All Questions
Tagged with spacetime event-horizon
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End points of event horizon
I am reading The Nature of Space and Time by S. W. Hawking. In the last paragraph on page 16 he said that:
event horizon may have past end points but don't have any future end points
I understand ...
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Can part of space be causally disconnected from the rest of the universe by being surrounded by black holes? [duplicate]
Is it possible for black hole event horizons to overlap and form a spherical wall around an island of space (that's not inside a black hole) while still being causally disconnected from the rest of ...
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How to Understand Negative Energy in the Ergoregion?
I am trying to understand the Penrose process and having trouble explaining negative energy in the ergoregion.
How I interpret it is:
Energy is the dot product between the four momentum of the object ...
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Time required to reach Black Hole's Event Horizon from outsider perspective?
Let's imagine a pair of particles that is entangled. One (call it $P_1$) is released and then falls to a black hole from a distant $x_0$, (for example $x_0=5r_s$) and velocity $v_0(=1/2c)$, while the ...
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Types of singularities
I am confused about the types of singularities. According to my limited knowledge there are two types of singularity. One is space like singularity ( a curvature singularity enclosed within a null ...
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How many null directions are there?
The metric signature of spacetime is usually given as ($3,1$), but spaces can also be ($3,n,1$). Null surfaces include photons and event horizons, which exist, so is $n$ actually $ > 1$ in the ...
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Event horizon in stationary spacetime
In the case of non-stationary spacetimes finding the event horizon is no easy task.
The stationary case should somehow be less involved or so it is in some well known cases, such as the Kerr spacetime....
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If I were to drop my phone into a black hole, would I be able to catch it?
Say, for the sake of argument, I am outside the event horizon of a black hole and accidentally drop my phone (or some other object) into the hole. If I were to enter the black hole, would I ever be ...
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Are black holes 4-dimensional balls of spacetime? If so, will they have 3-sphere surfaces?
If black holes are 4-dimensional balls of spacetime, they will have a 3-sphere surface with a 3-dimensional volume. Would this allow infalling matter to remain within this surface?
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Why isn't there an event horizon in the negative mass Schwarzschild metric?
The negative mass Schwarzschild metric has no event horizon.
Why isnt there a particular radius in which spactime flows outwards at the speed of light? This would imply a region of the solution for ...
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Penrose diagrams and Holographic Principle
What would the Penrose Diagram look like that represented a black hole (call it Black Hole-B), inside of a massive black hole (Black Hole-A), in our universe?
and, as inside of the Penrose diagram for ...
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Why do we defer to GR when describing black holes rather than rely on QM?
This is a broad question but it's well documented that GR and QM are very well tested in their own domains but they conflict around black holes.
Picture a neutron star slowly accreting matter until it'...
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If it's a common myth that a black hole contains a singularity, what does a black hole actually (likely) contain?
It's a common myth (especially in popsci) that a black hole contains a singularity. However, I cannot find an explanation for what we think a black hole actually does contain. The best I've seen is &...
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Are black holes the edge of our universe?
Are black holes the actual edge of the universe? Because spacetime is another dimension, I would assume the universe doesn’t have perceived corners or edges. At least humans cannot perceive it.
The ...
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Where does the parallel universe in the Penrose diagram come from?
In this diagram, as well as our universe, you have a parallel universe.
Where does this come from? Is this just a artifact of the diagram, or is it predicted by the maths in some sort.
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How exactly does Hawking radiation occur? [duplicate]
I understand some parts of the theory, I've read from here https://phys.libretexts.org/Bookshelves/Astronomy__Cosmology/Supplemental_Modules_(Astronomy_and_Cosmology)/Cosmology/Carlip/...
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Why are there multiple universes in the Reissner-Nordström solution?
I am trying to make sense of the Penrose diagram of a non extremal Reissner-Nordström spacetime, that is, the solution with two horizons. The coordinates are
$$ v'=\text{exp}\left(\frac{r_+-r_-}{2r_+^...
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Conformal Diagram for Astrophysical Black Hole
I have a question about the conformal diagram of an ‘astrophysical’ black hole which forms in finite time (but with no evaporation).
Usually I see the conformal diagram presented as something similar ...
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Curvature of space in a black hole
This is a very simplistic view from an interested structural design engineer (retired).
Mass curves space. Taking the case of a sphere of uniform density the point at which you have as much mass ...
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(1+1)d collapsing null-shell?
I am trying to understand the following Penrose diagram (from https://arxiv.org/abs/1507.03489)
According to the authors, it is depicting the formation of a (1+1)d black hole from a collapsing null ...
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Space falling faster than light after it falls inside the event horizon of a black hole?
Typing my question directly so people know what I am asking, afterwards providing background and context.
Q: What does it mean when space is falling, faster than light?
(I am specifically wondering ...
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Why are inner horizons Cauchy horizons?
I know that RN black hole has two horizons, one outer one and one inner one. The outer one is the event horizon.
As far as I know, a Cauchy horizon is the boundary of the domain of dependence of a ...
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Is it possible to extract energy from black hole and decrease event horizon size faster?
So imagine a black hole that is like 3 times the mass of the sun so that there can be a bigger gravitational gradient. Assume the black hole has no accretion disk, charge, or rotation for simplicity's ...
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Why do black holes remain? [closed]
When we think about black holes as not containing matter but being regions of warped spacetime, I can't think why they don't revert to being Euclidian space more quickly. This is because I can see how ...
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Why are distances to event horizons linear with mass when gravitational effects fall off as $1/r^2$?
