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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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Why exactly are singularities avoided or “deleted” in physics?

What is the real reason that make us reject singularities everytime we see them in a theory/model? For example, in GR, it is predicted that black holes singularities have infinite density. This makes ...
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Is entropy quantized in Loop Quantum Gravity?

From the Beckenstein-Hawking formula, we know that entropy is proportional to the area of the event horizon of a black-hole: $S\propto A$. From Loop Quantum Gravity, we know that length, area and ...
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Is a stable orbit possible inside the ergosphere of a Kerr (spinning) black hole?

I have heard that it's "impossible to hover" inside of an ergosphere, but everywhere I read this seemed to be speaking in the context of "relative to a stationary observer outside of the ergosphere". ...
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Does the Higgs VEV depend on the gravitational potential?

If I take the non-relativistic limit of the Klein-Gordon equation in a curved space-time I will get a contribution from the Newtonian gravitational potential. Shouldn't this gravitational contribution ...
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Could we use experimental evidence of micro black holes to prove the existence of additional spatial dimensions?

They are planning to create a new particle accelerator that can smash particles together at a much higher energy than the LHC. If in the process we observe the decay of a micro black hole, would that ...
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How do we calculate massive particle paths from schwaschield diagram?

Given the diagram here, we can easily trace out paths of light rays. But if I imagine a stationary particle moving upwards in time going through the center of the light cone, it appears as if it doesn'...
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Gravitational redshift and Energy of a photon

Let us assume that we have a large gravitational field, then the gravitational redshift can be expressed as, $$\frac {v_{\infty}} {v_e} = (1-r_s/R_e)^{1/2}$$ In this equation $v_{\infty}$ represents ...
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Can an invisible neutron star exist? [duplicate]

Basically a neutron/quark star that approaches it's Schwarzschild radius but doesn't surpass it. Its gravitational force is great enough to not let light escape from it, but not great enough to become ...
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How is black hole formed after a supernova explosion?

Does all the mass of a core collapsing star get scattered during a supernova explosion? If it does, then how can a black hole be formed without any remaining mass? Or are there remnants of the star ...
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Spacetime drag and galaxies' radial velocity mismatch

We know - or theorize - that a spinning black hole drags spacetime in its surroundings, producing measurable effects on the accretion disk (the minimum stable orbit radius). What I cannot grasp is why ...
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Advanced and Retarded Coordinates for Hawking Radiation

I am trying to solve the Klein Gordon equation near the horizon of a black hole. For a more accurate result I am using the Vaidya Metric in which the mass is a function of retarded time u. When ...
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Can a photon circle a black hole indefinitely?

Does a photon lose energy or redshift as it circles? Will it's wavelength be the rest wavelength with centripetal and gravitational forces exactly cancelling?
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Is a Radiation Structured BH a Possible Alternative to a Singularity? [closed]

A BH singularity implies an undefinable region of infinite density that cannot be explained by current scientific theory. There's got to be other explanations for the formation of a BH that do not ...
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Naked singularities and mass distribution of a black hole

Do naked singularities also appear mathematically if the mass of a black hole is uniformly spread around its Schwarzschild surface? I recently heard about the concept of a naked singularity and it ...
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What happens when the DeBroglie wavelength becomes smaller than the Planck length?

A particle around the size of a grain of salt would have a DeBroglie wavelength close the plank length. Does this mean quantum phenomena become hidden for larger particles since a DeBroglie wavelength ...
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Schwarzschild geometry for negative masses

Imagine that in a part of our universe there exists negative masses $M=-|M|<0$. The metric around this object -- say a black hole -- will be of the form $$ ds^2 = -\Big(1+\frac{2|M|}{r}\Big)dt^2 + ...
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Will I be destroying spherical symmetry if I write the mass of the body as a function of time?

Will I be destroying spherical symmetry if I write the mass of the body as a function of time? If yes, then how can I write a metric for a body with mass as a function of time?
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What gives mass to black hole?

