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

0
votes
0answers
20 views

Information of things inside a black hole [duplicate]

Can we get the information of things that are gone in black hole?
37
votes
4answers
4k views

When a star becomes a black hole, does its gravitational field strength become stronger?

I've seen in a documentary that when a star collapses and becomes a black hole, it starts to eat the planets around. But it has the same mass, so how does its gravitational field strength increase?
1
vote
1answer
86 views

Speed of light versus pull of gravity - Is $c$ really the limit? [duplicate]

The understanding I have is that the speed of light is considered to be the highest attainable speed in physics. Of course there are theories of tachyons but since those haven't been proven we'll ...
2
votes
1answer
49 views

Black Hole mass increase from infalling body

When a body falls into a black hole, the black hole's mass increases. Does this mass increase depend only on the rest mass of the infalling object, or is it affected by the velocity of the infalling ...
2
votes
4answers
488 views

Why is the Schwarzschild radius the radius of an event horizon?

I've been searching the web and many references without much success. My question is how do we know that, in the Schwarzschild black hole solution, the surface with coordinate $r=2M$ (in the geometric ...
1
vote
1answer
39 views

If MOND theory doesn't explain gravitational lensing, couldn't well placed black holes explain it?

When I first heard about star velocity as a function of its distance from the center of the galaxy and the difference between prediction and observation I immediately thought "there must be a ...
1
vote
2answers
72 views

Why aren't all large stars black holes?

Like all stars, large ones are stable as long as there is a sufficient amount of hydrogen (or helium) to fuse. This fusion process is what prevents them from collapsing in on themselves. However, ...
1
vote
2answers
49 views

Does the mass distribution matter in (Schwarzschild) black holes?

Is it possible that from the same initial mass different black hole radius will be created due to different mass distribution during black hole creation? If mass is concentrated more on the outside ...
3
votes
1answer
126 views

What is the Schwarzschild metric with proper radial distance?

Reading the marvellous book "The Membrane Paradigm" I stumbled upon a suggested change of variable that I'm not able to deal with. Starting with the usual Schwarzschild metric for the spatial ...
2
votes
2answers
250 views

Why does Matter occupies space? Matter and space are two things… If Space does not has mass, why should matter have space-ful nature?

This thought has completely changed my perspective towards matter. If the matter in a star can collapse to a point to form a Black hole, surely the true nature of matter should be able explain this ...
8
votes
0answers
102 views

Black hole (classical or quantum?)

Theoretically we can convert a body into black hole by compressing its mass below some radius (known as Schwarzschild Radius). Suppose such object after becoming black hole has a radius which is ...
6
votes
2answers
90 views

Inside a huge sphere, which is being converted to a black hole

Suppose the following scenario: You reside inside a huge stable spherical star with non-lethal environment at its core. The object is so huge and massive, that its radius is only slightly above its ...
2
votes
5answers
2k views

Why the center of our galaxy doesn't absorb us?

Depending on the theories, the center of our galaxy is a super massive black hole, this is easy to accept as a truth, but what I couldn't simply devour is how the solar system is orbiting around it ...
3
votes
2answers
329 views

Another faster-than-light question

Imagine we have something very heavy (i.e supermassive black hole) and some object that we can throw with 0.999999 speed of light (i.e proton). We are throwing our particle in the direction of hole. ...
1
vote
1answer
74 views

At what speed will objects hit a singularity? [duplicate]

I have read in numerous places like here that an object in free fall into a black hole will be travelling the speed of light when it passes the event horizon. How is it possible to go this fast? ...
0
votes
0answers
21 views

Blackholes gravity, why it has effect outside the event horizon? [duplicate]

If gravity is a curvature of space time, and the event horizon is the collapse of this curvature on itself, then if the blackhole gets more matter after it is formed we should see no effect at all ...
1
vote
1answer
127 views

A two-dimensional event horizon?

Today, I came across an article which says that the Universe began with the collapse of a 4-dimensional [spatial] Universe. Well, I don't know if there's any way to confirm these like it is with all ...
0
votes
1answer
28 views

What is the power of the most powerful quasar found?

