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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scalar field boundary condition in background with U(1) isometry

I have a simple question about Lewkowycz and Maldacena's paper In section 2, they consider the scalar field in BTZ background ground and require boundary condition of the scalar field, $\phi \sim e^...
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Is there high ring-down frequencies in LIGO's recent discovery?

This question is from Physics overflow: question in physicsoverflow. I am reading LIGO's new discovery of gravitational waves by black hole merger. During the merger, two phases are not hard to ...
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Can there exist an observer able to observe a collapse of a star into a black hole?

We know that an observer at infinity cannot see a star forming into a black hole as the matter will take progressively longer and longer time to compress (from this observer's point of view). Is ...
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Is it possible for a particle to orbit a singularity inside the event horizon [duplicate]

Event horizon just means a distance from singularity, where there is no "going-away" from the black hole. But is it possible, that there are particles with enough momentum to actually orbit the ...
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How would one attach black hole to starship in the Black hole Starship model? [closed]

A black hole starship is a theoretical idea for enabling interstellar travel by propelling a starship by creating an artificial black hole and using a parabolic reflector to reflect its Hawking ...
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Energy conservation around a black hole

In the Schwarzschild black hole, the Killing vector "time translation" $k^a$, so that the following quantity is conserved along a geodesic: $$E = -g_{ab}k^au^b = (1 - \frac{2GM}{r})\frac{dt}{d\tau}.$$...
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Why does imaginary periodic time of the Rindler space of black hole give Hawking temperature?

The metric of a Schwarzschild black hole in Rindler coordinates is $$ds^2= -\frac{\rho^2}{(4MG)^2} dt^2 +d\rho^2 + d\mathbf x_\perp$$ where $\rho$ gives us the distance to the horizon. If we switch ...
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Does a black hole ever fully form to an outside observer? [duplicate]

According to general relativity, I had understood that time appears to slow down when looking into high gravitational fields from afar, so that as a black hole forms, the light from a collapsing star ...
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Could dark energy and Hawking radiation be the same thing? [closed]

Specifically, could black holes effectively emit some kind of negative mass or other form of dark energy, so that the mass of the black hole itself would actually increase rather than decrease (...
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What happens if I slowly lower a dangling object into a black hole?

I could've sworn I've seen this question before, but I couldn't find it. Suppose I have an object on the end of a really long string. I can slowly lower it near the event horizon of a black hole, ...
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Could a black hole maintain a stable orbit around the Earth?

Could a rotating Black Hole of mass $1.24\times 10^{10}$ kg maintain a stable orbit around Earth, without significantly altering the path of the Earth or Moon? In addition to this, would the presence ...
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A doubt regarding Black Hole Complementarity

A friend was explaining Black Hole Complementarity to me, and at one point he said that to get a (horrendously) mixed quantum state, i.e. a thermal density matrix without a heat bath, one takes a ...
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How do we know the Schwarzschild solution contains an object of mass $M$?

The Schwarzschild metric is $$ds^2 = - \left( 1 - \frac{2GM}{r} \right) dt^2 + \left(1-\frac{2GM}{r}\right)^{-1} dr^2 + r^2 d\Omega^2.$$ In Carroll's GR book, it is claimed that $M$ is the mass of the ...
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Why would a black hole evaporate? [duplicate]

Hawking radiation doesn't make sense to me, with respect to black holes getting smaller. It would seem that any particle (or anti-particle) leaving the Schwartzchild radius would have a similar anti-...
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Non equilibrium black hole radiating away energy and angular momentum, with total energy and angular momentum conserved

The question refers to whether mass (i.e., energy) and angular momentum can be considered to have been carried away by the gravitational radiation in the hole settling down. And whether those entities ...
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Pair Production via Black Hole

I think this should be a straight forward question. Is it possible for a photon to pass near a black hole and be turned into a matter and anti-matter pair according to current theory? Edit: From the ...
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Are black holes hollow? [closed]

So when enough matter to create a black hole falls towards the center of its collective mass, at one point the mass/area ratio becomes high enough to form the event horizon of the black hole. Because ...
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Black hole gravity vs parent star gravity

In the cases of black holes that form from supernova and collapse of a massive star, I understand that in most of these cases, the star loses significant amounts of mass from the explosion. Presumably,...
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Utilising Black Holes as a potential energy source

I'm aware of the Penrose process and the basic physics behind that. Also, I know that the Blandford-Zjanek process (That is potentially responsible for the relativistic jets). Aside from these two, ...
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An explanation of Hawking Radiation

Could someone please provide an explanation for the origin of Hawking Radiation? (Ideally someone who I have been speaking with on the h-bar) Any advanced maths beyond basic calculus will most ...
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1answer
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If black hole is a “hole”, how can its position be pinpointed from every position in universe? [closed]

Ok, this is going to sound lame but, here goes: When we say that a black hole is situated at a distance of 1 light year(let) FROM EARTH, that means it may situated at a distance of 100 light years ...
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Eddington-Finkelstein coordinate

The Eddington-Finkelstein coordinates in case of Schwarzschild metric are defined as \begin{align} u&=t-r^*\\ v&=t+r^* \end{align} where $$r^*=r+2GM\ln\left|\frac{r}{2GM}-1\right|$$ The ...
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what is the metric of N-sheeted $AdS_3$?

Suppose the AdS$_3$ metric is given by $$ ds^2 =d\rho^2+cosh^2\rho d\psi^2 +sinh^2 \rho d\phi^2 $$ what is the n-sheeted space of it? Can the n-sheeted BTZ be constructed from it by identifications ...
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Gravitational waves from neutron star - neutron star merger

How is neutron star-neutron star merger different from Black Hole-Black Hole merger and what information can be extracted from gravitational waves emitted by these processes?
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Is this analogy of Hawking Radiation correct?

