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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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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 $$r_s=\frac{2mG}{c^2}(\frac m {kg}).$$ So if we insert in this formula the mass of an electron (a point particle, according to mainstream ...
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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 ...
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Timehole - Black hole physics [closed]

If someone could shed some light so that I can think straight again that would be appreciated.. Anywho, since watching a video on what the universe may look like in trillions of years (each atom ...
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What are the differences between anything at just above absolute zero and something just above the event horizon?

I believe an item that has a half-life would decay at the same rate regardless of temperature (maybe I am wrong on this assumption) but it would definitely slow down due to time dilation above an ...
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What would happen to a person's body if they entered a black hole? [duplicate]

One of the premises of the classic sci-fi show Farscape is that a person could travel through black holes, I'm curious as to what exactly would happen to a person body if you were to enter a black ...
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What if the usual predictability of the laws of physics fail in general relativity?

In Newtonian mechanics the future is predictable, If we know the initial state of a system exactly, then the laws of physics determine its state arbitrarily far into the future. What if the usual ...
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What is the stretched horizon of a black hole?

What is the stretched horizon of a black hole? Is it related to the proposed "firewall" at the event horizon?
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Do you see the outside world from inside the black hole (can you see past the horizon from inside it)? [duplicate]

I have read these questions: If you fall in a black hole, when do you go past the event horizon? Can matter really fall through an event horizon? How can anything ever fall into a black hole as ...
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Where is mass density information of a Black Hole?

Have read that all the information of a black hole is contained on the Event Horizon. Does that include mass density of interior as a function of position? The shape of an event horizon depends on the ...
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Black hole interior [closed]

Had a few questions about the "interior" of the black hole. I've read that all the mass of a black hole is concentrated at it's center and that you can't isolate an object to an area of space ...
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Event Horizons Vibrations

Can the event horizon of a black hole vibrate? If so, are there mechanisms that dampen the vibration? Consider a spherical, non-rotating, non-charged black hole located far away from other sources ...
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Can the horizon of a black hole move?

Because of time dilation we cannot observe a black hole forming in a finite amount of time. For the same reason I suppose we also cannot observe the horizon moving: everything happening on the horizon ...
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apparent or event horizon

Is there anyway to determine whether there is an event horizon or not? I read a paper that the apparent horizon can be solved by setting $\frac{dr}{dt}=0$ and then solving for r. Is there something ...
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Oppenheimer-Snyder Collapse

I have read the Oppenheimer-Snyder collapse is significant in the sense that it was the first work which treated the problem of black hole formation under gravitational collapse. The model assumes a ...
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A question on firewall paradox?

In the firewall paradox, one considers a Hawking particle pair after the Page time. Assuming information conservation, the emitted particle must be entangled with early Hawking radiation. Assuming ...
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Simple proof of the area theorem for static black holes?

Let $M_i$, $i=1,2,3$, be the relativistic masses of three (static) black holes and $E_i=M_i/c^2$, $i=1,2,3$ their corresponding relativistic energies. Consider the idealized situation that $E_3$ is ...
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Could you escape a black hole - using a black hole? [duplicate]

I'm pretty sure the answer to the question in title is "No". But why? Below is a naive Newtonian simulation I made. You can see that in the animation, both black hole's horizons seem to "recede" for ...
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Can a sufficiently large black hole have low time dilation near its event horizon?

First time posting here, please let me know if I miss anything! As I understand it, as the mass of a BH increases, its Event Horizon increases more than the rate of the mass being put in. In ...
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How is it possible to enter a black hole, given time dilation?

This is a bit of a complex question - example scenario: X and Y (who happen to be immune from radiation and gravitation spaghettification, don't draw this into the equation) are some distance from a ...
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Consequences of infinite redshift in the event horizon of a Schwarzschild black hole

I am familiar with the usual derivation of the fact that it is impossible for someone to observe an object enter a black hole, because of the fact that light gets infinitely redshifted as its source ...
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Is the Taub-NUT solution stationary?

The Wikipedia article about the Taub-NUT spacetime says that it was a first attempt in finding the Kerr solution. Since the Kerr spacetime is a stationary solution, meaning that it admits an ...
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Innermost stable circular orbit in Schwarzschild solution

I've been reading about GR recently and I can follow the derivation of a Schwarzschild solution to it's final and well known form in Schwarzschild coordinates. The orbit stability argument (for a ...
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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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Can you have an infinite number of particles that don't form any event horizons?

If you bring two particles of matter close together at some point the combined mass will correspond to black hole with event horizon that encloses both particles. With an infinite number of particles ...
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The $t$-coordinate bounds inside a black hole

In case of the Schwarzschild black hole, the $t$ coordinate inside the event horizon becomes spacelike. Hypothetically, if one had too much time inside a grand-super-massive black hole and decided to ...