Hole Definition and 1000 Threads

Acceleration due to gravity at the Event Horizon of a Black Hole.

I was learning GR and I got to the point where I learned about worldlines and their structure within a Schwarzschild black hole, that space-time itself is curved so that all future worldlines from an event point inward. Since a massive object cannot exert gravity into its past, how does the...

When matter is dense enough (most usually because its own gravity compresses it), it collapses into a black hole. The necessary density is defined by the Schwarzschild radius of that matter: If matter is inside its own Schwarzschild radius, it will collapse into a singularity (as far as we...

I assume the water to start flowing from rest at position 1, hence ##v_1 = 0##. Applying the continuity equation, ##A_1 v_1 = A_2 v_2##, we find the (wrong) result that the velocity at position 2 is ##v_2 = 0## also! (We assume that ##A_2## is small but finite) Hence, to answer the question...

Hi all, this is a new scenario I got thinking about after having received great feedback and corrections from other PF'ers in this thread. Thanks again for the great help! This new scenario is similar to the previous one, but with a twist including a mirror. And as I said in that thread, I am...

Hi all, I've just read this entire thread and watched the videos about black holes posted by @PeroK, which I liked very much (thanks @PeroK! :smile:). I am not particularly well aquainted with GR and my questions are concerning the often mentioned statement that an observer that passes the...

Probably a silly question I thought of last night but would appreciate some clarification. Matter falling towards a black hole joins the disk spinning around the black hole slowly inching towards the event horizon with each orbit of the black hole. When matter passes the event horizon does it...

As I understand it, as you get closer to a black hole no matter what trajectory you were on approaching it, you will get pulled into the disk of spinning matter around the black hole which I assume is around it's equator? I am just curious as to what is happening to the "space" in the disk, and...

Hi, These questions may seem basic, yet, I do not have an answer for that. The density of electrons and the density of holes in an intrinsic semiconductor are equal at equilibrium. From this, it seems that holes are simply lack of electrons that are given enough (thermal) energy to escape...

Summary:: If BH do exist, could a sufficiently small mass attached to a sufficiently strong string be lowered past the Schwarzschild radius, and pulled back out? If BH do exist (see https://arxiv.org/abs/1406.1525), could a sufficiently small mass attached to a sufficiently strong...

1. So for an asymptotically far-away observer, something falling towards a black hole will never reach it 2. However, the thing falling in will reach the event horizon is finite affine parameter 3. The Universe has a finite age for an asymptotically far away observer a) Does this mean that one...

Article: https://www.skyandtelescope.com/astronomy-news/heavyweight-black-hole-find-mystifies-astronomers/

Summary: The Big Band question. Why don’t we live in a black hole? Or do we? That actually might be a stupid question... so don’t hesitate to tell me that. If all mass of our universe was squeezed into something incredible small (compared the universe size) like maybe a tennis ball or maybe...

I read that after explosions supernovas can ”transform” into a neutron start or into a black hole? And now I’m curious of therer any factors which can predict what thing we’re going to get after the supernova’s explosion.

Due to strong gravitational tidal forces, any living observer will die and any macroscopic measuring apparatus will be destroyed before it reaches the black hole singularity at the center of a macroscopic black hole. Hence the black hole singularity cannot be observed, so from an experimental...

If I was looking at a black hole head on (from outside its Schwarzschild radius), would I be able to discern it? Would the stars etc from behind the hole be gravitationally lensed as to "obscure" the hole? Would any of this change when positioned ON, or within, its Schwarzschild radius? Excuse...

As a prior physics major (had to stop at my 3rd year of undergrad due to children and financial difficulties), there was something I was always extremely passionate about - finding feasible ways to travel faster than light. Obviously, the only feasible way seems to be to bend space-time, but I...

If I had two bullets one moving 0.000001 c and the other moving at .9 c for example, and they both went thru a tin foil square target, assuming they are traveling in vacuum, and that the tin foil square target will make a perfect aperture around the radius of the passing bullet, assuming the...

Summary: If a situation can exist where photons can escape black hole gravity, would “dark star” not be a more adequate name? It is commonly accepted that once there is enough material put together to create a sphere with a mass big enough to prevent light from escaping, that this object then...

For this assume General and Special Relativity and not some of the quantum enhancements that have been proposed for them. If I were an astronaut who just crossed the event horizon on a galaxy center type black hole (big enough so I would not be immediately spaghettified) and avoided the...

Suppose we have two boxes of same material and same size but one have a hole in it and if we raise the temperature with equal amount then does two box expand equally?

My approach was that I consider the pressure of cross section A first Pa= P + dgh Now by writting Bernoulli's Equation between the cross-section A and the opening : $$ Pa + 2dgh + 2dV^2/2 = Pb + 2dV'^2/2$$ Where Pb is the pressure of the opening which is equal to the atmospheric pressure...

Hello, I am running a MCNP calculation where there is a source in the center of a building. I have put a fmesh tally parallel to the ground so that I can see how the particles disperse throughout the building and outside the building. However, I only really care about the results from the fmesh...

If it all started with a big bang of incredible density and energy, does that mean there must be particles that can travel faster than light? How would anything escape the grip of that kind of gravity? I was under the impression that at a certain threshold of mass/density, it turns into a...

