All Questions
Tagged with black-holes causality
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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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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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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 ...
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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 ...
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What happens if $ a^2 > M^2 $ in Kerr metric?
(Boyer-Lindquist coordinates and $ c = G =1 $ taken)
As I know, line element in Kerr metric $ d s^2 = - \left( 1 - \frac{2Mr}{\rho^2} \right) d t^2 - \frac{4 M a r \sin^2 \theta}{\rho^2} d \phi d t + \...
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Carter-Robinson Theorem
There are uniqueness theorems that classify Black holes according to its mass, angular momentum and charge. One of the theorem is Carter-Robinson theorem which has many assumptions and then it says ...
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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:
$$g=-c^{2}d\tau^{2}=(1-\frac{2GM}{c^{2}r})c^{2}dt^{2}-(1-\frac{2GM}{c^{2}r})^{-1}dr^{2}+r^{...
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If I were to drop my phone into a black hole, would I be able to catch it?
Say, for the sake of argument, I am outside the event horizon of a black hole and accidentally drop my phone (or some other object) into the hole. If I were to enter the black hole, would I ever be ...
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Does a geodesic exist that will take someone across the event horizon?
I saw the movie "Interstellar" a few years back, and was amazed that Cooper was able to fall from 1 AU into a black hole before his daughter turned 110. Intuitively, I would think that there ...
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Light-like vector in Schwarzschild metric
If I shoot a light ray radially, assuming the Schwarzschild metric, what vector would I plug in to the metric tensor to get a relationship between the $t$ and $r$ coordinates? That is what vector do I ...
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Can a body escape a black hole by being thrusted? [duplicate]
I am told many time that nothing can escape black-hole because black-holes escape velocity is more than speed of light. But we know object don't necessarily have to exceed speed of light to escape a ...
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What is the radius of a black hole? [duplicate]
I know the formula for the event horizon is $$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 ...
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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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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 ...
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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 ...
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How much time does it take for the gravitons generated by a black hole singularity to travel before exerting gravity forces on other celestial bodies?
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 ...
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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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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 ...
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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 ...
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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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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
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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 ...
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Black hole metric of reflected shell of incoming light
At this point in Leonard Susskind's eighth lecture on general relativity, he begins a discussion about finding the metric of a black hole formed by an incoming, spherically symmetric shell of light. ...
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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 ...
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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 ...
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Self-coupling of gravity and gravitation escaping a black hole - contradiction?
The field equations are non-linear, that can be interpreted as gravity is coupling with itself, see for example here:
Non-linearity and self-coupling of gravity
I'm trying to understand what that ...
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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 ...
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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 ...
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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}...
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If light and gravity travels at the same speed then why light cant escape the gravitational pull of black holes?
Speed of gravity is infinite?
If fastest thing is light then how universe is spreading faster than the speed of light?
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When can information escape from a black hole through a warp bubble-like spacetime?
It is well known that nothing can escape a black hole, including gravitational radiation. Many questions have been asked here about this topic, such as:
Can gravitational waves escape a black hole?
...
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What would happen if a black hole disappeared? [closed]
Imagine if a black hole disappeared. Would spacetime act like a rubber band and propel objects that used to be caught in its gravitational field outwards - i.e. some kind of space time explosion? How ...
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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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Outer Apparent Horizon is a Null Hypersurface
Let $(N,h)$ be a 4 dimensional spacetime, $M$ be some spacelike hypersurface in $N$ and $S$ be some closed hypersurface in $M$. Let $K$ be some null normal field on $S$. This is the standard set up ...
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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?
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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 ...
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Near a white hole, what do lightcones look like?
In the vicinity of a Schwarzchild black hole, spacetime looks like this (at least according to a quick google search). Here, the centre of the black hole is supposed to be at $x=0$ and the event ...
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Black hole, ingoing light-like and causality
In Kruskal-Szekerers coordinates, for example, I've noticed that ingoing light-like trajectories, in the interval $ 0<r<r_s$, are decreasing in time $t$ so they travel in the past for an ...
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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 ...
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Penrose diagram for two black holes
There are well-known Penrose diagrams for black holes:
And for collapsing star:
Diagram for collapsing star is obtained by joining two Penrose diagrams:
Is it possible to join diagrams for two ...
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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.
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Another form of Black Hole Information Paradox?
Consider the Penrose Diagram of Collapsing Gravitational matter :
Any radial light ray (say P) originating from $\mathscr{I}^{-}$ is bound to end up in the Black Hole. The ...
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What kind of causality would be broken if black hole singularities would be real?
In his article "The Universe as a Whole" 1, physicist Dennis Sciama said
We therefore face a crisis in theoretical physics. Either classical general relativity breaks down, or effectively ...
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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 ...
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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?
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What happens when you get rid of a singularity? [closed]
There's a pretty common physics chestnut about what would happen if the sun just up and disappeared. "How long would it take for us to notice?" The answer is of course roughly eight minutes--...
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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 ...
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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 ...
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What happens when you remove the mass inside a black hole?
Imagine you could delete the sun from its existence at an instance. Information cannot travel faster than the speed of light so the earth will keep orbiting the sun for about 8 minutes without knowing ...
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Black holes, and faster than light communication
Lets say we have a black hole that has a diameter > 1 light year and we can somehow push and pull this black hole {though the push and pull itself can never exceed lightspeed of course}.Will the other ...
