What is Black hole: Definition and 1000 Discussions
A black hole is a region of spacetime where gravity is so strong that nothing—no particles or even electromagnetic radiation such as light—can escape from it. The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole. The boundary of no escape is called the event horizon. Although it has an enormous effect on the fate and circumstances of an object crossing it, according to general relativity it has no locally detectable features. In many ways, a black hole acts like an ideal black body, as it reflects no light. Moreover, quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass. This temperature is on the order of billionths of a kelvin for black holes of stellar mass, making it essentially impossible to observe directly.
Objects whose gravitational fields are too strong for light to escape were first considered in the 18th century by John Michell and Pierre-Simon Laplace. The first modern solution of general relativity that would characterize a black hole was found by Karl Schwarzschild in 1916, and its interpretation as a region of space from which nothing can escape was first published by David Finkelstein in 1958. Black holes were long considered a mathematical curiosity; it was not until the 1960s that theoretical work showed they were a generic prediction of general relativity. The discovery of neutron stars by Jocelyn Bell Burnell in 1967 sparked interest in gravitationally collapsed compact objects as a possible astrophysical reality. The first black hole known as such was Cygnus X-1, identified by several researchers independently in 1971.Black holes of stellar mass form when very massive stars collapse at the end of their life cycle. After a black hole has formed, it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses (M☉) may form. There is consensus that supermassive black holes exist in the centers of most galaxies.
The presence of a black hole can be inferred through its interaction with other matter and with electromagnetic radiation such as visible light. Matter that falls onto a black hole can form an external accretion disk heated by friction, forming quasars, some of the brightest objects in the universe. Stars passing too close to a supermassive black hole can be shred into streamers that shine very brightly before being "swallowed." If there are other stars orbiting a black hole, their orbits can be used to determine the black hole's mass and location. Such observations can be used to exclude possible alternatives such as neutron stars. In this way, astronomers have identified numerous stellar black hole candidates in binary systems, and established that the radio source known as Sagittarius A*, at the core of the Milky Way galaxy, contains a supermassive black hole of about 4.3 million solar masses.
On 11 February 2016, the LIGO Scientific Collaboration and the Virgo collaboration announced the first direct detection of gravitational waves, which also represented the first observation of a black hole merger. As of December 2018, eleven gravitational wave events have been observed that originated from ten merging black holes (along with one binary neutron star merger). On 10 April 2019, the first direct image of a black hole and its vicinity was published, following observations made by the Event Horizon Telescope (EHT) in 2017 of the supermassive black hole in Messier 87's galactic centre. In March 2021, the EHT Collaboration presented, for the first time, a polarized-based image of the black hole which may help better reveal the forces giving rise to quasars.
As of 2021, the nearest known body thought to be a black hole is around 1500 light-years away (see List of nearest black holes). Though only a couple dozen black holes have been found so far in the Milky Way, there are thought to be hundreds of millions, most of which are solitary and do not cause emission of radiation, so would only be detectable by gravitational lensing.
Supposing the total mass of a stationary, non rotating Neutron Star is just one Kg below the mass required to form a black hole. Based on the wiki reference below the Schwarzschild radius must be just beneath the surface of the Neutron Star sphere.
Now supposing an object with a mass of one Kg...
When two BHs collide the resulting single BH bulges and contorts until it settles down to a stable state.
1) Does this mean that during this 'settling' period the mass internal to the merged BH is not (yet) a singularity, but instead two 'singularities' spinning down around each other in...
I have heard from, many attractive looking physicists on Youtube, that if I fell into a black hole, as I fell, I'd be stretched out or be "spaghettified. Is this correct?
It seems to me that gravity compresses the free space, and that means, any object near a black hole should be compressed. If...
Stephen Hawking, in his book Brief Answers to the Big Questions (2018), wrote the following (pp. 106-107):
"If you fall towards a black hole feet first, gravity will pull harder on your feet than your head, because they are nearer the black hole. The result is that you will be stretched out...
Seeing the Unseeable: Capturing an Image of a Black Hole
A Free Science Lecture
Led by Dr. Sheperd Doeleman, the Event Horizon Telescope (EHT) project made history by capturing an image of the event horizon of a black hole, where gravity is so strong that even light cannot escape. You are...
[Moderator's note: Spin off from previous thread due to topic change.]
Just as an aside and not related to the OP, would a real particle with negative mass inside the event horizon follow the runaway motion? Would it be ejected?
Frame dragging around a spinning black hole is generally accepted in cosmology. Does the frame dragging result solely in space-time rotating forever around the black hole, or does frame dragging ultimately result in space-time vectoring towards and into the black hole?
An additional question...
If a black hole experiences infinitely slow time (IE time stops) due to their singularity, how are they able to move/orbit?
If supermassives occupy the centre of most/all galaxies. How are the galaxies moving if the black holes can't?
After binge watching Steins;Gate, it has had me thinking about black holes. In the show it mentioned the idea of microscopic black holes(CERN). That being said, if matter and energy have an "equivalence", and if the Schwarzschild radius depends on mass, then would it be possible to confine a...
Hi,
This question is so simple - sorry if the answer is also that simple...
It is pretty clear that every matter can cross the event horzion of a black hole. It is said that this process can be even very smooth if the black hole is big enough ("the bigger, the better for you", this is how I...
I am looking for a book/notes on the topics mentioned in the title that would be accessible to an undergrad. I have a background in grad quantum and statistical mechanics, but most resources I found on those topics assume a familiarity with QFT, string theory, gauge theory, and general...
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...
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...
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...
Dark matter passes through everything, but is only influenced by gravity, so in the case of a neutron star, since dark matter doesn't interfere with ordinary matter, it can just pass through it, but neutrinos might be stopped by it's density, as neutrinos can just pass through stars almost as if...
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 I have studied before, I found that Infinite Red Shift occurs where gtt = 0 but this exercise says that on Kerr's Black Hole it doesn't really work like that.
Right now I'm blocked because I didn't find anything on the internet about it so I don't know how to show this phenomenon. Any help...
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...
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...
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...
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...
I am trying to derive the radial momentum equation in the equatorial Kerr geometry obtained from the equation $$ (P+\rho)u^\nu u^r_{;\nu}+(g^{r\nu}+u^ru^\nu)P_{,r}=0 \qquad $$. Expressing the first term in the equation as $$ (P+\rho)u^\nu u^r_{;\nu}=(P+\rho)u^r u^r_{;r} $$ I obtained the...
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.
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?