# Tag Info

10

I think the problem with matter that only interacts gravitationally is that it's hard to get it all to stay in one place. Nebula slowly form stars and planets in part because of collisions between particles lead to larger particles, which tend to attract further particles. But particles that just wizz right through each-other can't coalesce without violating ...

6

Gamma rays are affected just like light rays, so they will be subject to a gravitational red shift and they will be bent by gravitational fields just as visible light is. It's important to be clear that in a gamma ray burst the gamma rays are not generated by the black hole. The process of forming the black hole heats the interior of the star to incredible ...

5

Once inside the Schwarzschild horizon, it is immaterial to observers outside whether what went in was "matter" (fermionic stuff with a rest mass) or "pure energy" (bosonic stuff): it all adds to the black hole's mass by the amount $\sqrt{m_0+(p^2/c^2)}$ where $m_0$ is the rest mass and $p$ its momentum (at infinity). After this, all the body's energy stays ...

5

Whenever an amount of mass (or mass equivalent in energy) finds itself inside a volume smaller than the event-horizon for a Schwarzschild black hole of that mass ($R = \frac{Gm}{c^2}$), then you have the necessary and sufficient condition for a black hole. In fact, the black hole may form before the matter reaches this point, provided that it will reach it ...

5

If you want, you can go and use the ansatz: $$ds^{2} = -A(r) dt^{2} + B(r) dr^{2} + 2C(r)\,dt\,dr + f(r)\left(d\theta^{2} + \sin^{2}\theta d\phi^{2}\right)$$ Where the functions only depend on $r$ due to the fact that $t$ generates a symmetry of the spacetime -- you are assuming a static spacetime. Note, however, that you are free to arbitrarily rescale ...

4

Black holes this small will have very high Hawking temperature: $$T_H = \frac{\hbar c^3}{8 \pi G M k_B} \approx 10^{20}\,\text{K},$$ So, before this black hole can fall down even the diameter of an atom it will evaporate through Hawking radiation. As a result, the 1 tonne of black hole mass would be converted into the energy of very high energy particles ...

4

It's important to appreciate that when you talk about a singularity at the Big Bang or the centre of a black hole you are really referring to a singularity in a particular mathematical model. A black hole (usually) means the Schwarzschild metric, and the equations that describe this become singular at $r = 0$. The Big bang means the FLRW metric, and the ...

4

I believe that both questions are easily answered even without a GR background. Saying that something is an artifact of spherical symmetry means, in this context, that it was expected that the singularity would not occur in cases where no perfect symmetry is assumed. Since perfect spherical symmetry only ever occurs in theory, this would solve the problem of ...

3

I think you're asking if there is some special cutoff density after which spacetime "collapses" and forms a black hole. If this is your question then the answer is no, there is no specific cutoff. Density unites are $\frac{\mathrm{mass}}{\mathrm{volume}}$ but the size of black holes is dependent on the mass and the size is not proportional to the volume ...

2

Assume the rocket is hovering above the hole. If the rocket loses thrust, an accelerometer on the rocket will measure less acceleration. Thus, an astronaut will feel less heavy. However, if the gravity of the hole increases, the accelerometer reading will not change; the astronaut will not feel any change in weight. But, in both cases, the rocket will ...

2

The maximum mass for a neutron star is the Tolman–Oppenheimer–Volkoff limit and is thought to be between 1.5 and 3 solar masses, the range being due to uncertainties of the equation of state of matter at these extreme densities. If the mass of a neutron star exceeds this limit then implosion to a black hole is assumed to be inevitable, there is no force that ...

2

The simplest answer is that the horizon doesn't hold the charged particles, but rather, the information about the charges of particles that have fallen into the horizon lives on the horizon itself. In simpler terms, the electric monopole moment of all of the matter inside of the horizon can be inferred from the geometry and field in a neighborhood of the ...

