Questions tagged [event-horizon]
An event horizon is a type of boundary such that any information past this boundary is inaccessible to the observer it is defined for. Common examples are the Schwarzschild radius of a black hole (which is defined commonly for all observers outside this radius) and the cosmological event horizon (which is defined as a radius from an observer)
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Crossing an event horizon… at the end of time
I am not sure if my representation of entering into a black hole is correct, and so I would like to ask if I got anything wrong.
The black hole is getting closer. I know exactly when I will cross the ...
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Should density be considered in the role of the strength of a gravitational field?
Ok, here me out. Black holes are usually formed from the compression of the mas released during the death of a supermassive star, however they have the same mass as the star (or even less). Yet when ...
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An object moving close to $c$ falling into a black hole
Assume there is an object moving at $0.99999999999........c$ (upto like a thousand 9s) towards the center of a non-rotating black hole in a straight line
Since it can't escape once it falls in where ...
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How to model a Black Hole inside a Black Hole?
If you put a Black Hole inside of a Black hole, then you get a spherically symmetric vacuum outside the inner Black Hole and the Schwarzschild metric is the only spherically symmetric vacuum in GR. ...
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Interpretation of geodesics in Kerr metric when approaching the event horizon
I'm learning about the Kerr metric and Kerr black holes from the book "General Relativity: An Introduction for Physicists, M.P.Hobson" and I'm interested in an interpretation for the ...
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How do we know that at the Schwarzschild radius the falling speed is $c$? [duplicate]
I see something strange: The kinetic energy required for a falling body to reach the Schwarzschild radius is $(\gamma-1)mc^{2}$, so with $\gamma$ tending to infinity at the Schwarzschild radius, the ...
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Would Hawking blackbody radiation theory still apply if there is no singularity in a black hole?
If it was shown that there was structure where a singularity should be, such as quarks and electrons, inside a black hole, would it still be expected to emit blackbody radiation as Stephen Hawking ...
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How can I (mathematically) see that, as you fell into a black hole, you would not see infinite time passing? [duplicate]
I'm currently taking a GR course and, as we were studying the Schwarzchild solution, we discussed how someone entering a black hole (Alice) could pass through the event horizon in finite proper time, ...
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Can something escape from inside of a black hole's event horizon by putting a sufficiently large mass next to it?
As far as I know, a lot of the maths surrounding black holes basically assumes that the black hole is the only mass in the entire universe. But what if something has fallen into the event horizon of a ...
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Black hole collision with a probe
I understand that for an outside observer it will take an infinite amount of time for an infalling object to reach the black-hole event horizon.
Consider a black hole that orbits around the center of ...
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What happens physically at the white hole - black hole transition in Kruskal-Szekeres coordinates?
Imagine you are an observer that starts on the timelike past $i^-$ of a maximally extended black hole spacetime and immediately shoots along a null ray hovering on the past horizon $\mathcal{H}^-$ of ...
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Are there other ways we know of a massive star becoming a Black Hole other than the star going supernova? [duplicate]
Are Black Holes in the past, but also in the present, possibly formed by a process other than "very massive stars going supernova"?
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Would a star with the same mass as a Black Hole appear as a dark star?
In the case of the Black Hole (BH) the surface is a few Km from the center of mass whereas in the case of an active star with the same mass as the BH, the surface can be be many millions of Km distant ...
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When two black holes spinning in opposite directions approach one another is the Kerr metric destroyed as two regions of frame-dragged space meet?
The kerr metric describes the frame-dragged space just outside a spinning uncharged black hole. I have read in popular science articles that frame dragging is like a stick spinning in treacle causing ...
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Are black holes …white holes? [closed]
Time dilation is infinite at the event horizon, after all.
As you approach the event horizon, your frame of reference slows asymptotically to match that of the black hole. While the universe around ...
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How can black hole mergers happen if we cannot see anything fall into a black hole? [duplicate]
If we cannot observe anything fall into a black hole as an external observer, how can we observe a black hole merger?
