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.

learn more… | top users | synonyms (1)

1
vote
0answers
15 views

Why does one particle in Hawking radiation have negative energy?

So as I understand it, Hawking radiation occurs when virtual antiparticle-particle pair are created near the event horizon of a black hole due to vacuum fluctuations because of Heisenberg uncertainty ...
0
votes
2answers
33 views

Can a binary star system create a stationary black hole?

Research over the last 200 years suggests that half or more of visible stars are part of multiple star systems. I apologise for the number of assumptions in my question but, because of the numbers of ...
2
votes
2answers
21 views

The difference between sub-Eddington and super-Eddington accretion disk models

I am studying accretion disks around active galactic nuclei (AGN). I have seen two sets of models, sub-Eddington and super-Eddington accretion disks. What do these terms refer to in this case? I ...
-1
votes
2answers
67 views

Universe as a black hole - how did everything escape? [duplicate]

While listening to physics lectures, they glossed over things escaping from black holes insinuating it doesn't happen other than radiation... When talking about the period between Planck Epoch & ...
1
vote
3answers
69 views

Ricci scalar for Black Hole

What is the value of Ricci scalar at $r=0$ inside Black Hole? Since $R_{\mu \nu}=0$ is vacuum solution and valid outside event Horizon of black hole where there is no mass energy density. But inside ...
1
vote
2answers
54 views

How do we know singularities/black holes actually bend time?

Recently I read up on some articles about singularities / black holes, how they form and how they affect their surroundings. Just to clarify, I am no physicist and I found those articles on the ...
0
votes
0answers
114 views

What is the mass density distribution of a Schwarzchild black hole?

According to Schwarzschild metric the density element $$ \frac{\mathrm dm}{\mathrm dV} $$ becomes infinite on the event horizon if there is mass $$ \mathrm dm > 0 $$very near at the outside of ...
-1
votes
1answer
53 views
30
votes
3answers
2k views

How much mass can colliding black holes lose as gravitational waves?

Concerning the recent detection of gravitational waves produced by colliding black holes, it has been reported that a significant percentage of the combined mass was lost in the resulting production ...
-4
votes
1answer
63 views

Is the universe actually formed according to the big bang theory? [closed]

Big-Bang pretty much explain the origin of universe, but still many people don't believe it. Isn't it possible that our universe come out of a white hole which have been pulled out from a black hole ...
19
votes
2answers
1k views

Chasing someone who has fallen into a black hole

Assume that my friend and I decided to explore a black hole. I parked the spaceship in a circular orbit safely away from the horizon. He puts on his spacesuit with a jet pack and carefully travels ...
3
votes
1answer
48 views

Killing Horizon for Kerr Black Hole

I have some confusion about Killing Horizon of BH. Since a Killing Horizon (KH) is a null hyper-surface at which killing vector $k^{\mu}$ is null; $k^{\mu}k_{\mu}=0.$ For time translation symmetry $...
0
votes
0answers
10 views

QPO and super-orbital periods

Are super-orbital periods from pulsars/neutron stars the same thing that the Quasi-Periodic Oscillations that apply not only to pulsar but also to more general compact objects like black holes?
0
votes
0answers
25 views

Killing Horizon for a space-time metric

How many Killing Horizon (KH) are possible for a particular S.T. metric. Because there is a surface gravity associated to each KH, which is constant over KH surface. Do a Black Hole have many surface ...
0
votes
0answers
35 views

If a neutron decays inside event horizon of a negatively charged black hole,can the electron from decay escape? [closed]

Suppose a neutron is decaying inside event horizon of a negatively charged black hole and an electron is produced, is that electron possible to escape from black hole due to charge repulsion with the ...
1
vote
1answer
54 views

Black Hole: mass density or energy density? [closed]

Recently, I read a quora post in which the OP asked the following question: Can a black hole be formed from accelerating a body and increasing its relativistic mass to the level of a Schwarzschild ...
0
votes
0answers
23 views

