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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What is inside the Schwarzschild radius of the Sun?

According to https://www.britannica.com/science/Schwarzschild-radius, the Schwarzschild radius for a black hole with the mass of the Sun is about 2 miles. My question is what is inside of it? Is ...
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Change in area of a disk near black hole?

I know that the length contraction can be determined by the formula: $L\sqrt{1-\frac{2GM}{Rc^2}}$ But how can we calculate the decrement in area due to the black hole?? Suppose we have a disc of area ...
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What does Schwarzschild refer to as Einstein's approximation regarding his exact solution?

In 1916, Schwarzschild published his $R$-metric solution that differs from the $r$-metric solution we are all familiar with. The relation between $R$ and $r$ is $R^3=r^3 + α^3$ with $r$ been the ...
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Do “almost black holes” exist?

The only things I read about so far in astrophysics are either black holes, developing black holes or not black holes at all. So I am wondering, is it physically possible to have an object that is ...
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How do astronomers detect the spin of the black holes?

There are various theoretical models for accretion disks and relativistic jets from black holes that considers the black hole to be either rotating (Kerr geometry) or non-rotating (Schwarzschild ...
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How can we see black holes form and gain mass if nothing can cross a horizon (when viewed from our reference frame)? [duplicate]

As things approach the horizon of a black hole, their time dilation approaches infinity. How then can we observe black holes gaining mass if, according to our reference frame, nothing can pass the ...
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Charged Black Hole in 1+2 dimensions without cosmological constant

Does a charged black hole in 2+1D exist? I am interested in a theory of the form: $$ S = \int d^3 x \sqrt{-g}\big(\cfrac{R}{2} - \cfrac{1}{2}F^2\big) $$ where $F^2 = F_{ab}F^{ab}$. The field equations ...
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Are the original Schwarzschild solution and Hilbert's solution exactly the same?

Schwarzschild original metric solution can be found here in equation (14): http://old.phys.huji.ac.il/~barak_kol/Courses/Black-holes/reading-papers/SchwarzschildTranslated.pdf Hilbert's metric ...
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Charged Black Hole: Self-Discharge

Palti writes in his introduction to the swampland (page 39) that a charged black hole (BH) could discharge itself, when the Hawking temperature $$ T_H=\frac{R_+-R_-}{4\pi R_+^2} $$ ($R_+$ and $R_-$ ...
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How do black holes get their magnetic fields?

I imagine the magnetic field comes from the accretion disk. But what causes it to "spring" upward?
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What would happen to a particle caught exactly in the middle of two merging black holes both of which are also at the ends of their lives?

If the two tiny, about-to-vanish black holes are of the same mass then such a particle should not go anywhere and remain stationary in the middle even as it gets enveloped inside the new unified event ...
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What happens to a hydrogen atom under an intense gravitational field (or intense acceleration)?

I'm curious how the observed quantum-mechanical properties of a hydrogen atom would change under very high gravitational fields. Do the energy levels change? Do the chemical bonding properties ...
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Is it possible that a black hole has a universe in itself? [duplicate]

Every black hole has a singularity. But the singularity is covered by a thing called 'Event horizon'or ' Schwarzchild Radius'. Our universe is originated from a naked singularity. By naked singularity,...
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Child Universes

I was watching a program on the future of the Universe and it got into the eventual heat death resulting from Hawking Radiation as the last hurdle till nothingness (assuming the unproven theory of ...
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Intuition for the Ricci tensor of a Reissner-Nordström black hole

The Reissner-Nordström metric is given by $$g = -h(r)\,{\rm d}t^2 + h(r)^{-1}\,{\rm d}r^2 + r^2\,{\rm d}\Omega^2,$$where ${\rm d}\Omega^2$ is the round metric on a unit sphere $\Bbb S^2$ and $h(r) = 1-...
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What happens if a magnetar collapses into a black hole?

