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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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Does time stop inside a black hole? How about at it's singularity?

I'm a layperson who just read Brief Answers to Big Question and in the book, Hawking proposes that: A typical black hole is a star so massive that it has collapsed in on itself. It’s so massive ...
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Black hole horizon states in standard derivations of Hawking Radiation

In all standard derivations of Hawking radiation given e.g. by Hawking, Parker and Wald, one has the so-called horizon states. The point is that when one is quantizing a scalar field on a black hole ...
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Tidal force of a black hole without using tetrad

One thing I have been somewhat curious about is computing geodesic deviation for tidal effect. Specifically, given separation vector $\xi^a$ between two nearby timelike geodesics and $V^c$ the ...
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Black hole absorbing photons?

If a black hole has a radius that is not that much smaller than the wavelength of light emitted by the sun, and is at the same temperature, shouldn't it be able to absorb photons as well as emit them? ...
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What is the difference between “black hole” and “division by zero”? [closed]

According to this answer: "In fact we really don't know what is inside a black hole." That sounds like: "In fact we really don't know what is a black hole" At first we have matter and gravity. ...
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Nothing can ever go through the event horizon of a black hole [duplicate]

Nothing can ever fall into a black hole, even from the perspective of something in the process of falling through (a process I am arguing can never be completed.) Here's my argument. As seen from ...
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Are black holes physical entities? [closed]

what exactly are black holes, now Ive been told that they are in fact physical entities but then i had the question what is it on the other side of it(as in like 2 sides of a disk) and i was told that ...
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Escape velocity of Kerr black Hole [duplicate]

Do the event horizon of Kerr black Hole have escape velocity is equal to the speed of light same Schwarzschild black hole?
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When will a person falling into a black hole be crashed by tidal forces?

Assume that a person can resist $30g\approx 300\frac m{s^2}$ (remark: the record is actually at 46.2). I want to solve for the distance $r$ at which a $1.5m$ tall slim person falling radially into a ...
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Do black holes violate the conservation of mass?

This question has confused me for quite some time now. I have searched it up online, and the basic answer is: 'Mass is a form of energy. When black holes die they release the amount of energy that ...
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Null Kruskal Coordinates and affine parameterization

Let $(M,g)$ be the Schwarzschild spacetime. The usual metric tensor in Schwarzschild coordinates reads: $$g=-f(r)dt^2+f(r)^{-1}dr^2+r^2d\Omega^2\quad f(r)=1-\frac{2M}{r}.$$ Now consider radial null ...
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Weird Black Hole definition

I'm quite new to the topic of Black Holes, and I can't understand the definition $$B = M-I^-(I^+),$$ where $M$ defines an asymptotically flat spacetime $(M,g_{\alpha\beta})$ (representing an isolated ...
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Why this constant is included in the tortoise coordinate?

In the Schwarzschild spacetime, the tortoise coordinate $r_\ast$ is defined by the property that $$\dfrac{dr_\ast}{dr}=\left(1-\dfrac{2M}{r}\right)^{-1}$$ Now, we cam integrate this. Multiply by $r$ ...
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What Is The Connection Between The Planck Force And Black-holes? [duplicate]

This question deals with a unique case where the gravitational force at the event-horizon of a Black-hole can be expressed by The Planck Force acting at the boundary of a Schwarzschild envelope. If ...
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Can anyone tell me how can draw shadow of black hole like in presented in Intersteller movie? Is there any code for it in Mathematica or in Python? [closed]

Equation of motion for photon $$ \Sigma \frac{dt}{d\lambda} = aL\left(1-\frac{r^2+a^2}{\Delta}\right) + \omega\left(\frac{\left(r^2+a^2\right)^2}{\Delta}-a^2 \sin ^2\theta\right)\ , $$ $$ \Sigma\frac{...
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Death of a black hole? [duplicate]

Is there a death of black holes? If yes, what is the average lifetime of a black hole and what happens when a black hole dies?
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How can one of the particles on a pair of virtual particles created by vacuum fluctuations have negative energy? How does it lower black hole mass?

Assuming Stephen Hawking’s 1975 “Particle Creation by Black Holes” : “One might picture this negative energy flux in the following way. Just outside the event horizon there will be virtual pairs of ...
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What is the closest distance for an undiscovered black hole?

How close can a stellar black hole, an inactive old one, get to our solar system before we notice it? And then, how long to get to the inner solar system at typical speeds? I know it is very unlikely ...
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Why do external observers see LIGO results if an object falling into a black hole never reaches the event horizon? [duplicate]

If I throw a clock towards a black hole, its time slows down, it is redshifted, and according to many theories it never reaches the event horizon from my point of view. How is it then, that a star can ...
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Relay information from near a black hole

I realize this must a very fundamental issue and perhaps already answered but I couldn’t find a double. Let A be an object near the event horizon - as seen by us - of a black hole, and B an object ...
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QFT Hilbert space and analysis of quantum black holes

QFT Hilbert space is infinite dimensional and it is known that given a region $A$ and its complement $A^c$ of the spacetime, the QFT Hilbert space $\textbf{does not}$ decompose into $\mathcal{H}_A \...
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Black hole fundamental laws [closed]

As the Laws of physics breakdown during inside a black hole, would we have to create new fundamental Laws?
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If a black hole cannot have a diameter smaller than Planck length (a finite and non-zero length), then how can we call it a singularity?

As title. If a black hole has a minimum finite diameter, than it should not mathematically be a singularity.
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Help with the river model of black holes

I've been reading the paper "The river model of black holes" by Andrew Hamilton et. al., but I've been unable to derive the paper's results on the tetrad frame connection coefficients. I must be doing ...
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Does the Jerk (derivative of acceleration) play a significant role in relativity?

