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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How can super massive black holes have a lower density than water?
I heard on a podcast recently that the supermassive black holes at the centre of some galaxies could have densities less than water, so in theory, they could float on the substance they were gobbling ...
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How does light behave within a black hole's event horizon?
If the event horizon of a black hole is the distance from the center from within which light cannot escape, imagine a person with a flashlight falls into the black hole.
He points his flashlight in a ...
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Fighting a black hole: Could a strong spherical shell inside an event horizon resist falling in to the singularity?
As a thought experiment imagine an incredibly strong spherical shell with a diameter a bit smaller than the event horizon of a particular large black hole. The shell is split into two hemispheres, ...
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How can a black hole produce sound?
I was reading this article from NASA -- it's NASA -- and literally found myself perplexed. The article describes the discovery that black holes emit a "note" that has physical ramifications ...
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Is there a way to split a black hole?
Classically, black holes can merge, becoming a single black hole with an horizon area greater than the sum of both merged components.
Is it thermodynamically / statistically possible to split a black ...
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Do all massive bodies emit Hawking or Unruh radiation?
It is known that any accelerated observer is subject to a heat bath due to Unruh radiation. The principle of equivalence suggests that any stationary observer on the surface of a massive body should ...
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How do we know the Schwarzschild solution contains an object of mass $M$?
The Schwarzschild metric is
$$ds^2 = - \left( 1 - \frac{2GM}{r} \right) dt^2 + \left(1-\frac{2GM}{r}\right)^{-1} dr^2 + r^2 d\Omega^2.$$
In all GR books, it is stated that $M$ is the mass of the black ...
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What is Hawking radiation and how does it cause a black hole to evaporate?
My understanding is that Hawking radiation isn't really radiated from a black hole, but rather occurs when a particle anti-particle pair spontaneously pop into existence, and before they can ...
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Does light really travel more slowly near a massive body?
It is a routine problem for beginners in general relativity to calculate the coordinate velocity of light for the Schwarzschild metric. Starting from the metric:
$$ ds^2 = -\left(1-\frac{r_s}{r}\...
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Thought Experiment - Poking a stick across a Black Hole's Event Horizon
The classical explanation of a black hole says that if you get to close, you reach a point - the event horizon radius - from which you cannot escape even travelling at the speed of light. Then they ...
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What is the capture cross-section of a black hole region for ultra-relativistic particles?
What is the capture cross-section of a black hole region for ultra-relativistic particles? I have read that it is
$$\sigma ~=~ \frac{27}{4}\pi R^{2}_{s}$$
for a Schwarzschild BH in the geometric ...
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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?
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What happens to an embedded magnetic field when a black hole is formed from rotating charged dust?
Black holes have no-hair so there are uniquely specified by a mass, charge and angular momentum. Imagine a cloud of charged rotating dust. There will be a magnetic field associated with the current ...
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Do new universes form on the other side of black holes?
I have four questions about black holes and universe formations.
Do new universes form on the other side of black holes?
Was our own universe formed by this process?
Was our big bang a black hole ...
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Another layman blackhole question, pulling one end of a string out from behind the event horizon
No long explanation is needed,
What would happen if I were to allow one end of a rope to fall past the event horizon of a black hole while I held the other end?
Would I be able to pull it out? Would ...
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