Questions tagged [black-holes]
A black hole is a region of spacetime from which nothing can escape. More formally, the future light cone of any observer within the black hole is completely contained in the black hole, and the black hole region is not within the past light cone of any observer that goes to spatial infinity in an infinite amount of time.
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If a mass moves close to the speed of light, does it turn into a black hole?
I'm a big fan of the podcast Astronomy Cast and a while back I was listening to a Q&A episode they did. A listener sent in a question that I found fascinating and have been wondering about ever ...
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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 far-away-...
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How does gravity escape a black hole?
My understanding is that light can not escape from within a black hole (within the event horizon). I've also heard that information cannot propagate faster than the speed of light. I assume that the ...
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An explanation of Hawking Radiation
Could someone please provide an explanation for the origin of Hawking Radiation? (Ideally someone who I have been speaking with on the h-bar)
Any advanced maths beyond basic calculus will most ...
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Can black holes form in a finite amount of time?
One thing I know about black holes is that an object gets closer to the event horizon, gravitation time dilation make it move more slower from an outside perspective, so that it looks like it take an ...
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Black holes and positive/negative-energy particles
I was reading Brian Greene's "Hidden Reality" and came to the part about Hawking Radiation. Quantum jitters that occur near the event horizon of a black hole, which create both positive-energy ...
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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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Does someone falling into a black hole see the end of the universe?
This question was prompted by Can matter really fall through an event horizon?. Notoriously, if you calculate the Schwarzschild coordinate time for anything, matter or light, to reach 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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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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Does any particle ever reach any singularity inside the black hole?
I am not a professional physicist, so I may say something rubbish in here, but this question has always popped in my mind every time I read or hear anyone speak of particles hitting singularities and "...
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Is a black hole singularity a single point?
General relativity is expressed in terms of differential geometry, which allows you to do interesting things with the coordinates: multiple coordinates may refer to a single point, eg. the ...
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Why singularity in a black hole, and not just "very dense"?
Why does there have to be a singularity in a black hole, and not just a very dense lump of matter of finite size? If there's any such thing as granularity of space, couldn't the "singularity" be just ...
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Why does Stephen Hawking say black holes don't exist?
Recently, I read in the journal Nature that Stephen Hawking wrote a paper claiming that black holes do not exist. How is this possible? Please explain it to me because I didn't understand what he ...
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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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Will free-fall object into black hole exceed speed of light $c$ before hitting black hole surface?
In Newtonian mechanics, 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 ...
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Can matter really fall through an event horizon?
This question is closely related to Event horizons without singularities from about a year ago (May 2012), which John Rennie answered nicely and persuasively.
My variant of the question is this: ...
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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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Will an object always fall at an infinite speed in a black hole? [duplicate]
Most of you if not everybody will agree that the stronger the gravitational pull, the faster an object will fall. For example, on a planet with 50 times the gravity of Earth, any object will hit the ...
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What will the universe look like for anyone falling into a black hole?
I've heard that, from the perspective of an external observer, something falling into a black hole will eventually look "frozen": light waves will move to the infrared and further into lower ...
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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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Detection of the Electric Charge of a Black Hole: How can an electromagnetic field escape the event horizon of a Reissner-Nordström black hole?
By the "No Hair Theorem", three quantities "define" a black hole; Mass, Angular Momentum, and Charge. The first is easy enough to determine, look at the radius of the event horizon and you can use the ...
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Why did the universe not collapse to a black hole shortly after the big bang?
Wasn't the density of the universe at the moment after the Big Bang so great as to create a black hole? If the answer is that the universe/space-time can expand anyway what does it imply about what ...
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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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How close would you have to be to the merger of two black holes, for the effects of gravitational waves to be detected without instruments?
Assume two black holes in the most common size range, spiraling into each other until they merge. The event releases significant amounts of energy via gravitational waves, which warp the space-time.
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Why is information indestructible?
I really can't understand what Leonard Susskind means when he says in the video Leonard Susskind on The World As Hologram that information is indestructible.
Is that information that is lost, through ...
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Why can't you escape a black hole?
I understand that the event horizon of a black hole forms at the radius from the singularity where the escape velocity is $c$. But it's also true that you don't have to go escape velocity to escape an ...
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How close does a photon have to get to a black hole to do a full loop?
How close does a photon have to get to a black hole to do ONE full loop? By full loop I mean it curves once around the black hole, and then it ends up on the same trajectory as it was one before it ...
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Are black holes really singularities?
