# Tag Info

14

Well, it can't (float), since a Black Hole is not a solid object that has any kind of surface. When someone says that a super massive black hole has less density than water, one probably means that since the density goes like $\frac{M}{R^3}$ where M is the mass and R is the typical size of the object, then for a black hole the typical size is the ...

13

A popular assumption about black holes is that their gravity grows beyond any limit so it beats all repulsive forces and the matter collapses into a singularity. [...] Is there any evidence for this assumption? It's not an assumption, it's a calculation plus a theorem, the Penrose singularity theorem. The calculation is the Tolman-Oppenheimer-Volkoff ...

8

Black holes and "anti"-black holes are the same objects. A black hole resulting from the collapse of normal matter, and a black hole resulting from the collapse of antimatter, are indistinguishable. Recall that black holes only have charge, mass, and spin and there is no way to tell that a black hole originally was matter or not (e.g., we can't measure B or ...

7

It is true that, from an outside perspective, nothing can ever pass the event horizon. I will attempt to describe the situation as best I can, to the best of my knowledge. First, let's imagine a classical black hole. By "classical" I mean a black-hole solution to Einstein's equations, which we imagine not to emit Hawking radiation (for now). Such an ...

7

Suppose you have some collection of matter that is so dense it has an event horizon where the escape velocity is greater than the speed of light. The escape velocity is obviously due to the strong gravitational field of the matter inside the event horizon, and equally obviously that matter is also pulled by it's own gravity towards it's centre of mass. Also ...

7

As dmckee says in his comment, the answer is no, a stationary spherical shell isn't possible. This is because not even the interparticle forces in neutronium are strong enough to support it. The problem is that once inside the event horizon there is no way to travel away from the singularity, or even maintain your distance from it, without travelling faster ...

6

Indeed, nothing can get under the horizon. The stuff close to the event horizon does move outwards as the BH radius increases. Even more with any BH deformations such as waves on its surface, the tidal deformations or the change of the rotation speed, all the oblects close enough to the horizon remain "sticked" to it and follow all the changes of the BH ...

6

No, there's no detectable dispersion in gravitational lensing, at least not when the wavelength is much shorter than the curvature radius. The reason is simple to see: one may approximate the light by rays propagating along geodesics. They have to be null geodesics because the photons are massless. And given the location of the source and initial ...

6

Spacetime is made out of events. An event simply means a moment in time plus a point in space. Events can cause other events, e.g., if a spaceship flies from event A to event B, or a radio signal travels from A to B. It's also possible to have events that can't be causally linked, e.g., if B is 10 light-years away from A and 5 years in A's future, then A ...

6

There are actually several types of black-hole horizons with different definitions, each of which are sometimes called the event horizon. But the one that scientists typically mean when they use this phrase is more precisely called the "absolute horizon". Wikipedia's page is accurate, but not very complete. There is also a nice overview given by the ...

6

The final stages of star collapse include various stages, but three common ones to consider are white dwarfs, neutron stars, and black holes. White Dwarfs are formed when gravitational forces of the mass of the remnants of the star cannot overcome the repulsion of the electron degeneracy pressure. So think of gravity competing with the electromagnetic ...

6

I suspect you're not asking the question you're really interested in, because the answer to your question is really boring. If you jump into a black hole you'll see the event horizon retreating before you, and you'll never cross it. The distance you've travelled is an ambiguous quantity since of course in your frame you're stationary and have travelled no ...

5

Whatever happens, one should always remember that for any observer gravity manifests itself only through second order effecs in the distance to the observer. In other words, in the coordinates, comoving with any observer, metric is always flat along the observer's world line and is quadratic in spatial distance to the world line (see comoving Fermi ...

5

We are already living in a nearly empty de Sitter space - the cosmological constant already represents 73% of the energy density in the Universe - and the Universe won't experience any qualitative change in the future: the percentage will just approach 100%. However, once the space may be approximated as an empty de Sitter space, all moments of time are ...

