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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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How to derive Smarr formula for Kerr Black Hole?

Following is the Smarr formula for Kerr Black Hole $$M=\frac{\kappa A}{4\pi}+2 \Omega J $$ where $\kappa, \Omega$, $J$ and $A$ are surface gravity, angular velocity, angular momentum and surface area ...
Talha Ahmed's user avatar
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Necessity of Singularity in General Relativity

The Schwarzschild solution is the standard example used to describe a black hole, its important points being the event horizon and the central singularity. But this solution is derived by assuming an ...
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Can black holes fully merge, according to a distant observer? [duplicate]

According to a distant observer, an object falling into a black hole doesn't disappear behind the even horizon but rather red-shift due to time dilation. Does the same apply to merging black holes? ...
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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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What is Suvrat Raju's point of view towards information paradox? Is it accepted by others? [closed]

Wondering what's the gist of his idea https://arxiv.org/abs/2012.05770 https://www.suvratraju.net/classes/black-hole-information-paradox/materials/lecture-1-introduction-and-two-point-qft-cor
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BTZ black hole as a quotient of AdS space

I am trying to understand this paper 1 and trying to reproduce some calculations and had some questions about that. In section 3.2, page 12, eq. 3.9, the authors are writing normal geodesics of an ...
Goodfellow's user avatar
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Can you calculate the radius of a hypothetical singular surface inside a black hole from observing changes to its linear momentum?

Say there is a ball of unknown radius surrounded by a bubble. The ball represents a hypothetical singular surface inside a black hole and the bubble represents the event horizon. If you threw marbles ...
user414142's user avatar
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Is the size of a black hole singularity smaller than a fundamental particle?

I am wondering about the size of a black hole singularity. We know that a classical black hole is infinitely dense. I am not asking about size of event horizon. I am asking about actual size of the ...
Arpan Purkait's user avatar
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Energy of the gravitational field within a sphere of radius $R$ in the Schwarzschild metric

The Landau-Lifshitz energy-momentum pseudotensor $t^{μν}$ is defined by $$16πt^{μν} = -2G^{μν} - g^{-1} \left[ -g \left( g^{μν}g^{αβ} - g^{μα}g^{νβ} \right) \right]_{,αβ}$$ where $g=\text{det}[g^{μν}]...
Khun Chang's user avatar
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Question about the toroidal magnetic field in quasars and the role of an ergosphere

1) Introduction Most of astrophysical bodies (in this context: formed stars, young stellar objects and black holes) produce or are immersed in a non-zero magnetic field $\vec{B}$. It is common, since ...
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Memory effects from black hole migration in the early universe

Let's say that there is a black hole at the center of a galaxy. When this galaxy is merging with another galaxy, it moves through space. If I am looking at a black hole now, there is a certain region ...
Kaushal Timilsina's user avatar
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David Tong, notes on General Relativity, pg. 25

I was studying GR using David Tong's notes, which I find very compelling and easy to read. The material is explained clearly to a fault, and I would recommend anyone picking up GR to at least skim ...
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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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Orthogonal complement of null vector [closed]

I am trying to solve excercise 8.5 d) from Straumanns book on general relativity (here V is an n-dimensional Minkowski vector space): Prove that the orthogonal complement of a null vector is an $(n-1)$...
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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 ...
user3624007's user avatar
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Quantum field expansion and bogoliubov coefficients in the interior of a rotating black hole

I am trying to quantize a real scalar field in the interior of a rotating black hole (3+1 D, asymptotically flat). My question is regarding the modes of the radial part of the equation (obtained after ...
Ratul Thakur's user avatar
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Are we certain of the mass we calculate for supermassive black holes?

If astronomers have concluded that the rotation speed of objects held firmly within a galaxy but far from a galactic center is too great to be explained by the visible matter of the galaxy alone, what ...
mdswartz's user avatar
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Where is the mass in a Black Hole without a "central" curvature singularity?

