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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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Does a black hole singularity compress mass back into energy?

I was watching this PBS spacetime video which made the case that mass is just a form of bound energy. Essentially massless particles are confined in a small space giving particles the illusion of mass....
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How exactly does black hole evaporate?

Reading about Hawking's Radiation I understood that black holes lose energy over time - which is logical in some way (otherwise they would be there forever and heat death would never technically ...
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Can mass be ejected from a black hole within the Schwarzschild radius?

This goes far beyond my limited knowledge regarding the physics of black holes, but my colleague told me this today: I doubt that if I stand on the surface of a black hole - inside the ...
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Should stellar black holes contain a singularity? (in GR) [duplicate]

Consider a star, for simplicity a non rotating one. The corresponding spacetime will be similar to a Schwartzschild one (if the star is static and spherically symmetric). Outside the star we will ...
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Can a black hole event horizon form before the singularity for an outside observer?

This is not a duplicate, my question is not about whether BHs have or don't have a singularity, but my question is whether, because of time dilation, no matter has reached the center and formed a ...
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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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Dropping a rope in a black hole [duplicate]

If there is a spaceship hovering above a black hole so that it can get away, what would happen if the spaceship connected a rope to itself and dropped the rope in? Could you pull the rope out?
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Due to TIME stopping, are contemporary Black Holes really just Large SHELLS of Matter and not Singularities? [duplicate]

If TIME slows down as you approach a Black Hole, has there been sufficient time in our Universe (13.8 Billion Years since the Big Bang occurred) for ANY matter inside the Event Horizon of ANY Black ...
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When GR is not valid passing the event horizon?

A test body crosses the event horizon of a black hole. When can we say that GR breaks down? Only at the singularity? At any point CLOSE enough to the singularity? How much close? In summary, do we ...
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Proper time of an object crossing event horizon in Kruskal coordinates

So I am reading a paper on a certain black hole paradox. The specifics actually don't matter, but if you want context (p16): black hole thought experiment. An object falls into a black hole. The ...
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How slow can time get near a black hole?

The closer I get to a black hole, does time slow down proportionally? Does time stop near the event horizon?
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What is the formal difference between the light cone and a black hole?

A black hole can be loosely defined as a spatial closed surface from which nothing, not even light, can leave. The light cone of special relativity is in some sense similar to a black hole because by ...
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Can you go back and forth through time when inside a black hole?

Inside the event horizon of a Schwarzschild black hole the radial coordinate becomes timelike and the time coordinate becomes spacelike (they change signatures). I read that timelike coordinates are ...
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Is my analogy on Hawking Radiation correct? [duplicate]

I just want to make sure what I know is correct. Let me explain and if there is anything wrong (any simple detail) with what I said, please, correct me. When the particle crosses the event horizon, ...
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What happens when you dump antimatter onto a naked singularity?

Inspired by this question, what would happen if you dumped antimatter onto a naked singularity? The answers for the previous question suggest that once the antimatter crossed the event horizon, no ...
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What happens when a black hole evaporates?

I have read other articles here on the subject and I think my question is different.. When a singularity evaporates enough to reach just above the limit of mass required to make the singularity, what ...
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Dynamics of Event Horizons between head-on colliding non-rotating black holes [duplicate]

What happens to the event horizons of each black hole as the two identical non-rotating black holes collide head on? At a point exactly between them, is the curvature zero (do the individual ...
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Black holes in p-adic gravity/ultra-metric metric field? [closed]

As a radically different to beyond standard general relativity, at least from the type of geometry it deals with: Consider p-adic gravity and/or general relativity defined on certain ultra-metric ...
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How much of a wave function must reside inside event horizon for it to be consumed by the black hole?

Related to this question in astronomy SE (https://astronomy.stackexchange.com/q/30611/10813) and in particular my attempt to answer it, I started to wonder which fraction of a waveform (for eg a ...
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Why is frame dragging possible for perfectly spherical spinning blackhole?

Even factoring the bulging as it spin very fast, I imagine no fluctuations in the density of gravitons emitting along the equator so how can frame dragging occurs? I know a gravitational wave depends ...
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Why energy matters for light to escape event horizon?

I like to know why photon with sufficient energy can avoid being pulled into the event horizon? Most light will get trapped and forms into photon sphere while some can escape, isn't light supposed to ...
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Negative surface gravity and temperature

Is it possible to have negative surface gravity $\kappa$? Also, since $T=\frac{\kappa}{2\pi}$ then if you have a negative surface gravity then that implies negative temperature too right?
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Does the same hawking radiation mechanism of a black hole apply to all other event horizons, like the hubble horizon?