2

Micro black holes have been hypothesized in some large dimension string phenomenological models and are searched for in the experiments at the CERN LHC. The first approach to the decays was thermodynamic with Hawking radiation diminishing them rapidly. Their lifetimes are very short so there is no way to gather and contain them and experiment with feeding ...

2

What you call "curvature" seems to me to be only a graphic representation-- a map-- of the varying thickness of time-space, a curve which being continuous can be rendered by Calculus. Curvature is a well-defined notion in terms of what happens to a vector when it is parallel-transported around a small loop. Wikipedia has a pretty good motivation for ...

2

There is no mention of Euclidean space here, because the $2$-spheres are not Euclidean. They're not going to be embedded in Euclidean space either. The Schwarzschild $r$ coordinate is defined so as to make the areas of the family of $2$-spheres $4\pi r^2$. In other words, we're simply labeling the $2$-spheres by the their areas. Euclidean space would ...

2

To describe the final stages of black hole evaporation will require a theory of quantum gravity, and no such theory exists at the moment. So your question cannot be answered: we simply don't know what happens when a black hole disappears. I have seen a presentation (I'm afraid I don't have the link) where the final stages of evaporation were calculated ...

2

As dmckee said in his comment, the black hole would fall towards the center of the Earth. To specifically answer this portion of your question: How dense would rock have to be to form a barrier? There is absolutely no density of rock or anything else that would stop or even slow it down. Even if you created this microscopic black hole on the surface ...

1

Here is the picture taken from the book Frolov, V. V. P., & Novikov, I. D. (1998). Black hole physics: basic concepts and new developments (Vol. 96). Springer. Google books we see that for black holes of large enough mass the radiation will consist entirely of massless particles. For smaller masses electrons and positrons would appear, for even ...

1

I hope that somebody familiar with this research will contribute so that we all learn something. I found the following , it is in a separate column like a comment: Locality and unitarity are the central pillars of quantum field theory, but as the following thought experiments show, both break down in certain situations involving gravity. This suggests ...

1

I am assuming that matter is considered energy when it is broken down to its simple building blocks. If that's your criteria for being "considered energy", then at least classically the singularity of a black hole of any mass whatsoever will do this, simply because the gravitational tidal forces diverge to infinity near those types of (curvature) ...

1

There is no 'escape velocity' inside the black hole, the singularity is in the future of all observers crossing the horizon the black hole and is thus unavoidable. Historically, application of Newtonian dynamics to the light of massive bodies goes back to works of John Michell and Pierre-Simon Laplace (note that light then was thought to be corpuscular, ...

1

It is generally adopted that for a 'reasonable' class of 'regular' initial data the vacuum spacetime outside a collapsed object will settle down to a stationary Kerr black hole. This means that at late time the metric is just the regularly perturbed Kerr metric, and the perturbations decay. As was formulated by John Wheeler: 'Black holes have no hair'. So ...

1

In addition to Danu's answer, it is worth mentioning that there is another class of solutions in GR which arise as the artifacts of special symmetry: naked sigularities (that is, singularities without event horizons). The cosmic censorship hypothesis states that for a 'reasonable' matter, dynamical evolution from generic initial conditions can never produce ...

1

As something falls towards the event-horizon Lorentz transformation on the object becomes apparent to an outside observer. Specifically, space contracts increasingly tighter as the object nears the event-horizon; time for the object slows, slowing to the point that it either appears stopped relative to the age of the universe, or is actually stopped; all ...

1

The super massive black hole is creating a force acting on the material in the galaxy, but that material still has angular momentum which needs to be conserved. In a similar way, the earth is in orbit around the sun and it is bound in its orbit by the gravitational potential from the sun's mass. If you were to naively calculate the force on the earth as \$F= ...

1

Well, this is by no means settled, but that's ok (meaning any answer is likely going to be seen as controversial until further observation provide more data). John Rennie's answer doesn't appear to address whether or not singularities actually form in the first place; he does suggests that few skeptics would argue this, but in fact a few have (David ...

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