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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Synchronous Gauge for a Schwarschild Background

Consider solving Einstein's equations in the Schwarschild background $ds^2 = -f(r)dt^2 +\frac{dr^2}{f(r)} + r^2d\Omega^2$ with $f(r) = 1 - \frac{2mG}{r}$. A common choice of gauge is Regge-Wheeler ...
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What happens to light in the middle of a BH binary?

A BH binary is a system of two black holes orbiting each other in close orbit. https://en.wikipedia.org/wiki/Binary_black_hole Now on this wiki page, there is a simulation of the merging black holes,...
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Is there any connection between an object and its copy once passing through the event horizon of a black hole?

We know that once an object passes through the event horizon of a black hole, it copies itself onto the surface. Once it does this, is there any connection between the actual object and its copy? I ...
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CFT dual to a black hole

AdS/CFT is famous correspondence between gravity and CFT. What one can say about black holes using this correspondence? What CFT object correspondent for black hole? For charged black hole? ...
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How to reach a singularity? [duplicate]

In the instant a singularity tries to form, it simultaneously would try to create a infinite density/gravity/time well. This raises a doubt in my mind that the singularity ever does actually form, ...
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Would a “black hole bomb” using dyson sphere and superradiant scattering destroy the blackhole? [closed]

I watched part of this Kurzgesagt video (link sends you to the video part) https://youtu.be/ulCdoCfw-bY?t=323 and part of this pbs spacetime video (link sends you to the video part) https://youtu.be/...
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Calculate entropy of a sphere (?)

I saw these two videos (1 and 2) and in both of them Tony Padilla gives basically the same definition of maximum entropy in a region of space (a sphere). Now, searching the web I haven't found any ...
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What would happen if we link Earth and a blackhole by some kind of force and say something near the black hole? [closed]

Like if we somehow made a supermassive black hole around 5 light years away from earth whose 10 seconds are equal to 1 earth hour{(as time gets bended by gravity (Einstein's Theory of General and ...
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Why is Schwarzschild metric taught at all?

It seems like the Schwarzschild metric only has historical relevance. Since other coordinates like the (ingoing) Eddington-Finkelstein coordinates are far superior. It seems like the condition on the ...
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In Eddington–Finkelstein coordinates how do we show a projectile appears to slow down near the horizon?

In the Schwarzschild coordinates it is stated that it is clear that an object appears to slow down near the horizon form the perspective of an outside observer. How do we do this calculation in in ...
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Are black holes 0 dimensional? [duplicate]

If black holes are mass of the star concentrated on a single point then how can it be moving through a 3-dimensional space? How can they spin?
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Would a “black hole bomb” destroy the blackhole? [closed]

I watched this Kurzgesagt video https://youtu.be/ulCdoCfw-bY?t=323 and this pbs spacetime video https://youtu.be/UjgGdGzDFiM?t=633 I was curious as to if the black hole itself would be destroyed in ...
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Particles falling towards a singularity [duplicate]

This is a revision of my earlier question. Sorry. I'm a beginner here and on a learning curve regarding format. If the gravity well steepens to infinity at the point of a singularity, then so does ...
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Particles reaching a singularity [closed]

If the gravity well steepens to infinity at the point of a singularity, then so does the "time well". Understandingly, the inward falling energy/matter is not aware of this change in time relative to ...
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Will a Black Hole, observed instantaneously at positions of 180deg to each other be seen as the same? [closed]

I'm interested to know, If I can stand on the opposite side of a Black Hole? I don't mean, "Could I pass though a Black Hole and see what is on the otherside?" Could I travel in free space, via way ...
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Could a Dyson Sphere-like object be made to encase a black hole without collapsing?

In this question, I'm not asking about the feasibility of building a black hole Dyson Sphere or the effectiveness, I simply want to know if an object could stay in a locked position encasing a black ...
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Fall of temperature of a radiating body

While going through the idea for the hawking radiation,the argument was "As T is inversely proportional to the product of GM(planck), so as black hole radiates energy ,then temperature will ...
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Accretion disk velocity

Can matter in the accretion disk of a black hole, travel at superluminal velocities? If not, why? I know from the laws of special relativity that objects can never reach or exceed the speed of light, ...
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What if black holes consumed exotic matter? [closed]

I've just watched Kurzgesagt in a Nutshell and I've been thinking of exotic matter for a while. What if black holes tried to consume exotic matter?
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Can one derive charged black hole solution of GR using Kaluza-Klein?

Kaluza-Klein showed that a 5 dimensional gravitional theory with one dimension on a circle, is equivalent in the limit to a 4 dimensional gravitational theory with electromagnetism. So for the ...
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Alternative to Dark Energy

If Black Holes consume mass and re radiate it in the form of massless photons the mass of the universe is declining. Could this explain the accelerating expansion of the universe without the ...
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Symmetries of quasinormal modes of Kerr black hole

I have been reading this following paper on numerical evolution of the Teukolsky equation (see e.g. Eq 1 in their paper) for spin -2 fields about a spinning black hole (Kerr) solution. As the ...
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Recipe to find null coordinates

Given a metric $g_{\mu\nu}$, is there a general recipe to define the null coordinates for the system? For the Schwarzschild metric, I think we can find it by the following method. $$ds^2 = -f(r)dt^2 ...
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Are experiments being conducted towards the creation of micro black holes? [closed]

Apparently, there could be a way for our own particle colliders to create very transient black holes, if some things about the universe are true (as far as I understand, those have to do with the ...
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Maximum power (luminosity) limit in General Relativity

Is the following thought experiment - to determine the maximum limit for power $P_m$ in General Relativity - correct? It is by Craig J Hogan, a member of the team that discovered dark energy, so ...
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If gravitons exist, why do black holes have gravity? [duplicate]

If gravitons exist, shouldn't gravitons produced beyond the event horizon of a black hole be stuck inside? And if gravitons couldn't escape, wouldn't black holes appear to observers outside the event ...
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What is known about the temperature “inside” Black Hole?

