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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Quarks falling in a black hole?

All I know about quarks is that they make up protons and neutrons and you can't really pull a quark pair apart, you just end up with 2 quark pairs because of all the energy you added becomes 2 new ...
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193 views

How does Hawking radiation grow as a black hole evaporates?

The temperature of Hawking radiation is inversely proportional to the mass of a black hole, $T_{\rm H}\propto M_{\rm BH}^{-1}$, and so as the black hole shrinks the temperature of the radiation should ...
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Based on our current observations, what all prevents a formed black from experiencing a repulsive force to overcome its gravitational force…? [duplicate]

Edit: I don't think this is a duplicate. [***] Question: Based on our current observations, what prevents a black from experiencing any sort of repulsive force to overcome its gravitational force ...
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Where does the energy of a photon trying to escape a black hole go?

I've heard "light cannot escape a black hole" explained several ways. One is that if a photon inside the event horizon tries to escape a black hole it loses energy to gravity. As it loses energy its ...
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How are negative energy orbits around a Black Hole defined?

I have read several times that within the ergosphere or a Kerr Black Hole, it is possible to have particles that have an orbital energy value of less than 0. However, I do not understand the concept ...
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48 views

What effect would a singularity have on the fabric of space time?

Galaxies are always moving and it seems that there are SMBHs at the heart of most galaxies. If black holes actually twist up spacetime so much thru infinite density how do we know that after the ...
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1answer
85 views

If an object has a temperature, does it have to radiate?

I'm reading through a powerpoint presentation about Hawking Radiation (HR). They are explaining all of the reasons that built up to the postulate of HR, and one of the reasons is that if there is a ...
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1answer
323 views

Can tachyons escape the gravitational pull of a classical black hole?

Anything that crosses the event horizon of a black hole cannot escape the pull since it has crossed the Schwarzschild radius and thus, the escape velocity is greater than the speed of light, and since ...
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1answer
355 views

In the big crunch theory, when the big crunch singularity forms, can the resulting black hole decay through hawking's radiation?

I've been pondering about this and I couldn't really find the answer for this. The big crunch theory postulates that the universe will eventually stop expanding and reverse back in on its self into a ...
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1answer
135 views

Two spherically symmetric shells of matter collapsing into a black hole

Suppose I have a thin spherically symmetric shell of matter of (rest) mass $M$ that collapses to form a black hole, and later a second thin spherical shell of mass (rest) $M'$, concentric to the first,...
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1answer
247 views

What happened to the black hole firewall theory?

What happened to the black hole firewall theory? Back in 2012, some physicists apparently came up with strong evidence that one of three things must be wrong for black holes to work the way we thought ...
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Hawking radiation and energy-negative energy pair production [duplicate]

A black hole evaporates through hawking radiation, what I don't get is the requirement for an energy-negative energy pair production. Since it's the black hole's gravitational energy that's ...
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Black holes shouldn't really exist? [duplicate]

General relativity states that for an observer sufficiently far from the gravitational field of a blackhole, the space time geodesic nearr the event horizon is so long that we should never observe an ...
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58 views

Orbital period and velocity around a Kerr black hole relative to fixed stars

I've been trying to make progress on some of the smaller pieces of this question about the environment around a Kerr black hole. In order to calculate the effects of special relativistic Doppler shift ...
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1answer
58 views

How can black hole interior theories be falsified?

How can any theory of the interior of black holes be falsified if the interior cannot be observed?
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32 views

What if a black hole of normal matter and a black hole of antimatter collided? [duplicate]

This is a curiosity question. Considering same mass black holes. As nothing can come out of either black hole, would the annihilation result into a black hole of energy? Assuming the two black holes ...
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0answers
26 views

Why '1+log slicing condition' and 'Gamma Driver Shift Condition' were successful in black hole simulations?

The 1+log slicing and Gamma driver shift conditions are I want to know if there is a specific reason why these conditions were used most for Black Hole simulations in Numerical Relativty. And how ...
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1answer
70 views

Does a singularity appear the instant a black hole is formed? [duplicate]

Imagine a very heavy (tens of solar masses) star in its final moments before collapsing to form a black hole. The gravitational force exerted by the weight of the star overcomes the neutron degeneracy ...
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3answers
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Does the mass of a star change as it collapses into a black hole?

I know (I think!) that when a really big star collapses on itself it creates a black hole. My question: When a star collapses, is the mass equal to the mass of the star when it's not a black hole? Or ...
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0answers
50 views

Is the general definition for black holes wrong? [duplicate]

I often read the definition of event horizon of a black hole as the region where the escape velocity is bigger than the speed of light. However this would imply that if you're inside the event horizon ...
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2answers
970 views

Einstein Equation at the Singularity. Understanding the Dr. Michio Kaku's explanation

I'm trying to understand what Dr. Michio Kaku is exaplaining in this video: http://www.youtube.com/watch?v=hydDhUNvva8 I'm just able to rewrite this two equations. I think I've probably made ​​some ...
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1answer
35 views

How do black holes join each other? [duplicate]

If black holes can swallow any object, and can radiate energy, then how could black holes join each other and can form bigger black holes?
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Can gravitational waves observed far from a black hole tell us anything about the multipole moments of a dynamical horizon?

In a paper by Ashtekar et al in 2013 on the approach to the final state to a stationary black hole they study the evolution of the multipole moments of dynamical horizons, which relax away (except for ...
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1answer
48 views

Can you see external light approaching a black hole when you are within it?

We can't see a BH because light can't escape from it, but when you are within a BH (assuming you would stay alive) could you see any light from extrenal stars? If so, would the ligth be very bright or ...
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5answers
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Event Horizon violability?