5

The Hubble length $c/H_0$ does not coincide with the radius of the observable universe. Your calculation assumes a Hubble parameter that doesn't change over time. This is not correct: the Hubble parameter $H$ changes over time, and $H_0$ (the Hubble constant) indicates the current value of $H$. To refer to $H_0$ as a 'constant' is a bit of a misnomer, it ...

5

This is one of the common fallacies when it comes to both special and general relativity. A lot of people, when encountering SR for the first time, think that causality can be violated by using a long pole to send messages. Similarly, it is a common thought that one can "dip" a pole into a black hole and then pull it out. After all, solid things are solid, ...

4

This is a case of an unwisely chosen simile taken waaaay too far. This idea, that the entire universe could be inside the event horizon of not a supermassive, but rather a superduperultrahypermegastupendouslymassive black hole, is usually introduced in introductory classes about general relativity. The instructor in this case is trying to make clear that, ...

4

Yes indeed, in the circumstances you describe a horizon does form, and it's called a cosmological event horizon. Googling for this term will lots of articles on the subject, though for once Wikipedia has let me down and does not have a good article on the subject. However each galaxy wouldn't be behind it's own horizon as groups of galaxies tend to be ...

4

An event horizon is the theoretical boundary around a black hole beyond which an observer cannot be in contact (i.e) the region from which light or any other radiation cannot (classically) escape from the inside. In my opinion, it's NO because once an object has reached beyond the event horizon, we don't even know what happens to it inside. It can't even ...

4

You've heard a slightly garbled account of the physics inside a black hole event horizon, but what you've heard is not so far from the truth. The physics of (stationary) black holes is described by the Schwarzschild metric, though unless you're a GR nerd you'll find this a bit opaque. I'll try to describe what is going on in everyday terms, but you need to ...

4

A stationary uncharged black hole is described the the Schwarzschild metric: $$ds^2 = -\left(1-\frac{2GM}{c^2r}\right)dt^2 + \frac{dr^2}{\left(1-\frac{2GM}{c^2r}\right)} + r^2 (d\theta^2 + sin^2\theta d\phi^2)$$ The event horizon is at $r = 2GM/c^2$, where the $dr^2$ term goes to infinity, so it is a surface of constant $r$ i.e. it is indeed a sphere. ...

4

To add to John's answer: black hole with nonzero angular momentum is represented by Kerr metric. It's horizon is a spherical surface, but it also has a special surface: ergosphere that is oblate spheroid touching horizon at two 'poles'. The no-hair theorem of black hole physics precludes them from having more complicated shapes, because such shape would have ...

3

When matter and antimatter meets we see an explosion but no additional mass/energy is created. What happens is that matter is converted to energy while the total mass/energy is constant. So when the "matter" black hole and the "antimatter" black hole merge, the total mass/energy of the resultant black hole will be equal to the sum of the mass/energy of the ...

3

I would like to add a fact that, perhaps, is not controversial. Namely, that all the information about any infalling object will be available for the outside observer at any time. The information cannot get lost under the horizon, otherwise we have the information loss paradox. This means that it is theoretically possible for an outside observer to restore ...

3

There is no uniquely defined time axis is relativity. Locally, observers in different states of motion have time axes that don't coincide. Even if we fix a state of motion for the observer, there is both a forward and a backward time axis. A more meaningful question would be phrased in terms of the past and future light cones: ...

3

It seems to me that this question is quite trivial, so I hope I am not overlooking anything. For a horizon to form, the observer needs to be "as light-like as possible" (otherwise the slope of his world-line will be more than 45 degrees and union of his past light-cones will cover all of universe). And because there is no other way of achieving that than ...

3

Yes, P will observe a thermal radiation that is locally identical to the Hawking radiation and it is called the Unruh radiation. Its temperature is $a/2\pi$ in the $\hbar=c=k_B=1$ units. http://en.wikipedia.org/wiki/Unruh_radiation A few more words about the relationship of Unruh and Hawking radiation. Unruh found his derivation after Hawking found ...

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