A black hole is a volume from which photons, or any matter, can not 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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My own thoughts about singularities and spacetime [on hold]

I believe the assumptions about black holes are mostly wrong due to the idea that time dilates relative to an outside observer as you get closer to the singularity at the center of the black hole. ...
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28 views

Does the holographic principle for black holes mean that information is cloned?

When an object falls into a black hole, does the holographic principle say that the information is copied and stored on the event horizon while the original information falls in?
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Link between Hawking-Bekenstein Black hole entropy and entanglement entropy

I'm currently doing a project on two sided Ads-Schwarzschild black holes in the context of Ads/CFT. I want to show that the entanglement entropy between the two CFTs corresponds approximately to the ...
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25 views

Can antimatter becomes black hole? [duplicate]

I know it seems unlikely to accumulate sufficient amount of antimatter to let it collapse under its own weight to become a black hole(maybe the gravity works differently I don't know) since they will ...
2
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2answers
75 views

If you fall in a black hole, when do you go past the event horizon? [duplicate]

Say I fall into the event horizon of a black hole. As I cross the black hole, I would appear to outside onlookers to freeze in time, and would never move from that point again. In my perspective, time ...
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31 views

Would the asymmetric collapse of a star produce a naked singularity?

If this is a duplicate, I will remove it. A few years ago SciAm ran an article based around possible variants of stellar collapse. My apologies, I can't remember much more than their main point, that ...
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0answers
29 views

Is there any matter inside an event horison? [duplicate]

Consider a feeding, growing black hole. We never observe any matter to cross the event horizon, because time stops there. All matter would be "stuck" in a sphere around the event horizon, slowly ...
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0answers
26 views

Resources for Entanglement Entropy in Condensed Matter Systems and Black Hole Thermodynamics

Over the last month or so, I have set it upon myself to teach myself the AdS/CFT correspondence. In particular, I am interested in the connection between black hole entropy and entanglement entropy in ...
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26 views

Why gauge field should be vanishing on horizon?

When considering an AdS spacetime including a black hole, matter field and gauge field, the value of temporal component $A_t$ of the gauge potential $A_\mu$ on horizon always is set be zero, even the ...
3
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2answers
84 views

Is an event horizon absolute to all observers?

Recently I had discussion whether the event horizon of a black hole is absolute or relative to different (outside) observers. Does someone just 1m above the horizon (disregarding effects of tidal ...
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1answer
97 views
+100

Spinning micro blackholes power conversion

In the context of energy extraction of spinning black holes, there are two known mechanisms: the Penrose process and the Blandford-Znajek process. The former relies on fragmentation of accreting flow, ...
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1answer
52 views

Uncertainty principle within a neutron star or black hole

Take the time-energy uncertainty relation, $\Delta$$E$$\Delta$$T$$\ge$$\hbar/2$. My question is based on my confusion about the effect this relation may have within the interior of a highly ...
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1answer
29 views

neutron stars and black holes lifespan [closed]

1-What is the lifespan of a neutron star? 2-How is the lifespan of a black hole estimated based on it's mass? For example those in here: ...
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1answer
66 views

How long does a black hole last after it becomes white hole?

According to Stephen Hawking's theory of black holes, once a sufficient mass has been lost through evaporation, the escape velocity $3 \times 10^8$ meters per second so light is able to escape from ...
2
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1answer
33 views

When does light reach a shell observer in Schwarzschild metric?

I am trying to simulate the trajectory of light in the Schwarzschild metric (as seen by a far away observer) with fixed $\theta = \pi/2$. According to my source (Chapter 18, section 18.5) the ...
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35 views

Is there a numerical calculation for black hole - neutron star merges?

I think it might be even a more frequent event as the black hole merges. It would be similar to the black hole merges in the gravitational wave spectrum, but it could have a very clear neutrino and ...
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54 views

A General Relativity question? [closed]

The line element for the outside of a spherical star\black hole is given by the Schwarzchild line element : $$-c^{2}d\tau^{2} = ds^{2} = -\left(1-\frac{2m}{r}\right)c^{2}dt^{2} + ...
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36 views

How black hole lose mass? [duplicate]

Even in empty space virtual particles can constantly pop in and out of existence in pairs, when 1 of the pair fall into the black hole while the other escape this is hawking radiation but black hole ...
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1answer
33 views

What is unstable about the wormhole in the Reissner-Nordström solution?

