All Questions
Tagged with event-horizon causality
90 questions
0
votes
2
answers
116
views
Can something escape from inside of a black hole's event horizon by putting a sufficiently large mass next to it?
As far as I know, a lot of the maths surrounding black holes basically assumes that the black hole is the only mass in the entire universe. But what if something has fallen into the event horizon of a ...
- 103
0
votes
0
answers
43
views
How can you safely enter a black hole? [duplicate]
In a hard science fiction novel, suppose we want our heroes to enter a black hole. What laws of physics could we tweak to make it theoretically possible, albeit a technological feat?
- 101
0
votes
2
answers
123
views
Is it possible to transmit information from within a black hole via the momentum of a rocket activated after passing the event horizon?
Imagine a simple, non-rotating black hole and a massive rocket that is free-falling past the event horizon of the black hole, linearly towards the singularity. The rocket is massive enough that it ...
- 671
0
votes
1
answer
135
views
If information can't move outwards the singularity then how does the event horizon "know" it has to expand simultaneously along the whole surface?
I have read this question:
When matter is added, carefully and radially to make sure it does not add angular momentum, the horizon radius increases proportionately to the amount of mass added (but ...
- 29.3k
0
votes
0
answers
86
views
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 ...
- 185
5
votes
3
answers
161
views
Can a light signal from Earth reach a galaxy outside the Hubble Horizon?
Is this video on the FLRW metric (timestamp 29:00 minutes) mistaken in its claim that a light signal from Earth cannot catch up with a galaxy outside the Hubble horizon, due to the horizon receding at ...
- 10.1k
3
votes
1
answer
106
views
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 ...
- 63
4
votes
0
answers
67
views
Can wormhole inside a black hole become an escape?
I did not major in Physics so not sure if this is a proper question; but according to some Google search there do exist papers discussing wormhole inside black hole like this, which I am not able to ...
- 141
21
votes
5
answers
3k
views
Does an object approaching a black hole ever cross the combined event horizon of the black hole and itself?
Once you start studying black holes, one of the first things you'll probably hear is that from an outsider's perspective objects falling into the black hole take an infinite time to do so due to time ...
- 356
1
vote
0
answers
86
views
What is the meaning to the switch $dt^2\to-dt^2$ and $dr^2\to-dr^2$ in the Schwarzschild metric?
What is the meaning of the change $dt^2\to-dt^2$ and $dr^2\to-dr^2$ in the Schwarzschild metric, leading to:
$$g=-c^{2}d\tau^{2}=(1-\frac{2GM}{c^{2}r})c^{2}dt^{2}-(1-\frac{2GM}{c^{2}r})^{-1}dr^{2}+r^{...
2
votes
2
answers
166
views
If I were to drop my phone into a black hole, would I be able to catch it?
Say, for the sake of argument, I am outside the event horizon of a black hole and accidentally drop my phone (or some other object) into the hole. If I were to enter the black hole, would I ever be ...
- 121
1
vote
1
answer
179
views
Does a geodesic exist that will take someone across the event horizon?
I saw the movie "Interstellar" a few years back, and was amazed that Cooper was able to fall from 1 AU into a black hole before his daughter turned 110. Intuitively, I would think that there ...
- 69
3
votes
1
answer
442
views
Cosmological understanding from Penrose diagram of de Sitter spacetime
The conformal diagram of de Sitter spacetime looks like this
I think I understand the causal properties of this diagram. Someone who is static in the south pole can send messages only to the upper ...
- 4,737
2
votes
2
answers
180
views
Can a body escape a black hole by being thrusted? [duplicate]
I am told many time that nothing can escape black-hole because black-holes escape velocity is more than speed of light. But we know object don't necessarily have to exceed speed of light to escape a ...
- 41
2
votes
3
answers
120
views
What is the radius of a black hole? [duplicate]
I know the formula for the event horizon is $$R_s = 2GM/c^2.$$ At this distance the escape velocity equals the speed of light so nothing can escape from a black hole from this distance or less. But ...
- 369
4
votes
0
answers
88
views
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 ...
