A black hole is a volume from which photons, or any matter, can not 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.

learn more… | top users | synonyms (1)

3
votes
1answer
85 views

The Riemannian Penrose Inequality in higher dimensions

I am reading the proof of the Riemannian Penrose Inequality by Huisken and Ilmamen in The Inverse Mean Curvature Flow and the Riemannian Penrose Inequality and I was wondering why they restrict their ...
4
votes
3answers
399 views

Zero $g$ in a black hole?

At the very centre of the Earth, masses cancel each other out, creating an effective zero-$g$ environment. Would the same happen in a black hole?
12
votes
3answers
1k views

Why does a singularity need to exist at the center of a black hole?

Sorry if this is a very basic question, but I am not a physicist, but this is something that has always intrigued me. I have read that the properties of a black hole are its mass, spin and angular ...
10
votes
3answers
763 views

Using a black hole as a mirror?

http://www.youtube.com/watch?v=ENd8Sz0AFOk The YouTube video is a good example how the gravity of this merging binary black holes bend light around themselves.
1
vote
1answer
97 views

Black hole merger orbital speed

If a ship is close to a black hole and we see it getting slower to the point if could see them not for red shift would almost stop moving. Then how fast is the little black hole moving in this video? ...
0
votes
1answer
53 views

See behind the black hole

Why in this video does the 2nd black hole appears to change size and appear larger the farther away it gets? How can you see behind it? http://www.youtube.com/watch?v=ENd8Sz0AFOk
0
votes
0answers
67 views

Black hole and relativity [duplicate]

This link shows movement around the black whole. It seems impossible for them to move this fast. http://www.youtube.com/watch?v=ENd8Sz0AFOk If we were able to watch a ship of people go into a black ...
1
vote
0answers
14 views

Effect on mass near a black hole [duplicate]

When an object falls inside the event horizon with already huge terminal velocity directed towards the black hole, the gravity must accelerate to near light speeds which should almost increase its ...
1
vote
1answer
50 views

Do zero gravitational-force points produce Naked singularities?

In between two singularities, does a particle experience gravity? If a particle is exactly at the midpoint between two singularities, will its net experience of the gravitational force (ignoring the ...
1
vote
0answers
62 views

Not even light can escape black hole due to it's pathetic speed or infinite time dilation or extreme curvature of space? [closed]

The correct explanation that why black hole is black must be one of the following: (1) Escape velocity of black hole exceeds speed of light like a pitched baseball can never go into orbit. (2) From ...
1
vote
0answers
43 views

Gauss-Bonnet correction to Carnot's efficiency

Could any one tell explicitly, how did the author get $Tc$ Vs $\alpha$ Plot given in Figure 3 in Gauss-Bonnet Black Holes and Holographic Heat Engines Beyond Large N
0
votes
0answers
17 views

Is black hole a black body? [duplicate]

I have studied in my physics class today that black body is one which absorbs radiation of all wavelength incident on it , Its absorptance is unity , reflectance and transmittance is zero . I wonder ...
0
votes
0answers
37 views

What would you experience if the universe ceased existing whilst you were under extreme time dilation?

Hypothetical situation, if you were in the center of a supermassive black hole (and managed to not be immediately obliterated), time dilation would be so that billions of years may pass outside the ...
-4
votes
1answer
111 views

do we live in 3.5 dimension? [closed]

After watching interstellar movie i came to know that time is 4th dimension and watch some videos and read articles which says there is there is around 10 dimension(11 also). As x,y,z are 3 dimension ...
1
vote
1answer
117 views

Can you shine a laser on an object that has fallen into a black hole? [duplicate]

I've been having a small back and forth on another website about the nature of objects that fall into black holes. I know that they never reach the event horizon from the perspective of a distant ...
6
votes
1answer
119 views

A question about gravity [duplicate]

Some time ago I encouraged by 11 year old son to watch Cosmos: A Spacetime Odyssey with Neil DeGrasse Tyson, and subsequently Cosmos: A personal Voyage with Carl Sagan, as well as other astronomy ...
5
votes
2answers
239 views

Can a black hole have negative temperature?