Assume there is a neutron star with a magnetic field and no net electrical charge. What happens to the magnetic field if it gains some mass and collapses into a black hole?
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How does a quasar generate its magnetic fields?

Obviously black holes don't have magnetic poles so that must mean that the electromagnetic fields are generated by the accretion disk. The charged particles would be pulled towards the black hole via ...
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Are black holes perfect heat sinks?

Imagine a closed system with a black body and a black hole, where the black hole has a lower temperature. The black body will radiate heat and the BH will absorb that heat, gaining mass in the process....
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How can I convert a rate of black hole mergers into an estimate for the number density of black holes?

There's an estimate from the recent detection of intermediate-mass black hole mergers that there are roughly $0.13^{+0.3}_{-0.11} Gpc^{-3} yr^{-1}$ similar events a year. I'm wondering how to convert ...
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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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How do they calculate the distance merging massive bodies are away from earth using just the gravitational waves? [duplicate]

I guess the title sums up my question nicely. So, how do physicists/cosmologists/scientists determine the distance two or more merging bodies are away from earth given just their emitted gravitational ...
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What do black holes spin relative to?

What do black holes spin relative to?In other words, what is black hole spin measured in relation to? Spinning black holes are different from non-spinning black holes. For instance, they have ...
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Does the gravitational distortion of a black hole depend of its rotation?

I would like to know if there is a difference produced by the added spin of a black hole to its gravitational space-time distortion. I am considering the distortion of space-time from a point far ...
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Schrodinger's cat experiment in a black hole

Imagine Alice falling into a black hole with a Schrodinger's cat experiment setup. after passing the event horizon towards the singularity she performs an observation to see if the cat is dead or ...
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How to derive this relation?

According to A relativist's toolkit by Poisson, the expansion of null radial geodetic in the Schwarzschild spacetime is $$\theta=\dfrac{2}{r}$$ How to derive this expression? The expansion is defined ...
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Stress-Energy Tensor of Reissner-Nordstrom solution

I am trying to derive the R-N solution and i am following Blau's notes (to be found here http://www.blau.itp.unibe.ch/newlecturesGR.pdf) pages 677-679. With the same metric ansatz: $$ ds^2 = -A(r)dt^2 ...
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What happens to an astronaut (with a long rope trailing behind him), when he crosses the event horizon of a black hole?

Let's assume there is an astronaut with a very long rope trailing behind him. As he approaches a very large black hole, he can look back and see the rope behind him trailing off into the distance. ...
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If all that existed was a supermassive black hole and yourself would you ever have knowledge of its presence, how could you detect it?

So this question is a two-parter. To elaborate on the first question, let's say you are 10 light-years away from this black hole and it is the mass of the one in the center of our galaxy. 2nd question:...
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$2$nd law of thermodynamics of an isolated system as it passes into a black hole?

So I'm quite confused by this. Let's say I have $2$ friends $A$ and $B$ with metrics: $$ ds_A^2 = -dx_1^2 + dx_2^2$$ and $$ ds_B^2 = dx_2^2 - dx_1^2$$ Ideally I would imagine a symmetry in their ...
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Introductory References for the Black Hole Information Paradox

I am looking for introductory references for the Black Hole Information Paradox and trying to compile a sequential list through which someone familiar with the basics of QFT and GR can go to ...
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Creating a black hole with explosives? [closed]

Would it be possible to compress enough matter into a small enough space using explosives, using roughly the same concepts as a nuclear bomb? of course, this couldn't be done with traditional ...
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Hawking Radiation and Information loss

The Gravity program I watched brought something really interesting to my awareness. With black-hole Hawking radiation... everything gets sucked into the event horizon... we don't know what happens ...
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Does the apparent radius of the event horizon of a black hole depend on the distance of the observer from the black hole?

Does the apparent radius of the event horizon of a black hole get smaller and smaller as an observer approaches a black hole? If this were true, it would seem to allow communications to be relayed ...
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How does String Theory calculate the entropy of a black hole in a background-dependent way?

