Questions tagged [black-holes]

A black hole is a region of spacetime from which nothing can escape. More formally, the future light cone of any observer within the black hole is completely contained in the black hole, and the black hole region is not within the past light cone of any observer that goes to spatial infinity in an infinite amount of time.

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Can we detect evaporating micro black holes?

The energy released by an evaporating black hole in its last instants (of duration $t$) is equal to $$E = \sqrt[3]{\frac{t \hbar c^{10}}{ 5120 \pi G^2}}$$ As the math shows, most of the black hole's ...
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Is gravitational time dilation considered as a repulsive force (from the point of view of a far-away observer)?

An infalling particle does never reach the event horizon of a Schwarzschild black hole, from the point of view of the reference frame of a far-away observer. Gravitational time dilation slows down its ...
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Finding the transformation for the Newman-Penrose formalism

I am working with the Kerr metric (suppose it is $g_{\mu \nu}$) where I want to understand how the Teukolsky master equation is derived using the Newman-Penrose formalism. The original article can be ...
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If you travel fast enough towards a light source, will you fall into a kugelblitz?

A kugelblitz is simply a special case of black hole, one that's formed from photons instead of fermions. Thanks of course to mass-energy equivalence, enough photon energy in a small enough region can ...
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How do you determine the Integration Coefficients of the Einstein Field Equations?

I'm currently self-studying general relativity after having previously learned differential geometry and most of the mathematical basis required for it. I'm trying to connect the mathematics to the ...
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Can a rotating neutron star collapse to a black hole through a reduction in rotation?

It is well known that non-rotating neutron stars cannot grow without bound, since an increase in mass causes a proportional increase in density, and accretion beyond a critical limit would cause it to ...
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Is there a GR explanation for cosmological coupling causing mass increase as the universe expands?

A paper in November 2021 considered the hypothesis that things with mass, including black holes, may be coupled to cosmological expansion so that they slowly gain mass as the universe expands, similar ...
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Is it possible to harness energy from kugelblitz mergers? [closed]

As I understand it, the kugelblitz is a theoretical method of constructing microscopic black holes, wherein enough radiation is concentrated within one region that it can warp spacetime to become a ...
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Black hole as a quantum system and the area of its horizon

I recently saw the article Rev. Mod. Phys. 93, 035002, in which the authors write about the ``Central Dogma''. According to this dogma, when seen from outside of its horizon, a black hole is ...
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Why must black hole have entropy?

From what I understand 2nd law of thermodynamics is just saying that information cannot be destroyed, ok got it. When a massive star collapses under it's own weight, it turns into a black hole and all ...
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Spacetime inside the horizon of a black hole

According to Susskind a bit of information crossing the event horizon of a black hole instantaneously encounters the singularity. Also, time appears to gradually slow down for an object approaching ...
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Is there an explanation for why Hawking radiation causes a black hole to shrink *without* referring to particle/anti-particle pairs?

An observer far from a black hole sees thermal radiation at the Hawking temperature. As a consequence, we are told that the black hole loses energy over time: that is, it evaporates. This logical step ...
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Physical properties of an event horizon

This question may seem a little naïve, but I need help with the basics of black hole physics. I'm having difficulty understanding the topology of the event horizon. Apologies for any errors in ...
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Why is black hole information paradox a problem when there are no real black holes in the Universe?

We know stars take infinite time in our coordinates to actually collapse into a black hole due to time dilation. See, e.g., How can anything ever fall into a black hole as seen from an outside ...
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How hot is a primordial black hole and does it has angular momentum?

I am fascinated by these elusive phenomenon, they are formed in the early stage of the universe where it is not homogeneous and some regions are very dense enough to undergoes gravitational collapse. ...
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How does definition of event horizon imply that they are null?

I am familiar with the definition of a (future) event horizon as the boundary of the closure of the causal past of the future null infinity. I am aware that event horizons are null hypersurfaces -- ...
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Are Reissner-Nordström black holes repulsive?

