Tagged Questions
1
vote
1answer
45 views
How does gravity effects both time and light if they have no mass [duplicate]
I've been reading about how black holes can effect both time and light with gravity. So I was wondering, doesn't something have to have mass to be effected by gravity? And if so, does this mean both ...
1
vote
1answer
29 views
Can we build a synthetic event horizon?
If we imagine ourselves to be a civilization capable of manipulating very heavy masses in arbitrary spatial and momentum configurations (because we have access to large amounts of motive force, for ...
2
votes
2answers
88 views
Why don't black holes within a galaxy pull in the stars of the galaxy
visit http://www.nasa.gov/audience/forstudents/k-4/stories/what-is-a-black-hole-k4.html
If black holes can pull even light, why cant they pull the stars in the galaxy?
0
votes
0answers
69 views
Is there any proof that the speed of gravity is limited? [duplicate]
I must warn that though I'm argumenting with black holes I'm not asking how does gravity escape the black hole!. I want to know if the absolute speed of gravity waves were proven bu an experiment.
We ...
1
vote
2answers
91 views
What happens to things when things get crushed in a blackhole [duplicate]
When a black hole destroys things until they are smaller than molecules, where does it go and what happens when it clogs up?
-1
votes
1answer
54 views
What is mathematical definition of a strong gravity?
Mathematical definition of a weak gravity is simple $g=\frac{GM}{r^2}$
but what is mathematical definition of a strong gravity? (blackhole-like or close to a blackhole-like object)
0
votes
1answer
105 views
Do all black holes spin in the same direction?
My question is as stated above, do all black holes spin the same direction?
To my knowledge, the spin in the direction of the spin of the matter that created them. Another similar question was asked ...
3
votes
3answers
180 views
Can a black hole collapse in itself?
As we know that the black hole is a lump of highly dense matter, and that's the reason for it's so strong gravitational force. Bat let us assume that it has sucked up a huge amount of mass and it's ...
0
votes
1answer
57 views
Dense Spherical Black Hole Shell with a Region Inside
I'm going to propose a thought experiment, based on two ideas.
One: A uniform spherical shell, by the Shell Theorem, does not exert any gravitational force on objects existing in the interior of the ...
1
vote
3answers
121 views
What actually happens to a light ray on a Schwarzschild black hole horizon?
I know the Schwarzschild event horizon is a null surface generated by null geodesics. But what does that actually mean in terms of the path of a light ray that reaches it? Does that mean the geodesic ...
4
votes
4answers
223 views
Gravity from a singularity as distance approaches zero
If you had a singularity (that had mass but took up no space), what would happen to the acceleration of an object as it approached this singularity? I would assume that it would be infinite, since as ...
4
votes
2answers
137 views
Is Brian Cox right to claim that Gravity is a strong force for large masses, is it wrong, or is it only a matter of interpretation?
I watched a program of his in which it was claimed that since mass bends space in accordance to General Relativity, then in the case of very large stars it becomes a strong force to the point of being ...
0
votes
0answers
66 views
How to calculate the mass of the Cygnus X-1 black hole?
I have received a question about how to calculate the mass of Cygnus x-1 (black star).
Since we are able to find the the mass through this Wikipedia page I know that we can find the mass through ...
0
votes
1answer
82 views
Nothing escapes BHs, gravitons mediate gravity, so why do BHs gravitate? [duplicate]
Possible Duplicate:
How does gravity escape a black hole?
Nothing escapes black holes, gravitons mediate gravity, so why do black holes gravitate?
My question is, "where is the hole (no ...
3
votes
3answers
312 views
Gravity stronger than electromagnetic force in a black hole?
Well, the question has somewhat been answered before, but there's one part missing, which - I'd think - is in conflict with the physical laws.
The earlier reply says that the gravitational pull even ...
2
votes
2answers
200 views
Is the Schwarzschild black hole unphysical?
