All Questions
Tagged with spacetime black-holes
322
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End points of event horizon
I am reading The Nature of Space and Time by S. W. Hawking. In the last paragraph on page 16 he said that:
event horizon may have past end points but don't have any future end points
I understand ...
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Does gravity accelerate you towards the geodesic of light between you and the mass?
If there's a planet far away, you will accelerate straight towards it due to gravity. If you place a Schwarzschild black hole right in the middle between you and the planet (the distance between the ...
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Can part of space be causally disconnected from the rest of the universe by being surrounded by black holes? [duplicate]
Is it possible for black hole event horizons to overlap and form a spherical wall around an island of space (that's not inside a black hole) while still being causally disconnected from the rest of ...
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When you are in a gravitational field, do object far away get physically closer to you as you get closer to the mass?
An observer A is close to a black hole and an observer B one light year away. They are both remaining at constant radial distance from the black hole. A is at 2 Rs away from the center of the black ...
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How to Understand Negative Energy in the Ergoregion?
I am trying to understand the Penrose process and having trouble explaining negative energy in the ergoregion.
How I interpret it is:
Energy is the dot product between the four momentum of the object ...
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10
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How do black holes move if they are just regions in spacetime?
If black holes are just regions of spacetime, how can black holes even move? When matter moves through spacetime, it bends the spacetime around it, but if black holes are just regions of spacetime, ...
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Time required to reach Black Hole's Event Horizon from outsider perspective?
Let's imagine a pair of particles that is entangled. One (call it $P_1$) is released and then falls to a black hole from a distant $x_0$, (for example $x_0=5r_s$) and velocity $v_0(=1/2c)$, while the ...
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Types of singularities
I am confused about the types of singularities. According to my limited knowledge there are two types of singularity. One is space like singularity ( a curvature singularity enclosed within a null ...
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Keplerian Frequency of Schwarzschild Black hole
The Keplerian frequency/ Orbital frequency is the inverse of orbital period and for Schwarzschild black hole it is given by $$\frac{1}{2\pi}\sqrt{\frac{M}{r^3}}.$$ its unit is Hertz. Now To express ...
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What’s the condition to form an astrophysical wormhole?
According to GR, what’s the mechanism for a star to form a wormhole? How is it different from collapsing to a black hole? What’s the energy scale required?
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Carter-Robinson Theorem
There are uniqueness theorems that classify Black holes according to its mass, angular momentum and charge. One of the theorem is Carter-Robinson theorem which has many assumptions and then it says ...
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Intuition for the interior Killing vector fields in Schwarzschild?
The Schwarzschild metric represents a stationary (and static), spherically-symmetric, spacetime. These characteristics are manifested by the four Killing vector fields: one for time translation and ...
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How can Planck-length elements exists in spacetime? [duplicate]
My question is simple, how can the theory of finite-sized elements (Planck-sized elements) in spacetime be correct, when you find the number $\pi$ in the Schwartzchild representation of the black hole,...
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Flat space between colliding black holes
When 2 black holes approach each other, they both bend space in an opposite direction. There must always be a flat space between 2 colliding black holes.
However, I heard that they actually merge, ...
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Event horizon in stationary spacetime
In the case of non-stationary spacetimes finding the event horizon is no easy task.
The stationary case should somehow be less involved or so it is in some well known cases, such as the Kerr spacetime....
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What effect causes stars to be optically enlarged at the Einstein ring of a black hole?
Sadly i do not know what this effect is called, but it can be seen in some simulations;
Credit to Alessandro Roussel
The same effect can be seen on the Wiki page for black holes.
For a while now, me ...
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Metric of Einstein static universe (ESU) black hole
The Einstein static universe (ESU) has metric
$$ g = - dt^2 + d\chi^2 + \sin^2 \chi d\Omega^2 $$
With
$$ t \in \mathbb{R}, \chi \in (0,\pi) .$$
Is there a metric that describes an eternal black hole ...
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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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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 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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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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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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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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Time in the negative mass Schwarzschild solution
I have read that for the Schwarzschild metric solution with $M<0$, something odd happens with the time coordinate. For the constants of motion, $dt/d\tau=e(1 - 2GM/r)^{-1}$ with $M<0$ and $e$ a ...
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Do all of the distortions in the space caused by that object get lost? Or do the distortions get back to where they began? [closed]
According to general relativity, if an object keeps moving and warping space and then gets lost by entering a black hole, do all of the distortions in the space caused by that object get lost? Or do ...
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Penrose diagrams and Holographic Principle
What would the Penrose Diagram look like that represented a black hole (call it Black Hole-B), inside of a massive black hole (Black Hole-A), in our universe?
and, as inside of the Penrose diagram for ...
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Kruskal Diagram: 2D projection?
Is a Kruskal diagram a 2D flat space projection of Schwarzschild space-time diagram? If not, isn't it true that one could not draw one accurately on a paper?
BTW, I am not referring to Penrose ...
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Why do we defer to GR when describing black holes rather than rely on QM?
This is a broad question but it's well documented that GR and QM are very well tested in their own domains but they conflict around black holes.
Picture a neutron star slowly accreting matter until it'...
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If it's a common myth that a black hole contains a singularity, what does a black hole actually (likely) contain?
