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 ...

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What would the Sagittarius A black hole look like if we were in a spaceship just outside its event horizon?

Let's pretend that our spaceship is shielded well enough that being in the presence of the black hole won't kill us with radiation. I have read that the black hole is as large as the space between the ...
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Is an event horizon absolute to all observers?

Recently I had discussion whether the event horizon of a black hole is absolute or relative to different (outside) observers. Does someone just 1m above the horizon (disregarding effects of tidal ...
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What happens to the wavelength/frequency of a photon as it passes through an event horizon?

I've asked a similar question about photons and black holes but wanted to rephrase it more specifically, so here goes... Ever since I learned how a photon's wavelength and frequency are indivisibly ...
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Direction of Time on Event Horizon

Does the axis of Time point into a black hole or away from? Can you give a reference paper?
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Relativity and volume of black hole

Does Einstein's theory of special or general relativity incorporate volume? In a particle accelerator two particles smash together at relativistic speeds and "create" new particles (quarks etc) due ...
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Electromagnetic black hole?

So I was thinking about something for the past while Consider a large spherical foam-ball with homogeneous density. Where a foam ball is defined as an object that can absorb matter with 0 friction ...
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Relativistic Computation?

Is it possible to employ relativity to develop computational technology? Here is a really basic example: Build a Computer and Feed it the Problem (say the problem is projected to take 10 years to ...
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If black holes almost or barely collide

Consider black holes approaching each other at some relative velocity and varying impact parameters. For impact parameters above some value they will sail past each other. For impact parameters below ...
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Are gravitons bound by the event horizon?

I understand that photons, even when traveling at the speed of light, cannot escape the event horizon of a black hole. Are gravitons and other virtual particles traveling at the speed of light also ...
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What does “all future lies within the event horizon” mean?

I was trying to find an answer as to why light does not escape black holes and I stumbled upon this Phys.SE question. In the answer it said that: "Since all future lies within the event horizon, ...
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Does cosmic expansion imply the possibility of the universe splitting in half; multiple big-crunches?

There is an event horizon where cosmic expansion leads to superluminal recession speeds for sufficiently distant objects -- the Hubble Volume. 1) Does matter beyond the event horizon affect us ...
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Apparent Horizon vs. Event Horizon

I understand that an apparent horizon is the boundary of trapped codimension-2 surfaces in the spacetime (surfaces from which both ingoing and outgoing light rays must be converging). Meanwhile, an ...
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Is a black hole's mass uniformly distributed?

If you were to fly around a black hole, would the gravitational pull be uniform and centered on the singularity, regardless of your relative location? If yes, how can this be consistent with models ...
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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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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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Does Birkhoff's theorem hold inside the event horizon?

Can Birkhoff's theorem be used to say that the blackhole exterior and interior sections of Kruskal-Szekeres's solution (or coordinate transformations of it like Gullstrand–Painlevé coordinates, etc.) ...
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Hawking radiation: why do only negative mass particles fall into black holes? [closed]

So I have read other questions asked about this topic, but they focus more on positive or negative energies of the virtual particles, and I want to ask specifically about the negative mass issue. ...
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Does the maximally-extended Schwarzschild horizon see the entire past of the parallel universe?

I've seen similar discussions for the inner horizons of Kerr and Reissner-Nordstöm metrics, but I've never seen mention for the simple maximally extended Schwarzschild geometry: As an infalling ...
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Is the Casimir effect also present between two event horizons?

A follow-up to this question: Is the Casimir effect also present between two event horizons?
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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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CPT and Event Horizon

Is the example of neutrino entering the event horizon of BH quoted from this article a valid possible example of CPT violation due to the presence of event horizon in BH ? Please, note that there is ...
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How is black hole affected by gravity of nearby bodies?

I run into wikipedia articles about ergosphere of rotating black holes. What if some massive body passes nearby some black hole: is something like ergosphere produced, or is the event horizon ...
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Black-holes are in which state of matter?

Wikipedia says, A black hole grows by absorbing everything nearby, during its life-cycle. By absorbing other stars, objects, and by merging with other black-holes, they could form supermassive ...
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Is there a way to photograph the very edge of black hole?

According to Professor Stephen Hawking, black holes emit radiation, Hawking radiation. The bigger the black hole, the fainter the radiation. That means, I personally believe, if we built large enough ...
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Why is the Schwarzschild radius the radius of an event horizon?

I've been searching the web and many references without much success. My question is how do we know that, in the Schwarzschild black hole solution, the surface with coordinate $r=2M$ (in the geometric ...
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Dipping a Dyson Ring below the event horizon

The basic assumption about inescapability beyond the Event Horizon is that the necessary escape speed (orbital speed) would exceed speed of light, therefore no object can achieve it. Now, would it be ...
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What happens when Dark Matter comes in contact with the event horizon of a large Black Hole

Josh Hill, 9, Oakdale Elementary has always talked Theory of Relativity and Astrophysics etc., I can answer most but lately he has stumped me and has been begging me to ask a pro, so here it is.... ...
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Why “light cones” have different shapes near black holes?

