Questions tagged [black-holes]

A black hole is a volume from which matter cannot escape. More formally, the coordinate speed of light at the event horizon - the boundary of a black hole - is zero, as measured by a sufficiently separated observer.

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What causes a black hole ringdown and why can it prove the no-hair theorem?

When two black holes merge, they will produce the so-called ringdown before forming a new black hole, even the newly created black hole produces some sort of tones due to the force of the impact, but ...
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Can Schwarzschild Metric be derived from curvilinear coordinates in special relativity?

If we regard a simple scenario in which a particle is subjected to a constant force, it will take a hyperbolic path on a Minkowski diagram. If we assumed equally spaced particles started accelerating ...
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Does the no-hair theorem imply that dark matter cannot have any “charge”?

By "charge" I mean some kind of unique conserved property, similar to electric charge, color charge, baryon number, etc. The no-hair theorem states that black holes can only have three macroscopic ...
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Proper time difference in Schwarzschild geometry: stationary observer vs. observer on a circular orbit

Lets consider two observer, which are at a position $r=R$ around a spherical mass which is described by Schwarzschild geometry. At some point, observer $1$ make a full round around the object on a ...
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Why are the Schwarzchild or Eddington-Finkelstein not maximal?

In the book by d'Inverno, he writes the definition of a manifold being maximal as "A manifold endowed with an affine or metric geometry is said to be maximal if every geodesic emanating from an ...
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Is there a Black Hole solution, similar to Schwarzschild solution but modified to accout for black hole mass reduction due to evaporation?

Schwarzschild solution is permanent and unchanging in time. What would happen if we modify it so to account for the change of BH mass over time due to evaporation?
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Weightless at the center of mass of two merging black holes?

I have read this question: What is the general relativity explanation for why objects at the center of the Earth are weightless? When $r = 0$ the Christoffel symbol $\Gamma_{tt}^r$ is zero and ...
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Black holes, and faster than light communication

Lets say we have a black hole that has a diameter > 1 light year and we can somehow push and pull this black hole {though the push and pull itself can never exceed lightspeed of course}.Will the other ...
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Is Bekenstein-Hawking black hole entropy the maximum bound of the entropy of a black hole?

Is the entropy $S_{BH}=\frac{c^3 A}{4Gh}$, where A is the surface area of the event horizon of a black hole, the "real" entropy of the black hole or a maximum bound of that "real" entropy?
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Do black holes also exist on a quantum level? [closed]

I understand that quantum mechanics refutes the concept that black holes can not exist on a quantum level because matter can only be absorbed no emitted from a black hole. However, Hawking's radiation ...
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Where is the singularity located in a Kerr black hole?

In a rotating Kerr black hole is the ringlike singularity situated between the inner and the outer event horizon of the black hole?
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Deformation of Schwarzchild radius of two approaching black holes

I'm trying to visualise merging black holes. Here's my impression of a single black hole: Green is the strength of gravity outside the Schwarzchild radius, blue is the strength of gravity inside the ...
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What elements do black holes expel into relativistic jets?

What does current research say about the elements that can be created in the accretion disc of a black hole, and does the Relativistic Jet contain any of those elements? Curious as to whether the ...
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Quasinormal modes for certain black holes

Quasinormal modes for black holes are an important topic today. What would be the properties of a black hole whose quasinormal modes (all of them) have equal real part? And if all the imaginary parts ...
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Black hole creation details

I've had a couple of general relativity courses thus far but I'm still not quite sure what the answer to this question is. Let's assume we are on a perfectly spherical planet at the centre of a region ...
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Do we have any statistical measures / estimates of how much mass, as a fraction of the total, is contained in black holes?

Do we have any measures or estimates (from the visible universe or perhaps avg. per galaxy) of how much mass is contained in black holes vs everything else?
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Does a (charged) black hole evaporate its charge?