In particular, I'm thinking of spaghettification that occurs as an object falls into a black hole (as an extreme example). But what about tidal forces like the tidal heating taking place on Jovian ...
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black hole chirp physics

I have read that the chirp frequency in the first LIGO data is related to the period of the orbit of merging black holes. I have also read that that the frequency is related to time-dependent ...
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Can High Energy Light Escape a Black Hole after Low Energy Light Cannot

Perhaps there is another way to calculate the escape velocity, or perhaps light always has the same kinetic energy regardless of the energy of the light, but shouldn't the event horizon of a black ...
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What would happen if a planet's escape velocity exceeded the speed of light?

What would happen if a planet's escape velocity exceeded the speed of light? Would it also collapse into a black hole (by Heisenberg's uncertainty principle and Pauli's exclusion principle) or would ...
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Is there a connection between these two results on soft hair on black holes?

In 2016 Strominger, Hawking and Perry published the paper "Soft Hair on Black Holes" proposing new results that could have importance to the study of the black hole information problem. One ...
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Can an event horizon be distorted temporarily by a passing star?

If a star travels past a black hole and then continues on (i.e. does not merge into the black hole), how does it affect the shape of the event horizon of the black hole during this temporary period? ...
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Does a Black-hole Represent a Limit To The Value of Energy-density? [closed]

I've recently read an article that stated the singularity of a black hole could be a string, which is stipulated to be pure energy. If that's the case, does this mean that the singularity of a black ...
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Penrose diagram of Janus black holes

Recently I'm interested in the Janus black holes, it's a solution of three dimensional Einstein-scalar action which can be embedded in ten dimensional type IIB supergravity with appropriate ansatz. ...
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How did some very large supermassive black holes form in the time span of the universe?

I saw a program recently called "How the Universe Works" discussing Black Holes. One of the enigmas, it seems, is the existence of extraordinarily large 'supermassive' black holes -- black holes of ...
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How are the predictions of the Schwarzschild solution consistent with our observations of black holes? [duplicate]

I am fully aware that there may be many misunderstandings in the following. However, in recent lectures we covered the Schwarzschild solution, and after some interesting mathematics we reached ...
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Merging black holes — how fast do they hit?

Orbital mechanics says something falling from infinity hits at the escape velocity of the object it strikes. The escape velocity at the event horizon is lightspeed, it must be higher for the ...
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How do big black hole kicks out small black hole?

I read that when 2 black holes collide they will either merge into a single black holes or recoil and the less massive black hole will be thrown out, I'm thinking of gravitational slingshot whereby ...
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Why don't merging black holes disprove the no-hair theorem?

The no-hair theorem of black holes says they're completely categorised by their charge and angular momentum and mass. But imagine two black holes colliding. At some point their event horizons would ...
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What happens to a black hole with an extremely large net charge? [duplicate]

Suppose you repeatedly shoot low energy protons into a 4 solar mass non-rotating black hole, until it has so much net charge that a nearby proton experiences slightly more electrostatic repulsion than ...
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If a Black Hole Singularity is a 1 Dimensional Object, How Does it Affect 4 Dimensional Space

If the center of a black hole is an one-dimensional point, gravitational singularity, where density and gravity become infinite and space-time curves infinitely, then / why: If spacetime is warped ...
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What is the electromagnetic 4-vector potential in Reissner-Nordström coordinates?

If you have the usual Reissner-Nordström metric of a charged black hole, is the electromagnetic potential of the black hole still: $$A_0(r,\theta,\phi) = \frac{Q}{r}$$ in these units?
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Blackhole Entropy

What happens if substance with zero entropy falls into a Scharzschild blackhole? Since $$S=\frac{k_b A}{4L^2_p},$$ increase in mass should be accompanied by an increase in entropy. Its area can not ...
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Can you electrically charge a black hole so that it would repel electrons?

Assume we somehow drop LOTS of electrons into a black hole, without the black hole accreting any other matter or energy at the same time. I suppose the charge of the black hole would build up much ...
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Quantum tunneling into a black hole

The currently accepted answer to Throwing a micro black hole into the sun: does it collapse into a black hole or does it result in a supernova? states that a small black hole of mass approximately $10^...
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Is there an equation for the amount of energy from a collapsing star BEFORE it creates a black hole?

(I could be phrasing my question incorrectly, so bear with me as I try to explain it.) Recently I've been very interested in the geometry and physics of black holes, so I started messing around with ...
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How big would a black hole have to be to absorb the Sun?

How big would a slow moving black hole have to be to absorb or otherwise destroy the Sun?
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Capture cross-section for massive particle in Schwarzschild solution

In the Schwarzschild solution, a massive particle starts with non-relativistic velocity $v$ relative to a stationary observer and at position $r\to\infty$. I need to show that the capture cross-...
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Do black holes have a limit of mass?

If you have a bunch of gas and increase its mass, gravity combines it into a planet. If you have a planet and increase its mass, gravity forces the planet to undergo nuclear fusion, turning it into a ...
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Time direction in the BH parallel exterior region

In the III region of the maximally extended Schwarzschild solution, described in the Kruskal coordinates , Sch. time runs in the reversed direction as compared to the I - event horizon III-II of the ...
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Could the singularity of a black hole just be an iron / dark matter sphere? [duplicate]

A singularity would break physics wouldn't it? How does a massive star collapse and suddenly fall into a point in space with no dimensions what so ever when it had a core and perfect balance between ...
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If two event horizons approach each other, can I observe the collision?

As Alice is approaching the event horizon, for outsider Bob, Bob will see that Alice's time delayed and slowly stop. I heard that even Bob waits for long time, Bob will never be able to see Alice ...