Tagged Questions

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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Spaans, “On Quantum Contributions to Black Hole Growth”

This paper was posted to arxiv a couple of weeks ago: http://arxiv.org/abs/1309.1067 From the abstract: The effects of Wheeler's quantum foam on black hole growth are explored from an ...
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1answer
164 views

Which way do black hole jets spin?

The centers of black holes and quasars often have jets coming out the two poles of an accretion disk, say north and south. Is it known if the two jets spin in the same direction or opposite ...
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Holographic principle “inside-out view”

From the perspective inside a black hole: Is information about everything outside a black hole - the rest of the cosmos - represented on the inside of the (event) horizon too? NB. I realize it is the ...
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The speed of light through space and its effects on black holes

I was curious about a couple of things that don't quite make sense to me: (and probably won't when I get the technical answer, but I'll ask it anywise.) When light travels through space, is its ...
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1answer
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Energy of a black hole : a quizz question

Consider a standard eternal Schwarzschild black hole of mass $M$. One wants to speak about the energy $E$ of a black hole, so let 's present this as a quizz. 1) This is a non-sense to speak about ...
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Visual representations of black holes

Some black holes emit jets that are rapidly circling the poles of what appears to be a sphere at the center of the black hole, cf. Fig. 1. Is there really a sphere here and how far (relative to the ...
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Are electrons just incompletely evaporated black holes?

Imagine a black hole that is fast-approaching its final exponential throws of Hawking evaporation. Presumably, at all points in this end process there there will remain a region that identifiably ...
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85 views

If you flew into a black hole

Would you outlive everyone? I'm coming from the point of view that time would be experienced more slowly (although not from your point of view) the denser the gravity gets.
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517 views

Black hole complementarity - absorption of Hawking radiation

I try to understand two principles formulated by Leonard Susskind in his book The Black Hole War: 1, To any observer who remains outside a black hole, the stretched horizon appears to be a hot layer ...
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2answers
472 views

Einstein Equation at the Singularity. Understanding the Dr. Michio Kaku's explanation

I'm trying to understand what Dr. Michio Kaku is exaplaining in this video: http://www.youtube.com/watch?v=hydDhUNvva8 I'm just able to rewrite this two equations. I think I've probably made ​​some ...
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3answers
497 views

Are black holes really singularities?

A popular assumption about black holes is that their gravity grows beyond any limit so it beats all repulsive forces and the matter collapses into a singularity. Is there any evidence for this ...
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1answer
176 views

Behavior of black holes in higher- and lower-dimensional space-times

The behavior of black holes in 3+1 dimensional space-time as our own is rather well known: formation, event-horizon size, mass, spin, radiation etc. However, my question is what would black holes ...
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Why do we care about the maximally extended versions of spacetimes?

One can take a spacetime and maximally extend it, so that geodesics end only on singularities, where they have to end -- not on coordinate singularities, which are not physically significant. But when ...
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What does “quantum theory forbids promiscuous entanglements” mean?

The context is this article about black hole firewalls. The phrase appears on page 3. It appears to be saying that only pairs of particles can be entangled, never multiple particles, and that this ...
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1answer
253 views

What does “all future lies within the event horizon” mean?

I was trying to find an answer as to why light does not escape black holes and I stumbled upon this Phys.SE question. In the answer it said that: "Since all future lies within the event horizon, ...
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1answer
109 views

Are black holes stationary?

If light/energy with zero mass hits the speed limit, are black holes with infinite density at a universal standstill with everything moving relative to them? Am I barking up the wrong tree as they ...
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1answer
62 views

Scaling of non-gravitational energy in a black hole

When looking at a Schwarzschild black hole, for instance, we know that we may apply black hole thermodynamics. We may define a entropy of the black hole which scales like the area of the horizon : $$S ...
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Information Preservation and Burning Books

I recently read an article in the NY Times called A Black Hole Mystery Wrapped in a Firewall Paradox. I really liked the article, but reading one quote immediately made me think of asking Physics.SE a ...
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What is the entropy of a string?

In his The Black Hole War: My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics (p. 373) Susskind states that the entropy of a string is [...] proportional to its length. ...
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Information escape from a black hole

Is the following a possible scenario? If not, why not? Assume there is a supermassive black hole $Z$ isolated in inter-galactic space. Nearby and stationary relative to $Z$ is observer $A$. A number ...
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What happens when the black hole at a galactic core eats the galaxy? [duplicate]

I'm making several assumptions, not sure if any are correct: there is a black hole at the center of a galaxy the black hole is eating the galaxy Eventually the galaxy will be gone, right? Has ...
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1answer
96 views

Singularity in a black hole [duplicate]

This might be a naive question, but how can an object such as a black hole singularity have infinite density but finite mass? (For example, we can approximate the mass of a black hole based on ...
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1answer
160 views

Kerr Metric in Orthogonal form

I've seen the Kerr metric usually presented in the Boyer-Lindquist coordinates where there is a cross term in the $d\phi$ and $dt$ term. I've done a good bit of searching and cannot find any ...
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1answer
237 views

Einstein's Cosmic Speed Limit $c$ challenged by Black Holes/Massive Objects [duplicate]

To even suggest the great Einstein is incorrect on anything he has written is inconceivably absurd. Being fully aware of this I do indeed dare to not only suggest he is wrong but I cannot believe ...
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5answers
284 views

What generates such a huge amount of gravitational attraction in a black hole?

