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Questions tagged [event-horizon]

An event horizon is a type of boundary such that any information past this boundary is inaccessible to the observer it is defined for. Common examples are the Schwarzschild radius of a black hole (which is defined commonly for all observers outside this radius) and the cosmological even horizon (which is defined as a radius from an observer)

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Why is $\chi_{[\mu}\nabla_\nu \chi_{\sigma ]} = 0$ at the Killing horizon?

Let $\chi$ be a Killing vector field that is null along a Killing horizon $\Sigma$ Why is $\chi_{[\mu}\nabla_\nu \chi_{\sigma ]} = 0$ at $\Sigma$?
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Does time stop inside a black hole? How about at it's singularity?

I'm a layperson who just read Brief Answers to Big Question and in the book, Hawking proposes that: A typical black hole is a star so massive that it has collapsed in on itself. It’s so massive ...
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Nothing can ever go through the event horizon of a black hole [duplicate]

Nothing can ever fall into a black hole, even from the perspective of something in the process of falling through (a process I am arguing can never be completed.) Here's my argument. As seen from ...
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Escape velocity of Kerr black Hole [duplicate]

Do the event horizon of Kerr black Hole have escape velocity is equal to the speed of light same Schwarzschild black hole?
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Why do external observers see LIGO results if an object falling into a black hole never reaches the event horizon? [duplicate]

If I throw a clock towards a black hole, its time slows down, it is redshifted, and according to many theories it never reaches the event horizon from my point of view. How is it then, that a star can ...
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Black hole fundamental laws [closed]

As the Laws of physics breakdown during inside a black hole, would we have to create new fundamental Laws?
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If a black hole cannot have a diameter smaller than Planck length (a finite and non-zero length), then how can we call it a singularity?

As title. If a black hole has a minimum finite diameter, than it should not mathematically be a singularity.
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Help with the river model of black holes

I've been reading the paper "The river model of black holes" by Andrew Hamilton et. al., but I've been unable to derive the paper's results on the tetrad frame connection coefficients. I must be doing ...
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Can High Energy Light Escape a Black Hole after Low Energy Light Cannot

Perhaps there is another way to calculate the escape velocity, or perhaps light always has the same kinetic energy regardless of the energy of the light, but shouldn't the event horizon of a black ...
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What would happen if a planet's escape velocity exceeded the speed of light?

What would happen if a planet's escape velocity exceeded the speed of light? Would it also collapse into a black hole (by Heisenberg's uncertainty principle and Pauli's exclusion principle) or would ...
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Can an event horizon be distorted temporarily by a passing star?

If a star travels past a black hole and then continues on (i.e. does not merge into the black hole), how does it affect the shape of the event horizon of the black hole during this temporary period? ...
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Does a Black-hole Represent a Limit To The Value of Energy-density? [closed]

I've recently read an article that stated the singularity of a black hole could be a string, which is stipulated to be pure energy. If that's the case, does this mean that the singularity of a black ...
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Merging black holes — how fast do they hit?

Orbital mechanics says something falling from infinity hits at the escape velocity of the object it strikes. The escape velocity at the event horizon is lightspeed, it must be higher for the ...
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How do big black hole kicks out small black hole?

I read that when 2 black holes collide they will either merge into a single black holes or recoil and the less massive black hole will be thrown out, I'm thinking of gravitational slingshot whereby ...
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Why don't merging black holes disprove the no-hair theorem?

The no-hair theorem of black holes says they're completely categorised by their charge and angular momentum and mass. But imagine two black holes colliding. At some point their event horizons would ...
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If two event horizons approach each other, can I observe the collision?

As Alice is approaching the event horizon, for outsider Bob, Bob will see that Alice's time delayed and slowly stop. I heard that even Bob waits for long time, Bob will never be able to see Alice ...
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Growth of black holes and the no-hair theorem

Let's assume there is a black hole, and there are two starships (A and B), hovering above the black hole at the same distance from the event horizon, but on the opposite sides of the black hole. ...
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Event horizon and the existence of point particles

In this paper by David Kuap that first introduced the concept of Boson stars, he states that when the Einstein-Klein-Gordon system of equations is solved, the solutions obtained do not account for an ...
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Does space expansion mean the escape velocity at a black hole event horizon is strictly less than $c$?

The title pretty much sums it up. I could ask this question slightly differently; does the expansion of space cause the effective event horizon to shrink below what it would be for flat space? If so,...
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Would I be vaporised by incoming light if I crossed the event horizon of a black hole?

Approaching a black hole event horizon, I would see time pass more quickly outside. In practical terms, let's assume I am approaching a supermassive black hole whose tidal forces I might survive at ...
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What would be the Schwarzschild radius of an $n$-dimensional black hole? [duplicate]

The Schwarzschild radius of a 3 spatial dimensional black hole is 2GM/c^2. What would be the Schwarzschild radius of an n dimensional black hole according to Einstein's field equations having n x n ...
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Frame-dragging to detect Black Holes?

As I understand it, there are no radial geodesics ingoing to a Kerr black hole (BH) due to the effects of frame-dragging. Consider an isolated Kerr BH in space, and suppose we shine a laser on it "...
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Why was it cold during inflation?

During inflation, spacetime is approximately de Sitter and there is a cosmological horizon; in analogy with Hawking radiation, there should be a de Sitter temperature $T \sim H$. This temperature has ...
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Is a stable orbit possible inside the ergosphere of a Kerr (spinning) black hole?