A popular assumption about black holes is that their gravity grows beyond any limit so it beats all repulsive forces and the matter collapses into a singularity.
Is there any evidence for this ...
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How fast a (relatively) small black hole will consume the Earth?
This question appeared quite a time ago and was inspired, of course, by all the fuss around "LHC will destroy the Earth".
Consider a small black hole, that is somehow got inside the Earth. Under "...
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Do all black holes have a singularity?
If a large star goes supernova, but not enough mass collapses to form a black hole, it often forms a neutron star. My understanding is that this is the densest object that can exist because of the ...
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How can Quasars emit anything if they're black holes?
We've heard it many times, nothing can escape the gravity of a black hole, even light once it's past the event horizon. If this is true, how can a black hole emit anything? Quasars are massive black ...
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How can a singularity in a black hole rotate if it's just a point?
I guess nobody really knows the true nature of black holes, however, based on everything I know about black holes, there is a "singularity" at their center, which has finite mass but is infinitely ...
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Entering a black hole, jumping into another universe---with questions
I'm quite familiar with SR, but I have very limited understanding in GR, singularities, and black holes. My friend, which is well-read and is interested in general physics, said that we can "jump" ...
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How were the solar masses and distance of the GW150914 merger event calculated from the signal?
The GW150914 signal was observed, giving us the frequency and amplitude of the event. Because LIGO has two detectors a rough source location could be derived.
But how do these three factors allow for ...
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So Black Holes Actually Merge! In 1/5th of a Second - How?
I've read a lot of conflicting answers in these forums. However, today saw the awesome announcement of gravitational waves. Two black holes merged: http://www.slate.com/blogs/bad_astronomy/2016/02/11/...
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Does black hole formation contradict the Pauli exclusion principle?
A star's collapse can be halted by the degeneracy pressure of electrons or neutrons due to the Pauli exclusion principle. In extreme relativistic conditions, a star will continue to collapse ...
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Given that matter cannot escape a black hole, how did the big bang produce the universe we see today?
Extrapolation of the expansion of the Universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past.
If the matter contained within our ...
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Can we have a black hole without a singularity?
Assuming we have a sufficiently small and massive object such that it's escape velocity is greater than the speed of light, isn't this a black hole? It has an event horizon that light cannot escape, ...
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Questions re significance of 2016 and 2020 LIGO observations
Phys. Rev. Lett. 116, 061102 (2016) - "Observation of Gravitational Waves from a Binary Black Hole Merger"
(https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.061102) reports that ...
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Taking selfies while falling, would you be able to notice a horizon before hitting a singularity?
I am generally interested in the role of "pings"(0a) between participants (a.k.a. "signal roundtrips"(0b), as familiar for instance from Synge's "five point curvature detector") in the determination ...
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If you shoot a light beam behind the event horizon of a black hole, what happens to the light?
I have a couple of questions about light here, and sorry of they are silly..
So since anything that goes beyond the event horizon can't go out, so what if a light beam was pointed somewhere behind ...
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Why does a black hole have a finite mass?
I mean besides the obvious "it has to have finite mass or it would suck up the universe." A singularity is a dimensionless point in space with infinite density, if I'm not mistaken. If something is ...
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Binary Black Hole Solution of General Relativity?
This is rather a technical question for experts in General Relativity. An accessible link would be an accepable answer, although any additional discussion is welcome.
GR has well known solutions ...
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Black hole formation as seen by a distant observer [duplicate]
Possible Duplicate:
How can anything ever fall into a black hole as seen from an outside observer?
Is black hole formation observable for a distant observer in finite amount of time? Specifically,...
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Thought experiment - would you notice if you fell into a black hole?
I've heard many scientists, when giving interviews and the like, state that if one were falling into a black hole massive enough that the tidal forces at the event horizon weren't too extreme, that ...
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When a star becomes a black hole, does its gravitational field 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?
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Are electrons just incompletely evaporated black holes?
Imagine a black hole that is fast-approaching its final exponential throws of Hawking evaporation.
Presumably, at all points in this end process
there will remain a region that identifiably remains &...
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No hair theorem for black holes and the baryon number
The no hair theorem says that a black hole can be characterized by a small number of parameters that are visible from distance - mass, angular momentum and electric charge.
For me it is puzzling why ...
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Why can't light escape from a classical black hole?
Photons do not have (rest) mass (that's why they can move at the speed of "light").
So my question is: how can the gravity of a classical$^1$ black hole stop light from escaping?
--
$^1$ We ...