In the conformal diagram for the Reissner-Nordström solution (included below), the inner horizon of the black hole leads to a wormhole which has a singularity, but it is possible to exit it through a ...
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1answer
31 views

Observing from a black hole proximity

Suppose that you could get close enough to a black hole that time slowed down significantly for you, and you looked at a giant clock back on earth through a telescope. Would you see the earth clock go ...
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55 views

Why do virtual particles form at black hole horizon? [duplicate]

This question relates to Hawking radiation. I understand about virtual particles - pairs of positive and negative particles that form - but I can't find out why they form at the event horizon and in ...
2
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2answers
59 views

Schwarzschild metric: motivations and applications in physics

I have a mathematical background and I have just derived the expression of the Schwarzschild metric. Now I was wondering what were the motivations and applications in physics of this metric. Any info ...
2
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0answers
109 views

$AdS_2$ Black Hole [closed]

I know that $AdS_2$ black hole has the following metric: $$ ds^2=(r^2-a^2)dt^2+\frac{dr^2}{r^2-a^2}.\tag{1} $$ Here $a$ is constant. On the other hand I am regularly facing with the following ...
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1answer
39 views

What will happen to earth if it's tied to a black hole thru a really strong string

Imagine a very strong string (with infinity as as its breaking point) attached to earth very strongly and the other end is put inside a black hole, What will happen to earth? and the string? will it ...
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1answer
85 views

Understanding gravitational time dilation / Schwarzschild metric

I've had a look at the answers to these sorts of questions already, but feel like I'm still missing something. Starting with this question, and this one and even this one here. I'm looking at this ...
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1answer
53 views

Typical rotation speeds for black holes

I am curious as to how rapidly most black holes are spinning. A refinement of the question could be: how rapidly are typical supermassive black holes spinning? I ask because I have the impression ...
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39 views

Star vs Blackhole - Accretion Disk

Simple question: can we prove that what we call today a "star" is not a accretion disk of a "tiny" black hole that happens to rotate in all the 3 axis?
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45 views

Proof of Schwarzschild metric construction (O'neill chap 13)

I am struggling with a few steps of the proof in O'neill book $\textit{Semi-Riemannian Geometry, with applications to Relativity}$ on the construction of Schwarzschild's metric (chap13, Lemma1). Is ...
2
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3answers
90 views

Milky Way galaxy central black hole and time contraction of life story

We know that life is almost 4 billion years old on Earth. We also know that time contracts as we approach the event horizon of the massive black hole at the center of the Milky Way galaxy. Now the ...
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2answers
141 views

Physics in the movie 'INTERSTELLAR'?

have a question , this movie is praised by the scientific accuracy however how the hell does relativity justify that the character can be thrown in black hole and survive ? also if the robot can not ...
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1answer
93 views

Why I cannot cross the Einstein-Rosen bridge?

I have been told that no observer can ever cross the wormhole present in the Kruskal-Szekeres coordinates, connecting region 1 and 4 in the Kruskal-Szekeres diagram, usually called Einstein-Rosen ...
6
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61 views

Would a collision of two black holes emit any electromagnetic radiation?

The video linked in this answer shows how space-time would be distorted during such an event. No doubt such an event would be an extremely strong source of gravitational waves. But would anything be ...
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60 views

How does a rotating black hole look like? How would it be to descend into one?

This image from Wikipedia shows how a black hole would look like: A black circle that acts as a gravitational lens for light rays coming from behind. How would a rotating black hole look like? How ...
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63 views

Free-falling from rest into a Kerr black hole

Is it impossible for a particle (with zero angular momentum) to free-fall from rest at infinity into the ergosphere of a Kerr black hole? It seems like it is very easy to show this is the case, but ...
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Does asymptotically AdS mean as $z \to 0$ or as $z \to \infty$ in Poincare metric of AdS?

