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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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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Israel-Wilson-Perjés Solutions

I'm searching for a reference that gives explicitly the field strength (or at least the gauge fields) of the Israel-Wilson-Perjés Solution, using complex harmonic functions for the metric. In "...
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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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49 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
91 views

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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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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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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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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Curvature Invariants in General Relativity and Singularities

Suppose that I want to check if a given metric is singular or not. I'm interested in curvature singularities, not coordinate singularities, so I can look to scalars made with Ricci, Riemann and Weyl ...
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1answer
69 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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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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1answer
34 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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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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80 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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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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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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27 views

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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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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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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Is the question: 'Is our universe the inside of a black hole?' meaningful? [duplicate]

By meaningful I mean experimentally falsifiable.
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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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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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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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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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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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245 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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1answer
56 views

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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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
87 views

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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29 views

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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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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How can the energy of a spinning black hole be converted into magnetic field?

I've read about the Blandford-Znajek process, and what I understood is that the black hole is treated as if it were a conducting sphere spinning rapidly with an accretion disk around it, and the ...
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Do black holes really exist, theoretically?

Do black holes really exist? Casting aside all the experimental proof, how do we arrive at the idea of a black hole theoretically using any metric (say Robertson-Walker) other than the Schwarzchild ...
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Why does an evaporating black hole always stay a black hole?

Stars can only collaps and form black holes if their masses are above the Chandrasekhar limit, $M>M_{\rm Pl}^3/M_{\rm hydrogen}^2$. When the universe eventually cools down enough, the black holes ...
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Closed timelike curves in the Kerr metric

I just read in Landau-Lifshitz that the Kerr metric admits closed timelike curves in the region $r \in (0, r_{hor})$ where $r_{hor}$ is the event-horizon ( I am talking about the case $|M|>|a|$ (...
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Kerr-Newman Black Holes' Phase Transitions in De Sitter Space

At $J=km^2$ Kerr black holes' specific heat shows infinite discontinuity. Therefore as Kerr black holes' angular momentum gets close to $km^2$ where $k$ is determined by Kerr-Newman metric and $m$ is ...
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1answer
80 views

Why can't our expanding universe be an imploding universe that appears expanding relative to us? [closed]

Galaxies moving away from us is our proof of an expanding universe. However, if the universe was contracting towards a black hole we would see a galaxy ahead of us accelerating faster towards that ...
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51 views

Minimum gravity required to suck light? [closed]

I was studying about gravitational forces of black holes when I came to a thought of what is the minimum gravity required to capture light in vacuum?BY capturing I mean the inability of light to ...
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Is white hole has minimum entropy in a region?

From here I know black hole has maximum in the region, how about white holes, can we deduce white hole has the minimum entropy in the region?
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If we increase the mass of a black hole by a factor of $k$, by what factor will the surface area of the event horizon change?

If we increase the mass of a black hole by a factor of $k$, by what factor will the surface area of the event horizon change? Given any number of identical black holes each with mass = $\mathrm{m_0}$ ...
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Conformal infinity in the Hawking-Hunter-Taylor-Robinson metric

I have been trying to follow some of the computations of this paper: http://arxiv.org/abs/hep-th/0408217 and particularly I couldn't derive the asymptotic form of the Kerr-AdS background (3.27) using ...
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1answer
54 views

Spaghettification on an atomic scale?

Spaghettification occurs when an object approaches a singularity. As one comes close enough to the singularity, the gravity at the feet (if this is a human) is greater than that at the head, ...
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65 views

Are Black Holes really spinning? [duplicate]

I've had this question: are black holes really spinning? Well if they are not spinning then why would the galaxy around them spin at all? They must spin! Or? Do they spin like a ball? What if they ...
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2answers
118 views

In what other fields of physics does “math break down”? [closed]

I've heard numerous articles and videos state that when it comes to black holes and singularities our "math breaks down". Is there any other areas in physics where math similarly "breaks down", where ...
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2answers
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The time it takes for a black hole to devour a star

It has been suggested here: How long does it take a black hole to eat a star? that it can take (at best) a rather short amount of time for a supermassive black hole to eat a star as viewed by a ...
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1answer
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How much energy is released by an evaporating black hole in the last second of its life?

From what I understand, due to Hawking radiation, black holes lose mass in the form of energy (electromagnetic radiation), with these characteristics: The larger the black hole, the less energy it ...
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A star starts at rest at infinity, how do you calculate the kinetic energy of the star when it crosses the event horizon of a black hole? [duplicate]

A star that begins very, very far away from a black hole (infinity) is brought in by the black hole's gravitational pull. How do you calculate the kinetic energy of the star as it passes the point ...