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1 vote

Why can't dark matter be black holes?

tl; dr: it can, but needs more observations/theoretical reasons to be convincing. Black holes can come in a variety of mass ranges, from stellar-mass ones to supermassive black holes. One feature for ...
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3 votes

Why light can't escape a black hole but can escape a star with same mass?

There are a lot of ways to explain this, with various levels of accuracy. I'm going to choose a way to justify this, in the spirit that the original question seems to not come from a place with a lot ...
8 votes

Why light can't escape a black hole but can escape a star with same mass?

yes, it is because the black hole is much more dense which means it packs the same mass & gravitational pull into a much smaller diameter. This means its surface gravity yields an escape velocity ...
3 votes

Why light can't escape a black hole but can escape a star with same mass?

Yes, it's just because the black hole is smaller. The only mass factor relevant to the trajectory of a particle in a spherically symmetric mass distribution is the total mass that is closer to the ...
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0 votes

Why apparent horizon area provides a lower bound for event horizon area?

Let me add a comment. Penrose inequality arise from this collapsing shell set-up; however, later this inequality become of interest independent of collapsing shell, as Gibbons put it in "...
10 votes

If massless objects ALWAYS travel at the speed of light and gluons are massless, how are they trapped within hadrons without a need for event horizon?

Now, to make the light trapped within a small region of spacetime, we need curvature so big that it causes an event horizon, so now we have a Black Hole Photons do not carry an electric charge. So ...
13 votes

If massless objects ALWAYS travel at the speed of light and gluons are massless, how are they trapped within hadrons without a need for event horizon?

That quarks and gluons are trapped inside hadrons is called color confinement. As far as I know: it is unrelated to black holes but I don't read Phys Rev D... it's possible it has been suggested ...
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0 votes

Star gets eaten and spit out?

You are talking about Hawking radiation, which makes black holes evaporate. While General Relativity explains how BHs form through gravitational collapse, it's quantum mechanics that explains their ...
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2 votes

Star gets eaten and spit out?

Anything that passes into the event horizon of an astronomical black hole is gone from our observations. It won't be seen again. But material can orbit a black hole without falling in. If there is ...
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2 votes

Typically energy of black hole compared to a planet or star of the same mass

You ask about "the typical energy of a black hole". All the black holes that we know about have formed from the collapse of massive stars or the merger of stellar black holes, so they have ...
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0 votes

Is there any evidence black holes can grow (gain mass) by accretion?

Mass accumulation rates typically are limited by the Eddington luminosity (where the light pressure starts blowing away the accreting material) giving a rate $\approx 1.5\cdot10^{17}(M/M_\odot)$ g/s ...
0 votes
Accepted

Why apparent horizon area provides a lower bound for event horizon area?

In Penrose's setup, the spacetime interior to the null hypersurface $H$ is always Minkowski.Since the area of a cross-section $S$ of $H$ has to be equal when measured from the interior and exterior ...
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0 votes

Where is Schwarzschild solution valid?

Schwarzschild solution is valid only outside of matter's sphere and it is static - there is no time dependence. In case of black hole it ends on the even horizon. Behind it, time and space change ...
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3 votes

Where is Schwarzschild solution valid?

However, when we use kruskal coordinates, the coordinate system covers the inside of the spherical object (usually taken to be a black hole). The interior (by which I mean, $r<r_S$) of a ...
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0 votes

How compute the mass of AdS-Schwarzschild by ADM mass formula?

I think the issue is that you’re using a formula for the ADM mass which assumes unit lapse. Consider a spherically symmetric spacetime of the form \begin{align} {\rm d}s^2 & = -f(r) \, {\rm d}t^2 +...
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0 votes

Does black hole entropy change as a gravitational wave passes it?

Yes, gravitational radiation can change the entropy of a black hole, but it will depend on the behavior of these radiation on the event horizon. One way to study these effects is by considering the ...
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1 vote

What happens to the entropy of the pre-existing information on a black hole event horizon as more mass falls into the hole?

I'm not a black hole entropy specialist. But I'm noting down some of my observations, which may be helpful for you. Black hole entropy is related to the horizon area, $A$ of that black hole: $S_{...
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1 vote

Does black hole entropy change as a gravitational wave passes it?

Gravitational waves might lead to the growth of the black hole, and hence they can make the area increase. However, the area will not decrease again. A heuristic way of thinking about this is by ...
1 vote

What is the fundamental reason why information is on the event horizon of a black hole?

I'm going to answer the question you actually asked, but as you'll see, this is probably not quite the question you really wanted. A possibly intuitive argument for it is information conservation. The ...
1 vote
Accepted

Star collapses to a singularity or is collapsing into singularity?

Clarifying this point. the star cannot collapse to a point, as long as there is an outside observer. Dale has answered the "as long as there is an outside observer" , that cause and effect ...
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1 vote

Star collapses to a singularity or is collapsing into singularity?

Your line of thinking is not nonsense, but it isn’t exactly right either. The part that is not nonsense is that it is true that an observer outside the event horizon will never receive a signal from ...
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1 vote

How would a clock with hands behave if half of it is outside a black hole and half inside?

A clock on a path aimed to be tangent would not hit at a tangent. Everything, even light, is deflected inward. Gravity around a black hole is so strong that light fired tangentially at 2.6 times the ...
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0 votes

Do supermassive black holes at galactic centers and the galaxies containing them spin with the same axis?