The Poincare metric of AdS_3 is given by $ ds^2 = \frac{R^2}{z^2}(dz^2 - dx_0^2 + dx_1^2)$. Using the coordinate transformation $\rho = \log(z)$, we can write this as, $ds^2 = R^2 (d\rho^2 + e^{-2 ...
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1answer
51 views

Two spherically symmetric shells of matter collapsing into a black hole

Suppose I have a thin spherically symmetric shell of matter of (rest) mass $M$ that collapses to form a black hole, and later a second thin spherical shell of mass (rest) $M'$, concentric to the ...
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3answers
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One-body General Relativity

Does it make sense to talk in General Relativity about only 1 body ? Can you have a universe in which there is only one body and that body is producing a gravitational field ? I'm asking this because ...
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32 views

How can a black hole have a charge? [duplicate]

Under current particle theory, the four fundamental forces use a force-carrying particle. The particle for electromagnetism is the photon. By definition a black hole is a mass from which light can ...
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1answer
91 views

What is the smallest amount of neutrinos needed to create a black hole? [duplicate]

Is there some smallest amount of neutrinos needed to create a black hole? Note that this question is not at all the same as the question here If a 1kg mass was accelerated close to the speed of light ...
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How does blackholes become supermassive? [duplicate]

I am not an expert in this field. I have seen documentaries on the Universe and have a basic idea of how black holes come to existence. When black holes form from supernovae they have a initial mass ...
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31 views

Penrose diagram for Schwarzschild metric

Can someone show me a procedure (I mean a complete series of mathematical passages) to derive the Penrose diagram for Schwarzschild metric? I don't want to do that passing through the Kruskal-Szekeres ...
2
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3answers
50 views

Why is the photon-sphere around a Kerr Black Hole spherical and not ellipsoid?

Kerr Black Holes have usually (excluding extrema $a=0$, $a=1$) due to their spinning activity an ellipsoidal ergosphere. So why does the photon-sphere does not have an ellipsoidal form? ...
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1answer
58 views

Black Holes: Are “elementary” particles just decomposing into sub-particles that we don't yet understand? [closed]

You often hear that black holes are so strong in their gravitational pull that matter, even light cannot escape. But this seems to contradict the laws of conservation of energy. Is it possible that ...
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1answer
34 views

Is force exerted by Earth different for different mass?

We know Earth pulls every object by the same acceleration $g$ (say). force exerted by earth $F=m\cdot g$ (with $m$ the mass of the body), so, if $g$ is constant then $F$ is directly proportional to ...
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1answer
41 views

Singularities in the Reissner–Nordström metric

I am doing a presentation on black holes but I'm having trouble finding information on the Reissner–Nordström metric. From the metric ...
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35 views

Colliding black holes to an outside observer

We know that a particle approaching an event horizon will appear to an outside observer to slow down and never cross the horizon. What is observed by an outside observer when a singularity ...
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4answers
104 views

Any tips on evaluating Riemann tensor?

I am calculating the Riemann tensor for the Schwarzschild solution. I've calculated all 9 non-vanishing Christoffel symbols already. Now I need to evaluate the Riemann tensor and I find no easy way to ...
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74 views

Decay of matter

I was watching Stephen Hawking's documentary and in there he explained how he realized why black holes eventually disintegrate: There are ripples in space, an antiparticle and a particle get ...
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2answers
643 views

Is there a way to photograph the very edge of black hole?

According to Professor Stephen Hawking, black holes emit radiation, Hawking radiation. The bigger the black hole, the fainter the radiation. That means, I personally believe, if we built large enough ...
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Relation between black hole mass and size applied to observable universe [duplicate]

If I apply the relations given in http://xaonon.dyndns.org/hawking/hrcalc.js and http://www0.arch.cuhk.edu.hk/~hall/ag/sw/SpinCalc/SpinCalc.htm to the values for the mass of the observable universe ...