I'm not sure if I understand how the charged black holes work. The Reissner-Nordström metric is defined as $$ds^2=\left(1-\frac{2M}{r}+\frac{Q^2}{r^2}\right)dt^2-\left(1-\frac{2M}{r}+\frac{Q^2}{r^2}\...
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Is there any *global* timelike Killing vector in Schwarzschild geometry?

I have been dealing with the following issue related to the Schwarzschild geometry recently. When expressed as: $$ ds^{2}=-\left(1-\frac{2GM}{r}\right)dt^{2}+\frac{1}{1-\frac{2GM}{r}}dr^{2}+d\Omega_{2}...
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How much energy is there in curved spacetime of a black hole?

Given that gravitational waves transmit energy, the curvature of spacetime must, itself, contain energy. How do you calculate this energy? Specifically, I'm most interested in how one would calculate ...
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How close does a photon have to get to a black hole to do a full loop?

How close does a photon have to get to a black hole to do ONE full loop? By full loop I mean it curves once around the black hole and ends up on the same trajectory as it was one before it approached ...
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How can the infalling observer see the event horizon to retreat in front after the observer crossed it (and can't even notice it)?

I have read this question: so looking toward the singularity, you see the horizon retreating from you as you fall in - even after you've already crossed the horizon. Are black holes naked ...
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Escape velocity at an angle near a black hole

What's the formula for the velocity required to escape at an angle (not normal) near a black hole? I know for example that the required velocity not to impact the black hole at $90$ degrees is larger ...
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Can we hear sound in a vacuum, if we stand near a black hole?

So we can't hear sound in a vacuum because there's no air to propagate the vibration. But from what I understand, black holes make vibrations in space-time. So much so that scientists can detect black ...
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If frequency shift is positive then how to interpret it?

Let say lightning is happening at $r=r_{A}$ from center of gravity and our observer at $r=r_{B}$ with $r_{A}>r_{B}$ then if we get frequency shift is positive (by considering let say schwarzschild ...
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Can you modify the surrounding curvature of a black hole while keeping the Schwarzschild radius constant?

TL;DR: I'm attempting to make a black hole rendering, and I like the current size of the event-horizon but I want to reduce the curvature further away from it. But it seems like $g_{uv}$ only depends ...
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Escape velocity at 45 degrees near a black hole [duplicate]

what if the formula for the escape velocity at 45 degrees compared to 90 degrees when you are near enough to a black hole for relativistic effects to become significant? I know that near enough to the ...
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What would happen if you dumped negative mass into an extremal black hole? [closed]

What would happen if you dumped negative mass into a near-extremal black hole? It appears to me that doing this would reduce the Black hole's mass without reducing the angular momentum or charge? ...
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Kruskal coordinates from GR field equations

The "usual" way (as I've seen in most text books) of dealing with Schwarzschild spacetime is to start with Schwarzschild coordinates, then explore the nature of coordinate singularity at $r=...
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Does it really take an infinite time to reach a Black Hole's event Horizon? [duplicate]

Does it really take an infinite time to reach a Black Hole's event Horizon. The forumla for gravitaional time dialation is. $$t_0=t_1\sqrt{1-r_s/r}$$ So I used wolfram alpha to calculate the time that ...
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Why would the validity of UV/IR decoupling imply the explicit constructability of black hole remnants?

A paper written in 2020 by Harlow and Shaghoulian (reference 1) proposes a connection between unitary black hole evaporation and the non-existence of global symmetries in quantum gravity. In passing (...
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The entropy of Hawking Radiation

I am reading the review paper "The Entropy of Hawking radiation" by Almheiri et. al. They show the Page curve in the following figure: However, they do not show a formula of what they refer ...
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Schwarzschild black hole and two particles

I'm looking to solve a problem using general relativity theory to "get a taste" of it. I've little knowledge of the the theory so just want to "taste" it before diving deep into ...
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More volume with more mass?

Regarding to the Schwarzschild solution, is there more volume with more mass? Lets take for example a space shuttle (spheric for simplicity) in an stationary orbit at the position $r_1$. The black ...
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How does a Cosmic String accelerate a Black Hole?

