Questions tagged [black-holes]
A black hole is a region of spacetime from which nothing can escape. More formally, the future light cone of any observer within the black hole is completely contained in the black hole, and the black hole region is not within the past light cone of any observer that goes to spatial infinity in an infinite amount of time.
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Doesn't matter collapsing inside a black hole spin at the speed of light?
Once an event horizon is formed in the process of the gravitational collapse of a star, the original star collapses inside the event horizon. As far as we know, stars and in particular neutron stars, ...
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Should we expect lightning-like phenomena as matter enters the accretion disk of a black hole?
As matter enters the accretion disk it will rapidly turn into superheated plasma. I would expect this to coincide with lightning-like phenomena due to the electric potential difference between this ...
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Shouldn't all things falling into a black hole meet at the horizon?
Consider two observers starting to fall toward a black hole from rest at different distances from the black hole with their worldlines shown on the Kruskal-Szekeres chart below
On the diagram, it ...
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Can the gravitational singularities of black holes be solved by potential or self-energy?
In Newtonian Mechanics, the energy density of gravitational field is negative in comparison
with the positive energy density assigned to mass density, meaning that that the total positive energy of ...
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Calculating the Bekenstein-Hawking entropy for 1+1 black hole with dilaton background
According to this paper the Bekenstein-Hawking entropy of a 1+1 black hole which described by the $SL_k(2,\mathbb{R})/U(1)$ WZW cigar geometry is given by the following formula appearing in eq. (5.7):
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If I were to drop my phone into a black hole, would I be able to catch it?
Say, for the sake of argument, I am outside the event horizon of a black hole and accidentally drop my phone (or some other object) into the hole. If I were to enter the black hole, would I ever be ...
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Thermodynamics of supersymmetric black hole
For a 4-dimenstional supersymmetric Kerr Newman black hole, we have
$$\beta\Omega=2\pi i$$,
and the index
$$\text{Tr}[(-)^Fe^{-\beta(M-Q)}]=e^{S+\beta\Omega J}$$
with $S+\beta\Omega J$ being $\beta$ ...
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General relativity change of observer
I have a problem in calculating the module of the velocity of a particle measured by a static observer in a specific metric. This metric is
$$ds^2=(r^2-R^2)dt^2-\frac{dr^2}{r^2-R^2}-r^2d\varphi^2$$
...
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What is the mechanism for neutrino emissions from a black hole
In Black Holes and Time Warps, Kip Thorne describes a conference he and Stephen Hawking took in the 1970's where Hawking discussed black hole evaporation. He discussed radiating gravitational waves ...
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Ejected bodies, dynamical friction and dark energy?
I have a question after reading a couple of papers (https://arxiv.org/abs/1707.06220 and https://www.arxiv-vanity.com/papers/1102.0007/).
Here, the authors seem to indicate that when bodies like stars ...
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Why does time stop at the event horizon of a black hole and not at the singularity?
I understand that infalling objects reach the speed of light at the event horizon. Any associated clock would be observed to stop, according to SR. But isn't the key here 'observed'? An infalling ...
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Are black holes 4-dimensional balls of spacetime? If so, will they have 3-sphere surfaces?
If black holes are 4-dimensional balls of spacetime, they will have a 3-sphere surface with a 3-dimensional volume. Would this allow infalling matter to remain within this surface?
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Why does Roy Kerr claim that the Kerr black hole does not contain a singularity?
In a preprint posted on the arXiv, Roy Kerr claims that there is a widespread misunderstanding related to the singularity inside the black hole that bears his name.
Can anyone explain his argument in ...
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How can gravity swallow light? [duplicate]
"In physics, gravity is a fundamental interaction which causes mutual attraction between all things that have mass"
"Light or visible light is electromagnetic radiation that has no mass&...
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Obtaining Geodesic equation for Massive particles using Schwarzschild metric
Geodesic equations for the metric $$dS^2 = \left(1 - \dfrac{2GM}{r}\right)\dot t^2 - \dfrac{\dot r^2}{\left(1-\dfrac{2GM}{r}\right)} - r^2\left(\dot \phi^2\right),$$ would be
$$\left(1 - \frac{2GM}{r}\...
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Minimum Radius of a material body around a Schwarzschild Blackhole? (Topic: Circular Geodesics in Schwarzschild Spacetime)
The question is to show that $r=3M$ is the minimum radius of a circular orbit for a material body in the Schwarzschild spacetime.
