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Questions tagged [black-holes]

A black hole is a volume from which matter cannot escape. More formally, the coordinate speed of light at the event horizon - the boundary of a black hole - is zero, as measured by a sufficiently separated observer.

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Given a black hole spacetime, how do we actualy find the event horizon?

As for a definition, there are quite precise ones for what an event horizon is. One can define it as the boundary of the causal past of future null infinity, i.e., $\mathcal{H}=\partial J^-(\mathscr{I}...
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The event horizon of a black hole supposes an absolute zero in some field?

Based on the Schwarzschild black hole theory, if the gravitational pull of the singularity were to equalize the energy that a photon has by approaching the event horizon from a special angle, would it ...
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What is the cause of the vacuum of space?

It seems logical that if space is expanding and also being dragged into black holes throughout the universe, the result would be a vacuum in the universe. Is that a satisfactory explanation of why ...
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Pulling of Light by a Black Hole? [duplicate]

We all know that light has no mass at all. Yet we say that the gravitational pull of a Blackhole within Schwarzschild radius doesn't let light to pass. If the light has no mass, how can it experience ...
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Timehole - Black hole physics [closed]

If someone could shed some light so that I can think straight again that would be appreciated.. Anywho, since watching a video on what the universe may look like in trillions of years (each atom ...
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Is it true that all symmetric binary black hole mergings emit the same power, regardless of “size”?

I recently saw this posting on Twitter: Yet it seems to me that Kepler's Law is a Newtonian statement and does not intuitively carry into a GR realm. Hawking Radiation would immediately provide a ...
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Quantum state of a black hole

On page 8 of the paper "Soft Hair on Black Holes" the authors say: Let $|M\rangle$ denote the incoming quantum state of a black hole defined on $\mathcal{H}$. We take it to be formed with neutral ...
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Scientific interpretation of the sci-fi film “Interstellar” [closed]

Great scientists such as Kip Thorne have served as consulting physicists for the 2014 movie Interstellar, I watched it twice or thrice, But couldn't understand many of the scenes displayed. Does the ...
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Could the Higgs detection have been a mini black hole?

Is it a possibility that the detection of the Higgs boson was in fact a miniature black hole. Given that the decay products of the Higgs boson were two gamma ray photons and that black holes are also ...
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Is it likely that galaxy supercluster attractors are all supermassive blackholes?

Really any phenomena about matter pulling into something always results to one thing eventually with enough mass, a black hole. I understand that the Shapely attractor is oftentimes referred to as the ...
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Should all black holes have Bekenstein-Hawking entropy especially in string theory?

There are so many confusing descriptions of how Bekenstein-Hawking entropy binds black holes, and thus the question. I previously asked similar questions, but I realized questions were misleading and ...
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Why do numerical relativity papers show $\ddot{h}_x$ instead of $h_x$ for gravitational waves amplitude?

I am very confused about why numerical relativity papers, when talking about gravitational wave extraction, usually plot the real part of the fourth Weyl scalar, $\Psi_4$, instead of its double ...
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What are the differences between anything at just above absolute zero and something just above the event horizon?

I believe an item that has a half-life would decay at the same rate regardless of temperature (maybe I am wrong on this assumption) but it would definitely slow down due to time dilation above an ...
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What would happen to a person's body if they entered a black hole? [duplicate]

One of the premises of the classic sci-fi show Farscape is that a person could travel through black holes, I'm curious as to what exactly would happen to a person body if you were to enter a black ...
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What if the usual predictability of the laws of physics fail in general relativity?

In Newtonian mechanics the future is predictable, If we know the initial state of a system exactly, then the laws of physics determine its state arbitrarily far into the future. What if the usual ...
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What is the stretched horizon of a black hole?

What is the stretched horizon of a black hole? Is it related to the proposed "firewall" at the event horizon?
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Would a Schwarzschild black hole “appear” to be a sphere in all reference frames?

I am wondering if all observers would agree that a non-rotating, non-charged black hole is spherical (i.e. there is no reference frame where one would measure it to be oblong in one dimension). I ...
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How is a non-rotating black hole a special case of a rotating black hole?

The Schwarzschild metric for an eternal non-rotating black hole is a special case of the Kerr metric for an eternal rotating black hole. But the Penrose diagrams for the maximal analytic extensions (...
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Could light find itself in orbit around an object? [duplicate]

We know light can't escape a black hole but... Could light find itself in orbit around an object? Say the light is on the cusp of an event horizon. Could it end up in an orbit around the black hole ...
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Do you see the outside world from inside the black hole (can you see past the horizon from inside it)? [duplicate]

I have read these questions: If you fall in a black hole, when do you go past the event horizon? Can matter really fall through an event horizon? How can anything ever fall into a black hole as ...
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Could dark matter consist of the supermassive black holes at the centers of galaxies?

Inspired by this question about whether dark matter is matter, noting that dark matter tends to be clumped in galaxies near the center and less so on the edges, accepting that many (most?) galaxies ...
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Black Hole Speed of Information Paradox?

Suppose you have a non-charged, non-spinning, spherically symmetric super massive black hole. Suppose also you could manipulate a sizable mass distribution within 10% of the Schwarzchild radius ...
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Where is mass density information of a Black Hole?

Have read that all the information of a black hole is contained on the Event Horizon. Does that include mass density of interior as a function of position? The shape of an event horizon depends on the ...
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Examples of perturbative force-free electrodynamics on top of vacuum solutions?

Imagine I have a solution $A_{(0)\mu }$ for the four-vector potential of vacuum electrodynamics. That is, the solution fulfills source-free Maxwell equations $F^{\mu\nu}_{(0)\;\;;\nu} = 0$ with $F_{(0)...
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How can entropy of a quantum subsystem increase?

