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.

Filter by
Sorted by
Tagged with
-3
votes
1answer
64 views

Can black holes really evaporate? [duplicate]

Stephen hawkings black hole evaporation theory assumes evaporation of black holes by losing mass through virtual particle radiation. As far as i know, the theory isn't based on mathematical ...
1
vote
1answer
34 views

Teukolsky (Bardeen-Press) equation ingoing coordinates

EDIT: I can't seem to delete this question, so I've posted the solution below (I must have made an algebra error-someone checking all this would still be appreciated!). I've left the question as is. ...
0
votes
0answers
56 views

Unified theory of gravity [duplicate]

I have heard that a lot of times, but I have never completely understood what is a force theory unified with quantum mechanics, and also, why do we need this kind of gravitational theory to fully ...
1
vote
1answer
61 views

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 ...
3
votes
1answer
109 views

Newtons law of gravity universal?

Im a bit puzzled about the video from “One Minute Physics” claiming that Newtons law of gravity is not universal, but breaks down on very heavy objects and are not really confirmed on very light ones. ...
0
votes
0answers
32 views

Relativity paradox with extreme speed [duplicate]

Consider: The supermassive black hole in a galactic core. A neutron star of nearly maximum mass does a very close flyby (remember, galactic black holes don't have the incredibly vicious tides of an ...
0
votes
0answers
30 views

AGN accretion disk double peak explained?

I have a question to the following lecture: https://ned.ipac.caltech.edu/level5/Fabian4/Fab2_2.html In Figure 3 it says that a narrow annuli of a Newtonian disk should alrdy show a double peak ...
1
vote
0answers
26 views

About black holes paradox [duplicate]

About the information paradox in the black holes, isn't it possible in principle, that the "singularity" itself doesn't really exists, and its just the horizon plane, where the particles that falls ...
4
votes
1answer
229 views

Are black holes bound to merge once their event horizons touch? [duplicate]

Even if they move toward each other at relativistic velocities and not head-on? I looked at questions Black hole collision and the event horizon and Dynamics of Event Horizons between head-on ...
1
vote
1answer
114 views

What is exactly rotating in a rotating black hole?

I have read this: https://arxiv.org/abs/gr-qc/9404041 In General Relativity the black hole solutions which have so far been found form a four parameter family called the generalized Kerr-...
0
votes
0answers
21 views

Black hole surrounded by black holes [duplicate]

Is there any configuration of black holes that surround a central black hole that could lift it out of the singularity due to net effects on space-time? Perhaps a sphere of black holes of equal mass ...
1
vote
0answers
73 views

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?
0
votes
1answer
85 views

Silly doubt on penrose diagram for evaporating black hole

This question is based on the naive ideia of evaporation of a black hole stated as, After sufficient Hawking emission the black hole loses it's mass and therefore it's area, until, as a result, the ...
0
votes
1answer
76 views

Black hole mass

It has been taught to us that since the black holes have finite mass, their gravitational force is also finite, which totally fits. The Black hole anyhow has a singularity at it's center which is ...
-1
votes
1answer
32 views

What is the significance in determining the mass of a black hole? [closed]

Was just wondering the importance of the mass of a black hole
3
votes
0answers
52 views

How can the interior pressure of compact objects affect cosmology?

This paper suggests that dark energy concentrated in black hole interiors (they use an unconventional BH model) could act like a cosmological constant. Their claim is that to calculate the equation of ...
3
votes
5answers
286 views

Is it theoretically possible to escape a classical black hole with force?

A stone being thrown from the surface must have a velocity of $11.2$ km/sec if it wants to escape Earth's gravity. However, a stone that has a constant force being applied on it need not have an ...
4
votes
0answers
41 views

Quantum tunneling of dark energy from black holes

There was an article posted recently suggesting there is a very deep connection between dark energy and black holes. https://phys.org/news/2019-09-black-holes-dark-energy.html I was thinking this ...
10
votes
1answer
447 views

Is Schwarzschild's solution in his original paper consistent with current solutions? [duplicate]

I was reading the Schwarzschild's original paper where he derives the Schwarzschild metric for the first time(The english translated version found in arXiv : On the Gravitational Field of a Mass ...
1
vote
1answer
45 views

Does the strong force continue to act upon quarks after they fall into the event horizon of a black hole?

I'm assuming that when particles come in contact with the event horizon, they start traveling directly to the singularity, which is one point that might need clarification. Tangent particles would ...
1
vote
1answer
55 views

How to relate the Schwarzschild time $t$ to the status of the wormhole featured in the Kruskal diagram?

