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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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Modelling a matter dominated universe collapsing into a black hole

With the FLRW equations we can get solutions for a matter dominated closed universe in which the finale is an ultimate collapse, but this is only in terms of $a$ (the scale factor) and $t$ (time) and ...
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What is the minimum physical distance between two electrons?

If electrons repel, then it seems to me that energy is required to decrease the distance between two electrons. Since energy and mass are equivalent, adding energy to a system also adds some mass. ...
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What is the total mass of the accelerated viewpoint particle atmosphere of a black hole?

Kip S Thorne's "Black Holes & Time Warps", 1994 paperback, p.443, just above Figure 12.5: Surprisingly, from the accelerated viewpoint, the vacuum fluctuations consist not of virtual particles ...
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To what extent are the astronomically observed black hole candidates compatible with GR black holes?

Do they all fit Schwarzschild black holes? How people compare them with more complicate BH solutions as spinning BH solutions (even if they are not known analytically), say. I'd like more than ...
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What do the components of light velocity look like in polar coordinates?

The Schwarzschild solution makes use of polar coordinates, and I'm wondering how the different components of velocity of light change with the position. Might I get some examples of light velocity ...
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144 views

Entanglement and Black holes

If you have two entangled quantum states, One state falls into a black hole and you measure the other state, What can you say about the state that has fallen into the black hole? If you have billions ...
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Pair production intefering with gamma-ray laser black hole fabrication

A common "proposal" to make a micro black hole is to use on the order of 10^12 kg of gamma-ray lasing medium and focus all the light at a small point. However, intense light will interact with itself ...
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184 views

Falling into a black hole emitter vs observer

Let's say we are working with the Schwarzschild metric and we have an emitter of light falling into a Schwarzschild black hole. Suppose we define the quantity $$u=t- v$$ where $$dv/dr= 1/(1-r_{s}/r)$$...
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transition between extremal and nonextremal black hole states

Extremal black holes are at zero temperature, hence they do not radiate. my question is twofold: 1) is extremality of micro black holes a stable property? electric charge is quickly emitted from sub-...
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739 views

Nonlinear refraction index of vacuum above Schwinger limit

This question is more about trying to feel the waters in our current abilities to compute (or roughly estimate) the refraction index of vacuum, specifically when high numbers of electromagnetic quanta ...
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1answer
55 views

What and why is quantum information?

Can someone explain to a muggle like me what is "Quantum Information" concretely? I'm asking this because I don't get it why the Hawking radiation "destroying information" is so controversial. Quoting ...
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162 views

Observation of Hawking radiation

Is it conceivable that Hawking radiation could be observed using e.g. gravitational waves or imprints in the CMB?
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Is all of a black hole's mass situated arbitrarily close to the event horizon?

Forgive me if I'm thinking about black holes in completely the wrong way, but since time dilation increases to arbitrarily large amounts the closer you get to the event horizon of a black hole, wouldn'...
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190 views

Would the detection of advanced waves be possible in curved space-time?

In the transactional interpretation of quantum mechanics reference 1, inspired by the Wheeler–Feynman absorber theory, a transaction is formed between the emitter and absorber by a superposition of ...
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1answer
257 views

Travelling through a worm hole

In every documentary I've seen about worm holes, I have noticed that they usually say that according to the General Theory of Relativity, there could exist worm holes called traversable wormholes; ...
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Can we measure the amount by which LIGO black hole mergers warp space itself, relative to the mass of the merger?

In our day and age we can calculate the mass and maximum volume of a black hole. With LIGO we can also now calculate by how much is space warped, relative to the distance to a black hole merger, such ...
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What would the eternal black hole look like?

The white hole and black hole regions in a Kruskal diagram are said to be actually two different locations. Given the problems with white holes it might be a silly question but, hypothetically, what ...
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Measurement postulate and black hole information paradox: are they related somehow?

The measurement postulate states that whenever we make a quantum measurement, we select (projection) from the general superposition state a single pure state. Thus, as a general quantum state is a ...
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What would be the properties of an anti-black hole (concentration of negative mass)?

What would be the properties of an anti-black hole (concentration of negative mass)? Would it emit radiation? Would time go infinitely fast on its surface?
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Measurement of time span of a free fall into a black hole

Observations of events seen from different reference systems always confuse me: A good example is the free fall of a particle into a -- for simplicity -- non-rotating black hole. In the reference ...
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How long does it take for the image of an object falling into a black hole to disappear?

From what I've read on other questions and answers, an outside observer cannot see an object fall into a black hole, but rather the image of the object will look like it is stuck on the surface of the ...
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75 views

Entanglement of Black Holes with the outside universe

Black holes are thermal objects which means the density matrix of the black hole is in a thermal state. This means that the black hole has to be maximally entangled with the rest of the universe to ...
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How does the connection of constant-phase surface in Schwarzschild spacetime work?