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How to prove Hawking area theorem using Raychaudhuri equation?
I recently started studying black holes and I am trying to understand the Hawking area theorem and its mathematical basis (at least at a simplified level). Some resources say that the Raychaudhuri ...
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Toroidal planets to toroidal black holes -- what changes?
It's believed that a sufficiently quickly rotating planet-sized mass could be stable in a toroidal planet formation (though vanishingly unlikely to form naturally). However, assuming no cosmological ...
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Apparent impossibility for spherically symmetric wormhole throats to be devoid of horizons
On Matt Visser's Lorentzian Wormholes there is a brief discussion of "topologically trivial" wormholes, also known as baby universes, or Bag of Gold spacetimes by some authors. This image ...
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Event and cosmological horizons in black hole structures
Sometimes, we face two horizons in the dS black hole. How do we determine the difference between the event horizon and the cosmological event horizon? How is it calculated, and what is the difference? ...
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Why does the Schwarzschild solution describe a black hole?
I recently started studying general relativity and came across the following problem. Schwarzschild's solution is a solution to Einstein's equations in a vacuum, with a zero energy-momentum tensor. As ...
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How can you safely enter a black hole? [duplicate]
In a hard science fiction novel, suppose we want our heroes to enter a black hole. What laws of physics could we tweak to make it theoretically possible, albeit a technological feat?
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Time separation inside a black hole horizon
In Schwarzschild geometry, we often say that when $r<2M$, $r$ becomes timelike and $t$ becomes spacelike. While I understand that this refers to the metric and how it behaves for a radial worldline ...
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Proper time is invariant but undefined at the event horizon in schwarzschild metric. How can we remove this problem with a coordinate transformation?
We know that the line element gives us the proper time taken to traverse “nearby” coordinates.This is an invariant as well, so how can we remove the issue of undefined proper time at the event horizon ...
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What happens when you shine circularly polarized light at a pole of a rotating black hole?
Circularly polarized light carries spin angular momentum (SpAM 😉), so shining it into a pole of a spinning black hole from a point on the rotation axis of the BH should raise the angular momentum of ...
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Cosmological horizon and particle production
Typically, one studies particle production in de Sitter using a scalar field, and observing that one can rearrange the problem as one of a scalar field in Minkowski but with a time-dependent frequency ...
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Is it possible to transmit information from within a black hole via the momentum of a rocket activated after passing the event horizon?
Imagine a simple, non-rotating black hole and a massive rocket that is free-falling past the event horizon of the black hole, linearly towards the singularity. The rocket is massive enough that it ...
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How does a black hole look from the inside (isn't there a paradox, here)? [closed]
This question has been closed as not being "mainstream physics". However, this seems to be wrong, since I recently found an interview of Neil deGrasse-Tyson in which he mentions exactly what ...
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If information can't move outwards the singularity then how does the event horizon "know" it has to expand simultaneously along the whole surface?
I have read this question:
When matter is added, carefully and radially to make sure it does not add angular momentum, the horizon radius increases proportionately to the amount of mass added (but ...
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Can we be alive in spinning Black Holes? [duplicate]
I was studying about black holes and I get to know that we can be alive in black holes if the black hole is spinning. I didn't understand the reason. Can anyone explain.
I mean to ask that if is it ...
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Does Black Hole Formation Require Only Mass Density or Also Event Horizon Contact?
Black holes are firstly a solution of classical general relativity, suggesting that when a sufficiently large mass is in a sufficiently small region, a black hole would appear. This is supported by ...
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What is the thermal radius of the universe's "horizon"?
I have repeatedly come across the statement that every time there's a horizon (could be an event horizon of a black hole, or a Rindler horizon associated with acceleration), the vacuum state differs ...
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Is a black hole spherical?
Black holes are usually created when massive stars use up all their fuel and collapse due to gravitational collapse.
All stars rotate.
However, since angular momentum must be conserved even when they ...