Not all black holes have a curvature singularity at their center (an example). But in principle, I thought that the curvature singularity was a direct result of the fact that the mass is concentrated ...
Aleph12345's user avatar
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Photonic black holes

"Can a photon turn into a black hole?" - usually the answer to this question is - it can't, because it has zero rest mass. However, when we derive the Schwarzchild Metric initially the $2M$ ...
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Imagining Graham's number in your head collapses your head to a black hole

My question is how does the above phenomenon, as mentioned in Numberphile here, occur in a semi-quantitative way through physical laws. (i.e saying statistical mechanics implies that is not ...
Mahammad Yusifov's user avatar
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Different Bekenstein bound equations – what’s the difference?

Can someone help me understand the difference between the Beckenstein bound equations that I’ve come across? They all appear to have different dimensions. I’ve been told that if you include the ...
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Robin conditions from action principle

Consider the Lagrangian density $$L(\tilde{\phi}, \nabla \tilde{\phi}, \tilde{g}) = \tilde{g}^{\mu \nu} \nabla_{\mu} \tilde{\phi} \nabla_{\nu} \tilde{\phi} + \xi \tilde{R} \tilde{\phi}^2$$ with $\...
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Is there a database or catalog of all black hole candidates?

I am searching for a catalog, list, database, etc. That should show astronomical observations of known black hole candidates. I am primarily interested in knowing the mass of these candidates. I have ...
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WKB Approximation of the Quasinormal Mode Spectrum of the Poschl-Teller (PT) Potential

In Black Hole Spectroscopy, it is well known that the Pöschl-Teller (PT) potential behaves approximately, or similarly to the more complicated Regge-Wheeler (RW) Potential. The WKB Approximation has ...
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Extreme Mass Ratio Inspirals and GWs cycles

I was reading through the following paper GRMHD study of accreting massive black hole binaries in astrophysical environment: A review. Therein, we have the following image It is not quite clear how ...
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Binary black hole merging condition

Assuming two black holes with the same rest mass $m$ collid coming from infinity with velocity $v$ and impact parameter $b$. Lets ignore spin at first. For which values of $v$ and $b$ would these ...
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Black Hole Formation -- How Can an Event Horizon be Observed to Grow? [duplicate]

This is a question about black hole formation. To be clear, I’m not suggesting that black holes don’t form. It’s that I’m having trouble with the accepted explanation so there’s a flaw in my logic ...
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ADM mass calculation for the BTZ black hole

Considering a non-rotating and non-charged 2+1 dimensional black hole, known as the BTZ black hole which obtained by adding a negative cosmological constant $\Lambda=-\frac{1}{l^2},l\ne0$ to the ...
Daniel Vainshtein's user avatar
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Black hole production via light speed [duplicate]

As a test particle with rest mass approaches $c$ relativistic mass increases to infinity, does this mean that the energy required to continue the acceleration will create a black hole (as time also ...
John Pryme's user avatar
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Is the information or energy in a black hole converted to another form of energy during decay of Hawking radiation?

The Hawking temperature equation states that the temperature of a black hole is inversely proportional to it's mass, and and the black hole loses mass when it emits particles in the form of radiation, ...
Sumer's user avatar
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1 answer
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Conformal equivalent to Schwarzschild metric

Consider Schwarzschild spacetime in Eddington-Finkelstein coordinates $(v,r,\theta,\phi)$ $$g \enspace = \enspace -f(r) \, dv^2 + 2 \, dv \, dr + r^2 \, d\Omega^2 \quad , \qquad f(r) = 1 - \frac{2m}{r}...
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How come the magnetic field disappears when a neutron star becomes a black hole, while the rotation remains?

The only question I found is this one, but this considers as non-rotating neutron star collapsing: Our final and most comprehensive test is represented by the collapse to a BH of a magnetized ...
Árpád Szendrei's user avatar
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What is the difference between Hawking radiation and a black hole laser?

While reading this paper (https://arxiv.org/abs/1409.6550), I got a little bit puzzled: what is the difference between Hawking radiation and a black hole laser? Is it the same thing? From my ...
Andris Erglis's user avatar
2 votes
1 answer
161 views

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 ...
Zach's user avatar
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3 votes
1 answer
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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 ...
Gene's user avatar
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2 answers
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When do we talk about spaghettification or pancakification in black holes?

