Unanswered Questions
1,161 questions with no upvoted or accepted answers
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View of the sky from inside a black hole
Consider an observer located at radius $r_o$ from a Schwarzschild black hole of radius $r_s$. The observer may be inside the event horizon ($r_o < r_s$).
Suppose the observer receives a light ray ...
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Significance for LQG of Sen's result on entropy of black holes?
Sen 2013 says,
...we apply Euclidean gravity to compute logarithmic corrections to the entropy of various non-extremal black holes in different dimensions [...] For Schwarzschild black holes in ...
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1
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Aren't black holes required to exist forever in our frame of reference instead of evaporating?
I know that for an observer far away, nothing ever crosses a black hole horizon (due to time dilation), while in the frame of reference of a falling observer the horizon is nothing special on its way ...
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10
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2
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940
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Conservation of Komar mass
The definition of Komar mass in GR is associated with one asymptotically flat end. However, a hypersurface may contain more than one end, such as the spacelike Einstein-Rosen bridge in Kruskal ...
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295
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What really are exotic supersymmetric black holes?
I have just read (in the black holes chapter 14 on p244 of this book Ref.1) that in string theory, when one adds an (electric?) charge $Q$ to a static black hole, one can arrive at an exotic ...
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Can a nearly-extremal black hole be stable against Schwinger vacuum breakdown?
I was doing some basic algebra to estimate the range of possible masses $M$ and electric charge $Q$ for a nearly extremal Reissner-Noström black hole. I want to see if the logic is correct
the ...
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10
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259
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Can we have consistent histories inside a black hole?
A consistent history is a POVM set of observables corresponding to a time-ordered product of projection operators. For gauge theories, not any old operator will do, only gauge-invariant observables. ...
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3
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Quantum pressure and chemical potential for a Schwarzschild black hole?
Just as Hawking showed that even Schwarzschild black holes have a temperature, shouldn't they also have a pressure and chemical potential? Are there any analytical formulae of those as well as
$$ T_{...
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262
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Does Hawking radiation need an apparent horizon and when does it switch on during stellar collapse?
I've read that Hawking radiation is implicitly linked with the existence of an apparent horizon (1). This seems a slightly less onerous than linking Hawking radiation with a genuine bona fide event ...
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8
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283
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Does an evaporating black hole violate conservation of angular momentum?
Angular momentum is supposed to be conserved, but when a rotating black hole evaporates the Hawking radiation comes out in straight lines. Doesn't this violate conservation of angular momentum?
Does ...
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8
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Hayden and Preskill's paper "Black holes as mirrors" - Classical model of black hole
If someone's read the "black holes as mirrors" paper by Hayden and Preskill which can be found here , Can you please explain to me how the probability of failure in the classical model of the black ...
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Collapse of two large black holes in AdS
In $4d$ flat space, two black holes of mass $M$ can collapse to form another one of (roughly) mass $2M$. This process is spontaneous, as reflected by the fact that the black hole entropy $S=M^2$ ...
8
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135
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How do inflationary models predict the generation of gravitational waves during the inflationary period?
Recent results from the BICEP2 experiment have produced a lot of talk about the primordial gravitational waves produced during the inflationary period.
I would like to have some explanation about how ...
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343
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Geometric entropy vs entanglement entropy (dependent on curvature coupling parameter)
I have a quick question. In hep-th/9506066, Larsen and Wilczek calculated the geometric entropy (which I believe is just another name for entanglement entropy) for a non-minimally coupled scalar field ...
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7
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187
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Correct statement of Birkhoff's theorem (spherically symmetric does not imply static?)
If I understand correctly, the appropriate statement of Birkhoff's theorem in general relativity is that
The Schwarzschild metric is the unique spherically symmetric vacuum
solution.
(Or we might ...
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291
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Relation between maximally mixed state and thermal state
Hawking calculated the density matrix of the outgoing radiation to be a thermal state. I have heard people say this is a maximally mixed state. Is this because given a fixed average energy in the ...
