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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 much energy can be radiated as gravitational waves from black hole merger?

In the black hole merger, recently observed by LIGO, about 5% of energy was irradiated in form of gravitational waves. source of data Is there any theoretical limit to how much energy can be ...
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Sources for black hole geodesic orbits

I am looking for good sources that discuss both Kerr and Schwarzschild particle orbits (geodesics). Most sources write down the geodesic equations, constants of motion and the Hamiltonian, but do not ...
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Fluctuating string in AdS black hole

People frequently use fluctuating string in AdS black hole (see 1,2,3 etc) to study dynamics of a "free external quark" in quark-gluon-plasma (QGP) at finite temperature. The background black hole ...
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What is Hawking Hartle vacuum state and why does the following Euclidean path integral gives the wave functional of it?

I am studying the wave function of black hole via the paper by Sergey Solodukhkin, Entanglement entropy of black holes,arXiv:hep-th: 1104.3712. In the paper, equation (53) is as follows: $$\Psi[\psi_{-...
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Are temperature and chemical potential of a black hole independent quantities?

I am a bit confused about the independent parameters in a charged black hole in AdS spaces. From equation (63) of this lecture notes we see that the temperature (T) of the black hole has chemical ...
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Is there any good literature about conceptual designs of Quantum (Micro-) Black Holes Machines?

I did some research and found this book: Quantum Black Holes, 2014 http://link.springer.com/book/10.1007/978-3-642-38939-9 It seems good for a repetition of current status of (Quantum) Black Hole ...
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Location of event horizon in moving black hole

Say a black hole is travelling at $c/2$, does the shape of the event horizon change? What about the location of the event horizon? If it is travelling at a hypothetical $c$, does the event horizon ...
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Can I hide a charge behind a black hole?

Suppose that you are standing on one side of a black hole. I'm standing directly opposite you, on the other side of the BH, and I'm holding a charged particle. Is it possible for us to be positioned ...
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Why are chemical reactions the same at different gravity levels?

Are all chemical reactions the same at a quantum level in different levels of gravity? How does the mass of subatomic particles change atomic behaviour at different gravity, in terms of speed and ...
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170 views

Time functions in general relativity

In my general relativity notes a function $f$ is called time function, if $\nabla f$ is time-like past-pointing. Say that we are in Schwarzschild spacetime and I want to check if $f=t$ is a time ...
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No-hair theorems for naked singularities?

For black holes, we have no-hair theorems that say, under certain assumptions about the matter fields, that they are uniquely characterized by just a few parameters. Are there any such theorem for ...
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What is the metric of Vaidya black-hole horizon?

The metric of a Vaidya black hole in outgoing/retarted null coordinates are $$ds^2=-\left(1-\frac{2m(u)}{r^2}\right)du^2-2dudr+r^2\Big(d\theta^2+\sin^2\theta d\phi^2 \Big)$$ The eveolving horizon ...
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entropy relation between ads$_2$ black hole and near extremal ads RN black brane

It seems that the entropy for AdS$_2$ black hole is independent of the temperature $s=s_0$. While for near extremal AdS RN black brane, $s=s_0+ s(T/\mu)$. Should not these two entropies be the same ...
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Trapped Surfaces. Any good articles?

I'm currently writing a dissertation on trapped surfaces as minimal surfaces. I have exhausted all of the resources I have, and the internet is pretty limited (in that it is fairly repetitive on just ...
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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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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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Evolution of black holes ensemble

Background: I’ve read many times that arrow of time can be explained from extremely low entropy of the Universe at the Big Bang (http://preposterousuniverse.com/eternitytohere/faq.html). The argument ...
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Is it mathematically possible or topologically allowable for cutouts, or cavities, to exist in a 3-manifold?

A few weeks back, I posted a related question, Could metric expansion create holes, or cavities in the fabric of spacetime?, asking if metric stretching could create cutouts in the spacetime manifold. ...
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Could harm due to spaghettification be mitigated by entering a black hole at almost the speed of light?