I did the calculations and it is coming up to $r=6M$ but as per my ...
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Suppose the universe from our big bang is surrounded by many distant blackholes with each a mass of sextillions of suns. Could it explain dark energy?
Could dark energy "simply" be from multiple distant universe-scale mass black holes, that are there "all-around" the big bang?
Scattered apparently randomly, throughout infinity (...
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Why cant a repulsive event horizon of negative mass be theoretically constructed?
An event horizon appears in the Schwarzschild metric when considering a positive point mass in General Relativity.
But for a negative point mass in the negative mass Schwarzschild metric, which ...
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How are white holes described as outgoing spacetime and still have attractive gravity?
White holes are described by the time reversed Schwarzschild metric. A test particle which is gravitationally attracted to the black hole's event horizon in the Schwarzschild metric is also ...
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Distinction between behavior of spacetime inside a black hole during evaporation and the phase of transition to a white hoöe, in Loop Quantum Gravity
I was just watching: Carlo Rovelli (2023). What is a white hole? The Royal Institution (YouTube).
Rovelli mentions at 36 minutes that the transition to a white hole state is more likely than ...
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Relativistic Corrections to orbital elements
Given a massive compact object $M$ and a smaller object $m$ orbiting it in an elliptical orbit where $M \gg m$, Newtonian gravity describes the orbital elements $(a, e, i, \omega, \Omega, T)$, such as ...
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Killing Vectors and Horizons
Let $\chi^{a}$ be a Killing vector, we want to prove that the surface gravity of a stationary black hole is constant over the horizon. The proof can be found in In page 333 of Robert M. Wald's General ...
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What happens if we throw the observer in a black hole?
Sorry if this sounds like a silly question, but what would happen if a scientist observes Schrodinger's cat alive, but is then thrown into a black hole before he has leaked any information to the ...
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Eigenvalues of the geodesic deviation equation, curvature invariants, and singularities
The geodesic deviation equation tells us what tidal forces freely falling observers experience in a local Lorentz reference frame. The tidal deformation tensor is
$$E^{\alpha}_{\gamma}=R^{\alpha}_{\...
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Are there a more general metrics for the case of multiple masses? [duplicate]
I understand that for the case of one non-rotating mass with an electric charge the metric describing the spacetime around the mass is the Reissner-Nordström metric
$$-c^2d\tau^2=\left(1-\frac{2GM}{c^...
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Penrose Diagram for AdS black hole
I am trying to construct the Penrose diagram of a black hole in AdS space. Now, I thought I was on a good track, my diagram looked like this:
The grey lines are the surfaces of constant $t$ and ...
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What would happen if two super-massive black holes collide faster than the speed of light? [closed]
I know objects can't move through space faster than light. But there's no law against the expansion of the universe increasing the distance between two objects faster than the speed of light. But this ...
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Time dilation for different observer in black hole metric
If I have a 2d Schwarzschild metric
$$
dS^2 = -(1-\frac{r_s}{r})dt^2 + \frac{dr^2}{1-\frac{r_s}{r}}
$$
I want to find the relation between the time of an asymptotic observer $t$ and the proper time of ...
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Constant determinant of metric tensor of Schwarzschild solution in $(x, y, z, t)$ coordinates?
In spherical Schwarzschild coordinates, it's $A \cdot B$
= constant. Is there something similar in the Schwarzschild solution in $(x, y, z, t)$ coordinates? For example in Droste coordinates $g_{tt} \...
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Redshift near Kerr Black Hole
The question is related to Sean Carroll's Spacetime and Geometry ex 6.6.
Consider a Kerr black hole with an accretion disk of negligible mass. Particles in the disk follow geodesics. Some iron in the ...
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What's physical meaning of 2-point correlation function in holographic condensed matter?
Background: In AdS/CFT, we can do calculations in AdS spacetime, and get the result in CFT. When we consider RN-AdS black hole/brane, 2-point correlation functions in CFT can be obtained, which are ...
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Are two oppositely charged black holes, matter & antimatter?
Let's say we have 2 identical black holes, except that their charges are opposite in sign.
Can we consider one to be matter, and the other antimatter? After all, I heard that anti-particles are the ...
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How fast would a hypothetical microscopic quantum black hole evaporate with an effective mass of two protons?
