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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Ergosphere in different coordinates

I am currently working with the concept of an ergosphere and I was wondering if it has any meaning to consider the ergosphere after changing coordinates. I mean if someone looks only on the sign of ...
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If Hawking radiation is proved to be true won't black holes lose entropy which is against laws of thermodynamics?

Hawking radiation states that black holes continuously emit radiation. But if its the case won't entropy of black holes decrease which violates the laws of heat, entropy and thermodynamics?
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Bondi-Metzner-Sachs (BMS) related Question(s)

I started studying the BMS group in connection with the set of papers by A. Strominger et al., also related with the supposed solution of the "Black Hole Information Paradox" by S. W. Hawking ...
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Do particles in a black hole stay in constant orbit?

Would particles behave the same way as a satellite orbiting earth, only far more compact and rapid correlating to the gravitational force of the black hole?
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Schwarzschild metric: Change in coordinates corresponds to change in object?

I have been reading about the Schwarzschild metric in the book "General Relativity: An Introduction for Physicists" by Hobson, Efstathiou and Lasenby and it appears to say something counter intuitive. ...
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127 views

If nothing can escape a black hole, where does Hawking radiation come from? [duplicate]

Hawking radiation is a form of electromagnetic wave travelling at the speed of light. Hawking radiation is radiated from a black hole. If nothing can escape the gravitational pull of a black hole, how ...
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What equation (/solution) predicts the existence of black holes?

Where does our theoretical prediction of the existence of black holes come from? If it is (as I am guessing) from the Einstein Field Equations, which solution predicts it and why?
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Where is the event horizon in a black hole?

At the beginning I thought that the event horizon coincides with the surfaces, but then making a new name when you could just call it surface would seem a bit pointless. Then where is the event ...
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Why isn't the universe a singularity? [duplicate]

So, i'm just a high-schooler interested in physics, so don't judge me if I misinterpreted anything. But from my understanding of black holes, a singularity is created because any matter inside the ...
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Black Hole Information Paradox: Why Isn't the Position of A Black Hole Count as Part of Its “Information”?

It's often said that a black hole only has three properties: its mass its electric charge, and its angular momentum Why isn't it's position and velocity relative to other objects part of this ...
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Problem imagining how does a black hole merger look like from far away

Suppose we have two black holes moving on a path of direct frontal collision. Is it correct that from far away, due to time dilation, we can never "detect" that the two black holes merging or ...
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Why are black holes called singularities? [duplicate]

I have researched black holes but I can't seem to find the answer. Any help would be appreciated.
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Do objects' velocities slow down going into black holes? Would they appear to?

I always hear that time slows down as you enter a black hole, and when I hear this, I always mentally imagine all of a person's motions, including their net velocity relative to the black hole, ...
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Can a particle tunnel from inside a black hole?

Event horizon isn’t special from GTR standpoint, and at least in AdS/CFT correspondence gravity can be “removed” from consideration entirely. So can a particle whose wave function is completely inside ...
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Relation between dark matter and supermassive black holes located in the center of galaxies

This research (see link), conducted on 3000 elliptical galaxies (which are typically galaxy merger results), reports finding "distinct relationship between the mass of the dark matter halo and the ...
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Black holes have conservative potentials. Why don't things “swing” in and out?

Black holes attract objects via gravity, which is a conservative force. Thinking from conservation of energy, It seems like for an object moving toward them, they should be able to "swing" into and ...
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How can a black hole be a cause of a Big Bang? [closed]

Is it possible that Big Bang occurred as a result of the black hole formation - that is the matter compressed so densely that it "exploded" within the boundaries of its horizon and became a what we ...
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123 views

Black-holes, space time and their bending [closed]

I have been watching some videos related to how space-time is bent because of the mass (just "visually", not mathematically), and then I watched some videos about black holes. My own conclusions ...
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Clocks at rest in Kruskal coordinates

The proper time in Kruskal coordinates is given by $$d\tau^2=\frac{4}{e^rr}(dT^2-dX^2)$$ (where r is measured in units of Schwarzschild radii). Since at $T=+/-\infty$, $dT^2-dX^2$ goes to zero for an ...
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Do black holes violate the first law of thermodynamics?

When a black hole absorbs matter is it destroying that mass, thereby destroying energy, therefore violating the first law of thermodynamics?
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Is it theoretically possible to prevent or deter a star from becoming a black hole?

Suppose NASA decides too many black holes are being produced in a year, so it decides to do something about that. Given our current scientific knowledge, is there theoretically any way NASA could ...
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How small would an electron have to be in order to become a black hole?

According to quantum mechanics electrons don't behave like everyday objects. Would this prevent the formation of a black hole? If so why? Also, assuming they behave like everyday objects, at what ...
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When a neutron star accumulates enough mass, does a black hole start at its center?

With less dense bodies, such as the earth and the sun, the center has less gravity/density (since there's an equal amount of mass surrounding the center, pulling out on it from all directions). Does ...
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Are black holes in a binary system with white holes, and are they both wormholes? [closed]

What is a black hole? The general explanation is that a black hole is small region in space with such strong gravitational effects that nothing can escape and even light is trapped inside of the event ...
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Why can black hole evaporate if objects need infinity time to reach event horizon (as seen by a distant observer)?

I am new to black hole, but have a question about it: Object needs infinity time to reach event horizons (as seen by a distant observer). Particle-antiparticles separates, one of them drops into ...
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Falling into a black hole doesn't reveal the future, but does that mean it's not happening?