Stephen Hawking said that black hole also have temperature and it is related to its mass so in other words a black hole can also be shown to have a negative temperature! I know that nothing is colder ...
-1
votes
1answer
53 views

Can a black hole explode?

In the nature many objects and processes have a crytical point (like the more mass an object collects). Logically is that inside black holes the more violent things happen the more matter is inside. ...
0
votes
2answers
197 views

Coordinate Singularity in Metric

Suppose I have some metric $$ds^2=g(t)dt^2+\frac{1}{r}dr^2$$ which has a singularity at $r=0$. However, if I make the coordinate transformation $u=\frac{1}{r}$, then I get: $$ds^2=g(t)dt^2+r^3 ...
1
vote
5answers
246 views

Can we speed up the evaporation of black holes manually by accelerating it?

If we throw an object to pass near a black hole, to bypass it, it will change the speed of the black hole, just like gravitational assist for a space probe. Does an accelerating black hole evaporate ...
-2
votes
1answer
93 views

Do photon inside event horizon exceeds speed of light?

Event horizon is a region of space whereby nothing with information can escape, so I imagine that a photon falls into this region of space does it accelerates toward the singularity if it exists? Do ...
1
vote
0answers
31 views

Does curvature of space affects magnetic field?

I just want to know can strong gravity such as a black hole bends magnetic field lines?
1
vote
1answer
51 views

Bekenstein bound and an infinite universe

I have a question about the Bekenstein bound or the holographic principle. I make the following assumptions: The universe is infinite The universe is homogeneous These assumptions are at least ...
1
vote
0answers
45 views

Scaling dimension in Kerr/CFT

Let's consider the scattering of a scalar field around a Kerr-AdS Black Hole. In terms of the Kerr/CFT correspondence how can I get the relation between the scaling dimension and the mass of the field ...
1
vote
3answers
132 views

What happens 'inside' the event horizon of a black hole, in particular to photon speed

This is a follow-up question to two answers given here and here, where the speed in different coordinate systems is discussed when approaching the Schwarzschild radius. To cite, derived directly from ...
0
votes
1answer
80 views

Exact meaning of radial coordinate of the Schwarzschild metric

In this answer as well as on Wikipedia the radial coordinate of the Schwarzschild metric is described as follows: ...the r co-ordinate is the value you get by dividing the circumference of the ...
0
votes
1answer
50 views

Can you escape from a black hole in an expanding universe?

Suppose I have a mass $M$ crushed below the Schwarzschild limit. I'm assuming singularities are unphysical; the mass inside the black hole is a super dense and hot cloud of exotic matter, but it's ...
9
votes
3answers
237 views

Coupling between galaxy spin and central black hole spin

What is the relationship between the spin of a galaxy and the spin of its corresponding black hole? Associated questions: Do they always have the same axis of rotation? Do they always spin in the ...
0
votes
0answers
26 views

String spreading?

What is 'string spreading' (in the context of string theory)? Does it have implications for the observability of post-classical effective field theory of gravity terms near macroscopic black hole ...
-2
votes
1answer
49 views

Black hole as a heat engine [closed]

Could anyone tell, how to take high temperature limit for the equation no.10. to get the expansion given in equation no-20, for $r_+$, $D=4$, in this paper http://arxiv.org/abs/1511.08782 ?
0
votes
1answer
41 views

Physical interpretation of the Reissner-Nordstrom metric

The Reissner-Nordstrom metric (in spherical coordinates and c = 1) differs from the Schwarzschild metric in a additive term $$\frac{GQ^2}{4\pi\epsilon_0 r^2}$$, so the metric is $$ds^2 = \left(1 - ...
0
votes
1answer
61 views

Is there any evidence of black holes existing? [duplicate]

Yes, I am aware of observations of "massive objects", for example near the center of the galaxy. But is there any evidence that the object in question is a black hole, with all its commonly attributed ...
3
votes
0answers
26 views

Does the maximally-extended Schwarzschild horizon see the entire past of the parallel universe?

