Questions tagged [black-holes]

A black hole is a region of spacetime from which nothing can escape. More formally, the future light cone of any observer within the black hole is completely contained in the black hole, and the black hole region is not within the past light cone of any observer that goes to spatial infinity in an infinite amount of time.

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Torus shaped event horizon

Is there a solution to GR field equations for a rotating black hole that has a torus shaped event horizon? If so, when a craft flys thru the torus, it can pass thru a ring singularity while remaining ...
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Event horizon, centrifugal force, 2 dimensionality [closed]

Is it possible centrifugal force from a black hole’s spin creates the event horizon’s mass 2 dimensionality? Would theorized No-spin BHs have the same 2d information encoding?
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Quantum entanglement wins or black hole wins? [closed]

suppose: you have 2 particles "entangled" with each other, one of them is dropped by you into a black hole. Ignore any problems with the particle during its entry into the black hole. The ...
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Is there any known thing or physical object that absolutely cannot be destroyed? [closed]

What hasn't worked I thought of elementary particles, but with antiparticles they can be converted into photons (thus the particles and antiparticles vanish), and photons can be converted into heat (...
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How Existence Of Matter Is Possible Inside The Black Hole? [closed]

According to Chronology of the universe, origin of the universe initiated as per below sequence: Big bang (at 0 sec) occurs in the black hole (10^-35 m in size, Planck Length) at the center of the ...
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How do tidal forces on incomplete geodesics determine extendability?

Why can we be sure that the manifold with the metric $(M,g)$ does not have a geodesically complete extension if it has an incomplete timelike geodesic along which the tidal force blows up? Does this ...
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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 happens to the matter already at the very center of the star when it turns to a black hole?

I was wondering whether when an object collapses into a black hole, the matter in the position $r=0$, instantly becomes part of the singularity, or does it take time to fall into the singularity, and ...
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What does the discovery of pressure of black holes imply for spacetime thermodynamics?

As far as I know AdS/CFT and spacetime thermodynamics was motivated by the Bekenstein-Hawking Entropy formula of black holes. Recently however it was discovered that black holes have a pressure as ...
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Escaping photons from a forming black hole

is it possible for light to escape to infinity if it starts its journey from a region between the expanding event horizon and the schwarzschild radius of a collapsing star? if no, how does the black ...
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Will a black hole disappear?

Reading The brief history of time by Stephen Hawking and the chapter 7 BLACK HOLES AIN’T SO BLACK has below. Does it mean if his theory is correct, a black hall can disappear on its own? Because ...
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What portion of a neutron star's mass is in the form of gravitational energy? [closed]

The reason this question is being asked is that in the case of black holes around 100% of their mass is stored in their gravitational field and I am wondering if neutron stars come close. It is known ...
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How can the connections of the Schwarzchild Metric remain torsion free?

In Schwartzchild metric $\Gamma^r_{tt}\neq 0$, meaning that the basis $e_{t}$ changes in the direction of the R axis as we move along T axis. However, $\Gamma^r_{rt}= 0$, meaning that $e_{r}$ does not ...
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Destroying a black hole

Is there any (known? theoretical?) way to destroy a black-hole? "Destroy" means forcing it to disappear - before it evaporates through Hawking radiation. "Disappear" means that it ...
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Relating Minkowski and Schwarzschild

In the article https://arxiv.org/abs/gr-qc/0309072 the author finds a link between Minkowski coordinates of a freely falling observer and Schwarzschild coordinates. He makes use of a Galilean ...
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In pure GR can a black hole spontaneously appear?

In a universe without matter and just gravitation fields, can a black hole spontaneously appear? I would assume it could since such a black hole would evaporate purely into high energy gravitons. The ...
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How to determine the Delta-v for orbital changes around a Kerr black hole?

What's the Delta-v required to change between the Marginally bound orbit and the innermost stable circular orbit around a Kerr black hole.
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Can we measure gravitational constant at high accuracy using a black hole?

It is extremely difficult to fetch accurate value of gravitational constant. How about we have a black hole toy and laboratory for it? I am (currently) not a university student and need help for it.
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4-velocity in Schwarzschild metric

Is 4-velocity always equal to light speed in any metric? In Schwarzschild metric, the light speed with respect to a remote observer is $1 - r_{S}/r$. Should that be the 4-velocity of a falling ...
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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 ...
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Question about dilaton monopole interaction derivation

I am trying to understand how one derives the dilaton monopole interaction. In "Black holes and membranes in higher-dimensional theories with dilaton fields", Gibbons and Maeda mentioned ...
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Can I really see what is on the opposite side of a black hole?

This question is only about objects outside the event horizon. Both the observer and the object are just outside the event horizon. I have read this question: An observer can see the back side of the ...
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Invariance of binary black hole gravitational waves

Why BBH gravitational waves can be parameterized with the mass ratio? (and is not necessary the value of the two masses explicitly)
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Angular velocity of a coordinate system in General Relativity

In the textbook, The Classical Theory of Fields (Landau & Lifshitz), the authors express the line interval of the 3-dimensional absolute-space in the form $$dl^2=\gamma_{\alpha\beta}dx^\alpha dx^\...
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If I jump into a black hole one million years after another person, how do we end up relative to one another in space and time? [duplicate]

Imagine we live eternally and can survive tidal forces. Say, we are point-particles. From a fixed distance someone jumps into a black hole. I wait a million years and jump from the same position. How ...
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How massive objects affect distance measurements, or "But how far away is a black hole singularity, really?"