Black holes' gravitational effects fall off as $1/r^2$, but their event horizon grows linearly with increasing mass. $R$ (event horizon) grows the same rate as $M$ (mass of black hole). Okay lets ...
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Is it possible the Black Holes to be pure deformations in the fabric of spacetime and not an effect of super-dense matter?
Is there any theory in the literature that supports this hypothesis that BHs in their center do not have a super-dense matter singularity but are pure deformations in the fabric of spacetime itself or ...
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Using gravity to beat event horizon of black hole [duplicate]
So I know it's impossible for an object A to escape a black hole once it has crossed the event horizon, but what if it had help from the outside? Is it theoretically possible for a massive enough ...
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What distant observer would see if spaceship remains insitu just outside event horizon?
Imagine 2 spaceships found themselves just outside the event horizon of a blackhole, spaceship A tries to remain in place relative to the black hole while B accelerates around the blackhole. To a ...
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How can I make sense of black hole complementarity if the universe consist of one manifold and observers are not married to coordinates?
I'm reaching way over my head here, so bare in mind my knowledge base is at best upper undergraduate. This is, unfortunately, yet another byproduct of discussions in this page that is itself a ...
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Why does Leonard Susskind draw constant time slices around a black hole as lines passing through the origin at zero?
In this video Inside Black Holes by Leonard Susskind,
why does he draw the constant time slice as lines passing through the origin at zero?
Something seems to be contradicting to have constant time ...
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Do test particles initially comoving with a black hole accelerate away from it?
This question feels ridiculous, but I really am confused. If you Google Image search "schwarzchild light cones" it shows how, relative to the frame comoving with the singularity, the speed ...
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Spacetime inside the horizon of a black hole
According to Susskind a bit of information crossing the event horizon of a black hole instantaneously encounters the singularity. Also, time appears to gradually slow down for an object approaching ...
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How can the infalling observer see the event horizon to retreat in front after the observer crossed it (and can't even notice it)?
I have read this question:
so looking toward the singularity, you see the horizon retreating from you as you fall in - even after you've already crossed the horizon.
Are black holes naked ...
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If spacetime can expand faster than the speed of light, then can a black hole do that too?
I have read this question:
Yes, the expansion of space itself is allowed to exceed the speed-of-light limit because the speed-of-light limit only applies to regions where special relativity – a ...
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Can one define a flow of spacetime?
One often reads things like, 'At the event horizon, the flow of spacetime exceeds the local speed of light.' But is this actually correct? Can you mathematically define some sort of spacetime flow ...
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A Question About the Surface of a Black Hole Singularity
In Kip Thorne's book, Black Holes and Time Warps, he states that the mass of the core of a star shrinks until quantum gravity takes over. And then discusses that at this distance, the singularity ...
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Does someone falling into a black hole see the future falling objects? [duplicate]
For someone falling into a black hole space and time swaps signs - the time coordinate of an outside observer becomes his space coordinate. That means, for any plane parallel to the horizon and inside ...
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Event horizon as a Null hypersurface
I have two questions regarding the Event horizon as a null hypersurface.
I know that for a null hypersurface, the normal vector to the surface is null (zero-length). It means normal to the event ...
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What happens to the ergospheres of two colliding black holes, right before collision?
Let's assume we have two black holes with equal mass. They move towards each other, each heading towards the center of the other black hole. Both black holes rotate with equal speed. What happens to a ...
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An apparent paradox in General relativity using a binary black hole
Note: Quantum gravity effects are ignored in this question.
Imagine 2 black holes each with mass $m$ approaching each other and after some time the Event Horizons (EHs) of both black holes touch each ...
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Event horizon of a rotating black hole
For a non-rotating black hole, Schwarzschild radius itself forms the event horizon, but how do we find the event horizon of a rotating Kerr black hole?
user301349
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A naïve question about spacetime singularities
Very little that I know about general relativity is that there are solutions of its equations with singularities, and these are interpreted as black holes.
Mathematically, the most widespread kind of ...
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How to describe ‘when’ a black hole actually is? [closed]
If I look at any point in space I can think of it as being in the future because it takes me time to travel there. I can go there and an observer can watch me go there.
When I look at a black hole I ...
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"Different reality" inside a black hole
why does our perception of space and time entirely change inside a black hole? And why does time not stop inside it from the perspective of the inside observer, however, extremely slows down for the ...
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How can the universe have an event horizon?
As I understand it (correct me if I'm wrong), the universe has an event horizon, and we can't possibly know if there's anything beyond it. This is due to the expansion of the universe, that space is ...
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Are objects that fall into a black hole *always* silhoutted against the Hawking radiation coming out?
There are many questions on this site about whether you can really fall into a black hole - some seem to say clearly yes, some say clearly no. So let's try one more stab at this one with a narrower ...
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Why can't anything return after crossing the event horizon? [duplicate]
I know that the escape velocity exceeds the speed of light but, what stops a rocket which can thrust constantly from returning, like, to escape earth we don't have to exceed escape velocity if we have ...
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Do black holes exist in 1+1 dimensional spacetime?
I'm currently working in 1+1 dimensional spacetime and would like to know if black holes can exist in such a manifold? I think they can because the Schwarzchild metric has the coordinate singularity, ...
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What closed 3D space looks and behaves like? (Relativistic Black Hole)
So I wanted to ask a question that is a copy of Why can't you escape a black hole?
From the answers, the conclusion I draw is: it's impossible to escape a black hole.
any trajectory inside the ...
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Does the singularity travel along the time axis?
Let's assume that all matter travels along t axis roughly at the speed of light, in the direction of worldline for rest mass.
Let's also assume that there is a ...
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