- 65
3
votes
2
answers
109
views
Influence of Schwarzschild radius and event horizon of ordinary objects
As far as I know, every body (with mass) has a Schwarzschild radius and therefore an event horizon. Thus, take an orange with radius $R$ and mass $M$ for example, then the Schwarzschild radius is ...
- 41
0
votes
0
answers
23
views
Is it possible for a black hole singularity to interact gravitationally with other celestial bodies (if we analyze it using the concept of gravitons)? [duplicate]
It is known to all that the travelling speed of gravitons (the propagation speed of gravitational field) is not instant. So for black holes, the gravitons (the gravitational field) generated by the ...
1
vote
1
answer
130
views
(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 ...
- 339
3
votes
1
answer
334
views
Can we identify a given metric as a black hole solution?
Given a metric $g_{\mu \nu}(x)$, can we identify whether it corresponds to a black hole? To be more precise, can we perform some calculations or define certain parameters of the metric which can help ...
- 93
1
vote
1
answer
77
views
Could light escape a chaotic event horizon?
In classical general relativity, one can think of an event horizon a tube of light cones in (3+1)D Minkowski space-time, in which the future-cone of each lightcone is inside the 4D region and the past-...
user84158
1
vote
0
answers
96
views
What specifically prevents the geodesics in Kerr BH from continuing beyond the second world line located inside the inner event horizon?
In a Kerr black hole, the inner event horizon is also called the Cauchy horizon. According to the answer to the question linked here, why are inner horizons Cauchy horizons, the Cauchy horizon only ...
5
votes
1
answer
951
views
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 ...
- 359
0
votes
2
answers
125
views
What prevents two particles that made a black hole to unmake it?
Assume you have two high energy particles approaching each other and forming a black hole even before colliding (but before a singularity is formed, which I am not sure that is possible). If the laws ...
user338734
5
votes
2
answers
413
views
How do we determine whether a surface is an event horizon? Exactly what to calculate to do this?
Event horizons play an important role in relativistic astrophysics, especially for black holes, but also for other spacetimes. It is fundamental to be able to determine whether a given surface is an ...
15
votes
2
answers
3k
views
If the escape velocity at the event horizon is the speed of light does it mean that slower bodies won't move away at all?
If we say that the escape velocity from a planet is say 10 km/s we think that a slower body will move away from that planet but will be eventually forced to fall back on the planet. In simple words we ...
- 2,275
0
votes
0
answers
117
views
How is the singularity of Schwarzschild space-like if a one can take a time like path to it?
It is known that when one crosses the event horizon of a Schwarzschild black hole, one cannot return and is destined to hit the $r=0$ horizon. My understanding is that this can be seen from the ...
3
votes
1
answer
360
views
Neutrinos and black holes
As a new user I could not comment on Do neutrinos of any flavor get trapped in black holes?
My question is that the Ice Cube facility has mapped Neutrinos coming from black holes, as stated in ...
0
votes
1
answer
233
views
Physical properties of an event horizon
This question may seem a little naïve, but I need help with the basics of black hole physics. I'm having difficulty understanding the topology of the event horizon. Apologies for any errors in ...
- 1
8
votes
3
answers
1k
views
Is there any *global* timelike Killing vector in Schwarzschild geometry?
I have been dealing with the following issue related to the Schwarzschild geometry recently. When expressed as:
$$
ds^{2}=-\left(1-\frac{2GM}{r}\right)dt^{2}+\frac{1}{1-\frac{2GM}{r}}dr^{2}+d\Omega_{2}...
- 663
1
vote
1
answer
85
views
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 ...
- 998
0
votes
0
answers
80
views
Do all null geodesics on a marginally trapped surface remain on or inside the surface?
The definition I've seen and used is essentially "if every congruence orthogonal to a spacelike two-surface has negative expansion, then that surface is said to be trapped." Does this ...
- 513
2
votes
0
answers
35
views
How can black holes emit detectable magnetic fields and gravity if nothing can escape the event horizon? [duplicate]
How can black holes emit detectable magnetic fields and gravity if nothing can escape the event horizon?
- 439
2
votes
2
answers
293
views
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 ...