Say that we wanted to send a probe into a black hole, perhaps hoping to see if it's actually a wormhole that might transport the probe elsewhere. Upon approaching a black hole, the probe would ...
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Does the Bekenstein entropy bound present problems for unitarity in cosmological models that invovle a collapse or bounce?

If we expect the Bekenstein bound, or something like it, to hold in a collapsing universe, will that not eventually force us to accept some loss of information, or is there something I'm missing?
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Are black hole states completely mixed?

A completely mixed state is a statistical mixture with no interference terms, and (QMD, McMahon, pg 229): $$\rho = \dfrac{1}{n}I$$ $$Tr(\rho^2) = \dfrac{1}{n}$$ Are black hole quantum states ...
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Event horizon area and entropy content of a black hole

I am aware that the entropy content of a black hole is proportional to the area of its event horizon, and that this can be boiled down to the bit content of a Planck area on the horizon, but I have ...
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Introduction to the black holes and mass positive theorem in a geometric point of view

I'm a math student with no physics background who want understand more about black holes and mass positive theorem in order to understand better the motivations of the Inverse Mean Curvature Flow (my ...
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The nearly unmoving heavy star?

The interior Schwarzschild metric $$c^2 {d \tau}^{2} = \frac{1}{4} \left( 3 \sqrt{1-\frac {r_s}{r_g}}-\sqrt{1-\frac{r^2 r_s}{r_g^3}} \right)^2 c^2 dt^2 - \left( 1-\frac{r^2 r_s}{r_g^3} \right)^{-1} ...
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Falling towards a black hole in a box

I've been studying General Relativity on a friend's notes. There's an exercise I can't quite grasp. An observer is located in a closed, windowless box. The box is falling radially towards a black ...
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Why are orbits $1.5r_{s} < r < 3r_{s}$ unstable around a Schwarzschild black hole?

The "Orbital motion" section of the Wikipedia entry corresponding to Schwarzschild metric reads: A particle orbiting in the Schwarzschild metric can have a stable circular orbit with $r > 3r_s$. ...
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Black Hole and Mach Principle

In the derivation of the spin–orbit interaction, we transform to a frame that is fixed on the electron. I have several questions about this: Will I see any radiation if I orbit around a static charge?...
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Weird Black Hole definition

I'm quite new to the topic of Black Holes, and I can't understand the definition $$B = M-I^-(I^+),$$ where $M$ defines an asymptotically flat spacetime $(M,g_{\alpha\beta})$ (representing an isolated ...
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Energy dissipation from a black hole by neutrinos

Could the high gravitation field of a black hole cause neutrinos to form a Fermi gas or superfluid? Could these neutrinos from the neutrino background of the big bang whose temperature and deBroglie ...
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164 views

Gravitational redshift and Energy of a photon

Let us assume that we have a large gravitational field, then the gravitational redshift can be expressed as, $$\frac {v_{\infty}} {v_e} = (1-r_s/R_e)^{1/2}$$ In this equation $v_{\infty}$ represents ...
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Pictures of Different Coordinate Systems in General Relativity

In General Relativity by Woodhouse there are the three following diagrams in Chapter 9 about Black Holes. Despite a (very brief) description of these diagrams in the book itself, I am struggling to ...
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Can mass still be observed in our universe after moving from a black-hole to a white-hole?

This question is based on the recent papers on LQG black-holes by Abhay Ashtekar. Quantum extension of the Kruskal spacetime and Quantum Transfiguration by Kruskal Black Holes When mass moves from ...
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Spacetime geometry of black hole formed by the scattering of two gravitons

In the paper "Breakdown of predictability in gravitational collapse" Stephen Hawking mentions the following on section 5: Consider, for example, the scattering of two gravitons. In this case the ...
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Could you actually see the event horizon of a Black Hole?

For years I thought to myself that surely you'd be able to see the event horizon of a Black Hole when looking at it against the surrounding light. I thought that it would be similar to this: Where ...
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Are there any black hole information loss solutions that do not resort to non-locality?