I had been fascinated to hear that anything can be swallowed by a black hole. But what causes such a huge amount of gravitational force?
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2answers
239 views

Entropy of a naked singularity

According to the wikipedia article http://en.wikipedia.org/wiki/Naked_singularity: "Some research has suggested that if loop quantum gravity is correct, then naked singularities could exist in nature, ...
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2answers
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Can we create a small black hole? [duplicate]

Suppose we create very large spherical body by using gamma rays generator and they will concentrate on a single point at the centre of sphere.We will place this spherical body thousands of kms above ...
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1answer
272 views

Can an object be infinitely small?

I read somewhere that the earth has to be smaller than 1 cm to become a black hole, according to Schwarzschild. Since big bang came from a singularity, I am wondering, is there any minimum volume for ...
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2answers
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What is the shape of a black hole?

I was thinking; what shape does a black hole have?. By 'Shape', I mean its form (e.g, circle , cylinder, sphere, torus, etc..). We usually think of black holes as if they're plugholes (e.g, a flat ...
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4answers
688 views

Hawking Radiation - Why it causes black holes to die? [duplicate]

If a particle is being expelled from the Black Hole and an antiparticle is being driven into it, shouldn't the opposite occur as well and in the same frequencies? I mean Black holes should emit ...
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2answers
405 views

Why does the event horizon of a black hole not look like a bright sphere?

All infalling matter-energy appears to an external observer as frozen in time at the event horizon. Why then is this horizon not extremely bright due to radiation that is able to escape radially? So ...
3
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1answer
74 views

Kerr throat solution derivative

I'm going through this article, since I'll need a part of it for my thesis. And I am trying to derive the Kerr throat solution, from which I should be able, with the change of coordinates get to ...
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1answer
212 views

White hole and Schwarzschild solution

What is the relation between white hole and the Schwarzschild solution commonly found in textbooks of physics and interpreted usually as black hole?
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1answer
357 views

Schwarzschild solution

I am calculating for many hours and I am really confused with this exercise. Consider a comoving observer sitting at constant spatial coordinates$(r∗,θ∗,φ*)$, around a Schwarzschild black hole of ...
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0answers
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Thermal radiation in the Unruh Effect

The following formula has been given in 't Hooft's black holes notes ($|\Omega \rangle$ is the vacuum state of Minkowski space, O is a operator): $$\langle \Omega| O|\Omega \rangle = \sum_{n \ge 0} ...
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1answer
480 views

A Hollow Black Hole

I was just reading a question about the gravity inside a hollow neutron star. It was a trivial question, obviously there is no force felt. But then it got me thinking. Suppose you had a hollow sphere ...
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2answers
184 views

Gravity in the center of a hollow neutron star [duplicate]

Imagine a hollow 100 metre diameter (for example) sphere made of incredible dense material (ie neutron star dust etc) but is self supporting (ie the central cavity). Assuming that the sphere skin is ...
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3answers
159 views

Understanding black holes [closed]

Basically I don't know anything about Physics but I am really curious about all these things. I was trying to understand what is a black hole, but unfortunately I couldn't figure out from online ...
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Significance for LQG of Sen's result on entropy of black holes?

Sen 2013 says, ...we apply Euclidean gravity to compute logarithmic corrections to the entropy of various non-extremal black holes in different dimensions [...] For Schwarzschild black holes in ...
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1answer
339 views

Gravitational field strength and Horizon in Rindler coordinates

I came across the following statements in 't Hooft's black holes notes, but not being able to justify them. The metric in the Rindler coordinates $x=\tilde{x}, y=\tilde{y}, z= \rho \cosh{\tau}, t= ...
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1answer
209 views

What would a sufficiently dense sphere of uranium do first: blow up, or form a black hole?

I think it is safe to assume that humans are a long way off from having either the technology to compress a normal-sized sphere of uranium below its Schwarzschild radius, or the technology to create a ...
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1answer
169 views

Angular momentum for the Kerr solution of a rotating blackhole

I am reading 't Hooft's noted on Black holes, where he quotes the Kerr metric for a black hole rotating about the z-axis as follows: He later says: "The parameter a can be identified with the ...
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0answers
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Graviton through the horizon and force felt outside a black hole [duplicate]

Gravitational force is mediated by graviton exchange. If I am standing outside a black hole, I can of course feel the attracting force towards the black hole. This should correspond to gravitons ...
2
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1answer
228 views

Do physicists believe the singularity theorems to be accurate?

This question is largely based on the last post by reddit user RobotRollCall who gave some fantastic explanations of phenomena in relativity on a layman's level. About a year ago, she said: The ...
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Singularity and Black Hole Complementarity

When looking at a (eternal) Schwarzschild Black Hole, we may identify two worlds. The region $R_1$ (right) - our world -, and the region $R_2$ (left) - an other world. The "black hole interior" ...
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1answer
326 views

Solving Klein-Gordon equation in the Rindler coordinates - the Unruh effect

I am reading 't Hooft's notes on Black Holes. I want to find the solutions of the Klein-Gordon equation $(\tilde{x},\tilde{y}, \rho, \tau)$ in the Rindler coordinates which are $$x=\tilde{x}\,\,\,\,\ ...
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1answer
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Can one black hole suck in another black hole?

In the recent news, scientists at NASA have found “unprecedented” black hole cluster near Andromeda’s central bulge. I wonder why doesn't all these black holes merge and such each other in until just ...
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1answer
180 views

A few questions related to frame dragging

I am trying to get my head around a few concepts related to frame dragging and related physics. In regards to black holes that have no charge and all their mass is tied up in rotational kinetic ...
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what happens to the angular velocity of star in star-black hole system?

What happens to the rotational and revolutionary angular velocities of star in star-black hole system as the star loses mass ?
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What do the components of light velocity look like in polar coordinates?

The Schwarzschild solution makes use of polar coordinates, and I'm wondering how the different components of velocity of light change with the position. Might I get some examples of light velocity ...