There are basically two types of charged black holes, Reissner–Nordström, and Kerr–Newman. Next, Hawking radiation itself is mostly electromagnetic. The numerous popular articles which say that ...
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Radiation efficiency of astrophysical objects using GR

The radiation efficiency $\epsilon$ of an object (for example a star or black hole) can be defined as the energy dissipated by some matter moving from infinity into a lower orbit divided by the total ...
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Geodesic equations in Schwarzschild geometry

I am a little bit confused about the following equations. In the lecture we derived the following four equations for a geodesic motion of a particle in Schwarzschild geometry using the Lagrangian ...
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Schwarzschild-like metric in black hole region [closed]

We know that in the black hole region $r<2M$ the metric takes the form $$ g = \Big(1-\frac{2M}{r}\Big)^{-1}\text{d}r^2 - \Big(1-\frac{2M}{r}\Big)\text{d}t^2 + r^2(\text{d}\theta^2 + \sin^2\theta \...
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Making a black hole using charge instead of mass?

You can just skip the background portion if you are interested in the real question. A bit of Background I was reading about gravity in my textbook and it mentioned about how in the recent years ...
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Is it possible to flyby a black hole at a distance of 1 m from event horizon and come back?

Since the kinetic energy is conserved, unless you cross the event horizon you should come back without any energy loss. In other words, a carefully constructed trajectory would allow an object to ...
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Can Light Penetrate the Schwarzschild radius of the Earth?

Suppose there is a narrow hole through the center of the earth. This hole passes through the Earth's Schwarzschild radius. My question Can light escape from Schwarzschild radius of Earth? ...
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The physical interpretation of the trip inside Kerr (black hole) spacetime

Consider then the Kerr metric$[1]$: $$ds^{2} = -\Bigg(1-\frac{2Mr}{\rho^2}\Bigg)dt^2 - \frac{2Marsin^2(\theta)}{\rho^2}[dt d\phi+d\phi dt] +$$ $$+\frac{\rho^2}{\Delta}dr^2+\rho^2d\theta^2+ \frac{sin^...
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Calculation regarding redshift and black hole

Reading this entry: http://www.scholarpedia.org/article/Bekenstein_bound I was wondering about one calculation mentioned. They say an object that descends adiabatically to almost a black hole's ...
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Slingshot around a black hole, dipping into event horizon

Suppose we have an object swinging by a black hole. My understanding is that if it remains outside the black hole, it just swings by and doesn't lose any energy (except maybe gravitational waves?). ...
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Infinity at event horizon boundary [duplicate]

It has been repeatedly explained to me that from the perspective of an outside observer a particle takes "infinite time" to cross the event horizon of a black hole. Also, according to these ...
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What is a cold black hole in astrophysics?

I found the word 'Cold Black hole' in some academic papers. I think it is concerned with thermodynamics of black hole but I don't know exactly what is cold black hole. Please help me to understand ...
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Can we tell how far away is the source of gravitational wave from the chirp alone? [duplicate]

From the analysis of chirp from Ligo and some other similar detector it is possible to tell the mass but can we also find out how far away too? or we need to look up into the sky and compare the ...
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Radial dependence of a spherical energy density, gravity, and black holes

To motivate my question, I'll share an example from Carroll. Carroll shows that if we have a simple model of a star as a perfect fluid characterized by some coordinate-dependent pressure $p$, energy ...
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What happens in the area between two black holes before they collide?

Before two black holes collide, the area between them must be under incredible stress. The effect could be measured by measuring the radiation from some object behind them.
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Black hole and the continuum of laws in normal space

I have heard it said that the laws of physics break down inside a black hole. I find it hard to believe that the law of conservation of angular momentum would not operate inside a black hole. Are some ...
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When and where is a black hole formed on the Penrose diagram

In the standard Penrose diagram of black hole forming from collpase (either a star or a matter shell), where can we locate the event where a black hole forms? Is it just the point at which the matter ...
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Does Birkhoff's theorem still apply to an ultra-dense spherical shell of matter falling into a black hole?