So I've been doing some research for a while now, and yesterday came across the video of PBS space time talking about what happens to quantum information in a black hole. In the thought experiment ...
Anais-Ellie Gucek's user avatar
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0 answers
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Can wormhole inside a black hole become an escape?

I did not major in Physics so not sure if this is a proper question; but according to some Google search there do exist papers discussing wormhole inside black hole like this, which I am not able to ...
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When it comes to getting closer to the Schwarzschild radius, how is discrete a limit?

From Keeton (2014) in Principles of Astrophysics: Using Gravity and Stellar Physics to Explore the Cosmos, Gravitational time dilation near a large, slowly rotating, nearly spherical body, such as the ...
olivierlambert's user avatar
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1 answer
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Why does the timelike killing vector become spacelike inside the ergoregion?

Why does the timelike killing vector become spacelike inside the ergoregion? Some textbooks make this claim and move on to explain negative energy, but I could not find any proof for this claim. I can'...
Gene's user avatar
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2 answers
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Realistic black holes

If I understand the answers provided in this Link Why singularity in a black hole, and not just "very dense"? Then the singularity at $r=0$ may just be a mathematical artifact, and may not ...
Precious Adegbite's user avatar
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1 answer
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Do we use transit photometry to look for a black hole star binary systems?

What would a light curve look like for a black hole transiting a star? Initially I thought it would bend all light essentially blacking out a star but we would probably still detect some however the ...
Joe's user avatar
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3 votes
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Can Black Holes with electroweak or strong interactions exists in General Relativity or in Supergravity?

During my Master's degree, we studied Black Holes as solutions of Einstein-Maxwell equations, and I was wondering if it would be possible to also add strong or electroweak forces in the classic non-...
Aleph12345's user avatar
16 votes
1 answer
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Can a Kerr black hole become super-extremal?

Let's assume there is a large Kerr black hole, which is almost extremal and would become extremal with the addition of a small amount of mass $M$ with spin $J$ to make the final $J=M$. What if this ...
Andrew's user avatar
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3 answers
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What is the proof that the Schwarzschild metric is not static inside the horizon?

In Lecture Notes on General Relativity, Sean M. Carroll shows that the Schwarzschild metric is not only stationary but also static (Chapter 7, page 169, Eq. 7.20 and following interpretation). On the ...
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Birkhoff's theorem and Schwarzschild vacuum solution [duplicate]

Birkhoff's theorem states that any spherically symmetric solution of the vacuum field equations must be static and asymptotically flat, but the well-known Schwarzschild solution satisfies these ...
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How could I calculate the time it will take for light and mass to go towards a black hole and come back, to and from constant radial distances?

If you have a "perfect mirror" and a "perfect trampoline" at some constant distance outside a black hole's event horizon: a) How would a shell observer at some distance farther ...
Zach's user avatar
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Planck Length Calculation

On this question, from long ago, the Planck length is calculated as the length at which the Reduced Compton Wavelength is equal to the Schwarzschild Radius. However, in the calculation, the scalar &...
Earl Whitney's user avatar
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Interior Solution for Black Hole in Particular

This paper seems to suggest that the interior metric for a black hole in particular (a.k.a not a different kind of spherically symmetric non-rotating body) is just the exterior Schwarzschild metric ...
user345249's user avatar
2 votes
1 answer
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What happens to quantum entanglement when one particle crosses the event horizon of a black hole?

I’m curious about the behavior of quantum entanglement in extreme scenarios. Specifically, what would happen if we sent one of a pair of entangled particles (say, a photon or electron) across the ...
Maksim Kudriavtsev's user avatar
4 votes
1 answer
169 views

How are objects inside a black hole affected by the gravity of objects outside the black hole?

There are many Q&As about whether something inside a black hole can escape the event horizon if another massive object gets close enough to pull it out. I realize the answer (I think universally ...
Peter Moore's user avatar

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