When galaxies were formed, the dense center would have created a spiral due to gravitational forces dominant at the center. Possibly a star had formed and as a part of evolution, the star has now ...
3 votes
Accepted

Synchrotron radiation and Hawking radiation

It is true that Hawking both proposed the virtual particle interpretation in the original paper, and immediately noted: It should be emphasized that these pictures of the mechanism responsible for ...
-2 votes

Why is light always used as an example while glorifying a black hole?

People focus on the inability of light to escape because the media focuses on it. It’s the most popular characteristic but there are much more interesting questions about light/photons than their ...
3 votes

Why is light always used as an example while glorifying a black hole?

Light is used as an example precisely because people usually assume it, among all other things, is free to travel at the highest possible speed unaffected by gravity. What other example would you use ...
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2 votes

Why is light always used as an example while glorifying a black hole?

Even though copy-pasting has always been and still is a widespread practice among those who want to sound intelligent, I don't believe that this is the case for this pretty superficial phrase. ...
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2 votes
Accepted

Why is light always used as an example while glorifying a black hole?

Light is composed of elementary particles called photons, which have zero mass and always travel with the velocity light has in vacuum, c. Photons can always "escape" a surface of a star, ...
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6 votes

What is the Schwarzschild metric in cylindrical coordinates?

As you noted, that's not the Schwarzschild metric in cylindrical coordinates. In spherical coordinates, where the corresponding Cartesian coordinates would be $$(x,y,z) = r (\sin θ \cos φ, \sin θ \sin ...
3 votes

Closest possible orbital radius for equal masses

If you have two objects with non-negligible mass there are no stable orbits due to loss of energy to gravitational radiation. When one of the two mass is much smaller than the other, there may still ...
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7 votes
Accepted

How do I use the Schwarzschild metric to calculate space curvature and time curvature seperately?

I quite dislike this particular turn of phrase - there is no meaningful sense in which curvature can be split into temporal and spatial parts. We can talk about pure spatial curvature in the sense ...
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1 vote

How much time does it take for the gravitons generated by a black hole singularity to travel before exerting gravity forces on other celestial bodies?

It is known to all that the travelling speed of gravitons (the propagation speed of gravitational field) is not instant. True. Changes to the gravitational field propagate at the (local) speed of ...
2 votes

Is there comprehensive catalogue of black holes and relativistic jets?

There is the BlackCat catalogue of black holes in X-ray binary systems (basically all the known Galactic black holes apart from the supermassive central one). The GWTC-3 catalogue is a compiled LIGO-...
1 vote

If two black holes orbit around each other should their tidal forces cause a shrinking of the closer parts of their event horizons?

An event horizon is not defined in terms of the strength of gravity or the absolute value of gravitational potential. It is the surface from behind which, photons cannot emerge and escape to be seen ...
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0 votes

Black hole image

They chose M87 because it was the black hole that should have had the largest angular size as viewed from the Earth. It was also a very massive black hole and the long characteristic timescale for ...
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0 votes

Black hole image

Because not all black holes are located in galaxies that face us head-on. It's not that easy to take a picture of a black hole when it's behind lightyears of stars, dust etc. M87 happened to be facing ...
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1 vote

Are stable orbits within the event horizon of a black hole possible?

This reference discusses orbits "inside" the Cauchy horizon. This region is unphysical (does not exist in real black holes) since the blueshift effect will convert the Cauchy horizon to a &...
6 votes
Accepted

Creation of Magnetic Monopoles using Black Holes

At that point of time, the magnetic field lines from north pole must not travel outside the event horizon to the south pole and the south pole outside must behave like a monopole. Is this thinking ...
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0 votes

How are the particles constituting a black hole entangled with Hawking radiation?

As mentioned in the answers linked in the comments to the post (see, for example, the answers to An explanation of Hawking Radiation), the notion of particle does not make sense in curved spacetime (...
3 votes
Accepted

Why is a primordial black hole's mass of the same order as the horizon mass in the early universe?

The early universe was dominated by radiation, that is, particles moving at or close to the speed of light. Such particles exert a huge amount of pressure, relative to their energy density. This ...
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1 vote
Accepted

What appears in the wake of a moving event horizon? For 2 merging black holes for example

First, remember that the notion of an "event horizon" is global statement about the causal structure of spacetime, rather than some local statement. As such talking about the movement of ...
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2 votes
Accepted

Is a black hole an ideal one-way function?

A black hole is not a function of any kind in the mathematical sense that a one-way function is. There's no connection. A one way function takes input in a way that's very difficult if not ...
0 votes

What is the pressure inside of a black hole?

Black holes have quantum pressure, which is one of their fundamental characteristics along with others like their Hawking temperature, which may be expressed mathematically as follows:  [1], $$P = \...
6 votes

How do we know the assumptions of the Schwarzschild solution are valid?

The spacetime be static is not a necessary assumption for obtaining the Schwarzschild solution. Birhoff's theorem tells us that any spherically symmetric vacuum solution to the Einstein equation is ...
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2 votes
Accepted

Is there no curvature due to mass when travelling along a sphere outside a black hole?

It's due to the way the radial coordinate $r$ is defined. It's natural for beginners to assume it's a radial distance, but it isn't. It is defined as the circumference of a circle centred on the black ...
19 votes
Accepted

How do we know the assumptions of the Schwarzschild solution are valid?

This is a very long-winded answer, so let me state the punchline up front: the boundary condition you pointed out really is physically suspect, and probably not realized in Nature -- however using it ...
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