I've read that a Cosmic String could accelerate a Black Hole, but I'm not exactly sure how that works. Is the Black Hole pulled along the length of the String? I've also heard it could accelerate ...
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With how much energy can a charged black hole launch a electron away with?

Lets say with have a black hole with a strong negative charge, then a say a neutron falls towards it. Just before the neutron reaches the black hole's event horizon it undergoes beta decay, this ...
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Would Newton's law in the solar system be independent from a presumed different fundamental gravitational law? [closed]

A thought experiment: Let's assume the gravitational field of a black hole without any neighbors would be stronger than the Schwarzschild metric in the Newtonian limit. Now, we let a solar system (sun ...
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How fast can manned spacecraft slingshot from Black Holes?

I've read of Black Holes launching particles at over 99% the speed of light. Could manned spacecraft use Black Holes to slingshot ourselves at this speed, or would the G forces kill us? Intuitively, I ...
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Should mathematics be the starting point of theoretical physics research or should it follow to support the hypothesis or experimental result? [closed]

In the supplied link, mathematical analysis was used to suggest that black holes exert some amount of pressure. Should mathematics be the starting point of theoretical physics/astrophysics research, ...
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If the Earth is placed between three spinning black holes, Will time on Earth slow down? Will we live longer?

Just randomly thought of placing the Earth equidistant from 3 spinning black holes. Will time on the Earth slow down? Will we age slower? Assuming that the Earth is not past the event horizon yet the ...
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Extracting rotational energy from a black hole: why does the direction in which we drop stuff in matter?

Imagine an advanced civilization living around a black hole, extracting energy from it for their needs. Two ways to do this: Drop matter in and use the light it emits as it heats up. Extract the ...
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Why doesn't a black hole have linear momentum?

The no hair theorem says that a black hole can have only three properties: mass, charge and angular momentum. But why don't we say that linear momentum is one of its properties? If we throw an object ...
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What's the big deal with information being lost in a black hole?

In the video here: https://www.youtube.com/watch?v=d_XuFkVdAYU (10:57), it is said that the loss of information when matter falls into a black hole is a paradox. He gives an example of throwing a ...
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If spacetime can expand faster than the speed of light, then can a black hole do that too?

I have read this question: Yes, the expansion of space itself is allowed to exceed the speed-of-light limit because the speed-of-light limit only applies to regions where special relativity – a ...
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When does the event horizon grow as measured by a distant observer? [duplicate]

As measured by a far away observer, it takes an infinite amount of time for a body to fall into a black hole (though the body itself takes finite time to reach the crunch). The event horizon grows ...
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Tensor Manipulation in Wald's General Relativity by Robert M. Wald at page 334

I don't understand the example, just after the "i.e.", at the end of the paragraph in the image. Why is it zero when the condition is fulfilled?
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Will an initially stationary spherical distribution of black holes flatten in time?

Assume a big number of black holes forming a rotating sphere held together by their mutual gravity initially. Will the spherical shape be a constant in time, or will a flat structure come about, like ...
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A black hole made of ordinary water?

Years ago I read a book about black holes. It said it is theoretically possible to make a black hole from ordinary-density materials, like tap water. It would take a lot -- about the size of a galaxy ...
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Null tetrad for a metric with most of the metric components non zero

I am working on a metric which is basically $g = g_0 + h_{\mu \nu}$ where $g_0$ is the Kerr metric up to order a (taking $a^2 = 0$) and $h_{\mu \nu}$ denotes perturbed metric. Hence the complete ...
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How close to a black hole can an object orbit elliptically?

How close to a black hole can an object orbit elliptically? I know circular orbits are no longer stable at distance less than 3 times the Schwarzschild radius. But what about elliptical orbits? Can an ...
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When does a black hole grow?

When does a black hole grow in size as measured by a distant observer? A falling body takes infinite amount of time to reach the horizon as measured by a remote observer, and the horizon of the black ...

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