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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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Where can I find visible/radio telescopic observations of the center of the Milky Way galaxy?

I recently stumbled across this time-lapse telescopic image (radio image, I believe) of the center of the galaxy. Here's a still image. Link to time-lapse video below. I haven't been able to find ...
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Three types of singularities in black holes?

Now, please don't dismiss this question because of its entertaining context. Today I attended a lecture by Kip Thorne and at some point he mentioned, almost offhand, that a few years ago it was found ...
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Black hole, gravitational redshift, Doppler effect

I'm studying up on the physics around black holes and got myself confused with the concepts of gravitational redship and Doppler effect. Imagine a spacecraft orbiting a non-spinning black hole. It ...
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How can time go in different directions in the Universe?

This is not a duplicate, I am not asking about any kind of time dilation caused by a BH. My question is about the direction of time (and if it is possible to have different directions for time, other ...
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Can mass be ejected from a black hole within the Schwarzschild radius?

This goes far beyond my limited knowledge regarding the physics of black holes, but my colleague told me this today: I doubt that if I stand on the surface of a black hole - inside the ...
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Stability of Schwarzschild and Reissner-Nordstrom spacetimes

I am interested to know what is the best we can say about stability of Schwarzschild and Reissner-Nordstrom black holes. I found some who study the behavior of perturbations that satisfy the ...
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Information content in black holes

Bekenstein-Hawking formula for entropy of a black hole tells us that information content in a black hole is proportional to its area which is in fact proportional to the mass^2 of the black hole. The ...
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Should stellar black holes contain a singularity? (in GR) [duplicate]

Consider a star, for simplicity a non rotating one. The corresponding spacetime will be similar to a Schwartzschild one (if the star is static and spherically symmetric). Outside the star we will ...
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Can a black hole event horizon form before the singularity for an outside observer?

This is not a duplicate, my question is not about whether BHs have or don't have a singularity, but my question is whether, because of time dilation, no matter has reached the center and formed a ...
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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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Could harm due to spaghettification be mitigated by entering a black hole at almost the speed of light?

Say that we wanted to send a probe into a black hole, perhaps hoping to see if it's actually a wormhole that might transport the probe elsewhere. Upon approaching a black hole, the probe would ...
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Time in our Universe versus time in Black Holes

The universe is immense and 13.8 Billion years has passed overall since it was formed in the Big Bang event. If time greatly slows down inside a Black Hole (BH), then logically very much less than 13....
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Dropping a rope in a black hole [duplicate]

If there is a spaceship hovering above a black hole so that it can get away, what would happen if the spaceship connected a rope to itself and dropped the rope in? Could you pull the rope out?
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Due to TIME stopping, are contemporary Black Holes really just Large SHELLS of Matter and not Singularities? [duplicate]

If TIME slows down as you approach a Black Hole, has there been sufficient time in our Universe (13.8 Billion Years since the Big Bang occurred) for ANY matter inside the Event Horizon of ANY Black ...
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Einstein Tensor doesn't vanish [on hold]

Hello my question is how is it possible that the Einstein tensor doesn't vanish in the Schwarzschild metric. For example $G_{11}= \frac{1}{2} g_{11} g^{22} g^{33} \partial_2 \partial_2g_{33}$ is one ...
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Hawking radiation doubt [duplicate]

My question is that according to hawking radiation two particles pop near a black hole one goes into the black hole and one escapes from it so we get the impression that the black hole is radiating ...
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Gravitational $N$-body mergers?

Beyond binary systems and its mergers, have numerical relativity groups computed an approximation to $N$-body GW signals/waveforms or is that a hard unsolved task?
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What causes the heating of a black hole's accretion disk?

Sheperd Doeleman said in a TED video that the accretion disk is heated by friction. Is that correct? I thought it might be adiabatic heating from the compression of the gasses.
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Why do Candelas and Howard say that $\sum_{n=1}^\infty \cos\left( n \kappa \epsilon \right) \ = \ - \frac{1}{2}$?

In the paper Vacuum $\langle \phi^2 \rangle$ in Schwarzschild Spacetime by Candalas and Howard, they say that for each non-zero $\epsilon$ it is true that $$ \sum_{n=1}^\infty \cos\left( n \kappa \...
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Radial infall in Schwarzschild

In Straumann's book on general relativity, one finds the following solution to the question of geodesic radial infall into the black hole: $$d\tau=(\frac{2m}{r}-\frac{2m}{R})^{-\frac{1}{2}}dr$$ To ...
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Proper time of an object crossing event horizon in Kruskal coordinates

So I am reading a paper on a certain black hole paradox. The specifics actually don't matter, but if you want context (p16): black hole thought experiment. An object falls into a black hole. The ...
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How slow can time get near a black hole?

The closer I get to a black hole, does time slow down proportionally? Does time stop near the event horizon?
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Why do photons get sucked into black holes? [duplicate]

As far as I am concerned photons don't have a mass so how it gets sucked into black holes or is it because of black holes huge mass bends time can some one explain why and what forces or what ever ...
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Is there a general relativistic mass in general relativity?

In general relativity the energy of a test-body moving in a spherically symmetric gravitational field can be written as: $$E=mc^2\left(\frac{\sqrt{1-\frac{2GM}{rc^2}}}{\sqrt{1-\frac{v^2}{c^2\left((1-\...
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What is the formal difference between the light cone and a black hole?

A black hole can be loosely defined as a spatial closed surface from which nothing, not even light, can leave. The light cone of special relativity is in some sense similar to a black hole because by ...
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What is the difference between fine-grained and coarse-grained entropy?