According to MIT Technology Review, the Solar System's conjectured 'ninth planet' might be a Black Hole. https://www.technologyreview.com/s/614441/is-planet-9-actually-a-primordial-black-hole/

NASA's website has a video and annotated image of a black hole simulation. It is here. I found the explanations of some aspects of the weirdness of black hole images interesting, but I'm no expert on these things.

From what I understand, a black hole can exist that was formed entirely from antimatter. If this antimatter black hole were to collide or merge with a black hole that has the exact same mass, but was made of regular matter, what would happen? Obviously the matter and antimatter would...

A wormhole collides with a black hole------------what would happen?

Hello! I am a bit confused about the circles of light around a black hole, that were present both in simulations and in that image of a real black hole. I understand that the gravitational field is so strong around the black hole that the light is forced to move in a circular path around (from...

Summary: I'd like to know if anyone else thinks of these things <mentor moved to general discussion>[/color] What if one day we can "warp" x light years away from Earth, build an enormous telescope, and look back at the Earth x years ago with resolution that rivals the American flag on the...

the radius of the hole should decrease from my perspective but the book says that the radius of the hole will increase. I am not able to understand why??

We know that black holes exist in the universe, but white holes don't, as far as we know, Although white holes are not an impossibility in GR. Is it reasonable to ask if the big bang singularity is in fact a white hole, and is the only white hole in the Universe? Is the place where everything...

suppose the universe started shrinking. because the density vs mass is a 1/M^2 factor, density can be made arbitrarily small given high enough mass. so it's not hard for all the mass in the universe to quickly reach black hole density. when critical density of the universal is reached, what...

if there's a really good rocket hovering just above a super massive black hole with very low tidal force, and it dips a rod just inside the event horizon, will the rod break? it seems a certainly but the tidal forces are very weak. is it like dipping ur feet into piranha infested water? so when...

How does a Kerr black hole look like from above? Is it the same as a Schwarzschild one? I could not find any convincing visualization for this question. By above I mean from a point r>0 and ϑ=0.

none

In a circular orbit, the 4-velocity is given by (I have already normalized it) $$ u^{\mu} = \left(1-\frac{3M}{r}\right)^{-\frac{1}{2}} (1,0,0,\Omega) $$Now, taking the covariant derivative, the only non vanishing term will be $$ a^{1} = \Gamma^{1}_{00}u^{0}u^{0} + \Gamma^{1}_{33}u^{3}u^{3} $$...

I just wandering about the following hypotetical process to get some information from the inside of a black hole. 1. Drop a satellite into the black hole on a nearly tangential orbit. 2. After it crossed the event horizon drop a large mass after it (for example a small black hole). 3. This mass...

I am new here so apologies in advance. When a virtual particle and anti particle appear at the event horizon of a black hole, before they destroy each other, they are split with one being sucked into the black hole and the other becoming exhaust. Are these the same particles as the quantum...

Hey, If we take a hypothetical black hole with the mass of thirty billion suns which has the schwarzschild radius of roughly the orbit of the venus and let a spaceship fly into it, would it be possible for the spaceship to do some reseach on the physical nature of the singularity and then...

What would happen if a black hole made entirely from anti-matter merges with another black hole made from normal matter? Since most of the product left over from the annihilation is in the form of light (gamma rays) would the black hole lose mass?

Because they evaporate, quantum micro black holes resemble unstable elementary particles: you can create them by smashing stable particles together, and then they will decay back into particles, as if it was all just another scattering process. Certainly in string theory, elementary particles...

It is known theoretical prediction that Black Holes must emit radiation "... its temperature and surface gravity are proportional to its mass divided by its area" (Kip Thorne). This is applied to Event Horizon (rg). But one can imagine inner Black Hole (e.g. inside r' < rg), it does not have...

Summary: Can a black hole merger be approximated as a daughter Schwarzschild black hole created when two parent Schwarzschild black holes pack close enough together to trigger a local Schwarzschild radius containing their combined mass? We know that the radius of the event horizon of a...

Suppose a black hole with a radius equal to the radius of the observable universe exists and a fleet of coasting spacecraft are moving directly away from the black hole (along a similar vector) at very close to the speed of light with an amount of kinetic energy A with respect to the...

Hello, I want to size my system to be able to get rid of fluids without any head buildup within the container. I am just a bit confused as to what formula I should use. My problem is summed up in the following schematic. Note that P1>P2, I have assumed H=10^(-4)m and my flow rate is 0.1 m3/s...

Suppose there is a 1 million solar mass black hole. A 1.022MeV photon near a nucleus produces an electron-positron pair a distance=A from the event horizon. The positron’s vector is directly “up” away from the singularity. It doesn’t have escape velocity. It reaches an apogee at distance=B from...

In a recent thread we discussed the idea that an object slowly dropped into a black hole, can have its rest mass recovered, as energy, if slowly brought to a halt at the event horizon. Once the object is dropped, it would be unrecoverable, and the BH would gain no new mass. I am under the...

While trying to dislodge some stubborn coffee debris in the holes of a wet espresso bottomless portafilter by blowing into it I discovered that it produced a tone at just over 3000Hz, with harmonics. I wondered, is there some way to use that to determine something about the hole size and...

I was reading another thread which has been closed, so I cannot ask this question there. The question is about a post by @PeterDonis (post #21). https://www.physicsforums.com/threads/what-happens-to-the-inertia-of-a-mass-falling-into-a-black-hole.970627/ There are no internal pressures inside a...