The maximally extended Schwarzschild spacetime as described by a Kruskal diagram features a wormhole which is dynamical. As Kruskal time $T$ elapses, a spacelike hypersurface hits the past singularity,...
4
votes
0answers
123 views

Why can't we accept that there's no black hole? [duplicate]

According to a distant observer, any mass never falls into the event horizon, they just appear to be moving slower towards it. So, why can't we tell people this : there are black holes, but it's just ...
0
votes
0answers
15 views

Can a particle turn into a black hole through acceleration? [duplicate]

If we accelerate a particle fast enough, could it turn into a black hole from our reference frame, due to the relativistic mass and dilation? If so, would it then shed some speed through Hawking ...
0
votes
0answers
53 views

Example of a metric of the form $ds^2 = g_{vv}(v,x)dv^2+2g_{vx}(v,x)dv\,dx+r^2(v,x)d\Omega^2$ with trapped region

Is there a known example of a spherically symmetric metric, especially one describing a spacetime containing trapped region/black hole, which takes the form $$ds^2 = g_{vv}(v,x)dv^2+2g_{vx}(v,x)dv\,dx+...
1
vote
0answers
38 views

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 ...
1
vote
0answers
44 views

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 ...
1
vote
1answer
42 views

Time reversal in Region III in the Kruskal-Szekeres coordinates

Here is a standard Kruskal-Szekeres diagram: [] The region III coordinate time $t$ is constant on lines through the origin going from $t = +\infty$ where regions III and IV intersect backwards to $t =...
1
vote
1answer
92 views

Is the Schwarzschild wormhole (Einstein-Rosen Bridge) time dependent?

In Kruskal-Szekeres Coordinates for the Schwarzschild blackhole, we know that the wormhole "opens up" joining regions I and III between Kruskal times $T = -1$ (vertex of the green hyperbola below) and ...
0
votes
0answers
38 views

Derivation of radial momentum equation in Kerr geometry

I am trying to derive the radial momentum equation in the equatorial plane of Kerr geometry obtained by Lasota (1994) which reads (eqn. 6 in page-343; I am using units in which $M=1$) as follows: $$uu'...
5
votes
3answers
854 views

What happens when a long-wavelength photon “hits” a small black hole? [duplicate]

It's not hard for a black hole to have a smaller Schwarszschild Radius than the wavelength of commonly existing electromagnetic radiation, such as a visible light, infrared, or microwave photon. ...
3
votes
1answer
63 views

Physical significance of angular velocity of orbits around Kerr black holes

For the Kerr metric $$ds^2=\left(g_{tt}-\frac{g_{t\phi}^2}{g_{\phi\phi}}\right)dt^2+g_{\phi\phi}\left(d\phi-\omega dt\right)^2+g_{rr}dr^2+g_{\theta\theta}d\theta^2$$ the angular momentum is defined as ...
5
votes
2answers
93 views

Is the ADM Mass for a boosted black hole $M$ or $\gamma M$?

If you were to take the metric for a Schwarzschild black hole and "boost" it, such that it were traveling with velocity $v$, would the ADM mass, corresponding to a time translation, be $M$ or $\gamma ...
5
votes
2answers
174 views

Blinking flashlight infalling into the black hole, as frequency → ∞

This is my third question from a series were I progressively refine my thought experiment. The others are: Can something (again) ever fall through the event horizon? Time of flashlight crossing the ...
1
vote
1answer
60 views

Black hole spin measurements

While measurements of black hole spin of supermassive black holes in AGN using the X-ray reflection method (as shown in the figure below), 90% confidence error bars on black hole spin and 1$\sigma$ ...
1
vote
2answers
70 views

Do the apparent Black Hole dimensions contract as the observer falls into Black Hole?

According the Special Relativity, all objects contract their length if moving fast. As the observer falls into black hole, his relative speed to it increases up to the speed of light. But from his ...
3
votes
3answers
126 views

Why are primordial black holes considered non-baryonic? [duplicate]

A very important question that has been bugging me is why do people consider primordial black holes to be non-baryonic. I know that such a statement is then used to saying that they can be considered ...
1
vote
0answers
76 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 ...
0
votes
0answers
44 views

How can a non-expert think of Black Hole microstates?

For someone familiar with basic QFT and GR and semi-classical physics but doesn't know string theory nor AdS/CFT is there some intuitive way to think about what the microstate of a black hole means? ...
1
vote
0answers
29 views

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)...
21
votes
3answers
4k views

Can an object tethered to a spaceship be pulled out of event horizon? [duplicate]

An object cannot escape the event horizon by catapulting it outside the black hole. However, what if instead of relying on escape velocity, the object was tethered to a ship orbiting well outside the ...
0
votes
1answer
79 views

Why spacetime fabric don't tears due to mass of heavy black hole?

In GR, All objects create curvatures in the space-time fabric. Why space-time fabric doesn't tear due to the mass of a heavy black hole? What is it made of?
3
votes
0answers
96 views

Black hole nucleus in hydrogen

The hydrogen atom gets the spectrum it has because you analyze the Schrodinger equation in spherical symmetry with the potential given by $V=-\frac{1}{4\pi\epsilon_0}\frac{e^2}{r}$. Yet the same ...
2
votes
0answers
69 views

Is it cold inside a black hole? [duplicate]

What is the temperature inside a black hole? It may be very cold, or very hot. Or just the surrounding temperature. Or maybe neither, because the concept of temperature makes no sense inside somehow?...
3
votes
0answers
51 views

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 ...
-4
votes
2answers
93 views

What is the Equation? [closed]

I am refreshing my Physics skills and I cannot remember what this equation is. Will someone please tell me what this formula is for? It sort of looks like Stephen Hawkings equation for black holes. $$...
1
vote
3answers
122 views

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?
0
votes
1answer
93 views

Greatest relativistc mass possible for a single photon [closed]

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 ...
2
votes
0answers
60 views

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 ...
0
votes
2answers
97 views

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 ...
4
votes
1answer
106 views

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 ...