43
votes
Why light can't escape a black hole but can escape a star with same mass?
yes, it is because the black hole is much more dense which means it packs the same mass & gravitational pull into a much smaller diameter. This means its surface gravity yields an escape velocity ...
- 90k
29
votes
How can a grain of sand be "spaghettified" when nearing a black hole?
Spagghetification occurs when the gravitational potential energy on the side of the grain of sand closer to the center of the black hole is much larger than the potential energy on the other side. The ...
- 46.4k
25
votes
Was Stephen Hawking's explanation of Hawking Radiation in "A Brief History of Time" not entirely accurate?
Hawking's explanation in A Brief History of Time is pictorial. It is meant to illustrate the effect, but doesn't really explain it. He actually introduced this illustration in the original article, ...
- 16.3k
23
votes
What does it mean for the laws of physics to "break down" at a singularity?
The "laws of physics" are mathematical models that describe the behavior of things in the universe. We use the laws to determine things like how matter moves and how fields interact by ...
- 6,826
19
votes
Accepted
How do we know the assumptions of the Schwarzschild solution are valid?
This is a very long-winded answer, so let me state the punchline up front: the boundary condition you pointed out really is physically suspect, and probably not realized in Nature -- however using it ...
- 46.4k
17
votes
How can a grain of sand be "spaghettified" when nearing a black hole?
The difference in gravity between the "front" and the "rear" of a grain of sand sent towards a black hole should/must be negligible, considering how weak gravity as a force is, ...
17
votes
Are black holes neutron stars with more mass?
I would answer your question "yes and no" :)
"Yes": If you take a neutron star, and add mass to it, it will not stay a neutron star forever. Eventually, you will surpass the ...
- 46.4k
16
votes
Schwarzschild metric with negative mass
Yes, the Schwarzschild solution with $M<0$ is still a vacuum solution to the Einstein equations. Bondi explored some ideas about negative mass in GR in this paper, in which he says
As long as ...
- 67.5k
14
votes
If massless objects ALWAYS travel at the speed of light and gluons are massless, how are they trapped within hadrons without a need for event horizon?
That quarks and gluons are trapped inside hadrons is called color confinement. As far as I know: it is unrelated to black holes but I don't read Phys Rev D... it's possible it has been suggested ...
- 31.4k
14
votes
Why light can't escape a black hole but can escape a star with same mass?
There are a lot of ways to explain this, with various levels of accuracy. I'm going to choose a way to justify this, in the spirit that the original question seems to not come from a place with a lot ...
14
votes
Do black holes have a size?
To the best of our understanding, any matter inside a black hole cannot remain at a constant radius. So the sphere of "matter" in your diagram must be collapsing towards the singularity; ...
- 47k
13
votes
Was Stephen Hawking's explanation of Hawking Radiation in "A Brief History of Time" not entirely accurate?
All of the positive claims that you listed:
The radiation arises not from the event horizon exclusively, but from the entirety of the curved space around it
The curved space around the black hole is ...
- 25.3k
13
votes
Accepted
Hayward black hole
The Hayward metric is a non-singular black hole metric in the sense that a central singularity is absent. It satisfies the condition $g_{tt}=1/g_{rr}$ (hence it is sourced by some particular theory ...
- 2,420
12
votes
If massless objects ALWAYS travel at the speed of light and gluons are massless, how are they trapped within hadrons without a need for event horizon?
Now, to make the light trapped within a small region of spacetime, we
need curvature so big that it causes an event horizon, so now we have
a Black Hole
Photons do not carry an electric charge. So ...
- 8,147
11
votes
How are white holes described as outgoing spacetime and still have attractive gravity?
There is no contradiction. “Attractive gravity” is about acceleration, which is directed toward the massive body. While “inward-flowing spacetime” in the river model of black hole (or “outward-...
- 15.4k
10
votes
Do black holes have a size?
One of the important lessons of quantum mechanics is an operational (as opposed to philosophical) approach to measurement. If there is some property of a system that is really impossible to measure, ...
rob♦
- 86.2k
10
votes
How can two black holes merge without violating No Hair?
The no-hair theorem refers to stationary black hole solutions to the Einstein equations. If the black holes in question are undergoing dynamics, then the no-hair theorem doesn't apply. Specifically, ...
- 67.5k
9
votes
How much pressure (force) is needed to turn something into a black hole?
If you don't have enough mass to create a black hole through its self-gravitation then you need to apply a pressure that is greater than the neutron degeneracy pressure of degenerate matter. ...
- 47.3k
9
votes
Was Stephen Hawking's explanation of Hawking Radiation in "A Brief History of Time" not entirely accurate?
That Big Think article draws a number of incorrect conclusions from Hawkings particle-antiparticle picture of Hawking radiation, and then goes on to debunk these strawman conclusions and ascribing ...
- 10.3k
8
votes
Why light can't escape a black hole but can escape a star with same mass?
Yes, it's just because the black hole is smaller. The only mass factor relevant to the trajectory of a particle in a spherically symmetric mass distribution is the total mass that is closer to the ...
- 12.7k
8
votes
Falsifiability of a hypothesis related to a gravitational geon at the edge of Solar System that I postulate
Your question is definitely falsifiable. All you need to do is send a space probe to the edge of the solar system and see if it can still send signals. Voyager 1 is pretty far from the solar system - ...
- 6,291
7
votes
Accepted
How do I use the Schwarzschild metric to calculate space curvature and time curvature seperately?
I quite dislike this particular turn of phrase - there is no meaningful sense in which curvature can be split into temporal and spatial parts. We can talk about pure spatial curvature in the sense ...
- 67.5k
7
votes
Why light can't escape a black hole but can escape a star with same mass?
This relates to the shell theorem:
If the body is a spherically symmetric shell (i.e., a hollow ball), no net gravitational force is exerted by the shell on any object inside, regardless of the ...
- 1,076
7
votes
Can the Schwarzschild orbit equation be solved exactly?
Closed-form analytic solutions for bound timelike geodesics around a Schwarzschild black hole were first (to the best of my knowledge) given by Charles G. Darwin (grandson of) in 1959 in this paper (...
- 10.3k
6
votes
Is the Planck mass the minimum required to form a black hole?
There's a pop-science myth going around that Planck units are somehow known to be fundamental to physics. Like many myths, it's based on a kernel of truth: there are reasons to suspect that at the ...
- 7,760
6
votes
How do we know the assumptions of the Schwarzschild solution are valid?
The spacetime be static is not a necessary assumption for obtaining the Schwarzschild solution. Birhoff's theorem tells us that any spherically symmetric vacuum solution to the Einstein equation is ...
- 10.3k
6
votes
Accepted
Creation of Magnetic Monopoles using Black Holes
At that point of time, the magnetic field lines from north pole must not travel outside the event horizon to the south pole and the south pole outside must behave like a monopole. Is this thinking ...
- 94.4k
6
votes
Accepted
What is the Schwarzschild metric in cylindrical coordinates?
As you noted, that's not the Schwarzschild metric in cylindrical coordinates.
In spherical coordinates, where the corresponding Cartesian coordinates would be
$$(x,y,z) = r (\sin θ \cos φ, \sin θ \sin ...
- 1,675
6
votes
How can a grain of sand be "spaghettified" when nearing a black hole?
To answer the "elementary particle" tag of the question which is:
Second question, if grain of sand would spaghettify in these circumstances, would something much smaller like a hydrogen ...
- 232k
6
votes
Accepted
What portion of the universe is black holes?
The cosmic inventory of the mass of various types of object is discussed in a well-known review by Fukugita & Peebles (2004).
They estimate that the fraction of the total matter in the universe ...
- 128k
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