From my understanding of black hole evaporation, a pair of virtual particles manages to avoid annihilating itself when one of the particles in the pair passes the event horizon and the other escapes ...
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How do growing event horizons affect observations of objects falling into black holes? [closed]

There are many questions on this forum about objects falling into black holes. Most of them describe that to distant observers, the objects never quite reach the event horizons and appear to be frozen ...
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quantum fluctuations at the horizon

So suppose we have a black hole with hair, that is a background solution in our field theory that describes a black hole spacetime and in which a field coupled to gravity has a non zero configuration. ...
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What is inside a black hole? [duplicate]

I'm trying to understand what causes a black hole in terms of it's composition. We know a start can die and collapse into a neutron star, but if the neutro star is not dense enough to create a black ...
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Black hole and its density

I'm an amateur to astrophysics but I'm slowly teaching myself things and I have these three doubts. How does more compression relate to a stronger gravitational pull. Like, when we say that a black ...
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Why is there no jet visible in the M87* black hole radio image?

M87 is known to contain a jet that is likely created by the supermassive black hole at its center. The recently published radio image taken by the Event Horizon Telescope does not display any ...
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Does infalling material leaves a detectable imprint on the black hole's event horizon?

An external observer will never see anything cross the event horizon, and in most explanations the story ends here. But I wonder what happens with gravitational effects of the material? For example ...
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Gold nucleus vs. M87 surface gravity

I tried to solve this gedanken problem, but I get a nonsense solution (may be is non resoluble). Take a gold nucleus. Nuclear radius is about $R=7.2 \:\rm fm$. Consider it a solid rigid sphere (I ...
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Pushing a long stick into the event horizon [duplicate]

Imagine a very long continuous stick with one end being pushed into the event horizon, all outside observers must agree that this end of the ruler come to a complete halt while I'm still pushing at ...
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Schwarzschild Radius of the Universe Part 2 [duplicate]

There seems to be some confusion as to what I am asking, so I am editing this for clarity. I know the universe is not a black hole. But if I could take all of the matter, (seen and unseen including ...
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Is the M87 black hole really a black hole? [duplicate]

I apologize if this is a silly question, but... From what I've been told, new black holes cannot form, because time slows to nearly stopped as matter approaches the event horizon. Instead, a new ...
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Shouldn't we also see our galaxy clusters in that ring just beyond the shadow of M87? [duplicate]

I read that the image of the shadow taken by EHT is actually all the surfaces of the event horizon of M87 which extends to 2.6 schwarchild radius but wouldn't that also meant that we can also observe ...
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Does the entire Universe prevent transforming the observable Universe into a black hole?

Consider the for us observable Universe (the entire Universe is many orders larger). It is (I think) common knowledge that the Schwarzschild Radius (SR) exceeds the radius of our observable Universe (...
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How does Event Horizon Telescope filter to get only the radio waves emitted by the target black hole?

There are many stars and black holes in the universe. So, their emitting radio waves may be mixed in the universe. How did the event horizon telescope filters to get only the radio waves emitted by ...
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How do you calculate the black hole diameter of M87*?

The recent data from the EHT Consortium on the size and mass of the central black hole of M87, named M87*, are telling us that the diameter of the event horizon ...
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First Black Hole Picture Takeaways [closed]

We just have obtained the first "picture" of a black hole, the supermassive blackhole at the center of galaxy M87. As spelled in this approved answer, there were multiple unsolved questions to be ...
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Why was M87 targeted for the Event Horizon Telescope instead of Sagittarius A*?

The first image of a black hole has been released today, April 10th, 2019. The team targeted the black hole at the center of the M87 galaxy. Why didn't the team target Sagittarius A* at the center of ...
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Why isn't the circumferential light around the M87 black hole's event horizon symmetric?

After the revelation of the first black hole images, it seems there is a bias towards the south side. Is it because of measuring it from earth or is it something more fundamental in the understanding ...
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What is the volume inside the event horizon of a black hole?

As I understand it, we measure things like distance (and volume) by the speed of light. A meter is the distance light travels in 1/299792458 seconds. In "normal space" this is (barely) straightforward,...
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Do the physics inside a black hole event horizon allow a person to remain conscious/alive?

Descriptions of black holes often include stories of a person falling through the event horizon. Since the physics inside the event horizon is dramatically different than outside the EH, are ...
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Evolution of area during the ring-down of a black-hole merger

My training is in nuclear physics, so my intuition is trained by the liquid drop model, in which the nuclear fluid is incompressible. This leads to the wrong intuition when I try to apply it to the ...
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What would an external observer see happen to objects entering an evaporating black hole?

As I understand it, objects passing the event horizon of a black hole from their perspective will, from an external perspective, seem to keep approaching but never reach the event horizon. Given this, ...
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Do gravitational waves violate the information paradox? [duplicate]

I know when black holes collide, the remnant black hole has a lower mass than the sum of the original two masses. This is due to energy loss via gravitational wave emission. Does this violate the ...
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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 ...