My question basically boils down to: Is the singularity of a black hole infinitely far away? I think questions like mine have been asked before, but they encounter responses like "no meaningful ...
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Checking null event horizon for a metric

I know that an event Horizon is a null hypersurface defining the boundary of the black hole region of spacetime. The first question is should all the black holes in general relativity have a null ...
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Is there comprehensive catalogue of black holes and relativistic jets?

Basically I am looking for a database with RJs and BHs characteristics. In High Energy Physics there is Particle Data Group book which is updated regularly. Is there something similar?
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If the escape velocity for a body that is very close to a BH is near $c$ but not $c$ can it move away a little bit before it falls inside the BH?

The escape velocity from a common gravtational attractor means that a body lounched away from the attractor will never turn back. But in the case of a BH at the event horizon this escape velocity is $...
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Schwarzchild Basis Vectors

In Schwarzschild metric, are the basis vectors $e_r$ and $e_t$ always orthogonal? If not, we would get non diagonal $g_{tr}$ terms, which we don't. But if we differentiate $e_r$ with respect to time ...
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Should the empirical Newton's gravitational constant $G$ be considered as the independent fundamental physical constant?

It is known that the effect of gravitational waves generation in collisions of (black holes)/(neutron stars) pair follows from the predicted consequences of Einstein's General Theory of Relativity. ...
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How does time warp at the midpoint between 2 equal mass black holes? [closed]

Assuming large masses and 1km between event horizons. Is time very slowed? or what? I am making a video game that is approximating relativistic effects on time near massive bodies. I am not concerned ...
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Boundary condition on wave in Schwarzschild-Anti-deSitter spacetime at event horizon

I am trying to model the behaviour of a spherical symmetric wave near a black hole in asymptotically Anti-de-Sitter spacetime. The metric for such a spacetime in Eddington-Finkelstein-coordinates is $$...
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Can the number of each particle in the universe have changed after a black hole evaporation?

Imagine a black hole that has formed from a neutron star, so from neutrons only. The neutrons appeared from a proton and an electron while an electron neutrino was sent into the surrounding universe. ...
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Why is Hawking radiation composed of photons?

The existence of the event horizon disrupts the vacuum state of quantum fields, and hence it appears to an observer that a black hole is generating radiation in its surroundings. Why is it the ...
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Does all matter from which a black hole is formed, end up, infinitely stretched out on the radial space coordinate, and on a small time interval?

As a previous question I asked is closed let me give it another try. In the comments on that question two different "views" popped up.(I don't know how to call it else, since I'm not sure ...
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When falling freely into a black hole, how far does space extend out in front of you? [closed]

Falling freely into a black hole (assuming we survive or being point-sized) all particles in front of you are accelerating away from you in the direction of the singularity which seems flying towards ...
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Can singularities' spin be related to the quantum spin of elementary particles?

We do know that black holes can and sometimes do have angular momentum, as described by the Kerr metric. Though I have not found anything about the description of the angular momentum of the contained ...
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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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Are metrics of general relativity tested?

We tested general relativity with effects as gravitational lensing and existence of black holes (Schwarzschild metric). But there are other metrics, e.g. Kerr-Newman metric for a point mass with ...
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Speed of light problem [duplicate]

This question totally hypothetical and I'm aware we can't travel at speed of light but IF .... If you were to travel at the speed of light according to Einsteins theory of relativity you would gain ...
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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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Can we use entangled particles to find out what is going on inside a black hole?

If you throw one entangled particle inside a black hole can we know what is happening with it from looking into another entangled particle outside the black hole?
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Measured Data Question regarding Galaxies with or without Dark Matter and Supermassive black holes

My understanding is that all Ultra Diffuse Galaxies (UDGs) have no Dark Matter nor do they have Supermassive Black Holes. It is also my understanding that all other galaxies have both Dark Matter and ...
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Partial Transit of an Event Horizon [duplicate]

To all the cosmologists and black hole experts out there: What would happen to a large object (very fast moving asteroid, planetoid, etc.) that was on a trajectory to tangentially intersect an event ...
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Rotation of astrophysical black holes

It's usually stated that "astrophysical black holes are expected to have non-zero angular momentum, due to their formation via collapse of rotating stellar objects". In other words: rotating ...
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Why do we need Quantum Gravity [duplicate]

I know this question has been asked many times but I have not found a satisfying answer to it yet. I understand Gravity as an emergent phenomenon of mass/energy. I understand quantum mechanics as the ...
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Could there be a Planck sized black hole with positive charge in the center of each atom, moving so fast on its way that it has nearly any mass? [closed]

A few days ago, I asked if Planck-sized black holes at the centers of atoms might explain the positive charges in the Rutherford model of the atom. Someone told me the Planck mass is much heavier than ...
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What does the no-hair theorem state about the entropy of black holes?

So does the no hair theorem say that all classical black holes will have zero entropy? If yes, why?
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Does the event horizon always stay below you in the frame falling freely with you in a black hole?

Imagine you fall freely into a black hole. You place small test masses inside your frame with a fixed distance between them. What will we see? The only force inside your frame is the tidal force. The ...
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