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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How to prove that any particle trajectories cross the event horizon always perpendicularly? [closed]

I was reading a paper and the author states that "all particles approach the horizon with the same incident angle (that is, they approach perpendicularly) and thus their relative velocities go to ...
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In a black hole, why the speed of a photon matter to it to escape the black hole?

I read that a black hole doesn't attract light but bend it. By that, I understood that the photon is going forward but the space-time is so bent on itself that the photon goes forward infinitely into ...
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Wouldn't Millers planet be ripped to shreds by extreme tidal forces?

If you can survive lava-like temperatures there is another issue for the well-being of Miller's planet from Interstellar. Time passes on the planet 61000 times more slowly than for an observer far ...
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Is the black brane picture valid only at small string coupling?

As far as I am aware there are two pictures of branes: for $g_SN\gg1$: branes as solitonic solutions to the field equations of supergravity (i.e. black branes) for $g_SN\ll1$: branes as fundamental ...
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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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Wouldn't Miller's planet be fried by blueshifted radiation?

In Interstellar, wouldn't Miller's planet be fried by blueshifted radiation? The 61,000x time dilation multiplier would make even cosmic background radiation photons into extreme UV photons. I was ...
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Could we be sucked up entirely by a micro black hole?

The hot question about being ripped apart in a black hole (no one can deny physics is boring!) got me thinking. If you cross the event horizon of a supermassive bkack hole, nothing noticeable will ...
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Is Singularity corresponds to null space and are forces causing singularity are some transformation matrix?

Some random thoughts: (Disclaimer: i am a complete noob in physics). I was studying linear transformations from linear algebra and just got this thought that there might be some set of transformation ...
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Cosmology from within a black hole

Suppose an astronaut is travelling "into" a black hole and has already passed the Schwarzschild boundary (as described in this video). What will the universe look like to this astronaut, ...
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Does an observer experience stronger gravity on the way towards the singularity?

I have read this question: Clearly as you approach the singularity the tidal force on a body increases. Are the fundamental forces unified in a black hole? Based on the questions on this site, ...
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How fast would a clock falling into a blackhole tick relative to the reflection of a clock stationed far away?

Lets say you have two clocks, you attach one to a mirror and drop in into a black hole, while you keep the other next to you, point it towards the black hole and view it's reflection on the mirror. ...
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Why would black hole rip me apart?

According to my understanding of General Relativity, gravity is not a force and an observer which is falling freely under the influence of gravity should be considered inertial. Now, I have come ...
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Given the time dilation at the event horizon plus black hole evaporation, can anything really ever enter a black hole? [duplicate]

There are many thought experiments about what it would be like to fall into a black hole, spaghettififaction, the singularity at the center and so on. But to me it seems that no object could actually ...
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Are there any other significant radii around Schwarzschild black holes besides the four I know of?

A Schwarzschild (non-rotating, non-charged) black hole has an event horizon with radius $r_s = \frac{2 G M} {c^2}$, where $M$ is the mass of the black hole, $G$ is the gravitational constant, and $c$ ...
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How compact can a thin shell be without collapsing?

I've heard of https://en.wikipedia.org/wiki/Buchdahl%27s_theorem. It states that as the ratio between a spheres radius a it's Schwarzschild radius approaches $9/8$. the pressure at the center tends to ...
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Would this method work for generating sustained arbitrarily high time dilation in the center?

Let's say we have a series of a large number of spherical shells. Each shell has twice the mass and radius as the last. There may be thin poles or ropes to make sure inner shells don't move around. ...
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Dropping a mirror into a blackhole

Say you dropped a mirror into a black hole while observing at a distance and holding a clock such that the clock's face was pointing to the black hole. What is the latest time you would view on the ...
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Temperature of a rotating/charged black hole

How do you calculate the temperature of a rotating or charged black hole? How will rotation/charge affect the lifetime?
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How do asymptotic symmetries help to get information about black holes?

I've heard that asymptotic symmetries (like BMS) are used to get information about black holes just by looking at their fields. How does that work?
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What if a cosmic string is eaten by two black holes?

What happens if you have a long cosmic string, each end of which is swallowed by a different blackhole, and the black holes have trajectories that will take them further away from eachother? WHat ...
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What does and observer inside of a collapsing shell observe?

What does and observer inside of a collapsing shell observe? Lets say we have a shell of matter collapsing to a black hole. What would observers near the center see? How would the rest of the universe ...
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Can a human pass through the event horizon of a supermassive Kerr black hole without being shredded? [closed]

Let's say a supermassive Kerr black hole with a mass of $1.78\times 10^8 M_{\odot}$ and a spin of $a \approx 0.99999999999999 M$. What would the tidal forces be on a human crossing the event horizon? ...
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Is the gravitational tidal force equivalent to expanding space?

If you fall towards a black hole, the particles in front and the back of you, in the direction of the center, are accelerating away from you. So, seen from a freely frame, can we say that space is ...
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Conformal cyclic cosmology, black hole and Weyl curvature

Roger penrose in his theory "conformal cyclic cosmology" states that after a google there would be a giant black hole. Does black hole have Weyl curvature? If it does, then we know that ...
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How to calculate Proper Distance as an arc length in Schwarzschild metric?

I am trying to determine the method to calculate proper distance with constant time and radius in Schwarzschild Geometry. With only $\theta$ and $\phi$ being variable. I think it involves integrating ...
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Can Black hole eat white hole or white hole eat black hole? [duplicate]

No light can enter black holes, but is it possible for a white hole to enter a black hole? Or vice versa? Could there be black holes inside the white holes? Or were there black holes inside them that ...
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Where does all the energy in black holes go?