In a previous answer it was said that you wouldn't see the universe speed up as you fell into a black hole, which I accept as true. However, is being unable to see or interact with the outside ...
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The black hole paradox

I recently read in the news that Stephen Hawking claims to have solved the Black Hole Information paradox. I researched a bit about the paradox and the research that Stephen Hawking did to solve it. ...
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274 views

Certain doubts regarding the movie 'Interstellar' [closed]

In the movie, or rather a scientific fiction, Interstellar, we saw in the last part, that the actor falls into the black hole, travels in time, which is a dimension inside the black hole, and then, ...
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Black Holes and Einstein Field Equations

I recently watched a video where Michio Kaku argues that there must be something fundamentally wrong with the theory of General Relativity because it gives time t=0 as a solution at the centre of a ...
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Trying to give a (non-rotating) black hole an electric dipole

Suppose there is a Schwarzschild black hole. You add positrons to the north "pole" and electrons to the south "pole". From an external observer, objects that you add end up "stuck" to the horizon. ...
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86 views

Does spaghettification really happen with conditions found close to a black hole?

This website tries to explain why the term spaghettification doesn't actually occur when something gets close to a black hole. The argument of the author is that the equations we use to predict the ...
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Could the Big Bang have simply been two black holes hitting each other at 99% of the speed of light? [closed]

Given two equally massive black holes moving at 99% of light speed, it seems there would clearly be enough energy for most (99%?) of the mass of both to be ejected and / or radiated well beyond the ...
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Can something (again) ever fall through the event horizon?

Since I am more confused by the answers given in this site to the many variants and duplicates of this question, with some arguing that from the point of view of the falling observer, it happens in ...
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Does a wormhole on the event horizon keep working?

Recently the discovery of how to build a magnetic wormhole has been all over the news. These wormholes tunnel magnetic fields instantly from one point to another point. (Although the current model is ...
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Do matter that falls in a black hole go to the future and the end of the universe?

Black holes are black because everything that enters don't exists anymore. When you pass the event horizon, the time stops for you, all the future of the universe has passed for you and the universe ...
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What is the meaning of t=infinity at the black hole horizon?

Looking at the Kruskal diagram for black holes, it seems to me that at the horizon, all free falling objects cross the horizon at the same coordinate radius and time. Does this not mean that all ...
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Can I say that the two-body problem is also a vacuum solution for the special case of two orbiting black holes?

A two body system is one where two bodies orbit each other. In the case of two orbiting black holes, since an individual black hole is described by a vacuum solution, can I say that the two-body ...
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How can black holes be observed to grow? [duplicate]

If, from the reference frame of an observer at rest outside a black hole, it takes an infinite amount of time for an object to be observed to fall into a black hole, how can black holes ever be ...
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If black holes almost or barely collide

Consider black holes approaching each other at some relative velocity and varying impact parameters. For impact parameters above some value they will sail past each other. For impact parameters below ...
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Could black holes interact with dark matter at distances far greater than matter?

I watched this Nova episode on super massive black holes and learned that the speed at which the outer stars in a galaxy orbit correspond to the size of the black hole at the galaxy's center. They ...
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Is there any situation in which a primordial black hole-sized black hole form AFTER the “Big Bang?”

I have seen that primordial black holes may have formed at the start of the universe due to the density of matter. So, is there any way in which our current Universe could spawn a non-stellar mass ...
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Before we realized that supermassive black holes were at the center of large galaxies, how did the models explain galaxies?

So, I was watching this show on Netflix about supermassive black holes. I didn't realize that astronomers were surprised to find these black holes at the center of galaxies, because I cannot imagine ...
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Black Hole horizons and the Equivalence Principle?

In the reference frame of a freefalling observer, does crossing the event horizon not cause a contradiction between two classical principles that supposedly apply at the event horizon: the equivalence ...
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Can photons be converted to dark matter in black holes? [duplicate]

I'm a pharmacy student so if it's not a good theory let's just not laugh at me :) I was wondering if it is possible that when the light enters a black hole and gets trapped in its gravity, its energy ...
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If spacetime is folded upon itself [closed]

Say there are two black holes about a parsec apart. You theoretically fold spacetime upon itself so that the singularities of the black holes are "touching". So the picture link I left above shows ...
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Is Hawking radiation valid for a microscopic black hole?

A black hole evaporates by Hawking radiation. The computation of the evaporation time uses some approximations. Question: Is the evaporation time valid for a microscopic black hole? In particular, ...
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818 views

What the lifetime of a massless black hole of photons?

Let a bundle of photons very concentrated in a very small area so that the space-time is curved as a black hole, and the photons can't escape: this is what I call a massless black hole of photons. ...
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If a black hole is just warped spacetime, then where is the electric charge?

I've heard Kip Thorne repeatedly state that matter is destroyed when a black hole is created, that all you are left with is distorted spacetime. "The idea that black holes are made from very ...
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time span in a black hole vs earth [closed]

This is a thought experiment: I am an immortal and I live for 1 billion years on earth. My identical twin brother spends the same amount of time in/on a black hole. To him, did he spend exactly 1 ...
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Blackholes / density [duplicate]

If a star explodes to form a black hole how does the gravitational field become infinite from one state to the next? ie: it seems additional mass has been added or is this simply a function of ...