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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The change in mass of a black hole do to Hawking Radiation

Hawking radiation which is the result of virtual particles, which are always created in pairs,(particle/antiparticle) near enough to the event horizon of a black hole, that one of the pair falls in ...
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What is the Equation?

I am refreshing my Physics skills and I cannot remember what this equation is. I am hoping that someone can help me. It sort of looks like Stephen Hawkings equation for black holes. $$ S = \frac{C^3 ...
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Schwarzschild Radius of a Galaxy

If an ultra compact/dense Galaxy has a Schwarzschild radius same as it is own radius, how can it be observed from the outside of the Galaxy?
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Greatest relativistc mass possible for a single photon

I have been told that photons can not be blue shifted to the point where they become black holes, although a photon with a Planck scale wavelength would also have a relativistic mass equal to a black ...
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Event horizon area and entropy content of a black hole

I am aware that the entropy content of a black hole is proportional to the area of its event horizon, and that this can be boiled down to the bit content of a Planck area on the horizon, but I have ...
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Can anyone explain the Planck area?

First of all, I am not an expert. I was reading about the holographic principle and came across the Planck area. It says that Planck area is the square of Planck length and there were some pictures ...
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Are black holes affected by tidal forces?

I've recently read about the last parsec problem, which states that supermassive black holes should not be able to merge within the current age of the universe. Once they reach a distance of about one ...
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Is there an equation that describes the spacetime inside an event horizon?

For instance, how far would a photon travel once passing the event horizon until it meets the singularity. What is the circumference of an orbit as a function of the distance to the singularity?
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Inside the singuarity [on hold]

This thread may be off-stream physics but it is related to a very interesting subject . What happens to the centre of a black hole ? Do quantum effects become dominant or is GR totally correct? ...
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A few questions about white holes

White holes are seen in the maximally extended Schwarzschild solution, they are a region from the past that appear in the theory of eternal black holes, where the singularity exists in both infinite ...
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A quick clarification of Hawking Radiation

Good Morning, I have read the other posts on Hawking Radiation, and did not want to post a question to an older post... I just want a quick clarification of a point. Can you please clarify is this ...
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Black hole in timespace: point or “trench”? [duplicate]

I wonder what is the nature of a black hole as set of points of manifold. A single point (with infinite curvature) or a "trench" ? I thought that it is a "point", but since black holes can collide and ...
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Define event horizon using only the notions of events and causality

Does this work? Consider a set $B$ of events which satisfies If $x$ belongs to $B$ and $x$ causes $y$ then $y$ belongs to $B$. The event horizon of $B$ is the set of events that are not in $B$ but ...
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Does general relativity predict that the gravitational field of a black hole would be uniform about its center of mass?

Black holes are typically represented by extreme distortions of space, where space is so bent, that it reaches a single point (i.e., the singularity). Further, they are almost always depicted as ...
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When looking for a black hole, will we always find a collapsing star instead?

I know that from the point of view of an outside observer, the collapse of a star into a black hole never finishes, i.e. any observer will always see a (red-shifted) version of the star before its ...
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Feynman diagram for Hawking radiation?

I'm starting to wrap my head around Feynman diagrams, and the idea of "real" vs. "virtual" particles, but one area where this distinction seems to break down is in describing Hawking radiation, where ...
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Event-Horizon Shape in a Binary System

I understand that a solitary black hole has a spherical event horizon. I was wondering whether this still stands in a binary system of two (let's say equal mass) black holes? Could the proximity of ...
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Is the mass needed to create a black hole greater if the black hole is moving? [duplicate]

So my argument goes like this: say we have an object that is nearly at the density required for a black hole. Then we Lorentz boost into a new frame, the object is Lorentz contracted and therefore ...
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Outer-Minimizing Property in Majumdar-Papapetrou Slice

In the canonical space slice of the Schwarzchild spacetime, which is the manifold $\mathbb{R}\times S^2$ with the metric $ds^2=\left( 1 -\frac{2m}{r} \right)dr^2 + r^2 d\omega^2$ it is well known ...
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Black hole's accretion disc and loss of information

in popular science, the concept that information is lost beyond the black hole's event horizon is put forward. But we know that black holes also have a mass, which is a type of information, and they ...
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Four-velocity of radially infalling gas in Schwarzschild metric

I am reading this paper by Thorne, Flammang & Zytkow (1981), which discusses the dynamics of spherical accretion onto a black hole in the Schwarzschild metric ($c=G=1$ units): $$ds^2=-(1-2M/r)dt^...
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“Black hole spins at $X$% of the speed of light”, what does that mean?

I've seen a few news stories recently (example, example) about some black holes spinning at X% of the speed of light. What does that mean? What exactly is moving at that speed, and with respect to ...
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A question about black hole interior

I am reading Maldacena's paper "Eternal black holes in anti-de Sitter". In the first paragraph, he wrote something about the black hole interior (in AdS): The regions close to the spacetime ...
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Can two black holes on a collision course, escape the merger?

Let's consider in this hypothesis that the two black holes have similar masses and approximation speed 0,99 c. At the maximum approximation point, only their event horizons are invaded slightly. The ...
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Taylor Series Expansion [closed]

If $~f(x)~$ is any function and $~x~$ is the inverse function of $~z$, how will we Тaylor expand $~f(x)~$ in $~z~$? E.g., in the Gravitational lensing by a regular black hole, how to expand the eq $(...
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What happens if you poke a black hole with a stick? [duplicate]

Suppose I'm hovering 1 meter away from the horizon of a Schwarzschild black hole and I'm holding a 2 meter long metal rod. Now I thrust the rod towards the horizon? What do I observe? A satisfactory ...
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Big crunch scenario, would black holes form?