According to the analysis shown in this research here, see link the evaporation time can be calculated in seconds using this equation:
$$
\begin{array}{l}
t_{\text {evap }}=\left(\frac{5120 \pi G^2}{\...
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Hawking radiation reduces mass of Black Hole?
If hawking radiation is the result of a pair of virtual particles popping into existence just outside the event horizon, and then one falling into the black hole through the event horizon, and the ...
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Schwarzschild and Kerr solution in $(x, y, z, t)$ coordinates?
The Schwarzschild solution ('simple' black holes) and the Kerr solution (rotating black holes) are very well known in General Relativity.
The coordinates which are used to describe them are mostly ...
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What is the expression for strong field Schwarzschild circular orbit perihelion precession?
How can you calculate how large the perihelion shift should be under Schwarzschild conditions in general relativity for minimally disturbed circular orbits in the strong field limit?
I think there is ...
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Rotating Black Holes and Birkhoff's theorem
I found a few questions that are similar to mine, but I do not think that either of them answers exactly what I want:
Is there a Birkhoff-like theorem for stationary axisymmetric metrics?
or
...
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Examples of black hole analogues
After spending some time looking for different means and mediums to model blackholes, I came across sonic holes which are modeled through Bose-Einstein-Condensates.
However I have not been able to ...
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Why isn't total Energy used in Schwarzchild Metric instead of just mass?
The $M$ is Schwarzschild metric comes from Newtonian limit. But in the Newtonian limit the total energy also reduces to $M$. Is there any particular reason for using mass instead of total energy? ...
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Can there be black holes in a 1+1D spacetime? [duplicate]
In 2D Einstein tensor is always zero, that means no mass (or cosmological constant or other stress energy tensor component) is allowed. Nevertheless, we can get nonzero Riemann, even nonzero Ricci ...
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Does a geodesic exist that will take someone across the event horizon?
I saw the movie "Interstellar" a few years back, and was amazed that Cooper was able to fall from 1 AU into a black hole before his daughter turned 110. Intuitively, I would think that there ...
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Can I fly my spaceship within a black hole's ISCO and still escape? [duplicate]
Regarding black holes: I recently learned that the Schwarzschild radius (where nothing can escape) is not the only interesting limit. There is an "Innermost stable circular orbit" (ISCO) ...
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Photon speed for an observer at the photon sphere [closed]
I am asked to compute the orbital period of a photon, in the Scwarzschild spacetime, at the photon sphere for an observer at the same radius, $r^\star=3M$. I have computed the result, $\Delta T=6\pi M$...
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The Calculation of the Volume of a Photon Sphere around a Black Hole
So, I chose this question for my chemistry assignment which is "How many moles of photons will it take to fill up the volume of the photon sphere around Sagittarius A". I realized there is ...
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Is entropy dependent on mass alone?
The entropy of a non-rotating, non-charged black hole is only proportional to the surface area of its event horizon (Bekenstein, 1973).
The area of the event horizon of a spherical black hole is only ...
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Falling angle of a photon near the event horizon
I have been doing simulations using Mathematica of light and matter paths (plotted in the $r, \phi$ Schwarzschild coordinates) around a black hole, in the Schwarzschild metric. This was in order to ...
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Why isn't there an event horizon in the negative mass Schwarzschild metric?
The negative mass Schwarzschild metric has no event horizon.
Why isnt there a particular radius in which spactime flows outwards at the speed of light? This would imply a region of the solution for ...
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Can the Schwarzschild orbit equation be solved exactly?
The Schwarzschild orbit is
$U'' + U = 1 + \varepsilon \cdot U^2$.
I've only seen turning-point analysis that demonstrates shift of perihelion.
Is there an exact solution?
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In the Schwarzschild metric how is the inverse for the $g_{rr}$ component written?
In the Schwarzschild metric how is the inverse written for the component $g_{rr}$, which is $\frac{1}{1-\frac{2GM}{rc^2}}$?
Does $g^{rr}$ =$\left(\frac{2GM}{rc^2}-1\right)$, and therefore, $\left(\...
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Can light "phase through" a small black hole?
Suppose we have a small black hole, maybe $1\ \mathrm{cm}$ in diameter, and a ray of light with a wavelength of $10\ \mathrm{m}$ (or more, if necessary) is exactly aligned with it.
Suppose the black ...
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