Is the "event horizon" of a black hole potentially violable? Black holes are commonly described as being unidirectional (matter / energy goes in, but doesn't come out), but is the event horizon of a ...
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1answer
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Geometry inside the event horizon

I'm trying to understand intuitively the geometry as it would look to an observer entering the event horizon of a Schwarszchild black hole. I would appreciate any insights or corrections to the above. ...
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Can a rotating black hole have a donut-shaped event horizon? [closed]

It is conjectured that a rotating black hole has at its center a ring-shaped singularity. Thus, at the center of the ring-shaped singularity the gravitational field must be zero (similar to ...
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1answer
101 views

Does a black hole really slow down time?

When an object gets pulled into a black hole it seems to slow and stop, but could it be possibly be because the speed of light that hit the object and came back was slowing down as the object got ...
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2answers
209 views

Why is the mass of a Kerr black hole proportional to its angular momentum?

I'm a third year mathematics undergrad, and have just started the module General Relativity and spacetime geometry, I also have a keen interest in black holes. However I would like to know why and ...
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1answer
58 views

Information from inside a black hole

Now I'm hardly a physicist, but I am pretty interested in it. I was thinking about black holes and the movie Interstellar, and if you've seen it, then one of the central points about it is that they ...
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Is there any known dynamical process to obtain supertubes?

I'm looking for a dynamical process to produce supertubes. For example, microstates of the D1-D5 black hole system can be understood as supertubes. See, for example, http://arxiv.org/abs/hep-th/...
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Can you have black holes in your black holes?

Inspired by Are we inside a black hole?, can you have a black hole such that other black holes are in them? In particular, the event horizon of the larger black hole should completely enclose the ...
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36 views

Help needed to understand Kerr coordinate transformation

The (uncharged) Kerr metric for a black hole of mass $M$ and angular momentum $Ma$ takes the form $$ds^{2} = \Sigma\Big(\frac{dr^{2}}{\Delta} + d\theta^{2}\Big) + (r^{2} + a^{2})\text{sin}^{2}\theta ...
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Are we inside a black hole?

I was surprised to only recently notice that An object of any density can be large enough to fall within its own Schwarzschild radius. Of course! It turns out that supermassive black holes at ...
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1answer
220 views

Please justify invoking logical positivism to causal patches and black hole interiors in quantum gravity! [closed]

Logical positivism is often invoked to explain why external observers can't talk about black hole interiors, or why we can't talk about what happens outside our causal patch in inflationary models. ...
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1answer
45 views

Time-dependent time dilation for circular orbits around a Schwarzschild black hole?

Take two clocks, one in a circular orbit around a Schwarzschild black hole and another with a distant observer. The time dilation factor between the two clocks is said (at https://en.wikipedia.org/...
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How is it possible to have such massive black holes? [duplicate]

Recent observations discovered really massive black holes, up to $20-40$ billions Solar masses. Now, according to an recent study and various computer simulations (I'm sorry, I don't have any ...
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2answers
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Power of blueshifted light falling on observer in circular orbit around Schwarzschild black hole

This answer explains that the time dilation for an observer in a circular orbit around a Schwarzschild black hole, relative to a distant observer at rest relative to the black hole, is given by the ...
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2answers
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Wald's General Relativity, section 6.3 Page 144

I cannot understand how he reaches the conclusion in equation 6.3.36 and 6.3.37; even the terminology is somewhat confusing. This is a problem of bending of light under gravitational field. This is ...
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1answer
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What Color Are Black Holes Really? (Yes, a serious question)

So I got into a mini-debate in science class today because I proposed that black holes aren't really black, they only look black because light can't reflect off them. But if you were to take the ...
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Are black holes very dense matter or empty?

The popular description of black holes, especially outside the academia, is that they are highly dense objects; so dense that even light (as particle or as waves) cannot escape it once it falls inside ...
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why does apparent horizon coincides with the event horizon in stationary spacetime?

Regarding the statement that the apparent horizon coincides with the event horizon (well, the intersection of the event horizon with the Cauchy surface where the apparent horizon is defined) in ...
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What would the effects of the GW150914 gravity wave burst be on observers much closer that 1.3B LY? [duplicate]

The effects of the GW150914 gravity wave burst were barely observable with state of the art instruments, i.e. LIGO. What would the effects of GW150914 gravity wave burst be on observers much closer ...
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180 views

Singularity - exactly what is it?

In quantum mechanics and gravity theories one always meets the word "singularity" in connection with talks about black holes and the big bang, etc. Now in mathematics a singularity is well defined – ...
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1answer
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Classical Limit of Schwarzschild Metric

The orbit of a test particle orbiting a black hole can be described by the Lagrangian $$\mathcal{L} = -\frac{1}{2}\left(-\left(1-\frac{2 G m}{c^2 r}\right) \dot{t}^2 + \left(1-\frac{2 G m}{c^2 r}\...
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22 views

does falling towards blackhole make universe move faster? [duplicate]

If you fall into a blackhole would you begin to see the events in universe happening faster and faster as you move towards the hole? Also if you survive the fall through the event horizon as seen ...
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1answer
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Deriving a Schwarzschild radius using relativistic mass

Introduction I have shown below two different approaches to deriving the Schwarzschild radius. I know these are less rigorous than the derivation of the Schwarzschild solution however the $\frac{2GM}...
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Can Earth be converted into a black hole? If so, how? [duplicate]

A black hole is a region of spacetime exhibiting such strong gravitational effects that nothing—including particles and electromagnetic radiation such as light—can escape from inside it. The theory of ...
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1answer
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Why don't the black holes appear black in color in images of galaxies taken from HST?

According to NASA a black hole is anything but empty space. Rather, it is a great amount of matter packed into a very small area According to the documentary Space Unraveling The Cosmos about ...