It seems as though all solutions to the information paradox resort to non local effects. What solutions do we have that preserve locality?
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Can we use entangled black holes to form wormholes?

If any object can be made black hole by crushing it to its Schwarzschild radius, can we do the same process in two different points in the space keeping the quantum states of both the objects ...
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Boundary contributions to the BTZ black hole hamiltonian

I've been trying to understand every step of the (2+1 dimensional) BTZ black hole paper (https://arxiv.org/pdf/hep-th/9204099.pdf ) but I'm stuck in the obtention of the boundary terms in the ...
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Is the Temperature of a Black Hole a Relative Thing?

Because the laws of physics are the same in every reference frame, I assume the relativity of simultaneity [should] in principle apply to the black hole temperature and even evaporation rate. Surely a ...
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How many types of Singularities has string theory successfully resolved?

Except for proposing the fuzzball as a solution for black hole singularity, has string theory successfully explained/dealt with any other form of spacelike or timelike singularities? Kindly explain
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On the Schwarzchild black hole metric:

I've been going through the introduction on Ads/CFT by Horatiu Nastase (https://arxiv.org/abs/0712.0689). In chapter 5, "Black holes and p branes", in Eq. 5.2, it is mentioned the schwarzchild metric ...
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Deriving a path-integral expression for a thermal density matrix with position-dependent temperature

I've been fiddling with deriving a path-integral expression for a thermal partition function with a position-dependent temperature but I'm not sure how to get started on this. Concretely, I'm trying ...
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Black hole internal mathematics

I read the article about mathematicians-disprove-conjecture-made-to-save-black-holes and was wondering: could we compare the universe to Meteorology and geophysics on earth to answer some of questions ...
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What would happen if an object were to pass through the center of a ringularity?

A ringularity is defined by Wiktionary as a black hole with spin, that takes the shape of a ring with zero thickness. I know that the ergosphere imbues a lot of kinetic energy to objects that enter it,...
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Boundary times and bulk time in eternal black hole duality

In AdS/CFT, a particular duality is the correspondence between an eternal black hole in AdS spacetime (a large maximally extended AdS-Schwarzschild black hole) and the thermofield double state, \begin{...
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Nice expressions for geodesics in Penrose diagram for Schwarzschild spacetime?

If $u$ and $v$ are lightlike coordinates for 1+1-dimensional Minkowski space ($ds^2=dudv$), then we can compactify with a coordinate transformation of the form $$U=f(u)$$ $$V=f(v),$$ where $f$ is a ...
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Is it possible to have a state of the universe with no solution to what state it will evolve into in general relativity?

The math shows that it's possible to have a curved 4-dimensional Minkowski space such that there's a continuous bijection from it to $R^4$ and a closed light-like curve and some closed timelike curves ...
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Would naked singularities have a complex irreducible mass?

The formula for the irreducible mass, also known as the Christodoulou and Ruffini equation, is $$M_{\rm irr} = \frac{\sqrt{2 M^2-Q^2+2 M \sqrt{M^2-Q^2-a^2}}}{2}$$ where M is the mass equivalent of ...
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Reduce 10 Killing equations by the dependence of the metric to some coordinates

Each independent components of Killing vector for Schwarzschild that generates isometry are from the translation along $t$ and also from the fact that it is spherically symmetric, i.e \begin{eqnarray}...
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Effects on Niven Ring Around Kerr Black Hole

Suppose I have a Kerr black hole (rotating, uncharged) and I build a giant ring (a "Niven Ring") around it. I am interested in two related scenarios (I am confused about the second, but am also ...
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Physical reality of inner event horizon and inner ergosurface in a rotating black hole in D. Wiltshire et al. “The Kerr spacetime”

In chapter 1/The Kerr spacetime-a brief introduction by Matt Visser of D. Wiltshire, M. Visser, S.M. Scott "The Kerr spacetime - Rotating black holes in general relativity" the author presents a ...