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Exact meaning of the mass $M$ in the Kerr metric event horizon?
Posting this as I have so far not been able to find a straightforward answer to the following question. The formula for the outer event horizon of a kerr black hole is given by the following equation:
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Black hole event horison diameter to steller mass
are event horizon diameters linear as compared to stellar mass? it seems as if there were singularities involved billion star mass black holes would have a relatively smaller event horizon diameter ...
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Boundary conditions on Schwarzschild event horizon
Consider the variational problem for a scalar field in Schwarzschild spacetime $M$ with respect to Eddington-Finkelstein coordinates $(v,r, \theta, \varphi)$, i.e.
$$\delta I(\phi) = \int_M dV \ \Big(...
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What is the difference between the Penrose Conjecture and the Penrose Inequality?
The Penrose Inequality applies to specific initial data sets in general relativity and has been proven under certain conditions, particularly in asymptotically flat spacetimes with non-negative scalar ...
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What is the qualitative difference between the (generalized) Israel theorem and the no-hair theorem?
I know that the Israel theorem accounts for only non-rotating, non-electrically charged black holes, but as I understand the theorem was then generalized for rotating and charged black holes. And, as ...
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Necessity of Singularity in General Relativity
The Schwarzschild solution is the standard example used to describe a black hole, its important points being the event horizon and the central singularity. But this solution is derived by assuming an ...
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End points of event horizon
I am reading The Nature of Space and Time by S. W. Hawking. In the last paragraph on page 16 he said that:
event horizon may have past end points but don't have any future end points
I understand ...
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Can you calculate the radius of a hypothetical singular surface inside a black hole from observing changes to its linear momentum?
Say there is a ball of unknown radius surrounded by a bubble. The ball represents a hypothetical singular surface inside a black hole and the bubble represents the event horizon.
If you threw marbles ...
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Can part of space be causally disconnected from the rest of the universe by being surrounded by black holes? [duplicate]
Is it possible for black hole event horizons to overlap and form a spherical wall around an island of space (that's not inside a black hole) while still being causally disconnected from the rest of ...
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Can a light signal from Earth reach a galaxy outside the Hubble Horizon?
Is this video on the FLRW metric (timestamp 29:00 minutes) mistaken in its claim that a light signal from Earth cannot catch up with a galaxy outside the Hubble horizon, due to the horizon receding at ...
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Black Hole Formation -- How Can an Event Horizon be Observed to Grow? [duplicate]
This is a question about black hole formation. To be clear, I’m not suggesting that black holes don’t form. It’s that I’m having trouble with the accepted explanation so there’s a flaw in my logic ...
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Multipole moment in the power spectra of the CMB
Why is the multipole moment l in the power spectra of the CMB corresponds to the radius (instead of diameter) of the sound wave horizon of the baryon-photon fluid?
It seems to me that it should be the ...
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How to Understand Negative Energy in the Ergoregion?
I am trying to understand the Penrose process and having trouble explaining negative energy in the ergoregion.
How I interpret it is:
Energy is the dot product between the four momentum of the object ...
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When do we talk about spaghettification or pancakification in black holes?
So I've been doing some research for a while now, and yesterday came across the video of PBS space time talking about what happens to quantum information in a black hole. In the thought experiment ...
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Can wormhole inside a black hole become an escape?
I did not major in Physics so not sure if this is a proper question; but according to some Google search there do exist papers discussing wormhole inside black hole like this, which I am not able to ...
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When it comes to getting closer to the Schwarzschild radius, how is discrete a limit?
From Keeton (2014) in Principles of Astrophysics: Using Gravity and Stellar Physics to Explore the Cosmos, Gravitational time dilation near a large, slowly rotating, nearly spherical body, such as the ...
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Why does the timelike killing vector become spacelike inside the ergoregion?
Why does the timelike killing vector become spacelike inside the ergoregion?
Some textbooks make this claim and move on to explain negative energy, but I could not find any proof for this claim. I can'...
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