I have heard that it's "impossible to hover" inside of an ergosphere, but everywhere I read this seemed to be speaking in the context of "relative to a stationary observer outside of the ergosphere". ...
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Naked singularities and mass distribution of a black hole

Do naked singularities also appear mathematically if the mass of a black hole is uniformly spread around its Schwarzschild surface? I recently heard about the concept of a naked singularity and it ...
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Would someone be able to see where they are headed once they crossed the event horizon? [duplicate]

Let us say an explorer was studying a supermassive black hole and ended up blundering past the event horizon. From my understanding, which may be wrong, the only paths allowed after crossing the event ...
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Confusion of the motion of a photon inside the photon sphere

In lecture, I am told that if a photon is initiated outside the event horizon of a black hole, from the laboratory frame of reference, it will never reach the event horizon. What is confusing is that ...
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How do light rays move parallel at the event horizon and why is this necessary?

This whole question and subquestions are based on the assumption that light rays on the event horizon are normal to the event horizon, so my apologies if this is not correct In A Brief History of ...
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Why does Hawking radiation outside the Schwarzschild radius decrease a black hole's mass? [duplicate]

When a pair of virtual particles is created outside the event horizon of black hole, there is a chance for one of the particles to escape. Zero chance on the surface of the event horizon and my ...
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Spaghettification on quarks?

Imagine a nucleon falls into a black hole, I would expect the gravitational force acting on each quark to be drastically different but due colour confinement wouldn't there more pairs of quarks being ...
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Centrifugal force on spinning black hole?

I saw the term spinning black hole popping up everywhere so my question do spinning black hole behave similarly to say a planet where it bulge in the equatorial and compress at the poles? what ...
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Radius of Star, The Schwarzschild metric and Black Holes

From Section 9.1, in General Relativity by Woodhouse: For a normal star, the Schwartzchild radius is well inside the star itself. As it is not in the vacuum region of space-time, the Ricci tensor ...
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If you fell into a black whole with an accretion disc, but perpendicular

If you fell into a black whole with an accretion disc, but orthogonal to the plane of the disc - would you ever meet the disc? I'm not asking whether you survive long enough, and obviously we assume ...
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Are black holes disc shaped, or sphere? [duplicate]

Been wondering, are black holes disc shaped pulling in only one/two directions, or spheres pulling in all directions?
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Can matter actually cross the event horizon effectively falling into a Black hole? [duplicate]

The answers I have seen do not take into effect the growth of the radius of the event horizon as significant amounts of matter get close to the existing event horizon. I would have expected that ...
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Magnetar's magnetic field near a black hole (novice question)

Even light cannot escape the event horizon of a black hole. Now, imagine a magnetar orbits a black hole. The magnetar orbits too far to be ripped and consumed by the black-hole. However, its magnetic ...
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What percent of the Black Hole energy is in the gravitational field significantly outside the event horizon?

This question is sparked with an answer to another one. We know that fields have mass-energy and gravitational field is no exception, some share of BH mass should be contributed by its gravitational ...
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In any collapse does the horizon “emerges” from $r =0$?

Consider the Vaidya geometry describing the collapse of a massless null shockwave: $$ds^2=-\left(1-\dfrac{2M\theta(v)}{r}\right)dv^2+2dvdr+r^2(d\theta^2+\sin^2\theta d\phi^2).$$ It is claimed in ...
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Black Hole Formation Physics [duplicate]

Considering only Black Holes that form due to gravitational collapse since the big-bang, wouldn't it be true that that have not (in our reference frame), in fact, formed a singularity or even much of ...
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Penrose diagram for two black holes?

Is there a Penrose diagram for two black holes near each other. Perhaps they are colliding or circling each other? Or can this method only describe a single black hole.
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Does a person falling into a black hole “see” the black hole? [duplicate]

For a person not falling into a black hole they must be accelerating. They will see a black hole event horizon with Hawking radiation coming out of it. For a free-falling person falling into a black ...
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If space expansion keeps accelerating, what would happen when the Hubble length gets smaller than the radius of a blackhole?

I am wondering what would happen if our universe expansion keeps accelerating and finally one day, the Hubble distance gets smaller than the radius of a black hole. For example, at time t, two points ...
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Kerr Black hole EH and Ergosphere embedding

Goodmorning everyone. I would like to share with you a question that has been gripping me for some time, but which I have never been able to give a convincing answer. When representing the ergosphere ...
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How can our universe have black holes? [duplicate]

Assuming that we were never inside a blackhole at any point in the history of the universe, how can blackholes exist against Gravitational time dilation? For any point in space time in the known ...
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Doesn't the fact that elementary particles are not black holes prove they are not point structures?

De Schwarzschild radius of a mass $m$ is defined as $$r_s=\frac{2mG}{c^2}(m).$$ So if we insert in this formula the mass of an electron (a point particle, according to mainstream physics), which ...
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Black Hole Surface Area-Mass Equation

Lets suppose we have a BH with mass $M$ and surface area $A$. From the Schwarzschild raidus we can say that the Area of the BH is proportional to its mass $$A=4\pi r^2=\frac {16 \pi G^2M^2} {c^4}$$ ...
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Event Horizons and Events [duplicate]

How can two black holes with event horizons merge within the lifetime of the universe? Basically events are happening where events stop (to the outside observer) so does an event horizon actually ...
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Why do accelerating bodies have an event horizon?

I read in a book named "Three Roads to Quantum Gravity" written by Lee Smolin that any accelerating body always have some hidden region of space-time from which light is never able to catch up with ...
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How does one find the Vaidya black hole event horizon?

As for a definition, there are quite precise ones for what an event horizon is. One can define it as the boundary of the causal past of future null infinity, i.e., $\mathcal{H}=\partial J^-(\mathscr{I}...
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The event horizon of a black hole supposes an absolute zero in some field?

Based on the Schwarzschild black hole theory, if the gravitational pull of the singularity were to equalize the energy that a photon has by approaching the event horizon from a special angle, would it ...