It's a common myth (especially in popsci) that a black hole contains a singularity. However, I cannot find an explanation for what we think a black hole actually does contain. The best I've seen is &...
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Books/resources for the study of non-flat spacetime shadows
I'm preparing a research paper (and possible review) on shadow calculation in asymptotically non-flat spacetime. I have been searching the internet for some references and the only thing I found was ...
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Are black holes the edge of our universe?
Are black holes the actual edge of the universe? Because spacetime is another dimension, I would assume the universe doesn’t have perceived corners or edges. At least humans cannot perceive it.
The ...
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Why are there two interior regions in this diagram?
Why are there two interior regions in this diagram? There seems to be two inner horizons, and two wormhole regions.
Where do the two such regions comes from? What determines which reason someone falls ...
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Where does the parallel universe in the Penrose diagram come from?
In this diagram, as well as our universe, you have a parallel universe.
Where does this come from? Is this just a artifact of the diagram, or is it predicted by the maths in some sort.
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Question on apparent super efficiency of the magnetic penrose process
Accordingly to this paper $[1]$, the efficiency of the so called Magnetic Penrose Process (MPP) is, for supermassive black holes of mass $M∼10^{10} M_{\odot}$ immersed in a magnetic field having $B∼10^...
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How exactly does Hawking radiation occur? [duplicate]
I understand some parts of the theory, I've read from here https://phys.libretexts.org/Bookshelves/Astronomy__Cosmology/Supplemental_Modules_(Astronomy_and_Cosmology)/Cosmology/Carlip/...
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Does spacetime move? With respect to what?
Can spacetime itself rotate along a body, like a black hole? Would it move like a wave?
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Why are there multiple universes in the Reissner-Nordström solution?
I am trying to make sense of the Penrose diagram of a non extremal Reissner-Nordström spacetime, that is, the solution with two horizons. The coordinates are
$$ v'=\text{exp}\left(\frac{r_+-r_-}{2r_+^...
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How does according to modern physics collision between two massive black holes create new space or new spacetime?
Now (hypothesis) -- And the Physics of Time
In this page the author talks about (find in page) (in the context of LIGO Observatories-->) "In their most powerful event, reported just this past ...
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Question on non-diagonal Einstein Field Equations and the interpretation of Energy-momentum tensor components
Considering an EFE like:
$$\begin{pmatrix}G_{00}& 0 & 0& G_{03}\\0 &G_{11} &0 &0\\ 0&0 & G_{22}&0\\ G_{30}& 0& 0&G_{33} \end{pmatrix} = 8\pi\begin{...
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Question about the energy-momentum tensor and tetrad formalism
Given a metric tensor, you have an Einstein tensor associated with it. If your Einstein tensor is diagonal, you can "match it" with a diagonal energy-momentum tensor. More over, if your ...
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Orbital Velocity Formula in Schwarzschild metric
What is the orbital velocity formula in Schwarzschild metric assuming that the orbit is circular? this formula should also work for relativistic velocities.
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Conformal Diagram for Astrophysical Black Hole
I have a question about the conformal diagram of an ‘astrophysical’ black hole which forms in finite time (but with no evaporation).
Usually I see the conformal diagram presented as something similar ...
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When a new black hole forms does the universe shrink/contract?
I know that scientists debate if the universe is infinite, but for the purposes of this question let’s assume that the universe is finite and has a boundary. When a new black hole forms does the ...
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Curvature of space in a black hole
This is a very simplistic view from an interested structural design engineer (retired).
Mass curves space. Taking the case of a sphere of uniform density the point at which you have as much mass ...
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Has it been observed so far an object moving towards a BH along the trajectory of the polar astrophysical jet?
If we assume that the cause of the astrophysical jet is the magnetism of the BH than an object falling towards the BH in counterdirection of 'polar jet' emitions should not feel any strong deflection ...
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Where does the singularity go in an Einstein-Rosen Bridge?
I've been reading up on some material about black holes and Einstein-Rosen bridges. Generally it is said that a black hole is defined by the event horizon (boundary in space where the gravitational ...
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Sorts of relativistic effect around black holes [closed]
There are many effects around black holes. In particular it is possible to study the motion of geodesics, calculate tidal tensors, lense-thirring effetcs and so on.
So, beyond tidal effects, geodesic ...
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How do we know the assumptions of the Schwarzschild solution are valid?
The Wikipedia article on the derivation of the Schwarzschild solution (https://en.wikipedia.org/wiki/Derivation_of_the_Schwarzschild_solution) lists 4 assumptions. The second of which is:
A static ...
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About the temporal coordinate $t_*$ in Eddinton-Finkelstein coordinate
For the Schwarzschild solution $ds^2=-(1-\frac{2GM}{r})dt^2+(1-\frac{2GM}{r})^{-1}dr^2+r^2(d\theta^2+sin^2\theta d\phi^2)$, if we set $dr^2_*=(1-\frac{2GM}{r})^{-2}dr^2$(in other words, $r_*=r+2GMlog{|...
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How to derive the Kerr killing vector?
The Kerr metric have two killing vectors:
$$t^{\mu} \equiv (k_{t})^{\mu} = (1,0,0,0)\hspace{5mm} \mathrm{and}\hspace{5mm} \phi^{\mu} \equiv (k_{\phi})^{\mu} = (0,0,0,1). \tag{1}$$
In general, it is ...
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