Trying to find an answer to this question, I came across many sources that are in complete in contradiction. For example Wikipedia states that a typical quasar has a power of $10^{40}$ watts while ...
6
votes
0answers
94 views

What are the implications for the AdS/Cft program if AdS is unstable?

To my understanding recent progress in the study of the non linear stability of AdS spacetime suggest that $AdS$ might be unstable. If this is true, what are the physical and mathematical ...
5
votes
0answers
52 views

BTZ Black Hole Central Charge and Conformal Weight

I have been trying to reproduce a calculation (equation 4.12) in this paper http://arxiv.org/pdf/1107.2678v1.pdf by Carlip reviewing the derivation of the effective central charge of the BTZ Black ...
0
votes
1answer
55 views

Misner String Singularity

In correspondence to AdS black hole solutions, what does it mean by Misner string singularities? And when there are no Misner string singularities, what does this mean in terms of curvature ...
1
vote
1answer
72 views

Infinite number of universes? Black holes are guilty? [closed]

I was thinking, if the space is infinite, what if there are infinite number of spaces, inside our universe? I mean, everyone knows that black holes exist, but nobody knows what happens when you get ...
2
votes
1answer
112 views

What happens when electron degeneracy pressure is being overcome (right before an object turns into a black hole)?

I was wondering about hydrostatic equilibrium (the balance of radially inward and outward forces) in large (i.e. above the Chandrasekhar limit) stars. It is often said that when the gravitational ...
3
votes
1answer
577 views

Special relativity paradox and gravitation/acceleration equivalence

One of the features of the black hole complementarity is the following : According to an external observer, the infinite time dilation at the horizon itself makes it appear as if it takes an ...
0
votes
1answer
45 views

Spaghettification while still in orbit

THIS POST does a pretty good job of giving an idea as to what the differences in forces becomes as you approach a black hole. For a small enough black hole, you would reach the dangerous tidal force ...
0
votes
1answer
46 views

Quantum uncertainty of a singularity: A Black Hole Problem

A black hole in GR seems to have a sharply defined radius. Of zero, that is since all the mass has been compressed into the infinitely dense center point. However, shouldn't the fuzziness of quantum ...
13
votes
1answer
284 views

How is the logarithmic correction to the entropy of a non extremal black hole derived?

I`ve just read, that for non extremal black holes, there exists a logarithmic (and other) correction(s) to the well known term proportional to the area of the horizon such that $S = \frac{A}{4G} + K ...
3
votes
1answer
58 views

If gravitational radiation (or anything) cannot escape a black hole, how can it produce redshift or curve spacetime? [duplicate]

There is an apparent paradox in a Black hole. Keenan Pepper wrote: Electromagnetic radiation cannot escape a black hole, because it travels at the speed of light. Similarly, gravitational ...
1
vote
1answer
91 views

In black hole complementarity, does Bob literally observe Alice's death?

I'm interested in the picture of black hole complementarity where an observer (Bob) far away from the black hole watches another observer (Alice) fall in, and develops a very different picture of ...
7
votes
1answer
895 views

What happens to light that falls into a black hole?

When light enters a black hole, what happens to it? I imagine the photons will either fall into the singularity, or the light will orbit just inside the event horizon indefinitely. (Some background ...
19
votes
5answers
1k views

Why do we care about black hole interiors' physics?

Whatever happens in there is not falsifiable nor provable to the outside. If for (amusing) example the interior consisted of 10^100 Beatles clones playing "Number Nine" backwards, do we know how to ...
0
votes
3answers
59 views

Size of black hole

I am wondering about size of black hole. How is it possible that we have black holes of different sizes? As I know the singularity is point which is infinite small and is infinite dense. So my ...
3
votes
1answer
182 views

Light Deflection by a Black Hole

I think I've come across a paradox while studying general relativity. Wikipedia states that the deflection angle of light by a point mass is $4GM/(c^2b)$. ...
0
votes
3answers
72 views

Black Hole gravity pull [duplicate]

Please I want to Know that if the radius of the black hole = 0 then how it have its surface and how it absorbs things? $$\begin{align} g &= \frac{Gm}{0^2} \\ &= \infty \end{align}$$ then ...
0
votes
1answer
144 views

What are the conditions around Sgr A* for a space ship trying to plunge into the central black hole?