From what I see at SE, it appears that discussion of Black Hole temperature is about temperature at the Event Horizon. I am curious about what is known / theorized about temperature and energy density ...
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Derivation of the Hawking radiation equation

What is derivation of the Hawking radiation equation?I need full derivation step by step.Specially I don't understand what is cauchy surface and it's requirement in this derivation(only I know in the ...
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Equation for black hole modeled as changing index of refraction

A similar question was asked here but i would like to know what the exact formula is for index of refraction as a function of radius that will give the same bending of light as a non rotating black ...
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In which direction would gravitational waves be emitted when two black holes colide?

Imagine two black holes on the x-axis coming together at the origin (not rotating around each other, just falling towards each other). In which direction would the most intense gravitational waves be ...
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QFT in Curved Spacetime - Commutation Relations for Annihilation Operators

I am currently learning about QFT in curved spacetime using these notes. Given a subspace $S_p$ of positive frequency solutions to the Klein-Gordon equation, we defined the corresponding ...
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Does Black Hole mass increase widen gravitational radius?

This is a question about the gravitational radius of SMBH's As far as I know if our Sun collapsed to a BH it would still hold most of the present solar system together Some matter would be lost via ...
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How do singularities combine when two black holes combine?

If we solved GR in the case of two black holes colliding, does each individual black hole have a single pointlike singularity at their respective centres, and would those singularity combine? Trying ...
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Growing black holes and eternally falling objects

What happens to objects falling to a black hole (from outside observer's perspective)? There seem to be a consensus that because of time dilation, an object's fall towards event horizon will take a ...
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Sign of gravitational action

I am reading this paper on Black hole phase transition in AdS, but for the life of me I cannot get the signs right for the expression of the Action of a Black Hole in AdS (eq (2.9)). Consider the AdS ...
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Ways to calculate Hawking radiation of black hole

As the title suggest what are different ways by which one can calculate Hawking radiation of a black hole? I only know the standard bogoliubov transformation method.
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Adding angular momentum to an extremal black hole

What happens if a body carrying angular momentum falls into a black hole that is already extremal with respect to angular momentum? Is the excess angular momentum radiated away in gravitational waves?...
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Doubt regarding particle dynamics and hydrodynamics in Schwarzschild geometry

The effective potential for particle orbits in the equatorial plane of a Schwarzschild black hole in units $G=M=c=1$ is given by $$V_{\textrm{eff}}=\sqrt{\left(1-\frac{2}{r}\right)\left(1+\frac{l^2}{r^...
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Hawking radiation and evaporation of black holes [duplicate]

Why black holes lose mass and energy by evaporation?
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Does the Casimir effect violate the Generalized Second Law of Thermodynamics?

In order to avoid violation of the Generalized Second Law of Thermodynamics by classical non-minimally coupled scalar fields, Ford et al demonstrated that the quantum inequalities prevent the negative ...
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Does terminal velocity matter in terms of black holes?

I think we all know that black holes are considered inescapable but I was looking at terminal velocity and became interested in finding out if black holes follow the rules of terminal velocity. For ...
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Systematic way of finding the Killing vector of a specific mass

I have a specific black hole solution (in AdS space) with a particular energy given. The energy was computed in the paper by assuming the validity of the first law ($dE = T dS + \Omega dJ + \Phi dQ$) ...
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What is the limit of validity for WKB expansion for black hole Quasi-normal modes?

Which is the limit of validity for WKB expansion for black hole Quasi-normal modes? In many papers I see that the authors only report the overtone $n=1$. Is WKB expansion valid only for small $n$? As ...
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Do black holes move through space? [closed]

I know it was already asked here: Does a black hole move through space? What happens to other things around it? And it might be a very stupid question, but here it is: From a relativistic ...
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Criterion for a black hole in Anti-de Sitter background

Consider a Schwarzschild-Anti de Sitter (SAdS) metric $$ds^2=-(1-\frac{2M}{r}+ k\, r^2 )\, dt^2+\frac{dr^2}{1-\frac{2M}{r}+k \,r^2}+r^2 d\Omega_2^2, $$ with $M,k>0$. This solution has only ...
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What would cause a gravitational wave that is measured on Earth by “macro-sized” instrumentation?

LIGO has successfully measured the effect of gravitational waves moving through the Earth from a very distant source. Such measurements require extremely precise and calibrated instrumentation ...
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How do you estimate the accretion rate onto a black hole (or neutron star)?

Assume we have a heavy mass like black hole and there is an accretion disk around it. At a black hole (eg. pics of the bh. of m87) some parts of the given disk glows brighter than the others. How can ...
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Do black holes present a contradiction in our understanding of GR? Or… what happens to the observer after infinity? [closed]

There have been some lengthy discussions, here and here (among others), about whether, to the distant observer, black holes form in a finite time or an object falls into a black hole in a finite time. ...
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Can a photon be frozen (relatively stationary) in space? [closed]

There are a lot of questions on this site about photon speed, none of them answer my question: Why and how is the speed of light in vacuum constant, i.e., independent of reference frame? If ...
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Would 'seeding' two individual black holes with entangled particles make those two black holes become entangled?

As I understand it from watching some Susskind lectures, if you create two black holes from the same 'pool' of entangled particles, the result can be described by ER and EPR to be a pair of entangled ...

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