I am stuck while following the calculation of constant-phase surface of Hawking radiation in Hawking's original paper (Particle Creation by Black Holes, S.W. Hawking, Commun. Math. Phys. 43, 199 (1975)...
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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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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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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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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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61 views

Hawking temperature, uncertainty principle and size of a black hole

I was playing with the Hawking temperature formula $$T_H = \frac{\hbar\ c^3}{8 \pi\ G\ M\ k_B}$$ and I thought it would be interesting to associate a velocity to this temperature: $$k_B\ T_H = \frac{1}...
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Raychaudhuri equation for black holes

Since the Raychaudhuri equation is defined only for timeline and null geodesic congruences, is it valid to use his equation to describe the null generators of the event horizon of a black hole? ...
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Can baby universes solve the information paradox?

My question is do baby universes give us a satisfying explanation of the black hole information paradox without violating thermodynamics, or any other established/known physics? My understanding is ...
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Violation of the Law of Conservation of Charge in black hole?

Hey I'm a noob to physics however after reading about black holes and Hawking radiation. If a black hole made entirely out of negatively charged particles (say this black hole is made entirely of ...
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40 views

Gravitational $N$-body mergers?

Beyond binary systems and its mergers, have numerical relativity groups computed an approximation to $N$-body GW signals/waveforms or is that a hard unsolved task?
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Proper time of an object crossing event horizon in Kruskal coordinates

So I am reading a paper on a certain black hole paradox. The specifics actually don't matter, but if you want context (p16): black hole thought experiment. An object falls into a black hole. The ...
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Do black holes have a maximum temperature?

"Standard" black holes have usually a very low temperature if we assign their temperature to the Hawking radiation they are expected to emit and compare it to the one of an ideal black body. For a ...
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2answers
71 views

Is the potential energy of falling into a black hole already included into $E=mc^2$?

Each object in the Universe has a huge potential energy, because you can drop it into a black hole, accelerating it to nearly the speed of light. Is this potential energy already included into ...
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What mechanism in the tidal effects around a black hole is it that destroys materia?

I just read this Quora question https://www.quora.com/Why-doesnt-NASA-send-a-probe-into-a-black-hole-to-learn-more and although the basic premises for the question are flawed (travel time is ...
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1answer
40 views

Metric for accreting spherically symmetric spacetime?

I know this may be an over-simplification of the system but can we treat the metric below as something that represents accretion? $$\mathrm ds^2 = - \left(1-\frac{2m(t)}{r}\right) \mathrm dt^2 + \...
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Relativistic doppler effect vs Gravitational Redshift

I recently found a pretty unknown but remarkably clear and simple article (Cochran, 1989) on different doppler shift relationships for an uniformly accelerated observer in a 1+1D flat spacetime, using ...
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If I were to shake a black hole, does it grow bigger?

Imagine I were to shake a black hole rigorously, does it grow bigger? I think it will simply radiate all the kinetic energy gained from the shaking as gravitational wave right? In the normal case, if ...
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What's the difference, if any, between Soft Hair & Quantum Hair

In the early 90s, John Preskill, Sidney Coleman, Frank Wilzcek and Lawrence Krauss presented a series of papers [1][2][3] on Quantum Hair on Black Holes due to Cosmic strings in a number of ways ...
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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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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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1answer
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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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Where this interpretation for the field modes comes from?

I'm reading the book "Modeling Black Hole Evaporation" by Alessandro Fabbri and Jose Navarro-Salas, and in section 3.3.2 they talk about wavepackets at $\mathscr{I}^+$. It all starts like this: one ...
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Could a black hole and a white hole cause gravitational waves, if orbiting one another?

I have been puzzled by an idea that I came across while reading about white holes. But I've only just started learning about white holes, so my knowledge is severely limited. It seems I also can't ...
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Time Dilation and Particle Decay

I am teaching myself astronomy and I'm working on a basic problem regarding time dilation around black holes. The question is basically this: if a neutron were ejected from a nucleus at a distance of ...
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Advanced and Retarded Coordinates for Hawking Radiation

I am trying to solve the Klein Gordon equation near the horizon of a black hole. For a more accurate result I am using the Vaidya Metric in which the mass is a function of retarded time u. When ...
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Gravitational redshift discrepancy?

I want to compute the redshift of a signal emitted by a static observer in $r=R_1$, $\phi=\phi_1$and recieved by another static observ at $r=R_2$, $\phi=\phi_2$ with $R_2>R_1$, in Schwarzschild ...
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Would a cellular-automata universe allow singularities to exist?

It is usually assumed (almost always) that cosmological singularities (like black hole singularities) are impossible and do not exist in the universe. If the universe was like a cellular automata (...