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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Schwarzschild radius after inflation [duplicate]

if after inflation the universe was only a fraction of a centimeter, all matter (or its energy equivalent) would be contained within that small volume. If I apply General Relativity (GR), (unless I ...
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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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Event horizons without singularities

Someone answered this question by saying that black hole entropy conditions and no-hair theorems are asymptotic in nature -- the equations give an ideal solution which is approached quickly but never ...
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Gravitational waves as information carriers

Is it possible to utilize gravitational waves as a delivery system for information between two observers straddling the event horizon of a black hole? And why ?
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Why aren't all black holes the same “size”?

The center of a black hole is a singularity. By definition, a singularity has infinite density. So how can a black hole with a different mass or density be described?
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Topology of a black hole

How many dimensions are theorized for a black hole, in view of the fact that black holes are not observed directly.
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If nothing ever falls into a black hole, why is there a puzzle about information?

From an outside perspective, nothing can ever pass the event horizon. It just scooches asymptotically close to the event horizon. So (from our perspective on earth), when a black hole reduces in ...
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What does this depiction of a black hole in the movie Interstellar mean?

I was expecting a whirlpool in 3D and the matter glowing from friction as it nears the center, as I expected a event horizon to be negligible visually. How does this depiction work? How big is the ...
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How close can an observer approach the black hole in an unpowered flyby without falling into it?

In classical mechanics by choosing the right trajectory you can approach a planet arbitrarily closely, if there is no atmosphere or anything to slow you down, you can approach the surface then fly ...
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Binary Singularity?

When two black holes collide, if the singularities were very small they would never merge together. they would rotate very fast as something other than a disk. The event horizon would oscillate very ...
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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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Can we have a black hole without a singularity?

Assuming we have a sufficiently small and massive object such that it's escape velocity is greater than the speed of light, isn't this a black hole? It has an event horizon that light cannot escape, ...
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'Dipping' an object beyond an event horizon [duplicate]

Say I'm hovering just outside an event horizon, what would happen if I held onto a stick and dipped half of it beyond the EH? Would I be pulled in? Would the portion of the stick beyond the EH be cut ...
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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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How do we expect distance measurements to compare inside and outside the event horizon of a black hole?

I've read that as one approaches the event horizon of a black hole, time is dilated relative to time measured farther away from the event horizon (clocks tick slower near the event horizon). I've ...
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How much extra distance to an event horizon?

How much extra distance would I have to travel through space to get from Earth to a stellar mass event horizon? (compared to the same point in space without a black hole)
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Does inbound light slow down (for an external observer) as it approaches the event horizon?

As I understand, for an external observer nothing even enters a black hole because it takes infinite time in his frame of reference for something to actually reach the event horizon. I also read that ...
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The arrow of time and the cosmology of a black hole interior

When solving the Einstein field equations in Schwarzschild metric for an observer falling into a black hole the radial coordinate r of the black hole and time t switch roles in the equations when ...
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What would happen if a negative mass crossed the event horizon of a black hole?

If negative mass really existed and somehow a very fast traveling negative mass object reached near the black hole's event horizon. What would happen when it crosses the event horizon? According to ...
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Is it possible to assign a physical radius to a black hole?

The Schwarzschild metric is given by: $$c^2d\tau^2 = \left(1-\frac{r_s}{r}\right)c^2 dt^2-\left(1-\frac{r_s}{r}\right)^{-1}dr^2 - r^2 \left(d\theta^2 + \sin^2 \theta \, d\varphi^2\right).$$ The ...
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Does any particle ever reach any singularity inside the black hole?

I am not a professional physicist, so I may say something rubbish in here, but this question has always popped in my mind every time I read or hear anyone speak of particles hitting singularities and ...
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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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Euclidean black hole extrinsic curvature

I have read that the extrinsic curvature at the horizon of a euclidean black hole is zero? Does anybody know how this can be shown?
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Black Holes: How does a three dimensional object collapse into a singularity & Where does the matter go?

