dilaton field has not been ruled out? [closed]
It is generally assumed that solar system gravitational tests have ruled out a long-range Brans-Dicke dilaton field. But consider the following hypothesis: each massive particle in the Sun is ...
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How are the heavens affected by a spinning gyroscope?
Inspired by this answer. Make a gyroscopic device having counter-rotating wheels, one enormous, one tiny and let its components be at rest. Place it at (for the sake of argument) the centre of the ...
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Can we say something analogous to that GR is purely spin-2 with respect to other maximal symmetric background spacetimes (dS or AdS)?
With respect to a flat spacetime background, we say that linearised Einstein gravity is `pure spin-2'. For instance, here is a quote from Maggiore's textbook: We have therefore found that the ...
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Doppler Effects for Gravitational Waves
Let's consider a binary system of a neutron star (NS) and a white dwarf (WD). If the system has a non-negligible proper motion or it is under the influence of an external gravitional field, then the ...
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Does Earth accelerate towards the object or the object accelerates toward the Earth? (General Relativity)
Does Earth accelerate towards the object or the object accelerates toward the Earth? If objects move along the curvature of spacetime and crash on earth, could we also say that earth accelerates ...
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How can you determine gravity on a quantum scale if quantum objects are not dense enough to warp spacetime? [closed]
If gravity is a result of the warping/curving of space-time, then how can we possibly measure gravity on a quantum scale if these quantum objects are not dense enough to warp spacetime in any ...
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Is scalar field $\phi$ in $f(R)$ theory time-dependent only?
I'm reading a paper on $f(R)$ theory, see here. In this paper, scalar field $\phi$ is introduced from the conformal transformation by $$ \kappa \phi \equiv \sqrt{3/2} \ln F, \tag{2.31} $$ and the ...
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What should I vary the Tetradic Palatini Action with respect to in order to find its equation of motion?
I'm trying to calculate the EoM for the Tetradic Palatini Action. I've gained an understanding for tetrads/vierbeins, the spin connection, Omega tensor, and the action, but when it comes to applying ...
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