To obtain the Schwarzschild metric from Einstein equations of general relativity, we suppose that the energy density is a distribution :
$$ \rho (\vec{r}) = M \delta(\vec{r})$$
The Schwarzschild ...
-1
votes
3answers
182 views
How many pieces of toast would you need to make a black hole?
I am trying to find the following:
How many pieces of toast would you need to make a black hole?
From what I've learnt so far I need to find an equation for the compression force the massive amount ...
3
votes
1answer
135 views
Formation of a black hole and Hawking radiation
From the perspective of an outside observer it takes infinitely long for the black hole to form.
But if the black hole is no extremal black hole, it emits Hawking radiation.
So the outside observer ...
1
vote
1answer
201 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 ...
1
vote
1answer
423 views
is gravity always the weakest force [duplicate]
Possible Duplicate:
What does it mean to say “Gravity is the weakest of the forces”?
Strongest force in nature
It is usually taught that the gravitational force is the weakest ...
1
vote
1answer
133 views
How is the singularity in Newtonian gravity resolved?
In Who's afraid of a Black Hole? at time ~20:38, Michio Kaku makes a claim that $1/r$ when $r=0$ equals $\infty$ and when physicists see the $\infty$ they see a monstrosity. Coincidentally, Newton's ...
3
votes
2answers
213 views
What do you feel when crossing the event horizon?
I have heard the claim over and over that you won't feel anything when crossing the event horizon as the curvature is not very large. But the fundamental fact remains that information cannot pass ...
0
votes
1answer
91 views
picture of newtonian gravity and black hole
We assume that the earth planet is a black hole.
When a light beam is fired.
The only way that even light can move is A and B which means this is impossible that light move on other ways C, D, E
...
1
vote
1answer
180 views
Why a black hole sucks nearly everything, but emits gravitons?
What is so inherently different to the idea of gravitons from other particles, that a black hole draws everything possible including massless photons, but emits gravitons the more the stronger it ...
3
votes
2answers
214 views
Do black holes accelerate in spin as they obtain more mass?
It is known that - When a star collapses during the formation of the black hole, the black hole obtains the spin of the star which it collapsed from...
What I'd like to know is, If this spin ...
3
votes
1answer
217 views
Black-holes are in which state of matter?
Wikipedia says,
A Black hole grows by absorbing everything nearby, during its life-cycle. By absorbing other stars, objects, and by merging with other black-holes, they could form Super massive ...
2
votes
0answers
51 views
How can black holes have gravity? [duplicate]
Possible Duplicate:
How does gravity escape a black hole?
How can gravity get out of a black hole? If a black hole is so powerful that even light does not travel fast enough to get out, and ...
4
votes
2answers
727 views
What is the escape velocity of a Black Hole?
The escape velocity of Earth is $v=\sqrt{\frac {GM}{R}}$, 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 $c$.
What is the ...
0
votes
2answers
97 views
What is gravitational speed for a black hole? [duplicate]
Possible Duplicate:
What is the escape velocity of a Black Hole?
This is gravitational speed for earth: $$v=\sqrt {g_{e}r}.$$
What is gravitational speed for a black hole? I want an ...
2
votes
3answers
343 views
What happens to matter in extremely high gravity?
Though I am a software engineer, I have bit interest in sciences as well. I was reading about black holes and I thought if there is any existing research results on How matter gets affected because of ...
2
votes
1answer
142 views
Can a charged black hole interact via electromagnetism? [duplicate]
Possible Duplicate:
Detection of the Electric Charge of a Black Hole
Light cannot escape from a black hole. However light is also interpreted as the carrier of the electromagnetic force. So ...
7
votes
1answer
331 views
Can a black hole form due to Lorentz contraction? [duplicate]
Possible Duplicate:
If a 1kg mass was accelerated close to the speed of light would it turn into a black hole?
Imagine, a rod of length L is moving with velocity approaching the speed of ...
7
votes
3answers
547 views
Why is a black hole black?
In general relativity (ignoring Hawking radiation), why is a black hole black? Why nothing, not even light, can escape from inside a black hole? To make the question simpler, say, why is a ...