Imagine a big crunch scenario in which everything (or almost everything as for I have other doubts, eg about entropy...) rolls back. Shouldn't the contracting universe reach a point of density at ...
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Does an evaporating black hole violate conservation of angular momentum?

Angular momentum is supposed to be conserved, but when a rotating black hole evaporates the Hawking radiation comes out in straight lines. Doesn't this violate conservation of angular momentum? Does ...
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Could you extract energy from a rotational white hole?

I understand it is possible to do it in a black hole, and its also possible to reach the event horizon of a black hole and increase its size. But in a white hole, no wave (electromagnetic or ...
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Why radial coordinate of a particle must decrease continuously once it is inside the Schwarzschild radius?

Suppose we are inside the Schwarzschild radius of a black hole and throw a ball radially outward. It is said that the ball has no possibility to increase its radial coordinate. It must continuously ...
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How do we know we cannot escape the event horizon?

A doubt assails me: deepening the Schwarzschild and Kerr space-times I found a beautiful article of the University of Rome, which provides a method to identify an event horizon based on the analysis ...
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Evaporating black hole - slower than expected by simple description [duplicate]

In the simple description of how a black hole evaporates, one particle of a pair that spontaneously forms forms inside the event horizon and the other outside. The one inside can’t escape. The one ...
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On what parameters does the Event Horizon depend on?

In the Schwarzschild metric, I know it depends only on mass. But in rotational black holes, it also depends on the angular momentum. I know hawking radiation can decrease the mass of the black hole, ...
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What are Quasi-Periodic Oscilations and how can they be used to determine the mass of black holes?

Quasi-Periodic Oscillations (QPOs) are observed in almost all accreting systems and they provide an essential tool to explore accretion flows from compact objects. But I couldn't understand what ...
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Supertranslations & Superrotations and black holes

I am currently (attempting to) the black hole memory effect and I am really struggling to find introductory explanations which simply state “this is what is meant by a supertranslation on a black hole ...
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CFT correlators and effective string picture

I have been reading https://arxiv.org/abs/hep-th/9702015 by Maldacena and Strominger. Authors derive emission rate of Kerr-Newmann black hole via standard asymptotic matching first. Then rederive ...
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How do we get the image of a black hole? [duplicate]

According to Einstein's theory, nothing can travel faster then light and also a black hole doesn't allow light to escape but recently scientists got an image of a black hole. Here I am confused: if ...
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2nd law of thermodynamics inside an event horizon?

Since time and space switch roles inside a black hole, does entropy inside a black hole still need to increase? If not, maybe something very strange could pop into existence, only to be crushed ...
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BH evaporation vs CMB

This is not a duplicate, I am not asking about how hawking radiation works or how CMB works or whether BHs can really evaporate, or not. My quesion is about how CMB (considering space expansion's ...
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When we are talking about black hole evaporation - what exactly happens?

According to Wikipedia: Hawking radiation reduces the mass and energy of black holes and is therefore also known as black hole evaporation. Because of this, black holes that do not gain mass ...
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To a distant observer, is the trajectory of an object falling toward a black hole always reversible?

I think (pure intuition) that every object that falls towards a massive center, in the condition of free fall, should always be able to return to the starting point. Even if this massive center is a ...
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Queuing by the event horizon

I understand that an observer outside the event horizon (EH) of a black hole (BH) will not see anything disappearing from outside the EH - only the effects of the time dilution near the EH. Assume ...
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Mass distribution inside inner horizon of a charged black hole

I am trying to understand the spatial geometry inside the inner horizon of a charged black hole. I assume the charge accumulates exactly on the inner horizon (https://jila.colorado.edu/~ajsh/bh/rn....
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Spin connection Kruskal space-time

Would anyone give me some references where I can find the components of the spin connection in Kruskal space-time?
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Landau Lifshitz' field theory book question

I am currently reading the Landua-Lifshitz' field theory book when I came across a concept I don't quite fully understand. In Section 100, pg $323$ (The Centrally Symmetric Gravitational Field) after ...
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Four-momentum of a photon emitted from an accretion disk

Consider a non-rotating compact object surrounded by a revolving accretion disk. A distant observer sees the disk in an edge-on position. The center of the disk may be taken as the origin of $r-\theta-...
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Deriving Kretschmann scalar for Schwarzschild solution

I'm trying to derive kretschmann scalar for schwarzschild solution, which is \begin{equation} K=\frac{48M^{2}}{r^{6}} \end{equation} I know I have to compute $R_{abcd}R^{abcd}$, but it seems like an ...
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Would a micro black hole keep oscillating around the center of Earth forever?

If a micro black hole was dropped from the surface of the Earth, and that it was too small to absorb particles and eat the Earth, it would fall until emerging from the other side, then fall again and ...
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Time dilation in black hole collisions

I understand the time dilation around a black hole to imply that when an object touches the event horizon of a black hole, the signals sent from the object will take infinite time to reach an observer....
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Would a micro black hole have enough energy to heat and liquefy the Earth from within without consuming it first?

Let's say an alien race drops a microscopic 72135 metric ton black hole into the center of the earth. It's Schwarzchild radius is too small (only 0.0001071103 femtometers) so it won't actually be able ...