I like to know when a dying star collapsed into a black hole, is there anything inside or on the event horizon that is interacting with higgs field?
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Is it Possible to Explain a Black-hole by Assuming a Matter-to-radiation Transformation Rather Than The Formation of an 'Infinite' Singularity?

**Is it possible that at the final nanosecond in the formation of a black-hole, particles in the interior of a developing Schwarzschild envelope are colliding at speeds approaching that of light; but ...
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Do black holes have Arago spots?

I've been reading about Arago spots and the Aragoscope. Some fascinating concepts! Basically, due to wave diffraction, there is a bright spot in the centre of the shadow behind any circular object. ...
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Would someone be able to see where they are headed once they crossed the event horizon? [duplicate]

Let us say an explorer was studying a supermassive black hole and ended up blundering past the event horizon. From my understanding, which may be wrong, the only paths allowed after crossing the event ...
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Confusion of the motion of a photon inside the photon sphere

In lecture, I am told that if a photon is initiated outside the event horizon of a black hole, from the laboratory frame of reference, it will never reach the event horizon. What is confusing is that ...
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By what mechanisms, other than Hawking Radiation, do black holes lose mass? [closed]

The equation for the luminosity of a black hole is: $\frac{1}{m^2}\frac{ħc^6}{5120G^2}$. However, if you take the mass of your black hole, and divide it by the lifetime, the equation you get is: $\...
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Gravitational redshift discrepancy?

I want to compute the redshift of a signal emitted by a static observer in $r=R_1$, $\phi=\phi_1$and recieved by another static observ at $r=R_2$, $\phi=\phi_2$ with $R_2>R_1$, in Schwarzschild ...
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Superradiant scattering in ergosphere?

In a black hole there is a region of space where only energy can escape, the ergosphere. I read superradiant scattering can allow lights to gain energy much like gravity assist, what is the different ...
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Circular motion in schwarzschild metric

Hi i have an observer in circular motion in Schwatzschild metric, the velocity is described by the vector $$u^{\mu}=A(\delta_0^\mu+\Omega\delta_0^\mu)$$ and i have to fix A, compute the acceleration ...
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Propagation of gravitaional waves near black holes [duplicate]

As we know near black holes light gets strongly deflected. And if the gravity of the black hole is strong enough, the light can move in circles around the black hole. How the gravitational wave ...
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Is the total information in the (visible) universe compatible with the one on the surface of a black hole with the same radius?

Is the total information available in the universe compatible with the entropy we would find on the surface of an hypothetical black hole whose radius is the visible universe one ($\sim14$ Gpc)? We ...
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Running coupling constants within a highly compressed object

I wonder is it possible. in highly compressed objects, such as neutron stars and black holes, (I'm not sure that this applies to singularities), that the physical conditions within these objects ...
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Would a cellular-automata universe allow singularities to exist?

It is usually assumed (almost always) that cosmological singularities (like black hole singularities) are impossible and do not exist in the universe. If the universe was like a cellular automata (...
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I almost got the Schwarzschild solution [closed]

I was trying to derive the Schwarzschild metric, but i got the following solution: $$ds^2=(C_1-\frac{1}{r})dt^2-\frac{1}{(C_1-\frac{1}{r})}dr^2-r^2d\Omega^2.$$ The correct place for the $C_1$ should ...
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Why does Hawking radiation outside the Schwarzschild radius decrease a black hole's mass? [duplicate]

When a pair of virtual particles is created outside the event horizon of black hole, there is a chance for one of the particles to escape. Zero chance on the surface of the event horizon and my ...
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Are there super-super black holes in the center of super clusters?

If a galaxy contains a super-massive-black hole at it's center. Is there anything likely to be at the center of a super-cluster of galaxies? Or to put it another way, are there likely to be any ...
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Photon emittance of objects under the effect of time dilation

Light that is emitted from objects under the effect of time dilation (from out point of view) is redshifted that much is clear. However I am interested if another effect occurs that I never seem to be ...
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Spaghettification on quarks?