I've seen similar discussions for the inner horizons of Kerr and Reissner-Nordstöm metrics, but I've never seen mention for the simple maximally extended Schwarzschild geometry: As an infalling ...
2
votes
1answer
38 views

Direction of Gamma Ray Bursts

After a black hole consumes a star, why do gamma ray bursts only shoot out perpendicular to the spinning accretion disk? Why do they not shoot out parallel to the disk?
4
votes
2answers
121 views

Is black hole formation reversible if physics law holds even in time reverse?

As we know many situations still fulfill physics law if time is reversed, such as particle collision. But how about black hole formation? Suppose a star is turning to a black hole and starts to have ...
1
vote
1answer
55 views

Eccentric binary black holes

Comparable-mass binary black hole inspirals and mergers are expected to be an important source of gravitational wave signals for current and future ground-based detectors. It is generally expected ...
0
votes
2answers
134 views

Spin of black hole?

Black hole have 3 properties Mass, Charge, Angular Momentum. As there is Angular Momentum it's means black hole is spinning. How would we know it is spinning in clockwise or anti-clockwise ? (in ...
0
votes
1answer
62 views

Black hole accretion disks and quasars

Are 'accretion disk of a black hole' and 'quasar' synonymous? If not, what's the difference?
2
votes
1answer
36 views

Black hole corona and bipolar outflows

What are black hole coronas? Are they different from accretion disks? If so, how are they formed?
0
votes
0answers
19 views

Does 'Entropy-Sated' mean that no 'Order' can be produced from a black hole?

I believe I have a misconception somewhere that I need help to clear up. No-Hair indicates that a black hole created from highly ordered matter with low entropy reaches the same final state as ...
3
votes
1answer
80 views

Kerr spacetime and Carter time machine

In the Boyer-Lindquist-Block III (inside the inner event horizon) exists the so-called Carter time machine. There we can show that for every two points $p,q$ there exists a future-pointing timelike ...
0
votes
0answers
17 views

Acceleration measured when hovering over a non-rotating black hole [duplicate]

I’m wondering what would be the acceleration theoretically measured (for example by an accelerometer) locally by an astronaut whose (ideally pointlike) spacecraft is hovering steadly over a non ...
1
vote
0answers
41 views

Neck size of wormholes in shape dynamics

As far as a dilettante like me knows, in shape dynamics, black holes are really wormholes. What it the expected diameter of the necks of such wormholes (both for regular holes resulting from the ...
0
votes
2answers
83 views

What subatomic particle is a blackhole composed of?

In a black hole, what is the "state" in which the mass that is present is in? Are they there as tightly packed neutron or soup of quark or are particles broken down into 10D strings and coiled as ...
8
votes
1answer
768 views

The final parsec “problem”

Many and perhaps all galaxies seem to contain supermassive black holes of about $10^7 M_\odot$ at their centres. Determining their origins is of great astrophysical interest. In what I understand to ...
1
vote
1answer
61 views

How do we know that supermassive black holes formed in the early universe?

Could they have existed from the very beginning? Could they be leftover fragments of the cosmic egg?
2
votes
0answers
90 views

Gravitational mass defect

In nuclear physics we have a mass defect by the binding energy of the nuclides. A similar effect appears in the theory of gravitation induced by the gravitational binding energy, which reduces the ...
2
votes
1answer
85 views

Time independent Kerr metric

The Kerr metric expressed in terms of polar coordinates $r,\theta,\phi$, such that $x = r\sin(\theta)\cos(\phi)$, $y = r\sin(\theta)\sin(\phi)$, $z = r\cos(\theta)$. Then the Kerr metric is given as ...
0
votes
1answer
65 views

Head on collision of two black holes

LHC was built to collide two atomic particles to study contents within them. There are millions and billions of black holes present throughout galaxies. As collision between the galaxies is common in ...