There is theory that light cone shape does not depend on the reference frame in which it is viewed. So why we draw light cones near black hole differently? I thought that if I am observing (from the ...
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What is the relationship between the Schwarzschild radius and Black hole Singularity?

What is the relationship between the Schwarzschild radius and Black hole Singularity? Can the Planck length be the length of singularity? Or is the length of the Schwartzschild shorter than the ...
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Question about Black Holes

I'm not a physicist so I'm sorry if this question is stupid from your point of view, but I believe it's not. Let us make this thought experiment: Take a black hole and put near it (but at a safety ...
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Do all event horizons emit radiation?

So, the event horizon around a black hole emits radiation, and Rindler space is full of thermal energy. I guess I have two questions- does the Unruh effect have anything to do with radiation from the ...
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A two-dimensional event horizon?

Today, I came across an article which says that the Universe began with the collapse of a 4-dimensional [spatial] Universe. Well, I don't know if there's any way to confirm these like it is with all ...
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Why are black holes special?

A black hole is where it's mass is great enough that light can't escape at a radius above the surface of the mass? I've been told that strange things happen inside the event horizon such as ...
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Hawking Temperature of the BTZ Black Hole

The metric of the BTZ Black Hole is given by $$ ds^2 = - N^2 dt^2 + N^{-2} dr^2 +r^2(d\phi + N^\phi dt)^2 $$ with $$ N^2 = -M+ \frac{r^2}{l^2} + \frac{J^2}{4 r^2}, \ \ \ \ \ \ N^\phi = ...
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When do you see the second horizon as you fall into a black hole?

Suppose you are a massive observer falling into a Schwarzschild black hole (geodesically or not, it shouldn't matter). At any point on your personal worldline you can receive light signals from the ...
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Event horizons in Newtonian mechanics and Galilean relativity

I've been revisiting classical physics (in the sense of Newtonian mechanics and Galilean relativity) and I was thinking why can't we have an event horizon in classical physics? Is it because the ...
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Cosmic event horizon

I had a question regarding cosmic event horizon. Let's say that a far away solar system has just crossed the cosmic event horizon due to the expansion of the universe. In that solar system let us ...
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Misconceptions about the universe - Hubble Sphere

I recently watched Veritasium's (excellent) video "Misconceptions about the Universe". However, he said something (at 2min 25s) which I think is wrong. Can somebody please clarify this one way or the ...
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Two spherically symmetric shells of matter collapsing into a black hole

Suppose I have a thin spherically symmetric shell of matter of (rest) mass $M$ that collapses to form a black hole, and later a second thin spherical shell of mass (rest) $M'$, concentric to the ...
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Is the difference between an event horizon and a singularity merely perspective?

I know the question has been asked about how an event horizon is distinguishable from a singularity given that time must come to a stop at the event horizon, but I haven't been fully satisfied by the ...
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Can light be launched outwards from an event horizon? [duplicate]

There's something which I don't quite get about black holes and event horizons (feel free to tell me if I got something completely backwards at any step along the way): Say we have a singularity and ...
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If charged particles always attach to black hole event horizons, how can ordinary matter fall in?

(A friend at work kindly loaned me loaned me his copy of Kip S Thorne's "Black Holes & Time Warps". This may have been ill-advised... :) BH&TW 1994 paperback p.410 Figure 11.5: ... all ...
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event horizons are untraversable by observers far from the collapse?

Consider this a followup question of this one In the classical schwarszchild solution with an eternal black hole, the user falls through the event horizon in finite local time, but this event does ...
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What is really going on in the ergosphere of a Kerr black hole?

Considering the Kerr metric with $GM>a$, we can compute 2 event horizons: $r_\pm=GM\pm \sqrt{G^2M^2-a^2}$ These event horizons are null surfaces, and trajectories are timelike between $r_+$ and ...
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How can a black hole increase mass? [duplicate]

As far as I know, the time dilation at the event horizon of a black hole is infinite, so everything falls asymptotically towards, but never reaching, the horizon for an outside observer. How then can ...
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Why do black holes have a finite size? [duplicate]

There is one piece of information that makes me wonder why finitely sized black holes are even possible: to an observer from afar, anything that approaches a black hole's event horizon is never ...
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Why Hawking radiation emission rate inversely proportional to black hole mass? [duplicate]

The rate of Hawking radiation emission increases as the mass of the black hole decreases, what I don't understand is that if the emission method is related to zero-point energy particles appearing and ...
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Force of gravity in the event horizon [closed]

I recently posted a question about black holes and gravity (this) and I discovered the Schwarzschild radius: $r_s=\frac{2GM}{c^2}$. I thought with a friend of mine about the actual force (in Newtons) ...
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Do black holes exert an infinite amount of energy at the event horizon?

An interesting thought crossed my mind when reading about Hawkings' apparent horizon theory. If we assume that event horizons do actually exist, it would seem that black holes violate basic laws of ...
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Can a black hole actually grow, from the point of view of a distant observer? [duplicate]

Possible Duplicate: Black hole formation as seen by a distant observer I've read in several places that from the PoV of a distant observer it will take an infinite amount of time for new ...