Through reading of textbooks and other research papers, I have settled on the analogy of hawking radiation below (Written completely by myself) Within the ergosphere of the black hole, virtual ...
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Can electrons escape a black hole?

Assuming that when an electron that changes energy states in an atom, and moves to a different orbit around the nucleus, but does not move through the space between orbits when it changes states, an ...
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Big Bang not really the beginning of a completely new universe? [closed]

From what I know about the origins of the universe and the big bang, it is stated that it all started from an intensely hot and dense mass. This sounds like a singularity to me, which means a black ...
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Based on our current observations, what all prevents a formed black from experiencing a repulsive force to overcome its gravitational force…? [duplicate]

Edit: I don't think this is a duplicate. [***] Question: Based on our current observations, what prevents a black from experiencing any sort of repulsive force to overcome its gravitational force ...
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Where does the energy of a photon trying to escape a black hole go?

I've heard "light cannot escape a black hole" explained several ways. One is that if a photon inside the event horizon tries to escape a black hole it loses energy to gravity. As it loses energy its ...
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How are negative energy orbits around a Black Hole defined?

I have read several times that within the ergosphere or a Kerr Black Hole, it is possible to have particles that have an orbital energy value of less than 0. However, I do not understand the concept ...
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What effect would a singularity have on the fabric of space time?

Galaxies are always moving and it seems that there are SMBHs at the heart of most galaxies. If black holes actually twist up spacetime so much thru infinite density how do we know that after the ...
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If an object has a temperature, does it have to radiate?

I'm reading through a powerpoint presentation about Hawking Radiation (HR). They are explaining all of the reasons that built up to the postulate of HR, and one of the reasons is that if there is a ...
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Israel-Wilson-Perjés Solutions

I'm searching for a reference that gives explicitly the field strength (or at least the gauge fields) of the Israel-Wilson-Perjés Solution, using complex harmonic functions for the metric. In "...
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Hawking radiation and energy-negative energy pair production [duplicate]

A black hole evaporates through hawking radiation, what I don't get is the requirement for an energy-negative energy pair production. Since it's the black hole's gravitational energy that's ...
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Orbital period and velocity around a Kerr black hole relative to fixed stars

I've been trying to make progress on some of the smaller pieces of this question about the environment around a Kerr black hole. In order to calculate the effects of special relativistic Doppler shift ...
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1answer
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Black holes shouldn't really exist? [duplicate]

General relativity states that for an observer sufficiently far from the gravitational field of a blackhole, the space time geodesic nearr the event horizon is so long that we should never observe an ...
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What if a black hole of normal matter and a black hole of antimatter collided? [duplicate]

This is a curiosity question. Considering same mass black holes. As nothing can come out of either black hole, would the annihilation result into a black hole of energy? Assuming the two black holes ...
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How can black hole interior theories be falsified?

How can any theory of the interior of black holes be falsified if the interior cannot be observed?
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Why '1+log slicing condition' and 'Gamma Driver Shift Condition' were successful in black hole simulations?

The 1+log slicing and Gamma driver shift conditions are I want to know if there is a specific reason why these conditions were used most for Black Hole simulations in Numerical Relativty. And how ...
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Curvature Invariants in General Relativity and Singularities

Suppose that I want to check if a given metric is singular or not. I'm interested in curvature singularities, not coordinate singularities, so I can look to scalars made with Ricci, Riemann and Weyl ...
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Does a singularity appear the instant a black hole is formed? [duplicate]

Imagine a very heavy (tens of solar masses) star in its final moments before collapsing to form a black hole. The gravitational force exerted by the weight of the star overcomes the neutron degeneracy ...
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Is the general definition for black holes wrong? [duplicate]

I often read the definition of event horizon of a black hole as the region where the escape velocity is bigger than the speed of light. However this would imply that if you're inside the event horizon ...
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How do black holes join each other? [duplicate]

If black holes can swallow any object, and can radiate energy, then how could black holes join each other and can form bigger black holes?
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Can gravitational waves observed far from a black hole tell us anything about the multipole moments of a dynamical horizon?

In a paper by Ashtekar et al in 2013 on the approach to the final state to a stationary black hole they study the evolution of the multipole moments of dynamical horizons, which relax away (except for ...
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Can you see external light approaching a black hole when you are within it?

We can't see a BH because light can't escape from it, but when you are within a BH (assuming you would stay alive) could you see any light from extrenal stars? If so, would the ligth be very bright or ...
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101 views

Does a black hole really slow down time?

When an object gets pulled into a black hole it seems to slow and stop, but could it be possibly be because the speed of light that hit the object and came back was slowing down as the object got ...
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Can a rotating black hole have a donut-shaped event horizon? [closed]

It is conjectured that a rotating black hole has at its center a ring-shaped singularity. Thus, at the center of the ring-shaped singularity the gravitational field must be zero (similar to ...
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Information from inside a black hole

Now I'm hardly a physicist, but I am pretty interested in it. I was thinking about black holes and the movie Interstellar, and if you've seen it, then one of the central points about it is that they ...
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Is there any known dynamical process to obtain supertubes?

I'm looking for a dynamical process to produce supertubes. For example, microstates of the D1-D5 black hole system can be understood as supertubes. See, for example, http://arxiv.org/abs/hep-th/...
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Help needed to understand Kerr coordinate transformation

The (uncharged) Kerr metric for a black hole of mass $M$ and angular momentum $Ma$ takes the form $$ds^{2} = \Sigma\Big(\frac{dr^{2}}{\Delta} + d\theta^{2}\Big) + (r^{2} + a^{2})\text{sin}^{2}\theta ...