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.
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How to get Planck length
I know that what Planck length equals to.
The first question is, how do you get the formula
$$\ell_P~=~\sqrt\frac{\hbar G}{c^3}$$ that describes the Planck length?
The second question is, will any ...
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1answer
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A Query on the Trapped Null Surface
A a compact, orientable, spacelike surface always has 2 independent forward-in-time pointing, lightlike, normal directions. For example, a (spacelike) sphere in Minkowski space has lightlike ...
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2answers
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Why is matter drawn into a black hole condensed into a single point within the singularity? [duplicate]
Possible Duplicate:
Why is matter drawn into a black hole not condensed into a single point within the singularity?
When we speak of black holes and their associated singularity, why is ...
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2answers
87 views
Galaxies Center
It is believed (to my understanding) that at the center of all large galaxies are super massive black holes. Why is it then when you see photos of galaxies that the center is extremely bright if a ...
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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 ...
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203 views
Area law for Entropy in Loop Quantum Gravity
In connection with the long saga of the (claimed) microscopic
calculations of the Hawking-Bekenstein entropy in (3+1) Loop Quantum Gravity (LQG) and related approaches I have the following question: ...
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1answer
258 views
Can two particles remain entangled even if one is past the event horizon of a black hole?
Can two particles remain entangled even if one is past the event horizon of a black hole? If both particles are in the black hole?
What changes occur when the particle(s) crosses(cross) the event ...
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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 ...
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1answer
226 views
Why is matter drawn into a black hole not condensed into a single point within the singularity?
When we speak of black holes and their associated singularity, why is matter drawn into a black hole not condensed into a single point within the singularity?
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Is the Graveyard Really so Serious?
Calculations in relation to black holes are solely in consideration of spacetime curvature and its effects. They are in total alienation with respect to the action of inertial agents[external ...
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1answer
353 views
The Black Hole Problem
Consider a spherical body of uniform density $\rho$ and initial radius R. You can imagine this body containing another sphere of radius R/2 which touches the center and the periphery of the larger ...
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1answer
101 views
Entropy of a black hole can be zero?
Black holes are one of highest density objects in the universe. Because of this high density(m/v) more matter is highly compressed in a very small volume( i am guessing that molecules cant freely move ...
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Is a black hole's surface area invariant for distant intertial observers?
Let's imagine I'm very far from any massive objects, so my local space-time is Minkowskian. Off in the distance is a black hole, far enough away that it doesn't noticeably curve space-time near me, ...
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Will a black hole increase the speed of sound above the speed of light in this medium
For the sake of this question we are inside the EH and a sound wave enters from our perspective as the sound moves closer to us at the EH would it speed up.
Specifically how would the extreme ...
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0answers
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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 ...
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black hole no-hair theorems vs. entropy and surface area
I was revisiting some old popular science books a while ago and two statements struck me as incompatible.
No-hair theorems: a black hole is fully-described by just a few numbers (mass, spin etc) ...
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1answer
95 views
Kerr-Newman black holes and infinite charge
Recall the first law of BH thermodynamics
$ dM=\frac{\kappa}{8\pi} dA + \Omega dJ + \Phi dQ $
Now, let's consider the Reissner-Nordstrom solution $J=0$ such that $m>Q$ but only slightly greater. ...
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1answer
109 views
Classical black holes?
How big should the black hole be so we can consider it to be classical?
When they claim that we can not probe shorter distances than the Planck length, can it be true?
The argument says that, ...
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1answer
141 views
Do black holes play a role in quantum decoherence?
Sorry for such a vague question but I could have sworn I read somewhere that Hawking proposed the reason we might see a classically appearing universe is due to the possible role of black holes in ...
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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 ...
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1answer
67 views
Learning about predicted interactions between black holes and tachyons
I'm a hobbyist reader/student looking for some reading material on the predicted effects of tachyons and black holes (or other ultra-high gravity wells). Are there any good links or references people ...
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3answers
803 views
Why can't dark matter be black holes?
Since 90 % of matter is what we cannot see, why can't it be black-holes from early on? Is is possible to figure out that there are no black holes in the line of sight of various stars/galaxies we ...
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2answers
395 views
What is the mass density distribution of an electron?
I am wondering if the mass density profile $\rho(\vec{r})$ has been characterized for atomic particles such as quarks and electrons. I am currently taking an intro class in quantum mechanics, and I ...
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1answer
159 views
Why do the stars in the galaxy core move so fast?
There are a bunch of stars orbiting the black hole in the center of our galaxy.
These stars move at huge speed. Why do we see this? Why do the black hole not impose any noticeable time dilation on ...
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3answers
255 views
Information loss in a black hole
How does the Holographic Principle help to establish the fact that all the information is not lost in a black hole?
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1answer
227 views
Mass of Milky Way Supermassive Black Hole
I am looking at data from Andrea Ghez (2005; ADS link) which states the mass of the supermassive black hole in the MW. Here's what I don't understand: She reports the value as $3.7 \times 10^6 ...
