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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Can you modify the surrounding curvature of a black hole while keeping the Schwarzschild radius constant?

[SOLVED] 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 ...
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Escape velocity at 45 degrees near a black hole

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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Proving : $\Omega =\sqrt{GM}r^{-3/2}$ for the Schwarzchild space-time

If $\Omega =u^{\phi}/u^t$, I'm trying to prove that $$\Omega=\sqrt{GM}r^{-3/2}$$ for the Schwarzchild space-time. Here, $u^{\phi}$ and $u^t$ are respectively the azimuthal and temporal contravariant ...
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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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If vacuum decay was somehow triggered inside a black hole, would the rest of the universe notice?

If vacuum decay is triggered, it will spread (at maximum) at the speed of light. But the spacetime inside a black hole is causally disconnected from the space outside. Nothing that happens inside the ...
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How is information about the inners of a black hole stored on or near the event horizon?

In other words, why do black holes have no hair (ahum...no offense implied here!)? Is during the formation of the hole this information entangled on the horizon? Seen from the outside, the hole is ...
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Question related to Schwarzschild radius [duplicate]

I have a quick question about Schwarzschild radius $r_{s} = \frac{2GM}{c^{2}}$. As we know it can be derived from the equation of escape velocity by putting v = c, so does this implies that the escape ...
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Why is high energy required if we want to send a message in a short amount of time?

I am somewhat puzzled by the following statement "If Alice, after crossing the horizon, has less than a Planck time to communicate with Bob about the status of her qubits, then she is required to ...
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Could we move a spaceship by moving a Black Hole?

What if a spacecraft were dragged by a black hole, and the black hole were pushed by continuously firing lasers into it? Would this allow the ship to be accelerated faster than laser propulsion, which ...
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Black holes: Is merger inevitable when horizons touch?

I watched a simulation of the binary black hole merger of 2019 April 12 https://www.ligo.caltech.edu/video/ligo20200420v1 When the "apparent horizons" (their terminology; are those ...
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Can one define a flow of spacetime?

One often reads things like, 'At the event horizon, the flow of spacetime exceeds the local speed of light.' But is this actually correct? Can you mathematically define some sort of spacetime flow ...
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Can a Black Hole be moved by lasers?

I've read that an object colliding with a black hole will move it, as it would any other object. Could a laser continuously fired into a black hole move it?
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Is it experimentally verifiable if we are a inside a white hole that never fully formed?

There are a lot of questions on this site about white holes, but none of them specifically answer my question. Most of these questions talk about observing white holes from outside, but do not talk ...
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Black Hole limit from Morris-Thorne wormhole

I have to show that Morris-Thorne wormhole given by the metric $$ds^2=-e^{2 \Phi}dt^2+\frac{1}{1-\frac{b}{r}}dr+r^2(d \theta^2+\sin^2 \theta \ d \varphi^2)$$ has a black hole limit (or maybe flat or ...
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Doubt about the coordinates of a moving Schwarzschild black hole [duplicate]

My friend asked me about the geometry of a moving black hole with constant velocity. My first attempt to cast its metric tensor was: $1.$ Start with Schwarzschild metric in spherical coordinates $(t,...
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Energy released from forming a black hole?

Gravitational collapse release potential energy, in some cases a huge amount of it. In the case of a black hole, that collapse goes to "infinity" which suggests the release of "infinite&...
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Is there a coordinate system for the Schwarzchild spacetime where $r$ is proper length/time?

I don't see any when I search online. The way I would try to construct them would be as follows. Starting from the standard metric $$ds^2 = -\left(1 - \frac{r_s}{r'}\right)dt^2 + \left(1-\frac{r_s}{...
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Infalling Observer and Black Hole Information Loss?

I was thinking about the following scenario today, and I was hoping to find a more detailed explanation. Suppose you have an observer $A$ falling into a black hole and an observer $B$ at infinity. In ...
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Minkowski limit of Schwarzschild Metric

I've seen questions and answers dealing with similar topics, but none that seem to provide what I'm looking for. The Schwarzschild metric (and indeed any valid metric) should reduce to the Minkowski ...
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A little more detail on the definition of Angular Moment parameter for a Kerr Black Hole

In several places, such as Explain Kerr-Newmann Black Hole Spins in SI Units, the parameter $a$ is defined as: $$a = \frac{Jc}{GM^2}$$ My question is whether the angular momentum J can be defined in ...
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Is spacetime curved by relativistic mass?

The top answer to this question asserts that only rest mass curves spacetime. My poor understanding of the EFEs & special relativity is that $T_{\mu\nu}$ doesn't care about what whether energy ...
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Would a black hole act as a particle accelerator between the photon sphere and the event horizon?

I'm trying to understand the behavior of photons between the photon sphere and the event horizon. Here is what I think I understand so far : the radius of the PS is 1.5 times the Schwarzschild radius ...
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What would a merger look like from inside a black hole (and would we even notice it)?

I have read this question: We start with a rocket hovering at a constant distance away from the black hole At time t0 our observer leaves the rocket and starts falling towards the black hole. At time ...
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What is the approximate energy/ pressure needed to collapse a star into a black hole? [closed]

Other questions have covered the equations you need to calculate the pressure or energy needed for gravity to overcome the other forces and collapse a star. But, I have seen no examples of what ...
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Can information leak through between merging black holes during the merger?

There are other questions on this site about black hole mergers and information, but none of them answer my question. What happens to information when black holes merge? This does not give a ...
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The Logic Problems of Black Holes [closed]

As I always preference my questions, I am not a scientist or student. I am an engineer who loves physics. It is my understanding that, there are 2 competing thoughts behind what a black hole is. ...
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Black hole interior definition

We know how to define black holes as trapped surfaces and solutions from gravitational theories. However, if we define black holes as vacuum solutions/charged solutions to Einstein Field Equations, ...

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