- 207k
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147
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Resource Recommendation for black hole metrics in General Relativity
In classical textbooks for GR, Schwarzschild and Kerr spacetimes are adequately described. In which books or articles, it is mostly believed that Reissner–Nordstrom, Kerr–Newman, Schwarzschild–de ...
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A Universal Upper Limit on Mass Within a Radius $R$?
Since the universe has a positive cosmological constant, there is an upper limit on the mass of the black holes as evident from the so-called Schwarzschild-de Sitter metric:
$ds^2 = -f(r)dt^2 + \...
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308
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Is the $\alpha'$ expansion in string theory an asymptotic expansion?
The low-energy bosonic effective actions of string theory lead to Einstein-Hilbert gravity, along with scalars and $p$-form Maxwell fields. For example, the action for type IIA string theory is
$S = \...
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Is it possible to have fermions in Schwarzschild spacetime?
To my understanding Geroch proved that on 4-dimensional non-compact manifold a necessary and sufficient condition for a manifold to have a notion of spinors is to be parallelizabe .1 (General ...
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260
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BTZ Black Hole Central Charge and Conformal Weight
I have been trying to reproduce a calculation (equation 4.12) in this paper http://arxiv.org/pdf/1107.2678v1.pdf by Carlip reviewing the derivation of the effective central charge of the BTZ Black ...
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Energy-Momentum Tensor of a Gravitational Wave
In radiation gauge ($\gamma=0$), the Einstein field equation in vacuum for a perturbation $\gamma_{\mu\nu}:=g_{\mu\nu}-\eta_{\mu\nu}$ is given by $$ \boxed{ \partial^\alpha\partial_\alpha \gamma_{\mu\...
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715
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The surface area to volume ratio of a sphere and the Bekenstein bound
I am trying to relate the surface-area-to-volume-ratio of a sphere to the Bekenstein bound. Since the surface-area-to-volume-ratio decreases with increasing volume, one would surmise that, per unit of ...
- 207k
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233
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Do semiclassical GR and charge quantisation imply magnetic monopoles?
Assuming charge quantisation and semiclassical gravity, would the absence of magnetically charged black holes lead to a violation of locality, or some other inconsistency? If so, how?
(I am not ...
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6
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1
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How come the magnetic field disappears when a neutron star becomes a black hole, while the rotation remains?
The only question I found is this one, but this considers as non-rotating neutron star collapsing:
Our final and most comprehensive test is represented by the collapse to a BH of a magnetized ...
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6
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262
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Why are there multiple universes in the Reissner-Nordström solution?
I am trying to make sense of the Penrose diagram of a non extremal Reissner-Nordström spacetime, that is, the solution with two horizons. The coordinates are
$$ v'=\text{exp}\left(\frac{r_+-r_-}{2r_+^...
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0
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230
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Saddle point contributions to the gravitational path integral
In his lectures on black holes and quantum information, Tom Hartman states that the gravitational path integral can be approximated as
$$
Z(\beta) \approx \sum_{g_\text{cl}} e^{-I_E[g_\text{cl}, \phi]}...
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Improving physics simulation of black hole accretion disk?
I have recently been working on software which uses ray tracing/marching to render a black hole in the Schwarzchild metric. I've implemented most everything that I originally set out to do, and I am ...
6
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151
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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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122
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Growth of apparent horizons and null convergence condition
An apparent horion ( S. W. Hawking & G. F. R. Ellis (1975). The large scale structure of space-time.) in General Relativity is a surface where all null vectors are pointing "inwards", i....
6
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106
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How can blackholes be fast information scramblers?
I noticed that there was already a post discussing the fast scrambling property of black holes. But it seems no satisfactory answer was given.
As mentioned by L. Susskind et. al, the fast scrambling ...
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126
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Are there any SUSY black holes in D > 5?
Quoting from arXiv:0801.3471
Asymptotically flat BPS black hole solutions are known only for $d = 4,\,5$.
and I've seen this claim very timidly suggested in a couple of other places, but I couldn'...
