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Questions tagged [black-holes]

A black hole is a volume from which matter cannot 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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The age of black holes

I have a very small understanding of space-time however I have been watching some interviews and reading articles featuring theoretical physicist Kip Thorne and I have a few questions that I hope can ...
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Why gauge field should be vanishing on horizon?

When considering an AdS spacetime including a black hole, matter field and gauge field, the value of temporal component $A_t$ of the gauge potential $A_\mu$ on horizon always is set be zero, even the ...
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Star vs Blackhole - Accretion Disk

Simple question: can we prove that what we call today a "star" is not a accretion disk of a "tiny" black hole that happens to rotate in all the 3 axis?
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Does asymptotically AdS mean as $z \to 0$ or as $z \to \infty$ in Poincare metric of AdS?

The Poincare metric of AdS_3 is given by $ ds^2 = \frac{R^2}{z^2}(dz^2 - dx_0^2 + dx_1^2)$. Using the coordinate transformation $\rho = \log(z)$, we can write this as, $ds^2 = R^2 (d\rho^2 + e^{-2 \...
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Who can see the event/apparent horizon and Hawking effect?

As I understand, the Hawking effect is related with the presence both event and apparent horizon, i.e. if some kind of horizon exist, a loss/gain of information (unformally speaking) exists. My ...
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Quantum Gravity Singularity Question?

I am not a expert in QG (Quantum Gravity) or GR (General Relativity) for that matter so please forgive if I make any small mistakes, its just a curious question but I know that a singularity is a ...
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137 views

Topology of a black hole

How many dimensions are theorized for a black hole, in view of the fact that black holes are not observed directly.
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50 views

Increase in momentum of a particle falling into a black hole

From the point of view of an external observer at a fixed distance above a black hole, the linear momentum a free falling particle increases exponentially with Schwarzchild time. (This happens when ...
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120 views

Do all the spacelike curve terminate at the spatial infinity $i_0$ in the Penrose Diagram of a Schwarzchild black hole?

Let's restrict to the radial direction, so the metric can be expressed as $ds^2=-(1-r_S/r)dt^2+(1-r_S/r)^{-1}dr^2$ with $r_S$ the Schwarzchild radius. Expressed in Kruskal coordinates, the metric is ...
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609 views

Is the gravitational acceleration at the event horizon constant?

If the escape velocity at the event horizon of a black hole is equal to the speed of light, does this imply that the gravitational acceleration at the event horizon is also constant? For example, ...
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Loop quantum gravity- is the critical density sufficient to form a 'black hole'?

I think in loop quantum gravity the rate of expansion of the universe is found from: $H^2=\frac{8\pi G}{3}\rho\left(1-\frac{\rho}{\rho_c}\right)$ And I was wondering whether the critical density $...
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Primordial black hole: how dense does matter need to be to feed it?

Primordial black holes are a dark matter candidate. Suppose you put a primordial black hole the mass of an asteroid in Earth's core. Will it grow? NO! The black hole will be on the order of a proton ...
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Lorentz transformed Scharwzschild solution

A question that has always intrigued me is: "Imagine a star moving as it evolves into a black hole, Ignore the effect of debris from the supernova. Assume also that before the collapse, the star was ...
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149 views

Timelike Shell Collapsing into a Black Hole

Does anyone know where I can find the solution for a spherically symmetric thin shell of timelike matter falling into a Schwarzschild black hole? The matter should be pressureless, so that each ...
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216 views

Is the black hole surface area actually zero?

From quantum mechanics we know that entropy of a part can be smaller than the entropy of the whole. I wonder whether a similar rule works for GR as well. Quantum mechanics predicts that the entropy ...
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Deriving 'frame dragging' in elliptical coordinates?

I'm currently trying to derive the equation that demonstrates 'frame dragging'. I am familiar (of sorts) with the Kerr metric, but I am most comfortable working in cartesian/elliptical coordinates? ...
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How to use The Schwarzchild Metric formula to get distribution representing “free-fall”

Given formula: How I can use to calculate distribution of points in space, so if i choose path which contains most of the points I get path that close to "free-fall path". As far as I know i should ...
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What's the timescale until the Medium Crunch?