Cygnus X-1, HDE 226868 radial velocity and mass function

Where could I find the data of the Radial Velocity for the visible companion (an O class star) for the binary system of Cygnus X-1? Also for this system, we have the mass function $$ f(M)=\dfrac{PK_C^...
2
votes
0answers
36 views

black hole potential well

Let d be the distance (in some coordinate system) from a particle to the event horizon of a black hole. For a particle starting at distance d>0 above the event horizon to rise away to a very large (...
0
votes
2answers
70 views

Zero gravity at center of astronomical bodies [closed]

All theories agree that the net gravity at center of all astronomical bodies is zero. Are black holes exception to this? Is gravity zero at the center:) of singularity? Is anything like center of ...
0
votes
0answers
26 views

Removing temperature from black hole

Suppose I take the metric of a three dimensional AdS-Schwarzschild black brane (Note not actually a black hole!): $ds^2 = \frac{L^2 dr^2}{r^2 W} + \frac{r^2}{L^2} \left[ - W dt^2 + dz^2 + dx^2 \right]...
0
votes
0answers
24 views

How dense is the material in the core of a black hole? [duplicate]

If I would like to find out the answer of this question, I get two different one: The material is collapsed into a tiny dot. This density is called planck density, and it is so dense, all of the ...
19
votes
3answers
1k views

Why can't a nucleus become a black hole?

A nucleus is very small and very dense. Its density is approximately $2.3 \times 10^{17}~\mathrm{kg/m^3}.$ So why can't a nucleus itself become a black hole?
-1
votes
0answers
25 views

Black Hole mass

Where is the material that has past the event horizon of a black hole stored/kept? Does it become pure energy once it passes the event horizon? Does the singularity at the center of the black hole ...
1
vote
1answer
52 views

What's new about Hawking's theories in relation to 't Hooft's?

What's new about Hawking's theories in relation to 't Hooft's about resolving the black hole information paradox? Thanks "The information is stored in a super translation of the horizon that the ...
1
vote
0answers
22 views

Lower bound in the size of (naturally formed) black holes?

I suppose most of you will agree with me that Black Holes (BH) are among the coolest and most mysterious objects in our Universe. Theoretically predicted a century ago by the Einstein's theory of GR, ...
4
votes
3answers
121 views

Does a planet/star itself lose mass when it 'emits' gravitational waves?

Orbiting planets and stars can create gravitational waves, as seen with the LIGO. But are they also losing mass-energy. Beside that, the kinetic energy associated with their orbital angular momentum ...
0
votes
1answer
49 views

Does fusion continue to happen as a star collapses into a black hole (beyond our view)?

I know that we can't actually see a black hole form, because of time dilation, and we would see a frozen image just before the final collapse past the event horizon, slowly redshifting to nothing... ...
0
votes
1answer
64 views

Computation of photon trajectory near a black hole [closed]

I've been given the task to represent the trajectory of a photon near a Schwarzschild Black hole. I've read the physics behind it, and I've derived the equation of photon orbit near a schwarzschild ...
1
vote
0answers
28 views

Capacitors in space [closed]

I'm new here and I have a question for all of you. What happens to a capacitor in open space? There are different kind of them, which one of these is better to use there? And... We know that if we ...
7
votes
2answers
85 views

Can a black hole fall into another black hole's innermost stable orbit?

Can a black hole, in principle, fall into another black hole's innermost stable circular orbit?
0
votes
0answers
27 views

Fast moving black hole with low rest mass [duplicate]

Sorry for probably very stupid questions but.. If the massive body with small mass (one of the twins, for example) accelerates until the speed close to the speed of light, the mass of this body as ...
0
votes
1answer
61 views

Position / velocity / acceleration vs time graphs when falling towards a black hole

In Newtonian physics, as far as I understand it, for a small object falling towards a massive object, the graphs of position, speed and acceleration vs time look like this: (If the massive object ...
1
vote
0answers
16 views

How does the mass of black holes become so much larger than an average star [duplicate]

How does the mass of black holes become so much larger than the mass of an average star? Since black holes are formed from the collapse of large stars, how come the mass of black holes is of ...
5
votes
0answers
135 views

View of the sky from inside a black hole

Consider an observer located at radius $r_o$ from a Schwarzschild black hole of radius $r_s$. The observer may be inside the event horizon ($r_o < r_s$). Suppose the observer receives a light ray ...
4
votes
2answers
88 views

How Smooth is an Event Horizon?