The temperature inside of a black hole is almost absolute zero, so particles inside a black hole have almost zero motion. So if they don't give out any heat or light, where does all the energy it ...
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The Derivation of the Schwarzschild Solution

I went to this site to find the solution. However, I have a few questions about where these equations come from. In the category of Assumptions and Notations, what equation gives you $\partial_tg_{\...
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Is it possible the Black Holes to be pure deformations in the fabric of spacetime and not an effect of super-dense matter?

Is there any theory in the literature that supports this hypothesis that BHs in their center do not have a super-dense matter singularity but are pure deformations in the fabric of spacetime itself or ...
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Smooth vs analytic spacetimes

Recently in more technical settings (I was learning algebraic QFT), I encountered the term "real analytic" manifolds (Lorentzian manifolds, to be precise). This is in contrast to smooth ...
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About the non-intuitive announcement at 12 May 2022 of the EHT team that spin axis of Sgr A* Black Hole facing Earth?

What is this all about?: At 12 May 2022 at the ESO official announcement live streaming event the EHT representatives claimed that the Sgr A* BH accretion disc spin axis is sort of facing Earth's ...
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Is there an equation for retrograde equatorial orbital radii around a Kerr black hole as compared to a Schwarzschild black hole?

A non-rotating, non-charged, Schwarzschild black hole has an event horizon with radius $r_s = \frac{2 G M} {c^2}$, where $M$ is the mass of the black hole, $G$ is the gravitational constant, and $c$ ...
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What does a black hole accretion disk look like edge on?

We have images of two black holes so far, with representations showing a deep black center where in-falling matter vanishes- how would that look side on? Would it be possible to even obtain an image ...
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If an object is accelerated fast enough, will it become a black hole? [duplicate]

I know that if an object is accelerated, its relativistic mass increases. Does this mean that if it is accelerated fast enough, it will become so massive that it becomes a black hole?
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What are the three bright spots in the Sagittarius A black hole picture? [duplicate]

Today the first picture of the Sagittarius A black hole was released. Although somewhat similar to the first picture of the M87 black hole released three years ago, to my eye there's a notable ...
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Why are there three bright spots in the first picture of Sagittarius A*?

Why are there 3 distinct bright spots? The old blackhole picture had half bright and half dark, which I believe is a result of the different relative velocities of particles orbiting it, (towards or ...
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Normal vector of a hypersurface in AdS Schwarzschild Black Hole

Given an AdS-Schwarzschild black hole spacetime in the coordinates $(t,z,x,x_i)$, $$ds^2 = \frac{1}{z^2} \left( -f(z) dt^2 + \frac{dz^2}{f(z)} + dx^2 +\sum_{i=1}^{d-1} (dx_i)^2 \right), \qquad f(z) = ...
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Can the stretched horizon be entangled with an external qubit even if the qubit is unentangled with the black hole?

Suppose at time $t_0$, we have a black hole unentangled with an external mass and an external qubit $\left( |0\rangle+|1\rangle \right)/\sqrt 2$. We then perform a controlled gate operation on the ...
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Calculating divergence and flux of geodesic word lines

Given a family of neighbouring geodesic word lines, is there a way of calculating properties such as their divergence or flux? maybe by converting the tangent vectors of the world lines to a vector ...
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Could bremsstrahlung provide evidence that a Black Hole (BH) forms the underlying geometry of the electron?

There are several proposals that model electrons as BHs. For example, A. Burinskii proposes a spinning soliton on a Kerr-Newman BH geometry (enhanced to an MIT bag model in “New path to unification of ...
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Fundamental-ness of fundamental forces

Example : Consider a black hole and it's event horizon of radius R surrounding it . Suppose in the direction of a diameter of the black hole ( or event horizon) , there are two charged particles. ...
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Carroll's derivation of Hawking radiation

In Carroll's Spacetime and Geometry: An Introduction to General Relativity, in his derivation of Hawking radiation Carroll makes the following statement: "as observed over length and timescales ...
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Schwarzschild-deSitter horizon singularity

The Schwarzschild-deSitter spacetime in Euclidean signature is given by: $$ ds_{E}^{2}=\left(1-\frac{2M}{r}-\frac{H^{2}r^{2}}{3}\right)d\tau^{2}+\frac{dr^{2}}{\left(1-\frac{2M}{r}-\frac{H^{2}r^{2}}{3}\...
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A variation of a Bekenstein's thought experiment

Consider a glass of water with mass $m$ and temperature $T_w$ released very close to the black hole horizon. The black hole being at temperature $T_{b} = \frac{1}{8\pi G M}$. Now, the final state is a ...
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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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Do the apparent infinities at the center of black holes disappear if instead a phase-change takes the place of its singularity?

When I watch physics documentaries that discuss black holes, they talk of impossible infinities in the singularity at a black hole's centre. I was wondering about the bubbles of water vapour in ...
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Help in understanding how general relativity describes space-time near black hole poles that emit astrophysical jets

I am aware that General relativity does not explicitly deal with 'expanding' or growing and due to that reason 'cosmologically pressurised' aether (proposed by the 'Theory of a river flow' but that ...
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Current status and reference request for the Black Hole Stability Conjecture

The Black Hole Stability Conjecture is one of the most important open problems in the field of General Relativity, and remains open in spite of the advances in the past half century. To test Einstein’...
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Why are we so sure that there is a singularity inside the event horizon of a black hole? [duplicate]

The Schwarzchild solution is derived in vacuum, and we find that light cones always move inward when r < 2M. So if a spherically symmetric, non rotating celestial object of mass M has a radius less ...
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Kerr Solution metric ansatz for EFEs

The Schwarzschild metric ansatz is given by $$ds^2=-A(r)dt^2+B(r)dr^2+r^2d \Omega^2$$ where upon applying the Einstein Field Equations $$G_{\mu\nu}=\kappa T_{\mu\nu}$$ we obtain the normal ...
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