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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Fine structure parameter $\alpha$ at event horizon

As magnetic field lines collect at a black hole event horizont (for no field lines will be present in the black hole) then ratio of electric field to magnetic field will change, so by this change in ...
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Photons hitting the Black Hole Singularity

I was watching one of Dr. Susskind's Lectures on String theory and he was talking about black hole singularities (see the 40 minute mark of this lecture ...
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The speed of light through space and its effects on black holes

I was curious about a couple of things that don't quite make sense to me: (and probably won't when I get the technical answer, but I'll ask it anywise.) When light travels through space, is its ...
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Are black holes stationary?

If light/energy with zero mass hits the speed limit, are black holes with infinite density at a universal standstill with everything moving relative to them? Am I barking up the wrong tree as they ...
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399 views

Firewall's grandfather paradox

See What are cosmological "firewalls"?. Alice is in freefall in her spacecraft just above the horizon of a gigantic black hole. She measures whether or not the near modes of the horizon ...
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Light orbiting a massive body [duplicate]

Possible Duplicate: Can a photon be made to orbit a known (or undiscovered theoretical) body? How massive would a black hole have to be for light to orbit it at 1km away from the ...
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picture of newtonian gravity and black hole

We assume that the earth planet is a black hole. When a light beam is fired. The only way that even light can move is A and B which means this is impossible that light move on other ways C, D, E ...
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Do wormhole exits require matter? If not, would they change the edge of the universe?

As a layperson who watches way too many theoretical physics and astronomy documentaries, I've seen the folded-piece-of-paper analogy of wormholes a bajillion times. Usually they explain that black ...
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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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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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How do gravitational waves and black holes interact?

Following the first detection of gravitational waves yesterday (11 Feb. 2016) by LIGO, I have a couple of questions about how gravitational waves and black holes interact. Assume that there is a ...
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Flirting with a large black hole event horizon

I read with large black holes the tidal forces at the event horizon aren't particularly great, so you may not feel anything particularly special when you cross them. I also understand that if one is ...
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How much light is necessary to form a black hole [duplicate]

I suppose that enough light in a small enough volume could create a black hole. What is the good quantity that can tell when light can or cannot make a black hole? Energy density? But there must be ...
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Can black holes with the same mass evaporate with different speeds?

I am curious about the following observations: (1) For a normal Schwarzschild black hole, it evaporates according to $dm/dt=-1/m^2$; (2) We have eternal black holes which do not evaporate (though ...
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Why can't we see black holes?

Is it all because they are so black? But even our tech is not able to see black holes. We can only tell where black holes are, when they are eating stars. Or it's because they consume light ?
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Black hole starship: Would micro black-hole Hawking radiation emit anti-matter?

My understanding of Hawking radiation is that virtual particle pairs of matter and anti-matter are created from vacuum energy and one of those virtual particles may escape while the other enters the ...
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Shapes in a black hole

A question around an answer of Timaeus What happens to a particle in the exact center of a Kerr black hole?-answer Outside both is a normal type region of spacetime. In between the two the $r$ ...
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Two black holes prepared from different initial states

I have asked a similar question, I would like to reformulate it in more details. Here is a thought experiment: Assuming we can create black holes by squeezing photons, we can then prepare two ...
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Can a massive imploding sphere smash the holographic principle and conservation of information into oblivion?

Let $\text{BH}_1$ be a black hole of mass $M_1$, zero charge and angular momentum, and event horizon radius $R_1=\frac{2GM_1}{c^2}$. Alice is loitering at distance (all distances here are from the ...
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Hawking radiation and black hole evaporation [duplicate]

As I understand it, Hawking radiation predicts black hole evaporation because of quantum effects. After the creation of a particle/anti-particle near the horizon of the black hole, if the one gets ...
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Hawking paradox [duplicate]

Over the centuries, physicists "gave up" on conservation of elements, conservation of mass and then conservation of energy, which now we know to be related to symmetries. How can we be sure that ...
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When is the event horizon a Killing horizon?

I know the definition of both (event horizon is closure of causal past of future null infinity whilst Killing horizon is a null surface where some Killing vector becomes null e.g. the surface where it ...
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Clarification on hawking radiation [duplicate]

I read that a pair of entangled quantum particles can spontaneously pop into existence for a brief moment and especially near to the event horizon whereby one of the entangled quantum particle passes ...
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Is the information that a human brain (and body) “contains” superflorous to the information contained within the atoms of the body?

Layman here. I recently watched this video talking about Stephen Hawking's theory of hairy black holes. The speaker mentions, as I've heard before, that once "something" has entered a blackhole its ...
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Acceleration of a static observer in Schwarzschild measured in locally inertial frame [closed]

Consider a static observer at $r=R>2M$ with four velocity given by $$U^\mu=\frac{dx^\mu}{d\tau}=(1-\frac{2M}{R})^{-1/2}\delta^\mu_0$$ in the Schwarzschild metric ...
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How do black holes form without violating the uncertainty principle? [duplicate]

Many videos on YouTube while discussing black holes mention that it's born out of a heavy star when it collapses into a single point and that infinitely curves spacetime around it. When all the mass ...
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Black hole and relativity [duplicate]

This link shows movement around the black whole. It seems impossible for them to move this fast. http://www.youtube.com/watch?v=ENd8Sz0AFOk If we were able to watch a ship of people go into a black ...
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Is black hole a black body? [duplicate]

I have studied in my physics class today that black body is one which absorbs radiation of all wavelength incident on it , Its absorptance is unity , reflectance and transmittance is zero . I wonder ...
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What would you experience if the universe ceased existing whilst you were under extreme time dilation?