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1answer
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Are black holes (and the universe in general) said to exhibit Ostwald's Ripening?
Ostwald's Ripening is a phenomenon where the surface area to volume ratio of droplets causes small particles to shrink until they disappear and for droplets above a certain volume to continuously ...
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Are we inside a black hole?
I was surprised to only recently notice that
An object of any density can be large enough to fall within its own
Schwarzschild radius.
Of course! It turns out that supermassive black holes at ...
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2answers
523 views
How do we determine the temperature of a Black Hole?
How do we determine the temperature of a Black Hole?
Since we cannot see a Black Hole, which I presume, is because it absorbs light, would it not also prevent radiation from escaping, making ...
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3answers
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Is there a correlation between the mass of a supermassive black hole and the mass of its host galaxy?
I would love to know! I can find lots about the mass-velocity dispersion relation. There's a mass-luminosity relation (but not really tight). I hope you can include references, as I'm spending ...
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2answers
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What does velocity dispersion (sigma) reveal about a galaxy?
I'm getting hung up on this term. In studying SMBHs, I see that velocity dispersion strongly correlates with mass. Just what is the velocity dispersion? How can the velocity dispersion of the galaxy ...
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2answers
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Computing the path of photons near a black hole
For a simulation, I want to compute the path that light follows near a black hole.
Non-relativistically, a massive point particle in a central newtonian gravitational field follows either an ellipse, ...
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1answer
282 views
Do spacelike junctions in the Thin-Shell Formalism imply energy nonconservation and counterintuitive wormholes?
The Thin Shell Formalism (MTW 1973 p.551ff) is used to properly paste together different vacuum solutions to the Einstein equations. At the junction of the two solutions is a hypersurface of matter – ...
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Intensity of Hawking radiation for different observers relative to a black hole
Consider three observers in different states of motion relative to a black hole:
Observer A is far away from the black hole and stationary relative to it;
Observer B is suspended some distance ...
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1answer
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How will a super massive black hole effect our galaxy?
I've recently learned that the general consensus is that several (if not, most) galaxies have super massive black holes in their center, in particular the Milky Way. This, at least to me, makes ...
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5answers
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Theoretically, can a black hole collapse until it gets smaller than its Schwarzschild radius?
So, the concept is this:
When a supergiant star collapses, its nucleus becomes a black hole. So, what if that black hole collapses again?
I don't know if it is possible, but what if it happens?
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2answers
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Can a neutron star compress until it's converted to a black hole?
The Universe's densest objects are black holes. In the second place, there are neutron stars.
So, if a neutron star compresses to its Schwarzschild radius, would it appear as a black hole? That black ...
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Black Hole Photon Sphere
The photon sphere is a spherical region in space where photons are forced to travel in an orbit at $r = \frac{3GM}{c^{2}}$.
Is it possible to detect these spheres?
What happens if I fall through ...
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2answers
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Can black holes be created on a miniature scale?
A black hole is so powerful to suck everything into itself. So is it possible that mini black holes can be created? If not then we could have actively disproved the rumors spread during LHC ...
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2answers
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$2+1$ dimensional physics theory of our universe?
Is there any physics theory that depicts our universe as $2+1$ dimensional?
I heard that black holes seem to suggest that the world might be $2+1$ dimensional, so I am curious whether such theory ...
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From where (in space-time) does Hawking radiation originate?
According to my understanding of black hole thermodynamics, if I observe a black hole from a safe distance I should observe black body radiation emanating from it, with a temperature determined by its ...
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3answers
241 views
What is exactly the density of a black hole and how can it be calculated?
How do scientists calculate that density? What data do they have to calculate that?
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3answers
260 views
Can we see a spaceship falling into a black hole and entering the event horizon?
Or it pauses in time because the spaceship reaches the speed of light, c?
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3answers
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From the perspective of an observer inside a black hole's horizon, where does the energy for Hawking radiation come from?
Would energy be seen to "flow" to the outside of the black hole? Through what mechanism?
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1answer
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Hawking radiation: direct matter -> energy conversion?
When a black hole evaporates, does it turn all the matter that has fallen in directly to energy, or will it somehow throw back out the same kind of matter (normal or anti) that went in?
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What makes a supermassive black hole move through space?
In this example of a Neutrino jet, it seems that the supermassive black hole moves around the space and meets up with a interstellar object sometimes.
Source of the above image and a larger ...
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2answers
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Deriving Birkhoff's Theorem
I am trying to derive Birkhoff's theorem in GR as an exercise: a spherically symmetric gravitational field is static in the vacuum area. I managed to prove that $g_{00}$ is independent of t in the ...
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2answers
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Have scientists been able to find an example of a possible White Hole?
We went to a planetarium last night last night and watched "Black Holes" narrated by Liam Neeson. If I recall correctly, he said that scientists believe that they have identified a number of Black ...
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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 ...
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If an anti-matter singularity and a normal matter singularity, of equal masses, collided would we (outside the event horizon) see an explosion?
If an anti-matter singularity and a normal matter singularity, of equal masses, collided would we (outside the event horizon) see an explosion?