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Could the recently discovered "Dancing Black Holes" be the experimental signature of Hawking Radiation?
Science just reported two gravitationally bound massive black holes that are slowly receding from each other. Under normal circumstances two such bound black holes would be approaching each other as ...
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290
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What does black hole formation and evaporation actually look like as viewed from far away?
Many people on Physics SE (myself included) have been confused about what black hole formation and evaporation look like when viewed from far away. For example:
Does any particle ever reach any ...
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What is Hawking Hartle vacuum state and why does the following Euclidean path integral gives the wave functional of it?
I am studying the wave function of black hole via the paper by Sergey Solodukhkin, Entanglement entropy of black holes,arXiv:hep-th: 1104.3712. In the paper, equation (53) is as follows:
$$\Psi[\psi_{-...
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Situation after Saini & Stojkovic's paper on unitarity in gravitational collapse and non-formation of black holes?
In their paper, Anshul Saini and Dejan Stojkovic [1] claimed that by calculations it is possible to demonstrate that in a gravitational collapse of a disk, an event horizon is never made for a far ...
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2
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478
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Hayden-Preskill informational mirrors and decryption
I do have a question about an assumption made in the very interesting Hayden-Preskill paper of black holes as informational mirrors. Alice throws her top secret quantum diary which is $k$ qubits long ...
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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 undergo ...
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1
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Black Holes and QED
So, in quantum electrodynamics (at least to my rudimentary knowledge), the electromagnetic force is mediated by photons.
On the other hand, in classical general relativity, the Kerr Black Hole ...
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2
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Is the Schwarzschild singularity a limit of the Kerr singularity?
In a Schwarzschild black hole, the singularity is spacelike. In a Kerr black hole, it is timelike.
Is there any continuous transformation between those solutions? Can the Schwarzschild solution be ...
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Entanglement island distance = quantum width of BH horizon?
In this recent paper, Bousso and Penington (B&P) finds that the protrusion distance outside the horizon of an entanglement island from a 4D Schwarzschild black hole is $\sqrt {l_P r_{hor}}$, where ...
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What is the volume inside the inner event horizon of a Kerr black hole?
(Dokuchaev 2011) found periodic orbits of particles and photons inside a Kerr-Newman black hole with a maximally extended global geometry. They orbit inside the inner horizon $r<r_-$ where the $r$ ...
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450
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Kinnersley’s “photon rocket” and gravitational radiation
In this paper by Carlip, a comparison is made between electromagnetic and gravitational aberration.
For the latter case, he takes as a study subject the Kinnersley’s “photon rocket”, an exact ...
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On the embedding of the Schwarzschild metric in six dimensions
At every point of the 4-D space-time, it's metric, being a symmetric 2-tensor, has $\frac{D(D+1)}{2}=10$ independent components. From this we can subtract four degrees of freedom according to the four ...
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313
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Can one distinguish between near and far field for gravitational radiation?
For electromagnetic radiation one usually distinguishes between near and far field. The wave equation for the far field are based on Maxwell equations for the vacuum and predict that the radiated ...
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Is the Casimir effect also present between two event horizons?
A follow-up to this question:
Is the Casimir effect also present between two event horizons?
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Further explanation of the Penrose Conjecture
I'm currently a third year maths undergrad, writing a dissertation on the application of minimal surfaces in space.
I have recently come across the Penrose Conjecture that the mass of a spacetime is:
...
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Why can apparent horizon be computed based on its local geometry?
Why can apparent horizon be computed based on its local geometry? In the paper titled Black Holes, Geometric Flows, and the Penrose Inequality in General Relativity by Hubert L. Bray, has been written:...
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Euclidean black hole extrinsic curvature
I have read that the extrinsic curvature at the horizon of a euclidean black hole is zero? Does anybody know how this can be shown?
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Singularity and Black Hole Complementarity
When looking at a (eternal) Schwarzschild Black Hole, we may identify two worlds. The region $R_1$ (right) - our world -, and the region $R_2$ (left) - an other world.
The "black hole interior" ...
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