A black hole comes into existence as the result of the core collapse of enormous stars, which lose quite some mass in a supernova explosion. However, supermassive black holes are still by any means ...
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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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Would the inside of a black hole be like a giant mirror?

As any light reflected or emitted from objects inside a black hole (if it is possible to be there) does not leave the event horizon and comes back inside, would it be like seeing yourself? What I ...
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Falling into a black hole

I've heard it mentioned many times that "nothing special" happens for an infalling observer who crosses the event horizon of a black hole, but I've never been completely satisfied with that statement. ...
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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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Is the event horizon also the boundary region of the mass contained by a black hole?

I was recently playing around with classic Newtonian mechanics and calculated that the Earth would have to be compressed to a spherical region of 8.8 millimeters (Its Schwarzchild radius) to turn its ...
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How can anything ever fall into a black hole as seen from an outside observer?

The event horizon of a black hole is where gravity is such that not even light can escape. This is also the point I understand that according to Einstein time dilation will be infinite for a ...
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Is the gravitational acceleration at the event horizon constant?

If the escape velocity at the event horizon of a black hole is equal to the speed of light, does this imply that the gravitational acceleration at the event horizon is also constant? For example, ...
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If NASA could send a camera into a black hole, could we then see what's inside the black hole?

Inspired by Stephen Hawking I recently tripped upon an idea of what is really inside a black hole. I thought if NASA (or any other space agency) could send a super protected camera into a black hole, ...
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What is the shape of a black hole?

I was thinking; what shape does a black hole have?. By 'Shape', I mean its form (e.g, circle , cylinder, sphere, torus, etc..). We usually think of black holes as if they're plugholes (e.g, a flat ...
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How can black holes be point-like but have a size?

Presumably all singularities are the same size: a point. What makes one black hole bigger than another? Also, a singularity will only have the same mass (or less) than the collapsed star, so what ...
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When does a singularity start to exist during a black hole formation?

Excuse me if the question is naïve. I am not a physicist. Let's imagine formation of a black hole from a homogeneous collapsing star. At certain moment when enough of matter is inside of a small ...
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Is relativistic event horizon half of Newtonian event horizon?

Is Relativistic event horizon half of Newtonian event horizon? relativistic escape velocity formula (from $m\phi=E-E_0$) is $v_e=\sqrt{2\phi-(\frac{\phi}{c})^2}$ and the Newtonian version of the ...
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What happens to objects pulled in by black hole? [closed]

What happens to the particles/elements/objects that sink into the gravitational pull and ultimately go to the interior part of a black hole? If, according to popular notion, it is crushed by the ...
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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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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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Another faster-than-light question

Imagine we have something very heavy (i.e supermassive black hole) and some object that we can throw with 0.999999 speed of light (i.e proton). We are throwing our particle in the direction of hole. ...
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At what speed will objects hit a singularity? [duplicate]

I have read in numerous places like here that an object in free fall into a black hole will be travelling the speed of light when it passes the event horizon. How is it possible to go this fast? ...
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Blackholes gravity, why it has effect outside the event horizon? [duplicate]

If gravity is a curvature of space time, and the event horizon is the collapse of this curvature on itself, then if the blackhole gets more matter after it is formed we should see no effect at all ...
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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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Size of black hole

I am wondering about size of black hole. How is it possible that we have black holes of different sizes? As I know the singularity is point which is infinite small and is infinite dense. So my ...
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Could a ship equipped with Alcubierre drive theoretically escape from a black hole?

Could a ship equipped with Alcubierre drive theoretically escape from a black hole? Also, could it reach parts of the universe that are receding faster than the speed of light from us?
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Why does angular momentum shorten the Schwarzschild Radius of a black hole?

Angular momentum causes the event horizon of a black hole to recede. At maximum angular momentum, $J=GM^2/c$, the Schwarzschild radius is half of what it would be if the black hole wasn't spinning. ...
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Is it possible that black holes are also neutron stars, but so dark that we cannot see them?

Since the concept of the singularity in a black hole leads to infinite densities, I wonder if it is really certain that black holes exist? Is there a possibility that massive objects (which are ...
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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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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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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 ...