The possible problems for a space ship near an active galactic nucleus are the following: tidal forces, hot plasma accreting into the black hole and "jetting" out, radiation, and magnetic fields. If I ...
1
vote
1answer
32 views

Long string model of black hole

I wonder if anyone can advise me literature about "long string model" of black hole. I've tried Google, and of course arXiv.org. Still I couldn't find anything about this theme. I would be also very ...
4
votes
1answer
1k views

How could we travel to the nearest supermassive Black hole?

I have just watched a trailer for the upcoming movie Interstellar and started to wonder about some physics involved. In the end of the trailer, they are obviously plunging into a Black hole hoping to ...
5
votes
2answers
3k views

What is the escape velocity of a Black Hole?

The escape velocity of Earth is $v=\sqrt{\frac {2GM}{R}}$, where $M$ is the mass of the Earth and $R$ it's radius (approximating it as a sphere), and is much less than light speed $c$. But I want to ...
1
vote
2answers
58 views

Calculating the size of the “lensing sphere” of a black hole

Given a black hole of some size, say $10^8$ solar masses, how can the size of its sphere of influence of light be calculated? To clarify, ultimately I'd like to be able to calculate the apparent ...
0
votes
2answers
44 views

Charge Distribution in Reissner-Nordström Black Holes

In an electrically charged black hole, such as the one described by the Reissner-Nordström metric (i.e. with no angular momentum), where would the electric charge be situated (neglecting any charged ...
0
votes
1answer
52 views

Maximum curvature in a black hole

If the curvature inside the horizon of a black-hole is not infinite in some quantum gravity theories (as in Loop quantum gravity), then what is the expression of the maximum value of the curvature ...
8
votes
1answer
164 views

Which way do black hole jets spin?

The centers of black holes and quasars often have jets coming out the two poles of an accretion disk, say north and south. Is it known if the two jets spin in the same direction or opposite ...
4
votes
3answers
402 views

Could a ship equipped with Alcubierre drive theoretically escape from a black hole?

Could a ship equipped with Alcubierre drive theoretically escape from a black hole? Also, could it reach parts of the universe that are receding faster than the speed of light from us?
1
vote
1answer
337 views

How is the mass of black hole at the center of our galaxy measured?

I've been watching a video about dark mater and a lot of the mass is missing in our universe. Astronomers got to this by measuring the speed that stars orbit the center of the galaxy and when they did ...
6
votes
1answer
119 views

What prevents stars in globular clusters from merging over time to form a black hole?

Globular clusters are apparently very very old, and the density of these clusters appears to increase as one approaches the center of a cluster. Orbits are bound to be chaotic, since there is no ...
5
votes
3answers
329 views

How close can two extremal black holes with the same charge can get?

Here's a puzzle I have been pondering over. If we have two extremal black holes with the same charge, the electrostatic repulsion between them ought to cancel the gravitational attraction between ...
8
votes
2answers
124 views

Why does angular momentum shorten the Schwarzschild Radius of a black hole?

Angular momentum causes the event horizon of a black hole to recede. At maximum angular momentum, $J=GM^2/c$, the Schwarzschild radius is half of what it would be if the black hole wasn't spinning. ...
6
votes
0answers
139 views

Can we have consistent histories inside a black hole?

A consistent history is a POVM set of observables corresponding to a time-ordered product of projection operators. For gauge theories, not any old operator will do, only gauge-invariant observables. ...
1
vote
1answer
58 views

Black holes: Behaviour of light near an event horizon

Electromagnetic waves travel mostly in vacuum medium, in outer space, but sometimes in gaseous media, such as in gaseous atmospheres involved in nebulae. If electromagnetic-waves come anywhere in the ...
8
votes
6answers
2k views

What methods can astronomers use to find a black hole?

How can astronomers say, we know there are black holes at the centre of each galaxy? What methods of indirect detection are there to know where and how big a black hole is?