So I am not a native English speaker, but I understand the words "coarse" and "fine" grained. But I really don't know what to make of them in the context of entropy. I have encountered this many times ...
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Could the persistent effects of gravitational waves solve the black hole information paradox?

I am not bothered that much by the fact that two observers describe the same phenomenon differently. Something similar , in principle ,  happens with simultaneity in special relativity,  and special ...
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Pauli Exclusion and Black Holes [duplicate]

Pauli exclusion principle states that 2 identical electrons cannot be in the same state, where state includes a spacial component. I have heard that, in order to avoid being in the same state, in a ...
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Black hole gravity and light [duplicate]

When an object is squeezed to it’s swartzschild radius it becomes a black hole (made by density) and it’s mass does not change (it’s gravity doesn’t change), but if it’s mass doesn’t change (it’s ...
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If gravity turns out to be mediated by exchange particles, would that imply a problem with gravitational fields around a black hole?

In general relativity, gravity is a distortion of spacetime due to mass. Its effects travel (if that's the right word) at the speed of light. In the SM all 3 other known interactions are mediated by ...
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Vertical equilibrium condition in thin adiabatic black hole accretion flows

In adiabatic thin accretion accretion flows around black holes, we consider the vertical equilibrium condition where the thickness of the flow is very small as compared to the radial coordinate, i.e., ...
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Can you go back and forth through time when inside a black hole?

Inside the event horizon of a Schwarzschild black hole the radial coordinate becomes timelike and the time coordinate becomes spacelike (they change signatures). I read that timelike coordinates are ...
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Is my analogy on Hawking Radiation correct? [duplicate]

I just want to make sure what I know is correct. Let me explain and if there is anything wrong (any simple detail) with what I said, please, correct me. When the particle crosses the event horizon, ...
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M87 Black Hole's Radius

I'm curious about the actual radius of M87 black hole. I know they've recently found out its Schwarzschild radius is around $2\times10^{13} $m, with a mass of $6.5$ billion solar masses. But I wonder ...
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Energy of an object near a black hole

I'm starting to read about black holes and am confused with some concepts. So here is one (simple?) question that I can't wrap my head around: What would be the energy (Joules) released by a 1kg mass ...
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What do orbits around a black hole look like doing the calculations in isotropic coordinates?

JPL and others calculating ephemerides in the solar system are using a method based on taking the Schwarzschild solution, not as expressed in the most common Schwarzschild coordinates but in the ...
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Why are orbits $1.5r_{s} < r < 3r_{s}$ unstable around a Schwarzschild black hole?

The "Orbital motion" section of the Wikipedia entry corresponding to Schwarzschild metric reads: A particle orbiting in the Schwarzschild metric can have a stable circular orbit with $r > 3r_s$. ...
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Point-like particle vs naked singularity? [duplicate]

In general, physics seems to consider elementary particles such as electrons to be point-like. On the other hand, naked singularities seem to cause all sorts of trouble, including closed time-like ...
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Answering simple questions about black holes to someone not knowing GR

In the Newtonian description of a black hole, it is a gravitating astrophysical object for which the escape velocity exceeds the speed of light in vacuum ($c$). But what does this actually mean for a ...
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How do inspiraling black holes get closer?

In Newtonian mechanics, binaries are stable. We here on earth are very glad that it will not emit its angular momentum and spiral into the sun. What is different about the black holes and neutron ...
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Do black holes have a maximum temperature?

"Standard" black holes have usually a very low temperature if we assign their temperature to the Hawking radiation they are expected to emit and compare it to the one of an ideal black body. For a ...
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What happens when you dump antimatter onto a naked singularity?

Inspired by this question, what would happen if you dumped antimatter onto a naked singularity? The answers for the previous question suggest that once the antimatter crossed the event horizon, no ...
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Could the black hole photo be a gravastar?

Mazur-Mottola gravastars would in many ways appear identical to black holes. Does the new radio-photograph of M87* taken by the Event Horizon Telescope eliminate M87* as a gravastar candidate, and ...
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What happens when a black hole evaporates?

I have read other articles here on the subject and I think my question is different.. When a singularity evaporates enough to reach just above the limit of mass required to make the singularity, what ...
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In an anti-matter universe, would an anti-matter black hole emit anti-matter gamma radiation? [duplicate]

In an anti-matter universe, when an anti-matter black hole is consuming a large anti-matter star, would it emit anti-matter gamma radiation, or would gamma radiation be the same in either a matter or ...
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What happens if you dump antimatter into a black hole? [duplicate]

Let's say you have a 10 solar mass BH and dump 10 solar masses of antimatter into the BH... What would happen? Would I have a 20 solar mass BH? Would the BH explode Do we even understand what would ...
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How do LIGO and Virgo know that a gravitational wave has its origin in a neutron star or a black hole?

It is being said that gravitational-wave detectors are now able to distinguish neutron star waves from those originating from black holes. Two Questions: How do LIGO and Virgo know that a ...
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Dynamics of Event Horizons between head-on colliding non-rotating black holes [duplicate]

What happens to the event horizons of each black hole as the two identical non-rotating black holes collide head on? At a point exactly between them, is the curvature zero (do the individual ...
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Can there be temperature inside of a massive Black Hole?

Since it cannot radiate light and therefore neither heat, if any reactions which generate heat were to occur inside a Black Hole, that would seem to imply an infinitely high ever increasing ...
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Black holes in p-adic gravity/ultra-metric metric field? [closed]

As a radically different to beyond standard general relativity, at least from the type of geometry it deals with: Consider p-adic gravity and/or general relativity defined on certain ultra-metric ...