Hypothetical situation, if you were in the center of a supermassive black hole (and managed to not be immediately obliterated), time dilation would be so that billions of years may pass outside the ...
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Coordinate Singularity in Metric

Suppose I have some metric $$ds^2=g(t)dt^2+\frac{1}{r}dr^2$$ which has a singularity at $r=0$. However, if I make the coordinate transformation $u=\frac{1}{r}$, then I get: $$ds^2=g(t)dt^2+r^3 ...
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String spreading?

What is 'string spreading' (in the context of string theory)? Does it have implications for the observability of post-classical effective field theory of gravity terms near macroscopic black hole ...
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Does 'Entropy-Sated' mean that no 'Order' can be produced from a black hole?

I believe I have a misconception somewhere that I need help to clear up. No-Hair indicates that a black hole created from highly ordered matter with low entropy reaches the same final state as ...
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Acceleration measured when hovering over a non-rotating black hole [duplicate]

I’m wondering what would be the acceleration theoretically measured (for example by an accelerometer) locally by an astronaut whose (ideally pointlike) spacecraft is hovering steadly over a non ...
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What subatomic particle is a blackhole composed of?

In a black hole, what is the "state" in which the mass that is present is in? Are they there as tightly packed neutron or soup of quark or are particles broken down into 10D strings and coiled as ...
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if somehow we could survive in black hole would we survive the end of universe as time inside black hole would stop and would be infinite? [duplicate]

as time goes on slowing with increase in mass. how can black hole ever form in finite time. is infinite time has different meaning for different observer. if so would the event of universe dying be ...
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How to get null tetrad by metric?

How to get null tetrads ${l^a,n^a,m^a,\overline{m}^a}$ for this metric? This on is from Ryder's book (Introduction to general relativity) page 268 $g^{\mu\nu}=\begin{pmatrix} 0 & \frac{1}{c} ...
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Linear extent of an accretion disc

I have a homework assignment question on accretions discs (essentially an estimation of the number of electron scatterings, but this is just for background). There are a few parameters, one of them ...
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Angular momentum of rotating black holes

If a black hole rotates, how does the information about its angular momentum get passed up beyond its event horizon?
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How does time stoppage inside a blackhole affect physics? [duplicate]

From the gravitational time dilation theory we know that at event horizon of a black hole the time should stop completely. However all the physics phenomenon we know (at macro level) e.g laws of ...
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Cauchy-horizon (Kerr solution)

The Cauchy horizon, particularly the infinite blue shift and why the horizon becomes singular if a object passes him is not really understandable for me. Can anybody explain this mathematically ...
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Motion without matter

Suppose you have a vacuum spacetime with non zero cosmological constant then you can show that two test particles will move toward each other or apart depending on whether it is negative or positive ...
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Gravitational Time Dilation and Schwarzschild Coordinates

So I'm trying to use this equation for the time dilation of an object, but I don't know how to get the distance that I have (in meters) to a radial coordinate in terms of schwarzschild coordinates. ...
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Topology of Anti-de Sitter manifold with black hole

I'm interested in understanding the topology of space-time with a black hole. In other words how does having a black hole affect quantities such as the fundamental group, de-Rham cohomologies, Euler ...
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Is there a center of gravity in black hole?

I've come across this particular gif "http://i.imgur.com/AOCqg5j.gifv" If you can see the above gif, you can see the two perfectly spherical black holes merge together forming a single larger ...
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How should I interpret relativistic mass? [duplicate]

If I have a mass that gets accelerated to a near the speed of light, before it gets I would think its relativistic mass would expand its Schwarzschild radius enough to turn it into a black hole. I ...
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Do we know for sure that black holes must contain a singularity? [duplicate]

I've perhaps naively wondered for years whether there are any rules or proofs that allow us to know for sure that if all an object's mass is contained within its Schwarzschild radius it must be a ...
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High energy photon collapsing into its own black hole

If I did not mess up the math, a photon with a wave length of $$ \lambda = \sqrt{2\pi}\, l_p$$ where $l_p$ is the Planck length has a mass (energy/$c^2$) of $$ m = \sqrt{2\pi}\, m_p$$ where $m_p$ ...
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Gravitational Lensing by Sagittarius A in the center of the Milky Way

It was suggested that there is a black hole in the center of the Milky Way, called Sagittarius A. The whole region is heavily obscured by dust, although some very long wavelengths can break through. ...
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Archimede's Black Hole

So, I read Can a black hole be formed by radiation? and I think I get it. However, I do have a follow-up question. Would it be possible for an interstellar civilization to build huge mirrors around ...
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Can a black hole move at speed of light?

Black hole comes in all sizes ranging from microscopic black hole on a quantum scale to supermassive black hole that resides in the center of probably every galaxy. Photon have no rest mass hence they ...