1
vote
3answers
479 views
will free-fall object into black hole exceed speed of light before hitting blackhole surface?
imagine if we throw an object in against direction of gravity with speed v and it achieve max. height of h. now if we allow object to fall from that height h, it will eventually attain speed v when it ...
15
votes
8answers
2k views
How can anything ever fall into a black hole as seen from an outside observer?
The event horizon of a black hole is where gravity is such that not even light can escape. This is also the point I understand that according to Einstein time dilation will be infinite for a ...
1
vote
0answers
78 views
How complete is our understanding of general-relativistic solutions for extremal black holes?
Putting aside quantum mechanics (or at least putting aside the question of fermions), is our knowledge of extremal General-Relativity solutions good enough that we would be able to rule out a ...
3
votes
4answers
392 views
Can a black hole be explained by newtonian gravity?
In the simple explanation that a black hole appears when a big star
collapses under missing internal pressure and huge gravity, I can't see
any need to invoke relativity. Is this correct?
4
votes
1answer
289 views
Can Black Holes be the Dark Matter?
Seems to fit the definition: interacts with gravity, doesn’t radiate energy (except Hawking Radiation) and could create gravity lensing without absorbing very much of the light.
Could 80% of the ...
2
votes
3answers
278 views
Why doesn’t gravity break down in a large black hole?
By popular theory gravity didn’t exist at the start of the Big Bang, but came into existence some moments later. I think the other forces came into existence a little latter.
When a black hole ...
1
vote
3answers
208 views
Formation of black holes
If stars start with a finite density and light can escape from them, how can they be compacted to form a mass with infinite density which light cannot escape? The black hole will have the same mass as ...
3
votes
2answers
311 views
Black hole complementarity
Black hole complementarity states that two observers, one falling into a black hole, and one observing outside, experience two different histories but since they can not communicate there is no ...
6
votes
3answers
339 views
Charging a black hole?
What would happen if we have a black hole and we start shooting at it a single electron at a time, and go on doing it forever? Would the electrons start to bounce off eventually?
2
votes
1answer
457 views
Schwarzschild black hole geometry in Novikov coordinates
As I am a layman in gravitation I would really appreciate any suggestions on how to approach and tackle the following problem:
Choose a time-symmetric 3-geometry for the initial geometry of ...
9
votes
5answers
341 views
What happens to light and mass in the center of a black hole?
I know that black holes are "black" because nothing can escape it due to the massive gravity, but I am wondering if there are any theories as to what happens to the light or mass that enters a black ...
10
votes
5answers
6k views
How exactly does time slow down near a black hole?
How exactly does time slow down near a black hole? I have heard this as a possible way of time traveling, and I do understand that it is due in some way to the massive gravity around a black hole, but ...
12
votes
3answers
3k views
How do we determine the mass of a black hole?
Since by definition we cannot observe black holes directly, how do astronomers determine the mass of a black hole?
What observational techniques are there that would allow us to determine a black ...
17
votes
6answers
2k views
How can a black hole emit X-rays?
Considering that a black hole's gravity prevents light from escaping, how can a black hole emit X-rays? Visible light and X-rays are both electromagnetic radiation, shouldn't the black hole's gravity ...
3
votes
2answers
1k views
Strongest force in nature
Possible Duplicate:
What does it mean to say “Gravity is the weakest of the forces”?
It is said nuclear force is the strongest force in nature..
But it is not true near a black ...
7
votes
6answers
611 views
What prevents the accumulation of charge in a black hole?
What prevents a static black hole from accumulating more charge than its maximum? Is it just simple Coulomb repulsion?
Is the answer the same for rotating black holes?
Edit
What I understand from ...
3
votes
1answer
409 views
Gravitational redshift of virtual photons
There is well known gravitational redshift of real photons. What about gravitational redshift of virtual photons of charged neutron star? Is electrostatic force become weaker while mass of ...