Imagine a nucleon falls into a black hole, I would expect the gravitational force acting on each quark to be drastically different but due colour confinement wouldn't there more pairs of quarks being ...
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BH singularity? Infinite density

How can the density of a region of space go from finite density to infinite when there are no numbers larger than any Aleph0 number but smaller than any Aleph1 number (no decimal point in front of it, ...
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Why do macro black holes take ridiculous amounts of time to evaporate—unlike micro black holes which dissolve in even less than a second?

Why do macro black holes take ridiculous amounts of time to evaporate, considering that micro black holes dissolve in even less than a second? Does this mass-based behavior imply that the matter ...
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Centrifugal force on spinning black hole?

I saw the term spinning black hole popping up everywhere so my question do spinning black hole behave similarly to say a planet where it bulge in the equatorial and compress at the poles? what ...
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Black hole and lepton number conservation [duplicate]

Can falling of an electron into a black be treated as a process in which energy, charge, spin (angular momentum) and lepton number of the system are conserved? Imagine a small volume of space that at ...
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Is ER= EPR a Hidden Variable Theory & If NOT Why?

John Bell showed that entanglement cannot be a local hidden variable theory in 1964. Leonard Susskind and Juan Malcadena have proposed that non-traversable wormholes, Einstein-Rosen bridges (ER) are ...
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Pictures of Different Coordinate Systems in General Relativity

In General Relativity by Woodhouse there are the three following diagrams in Chapter 9 about Black Holes. Despite a (very brief) description of these diagrams in the book itself, I am struggling to ...
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Do binary black holes mergers emit light?

I am not referring to Hawking radiation and although I see there should be no way the collision can produce electromagnetic wave after all, a black hole is simply a region of space right?
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Super Heavy Brown Dwarf

As I understand black holes, they're considered "black" because the escape velocity is, or exceeds, the speed of light. Alternatively, the escaping light is redshifted infinitely. The Schwarzschild ...
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1answer
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Can 2 charged black holes repel each other?

Say there are 2 stellar black holes (equal mass and like charged), can they repel each other? I believe when people say things like charged black hole it is not the same as magnetar where it generates ...
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Time dilation, and curvature of space caused by two black holes of unequal masses

If you were to try to find the time dilation in a region of space near a black hole you would use the equation $$t_r=\sqrt{1-\frac{2GM}{rc^2}}$$ Would the time dilation from two black holes be this? ...
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Why is the information paradox restricted to black holes?

I am reading Hawking's "Brief answers". He complained that black holes destroy information (and was trying to find a way to avoid this). What I don't understand: Isn't deleting information quite a ...
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Radius of Star, The Schwarzschild metric and Black Holes

From Section 9.1, in General Relativity by Woodhouse: For a normal star, the Schwartzchild radius is well inside the star itself. As it is not in the vacuum region of space-time, the Ricci tensor ...
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Can mass still be observed in our universe after moving from a black-hole to a white-hole?

This question is based on the recent papers on LQG black-holes by Abhay Ashtekar. Quantum extension of the Kruskal spacetime and Quantum Transfiguration by Kruskal Black Holes When mass moves from ...
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Velocity of object falling into black hole measured in Schwarzschild co-ordinates, depending on co-ordinate time

This is a followup question to the answer given to a similar question where the velocity is given as $$v(r) = \left(1 - \frac{r_s}{r}\right)\sqrt{\frac{r_s}{r}}c \tag{1}$$ where $r$ is the radius ...
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Black Hole Evaporation with flux

I have a question regarding the black hole evaporation in dependency of the Schwarzschild radius $r_s = \frac{2GM}{c^2}$, flux $f = \frac{\partial E}{\partial t A}$ and the mass of the black hole. So ...