How does string theory get an agreement to the Hawking-Bekenstein equations to calculate the quantum entropy of a black hole in a background-dependent way? Is there any sort of area parameter in ...
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Why don't baby black holes give off a lot of Hawking radiation and vanish before having the chance to grow?

Is it because they eat more than they eject? Or something else? Edit ------- I mean black holes which have just started forming from stellar collapse. Should had mentioned this. Edit 2 ----- When you ...
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Would an interior inflation affect the size of the event horizon of a black hole?

Lets imagine that the interior of black holes are not singular, and they undergo an inflation. I know no information can escape a black hole, but the event horizon can grow in size, for instance, by ...
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How big should a star be to turn into a black hole?

My initial calculations show that if the radius of a star $-$ with a uniform mass density of $\rho$ $-$ is greater than $\frac{c}{2\sqrt{\pi G\rho/3}}$, the star would collapse into a black hole. ...
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Interpretation of $t$ coordinate for charged black hole in Kerr-Schild form

In GP coordinates, the $t$ coordinate matches the proper time of a free-falling observer starting at infinity with zero velocity. Now consider the Reissner–Nordström metric in Kerr–Schild form: $$ g_\...
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Schwarzschild solution derivation and determinant of the Schwarzschild metric tensor

At some point we are looking for the solution of the Einstein field equation for the Schwarzschild gravitation from $$ds^2 = A(r)dt^2 +B(r)dr^2+r^2d\theta+r^2\sin^2(\theta)d\phi^2\tag{1}$$ I am ...
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What determines intensity + range of a gravity field?

Related questions.. How does the Sun's gravity extend out millions of miles to influence the far reaches of our solar system? Reference: movie - "Interstellar", 2014 Nolan) / How does a ship ...
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If the photon is the force-carrier for the electromagnetic force, how does the electric charge of a black hole escape the event horizon? [duplicate]

When people speak of the electric charge of a black hole, do we actually mean it affects things outside of the event-horizon or is it just a property it has?
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Can a particle just below the event horizon of a tiny, fast shrinking black hole make a comeback?

A particle has just entered a minuscule (and non-rotating) black hole which is at the end of its life. The black hole is getting smaller really fast as it boils away ejecting tonnes of Hawking ...
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Why the horizon increases in the formation of blackhole with radiation

I am reading the lecture notes given by Professor Leonard Susskind. It's said the horizon appears and gets bigger as the in-falling photon sphere further falls in. Photon sphere here is a falling ...
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If I fall into a black hole, can I affect the event horizon after I pass through it?

I'm holding two very massive, dense objects, one in each hand at arms length. I fall through the event horizon of a very large black hole. The tidal forces at the event horizon are not large, so I ...
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Curvature of space near a black hole

(Rigorous theorists will have to forgive my phrasing on this question, I’m new to GR and most of my physics training is in quantum mechanics) In non-Euclidean geometry, we can speak of a spherical ...
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Physical Size of a Naked Singularity [closed]

I have watched a few videos about naked singularities, and it is my understanding that one is a singularity without an event horizon. My question, is could you ever have that with anything larger ...
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What is the physical shape of a rapidly spinning singularity?

Let's say I have a star 20x the mass of the sun. At the end of its life, it collapses into a black hole. Now correct me if I am wrong, but as it collapses it rotational speed dramatically increases ...
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Black Hole Formation

If you plot a space-time diagram of an object falling through the event horizon of a black hole, and draw the past and future "light cones" of the object at every point, wouldn't the point ...
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Existence of parallel vector field

Is there any known parallel vector field in a Schwarzschild spacetime? Or any method to identify parallel vector fields in any spacetime, given the metric $g$?
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How does Zerilli come up with this solution?

I am studying the paper "Gravitational field of a particle falling in a Scharzschild geometry analyzed in tensor harmonics" by Zerilli. The author calculates the gravitational radiation ...

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