In previous posts I discovered that the effects on space-time geometry of an ultra-dense stationary spherical shell yielded pretty simple equations but I imagine some sort of secondary non-linear ...
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What happens to the momentum when 2 black holes collide and no matter is present?

Imagine 2 non-spinning black holes with identical mass collides with each other at the same speed in the absence of other matter, I think the gravitational waves can only carry a little of the ...
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How do you calculate the acceleration due to gravity of a massive object (black hole) [duplicate]

Is there an online calculator to find the acceleration of gravity a massive object exerts from a specific distance? I know the effects of gravity kind of just fade out with distance but I'm asking for ...
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Using Lorentz Transformations in Black Hole

Can we use Lorentz Transformations to understand the space-time curvature around a black hole? To my understanding, the Lorentz Transformations enable us to understand time dilation, as in the case of ...
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Can a black hole accelerate matter / light particles beyond the speed of light? [duplicate]

We know that light cannot escape the gravitational pull of the event horizon of a blackhole. This made me wonder if falling into the event horizon would accelerate the matter / light particles beyond ...
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Can a charged (likely) binary black hole ( BBH) system be in a stable circular orbit (SCO)?

A BBH is a system consisting of two black holes in close orbit around each other. The last stable orbit or innermost stable circular orbit (ISCO) is the innermost complete orbit before the ...
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Relative Acceleration in Schwarzschild coordinates

Place an observer in spacetime such that the feet of the observer touches the Schwarzschild radius and the head is $x$ meters away from the Schwarzschild radius. I want to calculate the relative ...
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Boundary Conditions for Black Holes

Consider a metric with one degree of freedom lets say: $ds^{2} = -a(r)dt^{2} + a^{-1}(r)dr^{2} + r^{2}d\theta^{2} + r^{2}sin^{2}\theta d\phi^{2}$. Einstein's equations are second order differential ...
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Metric elements of the Schwarzchild metric

I am learning about the Schwarzchild metric $g_{\mu\nu}(x)$ for the spacetime geometry outside a spherically symmetric source with mass $M$. In the book by Cheng, a spherical coordinate system $(t,...
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Would a repulsive black hole interior expand along with the horizon or into a Wheeler's bag of gold?

Consider an embeded de Sitter space inside the Schwarzschild interior with negative energy density (repulsive). It is clear that it would not collapse but bounce. Does it bounce into a bag of gold (...
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Why can't you just call black holes “exceptions to the rule” in the black hole paradox?

My understanding of the black hole information paradox can be summarised as follows: Black holes emit hawking radiation in random directions. This means that they eventually "evaporate", and so all ...
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What is meant by “information” with regard to general relativity and/or black holes?

In The Universe in a Nutshell chapter 4, Hawking explains the warping of spacetime according to general relativity and introduces the basics of black holes. It surprised me to read about "information"...
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Formation of primordial black holes from density perturbations in the early universe

Recently I've been reading Carr and Hawking paper on the formation of primordial black holes "Primordial black holes in the universe" , but I couldn't understand part of it. It says that: A region ...
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Is Hawking radiation truly random or we just don't know that it is?

First off I am not a black hole scientist and my education is limited to special relativity so please treat this question appropriately. The way I understand the BH information paradox is that ...
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Quantum entanglement and black holes

If a particle pair has particle A fall into a black hole does that mean for particle B time stops?
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How do we see an object fall into a Black hole in gravitational waves? [duplicate]

If an object emitting light falls into a black hole, the conventional wisdom is that an observer (at infinity) will never actually see it fall in - they will see it moving slower and slower as it ...
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What does an external observer see if you shine a laser pointer while crossing an event horizon?

If I understand correctly, the event horizon of a black hole is the boundary beyond which we cannot perceive or describe any events, and all objects appear to freeze when falling in until their light ...

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