- 765
1
vote
1
answer
523
views
Can gravitational waves escape a black hole? [duplicate]
I know that one of the defining features of a black hole is that all matter, including light, cannot escape a black hole. I was wondering if gravitational waves can. If this is true perhaps we could ...
- 290
0
votes
1
answer
64
views
Schwarzschild radius violation?
I have a puzzling question concerning the crossing of the Schwarzschild boundary. If a person (lets say in some large robust spaceship) flys quickly between 2 super-massive rotating black holes within ...
0
votes
3
answers
279
views
Escape Velocity, Misattribution, and Black Holes
Escape velocity is the ballistic speed required to escape from a gravitational field to infinity, ignoring any third body dynamics. The operative word here being ballistic, meaning unpowered.
...
- 136
1
vote
2
answers
301
views
Can you fall into a black hole, and then back out again?
There is a innumerable number of questions on this site about black hole event horizons, the possibility of falling in and back out again, but none of them answer my question specifically.
I have read ...
- 29.3k
0
votes
1
answer
182
views
What is a naked singularity?
What is a naked singularity? Would a naked singularity be one that that the event horizon is so small that it is the same size as the singularity? what could make one that small?
- 2,754
12
votes
8
answers
3k
views
Does the radio (between two co-moving astronauts) stop working when crossing the event horizon?
There are a lot of questions about crossing the EH (event horizon) of a black hole on this site.
Some of them suggest, that when you cross the horizon, nothing special happens, you don't even notice ...
- 29.3k
6
votes
1
answer
485
views
Why can't light travel past the event horizon?
Since the event horizon is defined as the boundary within which the escape velocity is greater than the speed of light, and escape velocity is the speed required for that object to reach infinity away ...
2
votes
0
answers
320
views
Causal past of future null-like infinity for a black hole
I can't understand following statement from Carroll book
[Future] event horizon can be equivalently defined as the boundary of $J^-(\mathcal{I}^+)$, causal past of future null infinity.
I made use ...
- 3,073
0
votes
1
answer
50
views
Behavior of particles at very small scale of spacetime
If I act $\phi(x)$ on $|0\rangle$ I get a particle created at $x$. Now if I let my system evolve for some time and now I do the measurement on this system at $y$ and my $(x-y)^2$ is small let say of ...
- 3,073
1
vote
1
answer
736
views
What is a Past Event Horizon?
I've recently been informed about a few things from a reliable source:
1) Wormholes can't form via gravitational collapse
2) The event horizons in an ER bridge can't collide
I understand what a ...
- 998
2
votes
1
answer
251
views
What do I see looking forward inside the black hole? [closed]
I have read this question:
Future light cones inside black hole
What happens to light that falls into a black hole?
What would be the view like from inside a black hole looking towards the event ...
- 29.3k
1
vote
0
answers
151
views
What would the eternal black hole look like?
The white hole and black hole regions in a Kruskal diagram are said to be actually two different locations. Given the problems with white holes it might be a silly question but, hypothetically, what ...
- 781
4
votes
1
answer
331
views
Are black holes bound to merge once their event horizons touch? [duplicate]
Even if they move toward each other at relativistic velocities and not head-on?
I looked at questions Black hole collision and the event horizon and Dynamics of Event Horizons between head-on ...
4
votes
5
answers
642
views
Is it theoretically possible to escape a classical black hole with force?
A stone being thrown from the surface must have a velocity of $11.2$ km/sec if it wants to escape Earth's gravity. However, a stone that has a constant force being applied on it need not have an ...
- 165
1
vote
1
answer
83
views
Define event horizon using only the notions of events and causality
Does this work?
Consider a set $B$ of events which satisfies
If $x$ belongs to $B$ and $x$ causes $y$ then $y$ belongs to $B$. The event horizon of $B$ is the set of events that are not in $B$ but ...
- 121
10
votes
4
answers
999
views
Why radial coordinate of a particle must decrease continuously once it is inside the Schwarzschild radius?
Suppose we are inside the Schwarzschild radius of a black hole and throw a ball radially outward. It is said that the ball has no possibility to increase its radial coordinate. It must continuously ...
- 329