# 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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### How do we know the stars orbiting Sgr A* are orbiting a supermassive black hole and not just the center of mass of the Milky Way galaxy?

It is my understanding the best evidence we have for Sgr A* being the black hole at the center of our galaxy is the incredible velocities of the stars orbiting around it. But wouldn't the stars ...
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### Is there any connection between an object and its copy once passing through the event horizon of a black hole? [closed]

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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### Solution of Klein-Gordon field in Schwarzschild metric

I was trying to understand the behavior of scalar field in the vicinity of black hole and I set up the Klein-Gordon equation in the background of Schwarzschild metric. But I am having difficulty in ...
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### Synchronous Gauge for a Schwarschild Background

Consider solving Einstein's equations in the Schwarschild background $$ds^2 = -f(r)dt^2 +\frac{dr^2}{f(r)} + r^2d\Omega^2\tag{1},$$ with $f(r) = 1 - \frac{2mG}{r}$. A common choice of gauge is ...
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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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### Limit to the Oberth effect in a supermassive Black Hole flyby

To make this question clear, here are the details of the situation I wish to entertain. A spacecraft does a powered gravitational assist, where it fires engines in a near-approach to the body We ...
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### Would a 5-dimensional black hole actually break laws of physics?

I discovered a paper by, Figueras, Kunesch, and Tunyasuvunakool, "End Point of Black Ring Instabilities and the Weak Cosmic Censorship Conjecture." A Cambridge University press release is here. The ...
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### Can we exit the event horizon of merging black holes?

I have an intuitive scenario. Consider we have a spaceship just below the event horizon of a BH, which is merging with another black hole. Finally, the singularities merge and we have a single black ...
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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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### 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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### 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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### 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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### 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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### 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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### 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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### 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 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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### 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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### Definition of event horizon – gravity around black holes [closed]

Put simpler: A black hole is surrounded by a 'sphere' where, to an outside observer, the speed of light is (near) zero. What is the radius of that 'sphere' (for a non-rotating black hole) and how ...
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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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### 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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### 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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### 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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### 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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### Explain Kerr-Newmann Black Hole Spins in SI Units

I'm trying to run some calculations on Kerr-Newman black holes, but I'm having two major difficulties. First, most equations I've been able to find are only for Kerr black holes. Second, essentially ...
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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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### Can we have a black hole without a singularity?

Assuming we have a sufficiently small and massive object such that it's escape velocity is greater than the speed of light, isn't this a black hole? It has an event horizon that light cannot escape, ...
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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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### 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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### 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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### 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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### Is the singularity of a black hole homogenous?

When matter is dropped into a black hole, does the content of the matter impact the structure of the singularity? That is to say, other than the change in mass, do any of the singularity’s properties ...
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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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### What are BMS supertranslation symmetries?

What are BMS supertranslation symmetries? I am studying soft hair on black holes and I need to learn BMS supertranslation symmetries.
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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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### 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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### 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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### What happens to the accretion disk when two black holes merge?

I'm aware that accretion disks around black holes are formed from the swirling mass of matter that is slowly being stripped of its atoms, but what happens to it when two black holes merge? I was ...
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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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### 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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### Are black holes black?

GR was formulated before atomic clocks and radio signals between earth and space probes allowed actual measurements to be made. Atomic clocks count a beat frequency, but the frequency of emitted ...
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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 ...