Suppose that Hamiltonian of a global system is time-independent. Two subsystems encompass this global system. Adopt Heisenberg picture, and density matrix and state vector are constant. von Neumann ...
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Spacetime geometry of black hole formed by the scattering of two gravitons

In the paper "Breakdown of predictability in gravitational collapse" Stephen Hawking mentions the following on section 5: Consider, for example, the scattering of two gravitons. In this case the ...
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Smallest possible black hole containing any information?

My apologies for the meandering nature of this question. Using some back-of-the-envelope numbers: Planck length ≈ $10^{-35}$ m Planck area ≈ $10^{-70}$ m Radius of a black hole with a surface area ...
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At what rate does a photon blue shift as it falls toward a black hole and red shift as time slows close to a black hole?

Perhaps I am thinking about this incorrectly when a photon arrives at an event horizon, from an observers perspective time has essentially stopped for it and the photon has red shifted completely. ...
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Black hole interior [closed]

Had a few questions about the "interior" of the black hole. I've read that all the mass of a black hole is concentrated at it's center and that you can't isolate an object to an area of space ...
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The Large Hadron Collider and Black Holes

Last week, I went to Switzerland and visited the LHC. I took a tour, and the guide told us that it is possible for black holes to appear in the LHC, but they will be so small, that they would ...
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Event Horizons Vibrations

Can the event horizon of a black hole vibrate? If so, are there mechanisms that dampen the vibration? Consider a spherical, non-rotating, non-charged black hole located far away from other sources ...
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In string theory, what replaces the black hole singularity?

In string theory, what replaces the black hole singularity? I know that it is possible for a single string to have arbitrarily large energy. Is a black hole just a single string?
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Can Black Hole evaporation be used as a deterministic clock?

I was wondering if black hole evaporation can be used as a completely deterministic clock (e.g. a clock in which ticks are not dictated by quantum uncertainty). I read in the thread below that the ...
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From barotropic condition to density differentials

In my study of sound waves in a fluid I apply the barotropic condition, obtaining the barotropic equation: \begin{equation} \nabla h = \frac{1}{\rho} \nabla p \longrightarrow h(p) = \frac{p}{\rho} \...
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Is extracting energy via the Penrose Process only possible with Kerr black holes?

I have been learning about extracting energy via the Penrose Process. Everything I read mentions leveraging the ergosphere of a rotating black hole. But the ergosphere and the mechanisms of ...
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How does temperature correspond to number of particles produced?

I'm looking to calculate the number of particles per unit time produced by a Schwarzschild black hole through Hawking radiation. I know that the temperature of the black hole goes as $\kappa/2\pi$, ...
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Silly and unusual question about BH-BH binary

My question is quite simple. As we all know, we have detected gravitational waves. The first detection [https://en.wikipedia.org/wiki/First_observation_of_gravitational_waves#References] was about ...
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Does a disk galaxy wobble (galactic wobble)?

The center of mass of the solar system occasionally wanders up to 1/2 the solar diameter outside the sun. The sun rotates around this CM. Does something similar occur in spiral galaxies? If so, ...
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Wave equation for a static fluid that does not flow

From the fluid dynamics, applying a linear approximation, I arrived at the following master sound-wave equation: \begin{equation} \partial_t \left( - c_S^{-2} \rho_0\left( \partial_t \psi_1 + \mathbf{...
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Is there an equivalent of black body radiation for other particles?

So all objects composed of matter emit black body radiation, which is photons of various frequencies. Is there an equivalent of black body radiation for other processes? Is there gluon black body ...
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Can the horizon of a black hole move?

Because of time dilation we cannot observe a black hole forming in a finite amount of time. For the same reason I suppose we also cannot observe the horizon moving: everything happening on the horizon ...
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What would happen to the Earth, if the moon was a black hole? [closed]

Would it be a feasible scenario? I have read this question: What would happen to the Moon if Earth is turned into a black hole? Where Lubos Motl says: The extremal Kerr J=GM2/c∼RbhMc. Now, the ...
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Black hole evaporation and QFT scattering

In QFT we usually deal with scattering problems. On the asymptotic past we have free particles, they interact, and we compute the overlap with distinct free particle states on the asymptotic future to ...
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Periodicity trick for Kerr Black Holes

I am slightly confused concerning the euclidean section of a Kerr black hole. In page 5 of the following paper https://arxiv.org/abs/hep-th/9908022 it is said that in order to get the euclidean ...
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What prevents galaxies from collapsing towards the galactic center? [duplicate]

Galaxies have supermassive black holes at their cores exerting huge gravitational field. What is the reason that galaxies don't contract towards the center? What prevents this from happening.
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Oppenheimer-Snyder Collapse

I have read the Oppenheimer-Snyder collapse is significant in the sense that it was the first work which treated the problem of black hole formation under gravitational collapse. The model assumes a ...
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A question on firewall paradox?

In the firewall paradox, one considers a Hawking particle pair after the Page time. Assuming information conservation, the emitted particle must be entangled with early Hawking radiation. Assuming ...
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Is a GASER a theoretical possibility? [closed]

This question (basically an analogy ) is inspired by several questions and answers that appeared on stackexchange.  We first define GASER, for the purpose of this question. GASER - gravitational (...
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Simple proof of the area theorem for static black holes?

Let $M_i$, $i=1,2,3$, be the relativistic masses of three (static) black holes and $E_i=M_i/c^2$, $i=1,2,3$ their corresponding relativistic energies. Consider the idealized situation that $E_3$ is ...
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How to measure proper distance?

The proper distance from $R$ to $R+\Delta R$ in Schwarzschild metric is given by $$\displaystyle l = \int_R^{R+\Delta R} \frac{1}{\sqrt{1-\frac{r_s}{r}}} \; dr.$$ If static observer whose radial ...