For a non-rotating Black Hole, the event horizon can be described by the Schwarzschild metric as a sphere. Assuming external observer, away from the Black Hole and also assuming that there is no ...
6
votes
2answers
175 views

What happen if Hawking radiation is not found?

I'm not a physicist but reading from a few popsci books. I read that Hawking has equations that explain Hawking Radiation precisely, right? Is this equation a proof that the radiation is real or we ...
4
votes
2answers
220 views

Sending someone into the future using the sun?

Ok so for starters, i am not nearly as smart as the least-smart person on here, but I have been reading a lot about all sorts of stuff. We now know that the ability to "send" someone to the future is ...
3
votes
1answer
32 views

Does the Bekenstein entropy bound present problems for unitarity in cosmological models that invovle a collapse or bounce?

If we expect the Bekenstein bound, or something like it, to hold in a collapsing universe, will that not eventually force us to accept some loss of information, or is there something I'm missing?
-3
votes
2answers
66 views

Big Crunch, then new Big Bang = exact same Universe? [closed]

If after a Big Crunch, the new singularity explodes in a Big Bang, would we get the same Universe all over again? Since black holes retain all the information they've stored, would we get an exact ...
1
vote
0answers
105 views

Are strings in string theory actually little black holes? [closed]

I sometimes read that strings in string theory are actually little black holes, or can be interpreted that way. Is this true? How is that consistent with that the particle that a string represents ...
4
votes
2answers
133 views

What is the black hole information paradox really? [closed]

Preliminaries What is the black hole information paradox really? Is it a sophisticated way to ponder and debate the existence of an operator on the boundary that can tease out the interior of a ...
0
votes
0answers
26 views

Wave equation for odd spacetimes and source terms

It seems to be quite common practice, when solving the wave equation in spacetimes with odd topologies or horizons, to decompose the solution into a sum of the various origins (or destinations) of the ...
2
votes
0answers
34 views

How can matter be squeezed extremely dense [duplicate]

I'm fairly new to physics and currently reading A Brief History of Time by Stephen Hawking. Hawking is talking about black holes being the size of a few miles containing the mass of multiple times the ...
5
votes
2answers
139 views

What does string theory predict for the singularity inside a black hole?

The usual explanation for what's going on inside a black hole goes something like "General Relativity predicts a singularity with infinite curvature, but when matters gets so tightly compressed we ...
-5
votes
1answer
62 views

Black hole gravity at bottom (deepest end) [closed]

If black holes are hole in space and time then at bottom or below bottom there is no more gravity of black hole. As any hole has an end (Deepest end). Is this true ? ******** EDIT *********** http:/...
11
votes
3answers
2k views

Will a giant ball of protons form a black hole?

Suppose you have enough energy and resources to put together (in a momentarily static configuration in which they are all at rest at the same time) as many protons as you want to form a "proton star". ...
0
votes
1answer
51 views

Can light change the shape of a black hole?

If light orbits a black hole without ever getting closer or further from it, and the black hole pulls on the light from all directions nearly simultaneously, does the black hole extend at its equator? ...
1
vote
0answers
33 views

How to find tetrads corresponding to a given metric?

In General Relativity, sometimes doing calculations with a set of orthonormal tetrads $h^{a}_{\mu}$ where $h^{a}_{\mu}$$h^{b}_{\nu}$$\eta_{ab}=g_{\mu\nu}$ for some metric $g_{\mu\nu}$ is easier ...
1
vote
1answer
49 views

Why do stars having mass less than 3 X mass of sun do not collapse into a black hole

Why do stars having mass less than 3 X mass of sun do not collapse into black holes?? Is there a way to find out the radius of a black hole if we know the mass of the parent star and vice versa??
17
votes
2answers
2k views

What is black hole spin?

First of, congrats to the people at LIGO. In this article, the BBC notes that the latest LIGO results show that a new black hole was formed with a spin of $0.2$ (dimensionless number). What exactly ...