The future of the universe seems to be: clusters of galaxies drift apart due to space expansion until they all pass out of each other's cosmic event horizon. Gravitationally bound clusters overcome ...
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How connected thermodynamical stability and dynamical stability for black holes?

Criteria for thermodynamical stability is the convex of entropy. But for black hole entropy is non-additive.
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81 views

Silly doubt on penrose diagram for evaporating black hole

This question is based on the naive ideia of evaporation of a black hole stated as, After sufficient Hawking emission the black hole loses it's mass and therefore it's area, until, as a result, the ...
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560 views

Black hole accretion disks and quasars

Are 'accretion disk of a black hole' and 'quasar' synonymous? If not, what's the difference?
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197 views

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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124 views

Feynman diagram for Hawking radiation?

I'm starting to wrap my head around Feynman diagrams, and the idea of "real" vs. "virtual" particles, but one area where this distinction seems to break down is in describing Hawking radiation, where ...
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On Information-less systems

Is there any system in the universe that is information less, I.e., has or possesses no physical information. For example if I have a system of particles and if I throw this system into a black hole(...
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378 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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Is Hawking Radiation compatible with a GR solution with an Infalling Observer Crossing the Event Horizon?

EDIT: See my answer. Question is malformed as it was based on the assumption that something like Hawking radiation (i.e. mass draining) occurred prior to the formation of the event horizon based ...
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167 views

Quantum event horizon?

Is there a limit in an event horizon like there is with a blackhole and similarly with a causal quantum event that limits quantum behavior and segregates it form causal classical behaviour at this ...
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Should any matter form a singularity before crossing the event horizon?

I guess the destiny of any object that falling into a black hole will depend on its size and the density of the black hole it falling in. And from distant observer it should looks different. A small, ...
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104 views

Universe Expanding

Quick question that I was just hoping to get some clarification on. Current theories state that the universe is expanding; with those planets farther away moving away from us at a faster rate. Is it ...
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80 views

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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black holes and white holes

i have a question and i just couldn't get another way to get its answer. My question is regarding black hole and the possibility of a white hole. we know that even light cannot escape a black hole ...
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If all spacetime paths inside a black hole point towards the singularity, then does that mean entropy is decreasing?

Entropy is a concept of disorder. A drop of blue ink inside a glass of water only has 10^10 factorial possible ways of being an ink drop, but it has 10^11 factorial possible ways of being an ink soup. ...
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Singularities and Infinite Range of Gravity

I'm not sure if this has been answered in the past, but if a singularity (in a black hole) Really has Infinitely strong gravity, why is it that they do not affect other celestial objects across the ...
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Antimatter and annihilation of light and black light

Can we annihilate a photon with an opposite phase? As two photons annihilate to give two antimatter photons (with opposite charge), these photons must have some opposite property. When these opposite ...
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Can a charged particle be retrieved from beyond the event horizon?

So I read this question, which is similar, but not the same. My question involves a standard, non-charged, non-rotating Schwartzchild black hole. If a charged particle, (let's use an electron) passes ...
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111 views

Sphere of circumferential radius $r$

What do you then mean by constructing a sphere of circumferential radius $r$ centered on the black hole? Is that sphere a 2D surface?
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Matter beyond event horizon

Assuming that antimatter is matter with time arrow reversed, would it be right to say that matter beyond black hole event horizon then would become antimatter because of space and time axes exchanged? ...
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How can matter falling into a black hole make it grow?

None of the answers to this question or this question answer my question. Once an object falls past the event horizon, it can no longer act as a source of gravity. Also, if one tiny object falls into ...
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Black hole singularity with zero curvature (do you feel weightless)?

As described by GR, at the center of a black hole, may lie a gravitational singularity, where the spacetime curvature becomes infinite. https://en.wikipedia.org/wiki/Black_hole Though, by the ...
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Creating a Microscopic Black Hole

Creating a Bose-Einstein Condensate is real-world means of condensing as much matter into as small a space as possible. The amount of matter necessary to turn the BEC into a microscopic black hole is ...