What is Black holes: Definition and 993 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.

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  1. H

    B Can we see 2 black holes orbiting each other?

    as I understand time stops at the event horizon of a black hole for the far away observer; so can we actually see 2 black holes orbiting eachother? I also understood that the singularity is a moment in future. so what happens when 2 black holes orbit eachother with the singularites? from the...
  2. fresh_42

    A Do Black Holes have Singularities?

    Do Black Holes have Singularities? "There is no proof that black holes contain singularities when they are generated by real physical bodies. Roger Penrose claimed sixty years ago that trapped surfaces inevitably lead to light rays of finite affine length (FALL's). Penrose and Stephen Hawking...
  3. Vanadium 50

    I Where are the black holes?

    Where should the nearest black hole be? Something like 0.1% of stars end up as BH's, so that suggests about 100 million in the Milky Way. The easiest thing to do, instead of a complicated geometry problem, is to recognize that the cube root of 0.1% is 0.1, so that we expect BH's to have...
  4. Vanadium 50

    I Black Holes in Curved Spacetime

    We've been talking in another thread about supermassive black holes. That has me thinking about really, really big BH's - so large that the spacetime curvature and evolution of the universe matters. Let's start by defining the density of a black hole as its mass divided by the volume enclosed...
  5. KDP

    I Brian Cox and the end of the Universe?

    Professor Brian Cox was on the TV last night. He stated that eventually everything will end up in black holes. When there is nothing left to absorb they will start evaporating via Hawking radiation until eventually they all disappear in a small flash of light and then there will be eternal...
  6. Vick

    I What are the effects on a stationary observer for Kerr metric?

    A Kerr Black Hole (BH) is a spinning BH. There is an Event Horizon (EH) which is $$r_H^\pm = \frac{r_S \pm \sqrt{r_S^2 -4a^2}}{2}$$ where ##a=\frac{J}{Mc}## and ##r_S## is the Schwarzschild radius. My question is, suppose I'm in a spacecraft, not in orbit, but stationary at a distance ##r##. I...
  7. codebpr

    A Symmetry breaking in the AdS small/large black hole phase transition

    I am trying to reproduce the results from this paper where they find out the expression for the Landau functional to be $$\psi(x,t,p)=\frac{1}{4}(\frac{1}{x}+6x+px^3-4tx^2)$$ We plot the Landau functional v/s the order parameter($x$) at $p=0.5$ and obtain the Figure 4. from the paper as Now...
  8. jaketodd

    I Entanglement may reveal more about black holes? ...Even micro ones!

    Could an entangled particle (or larger entangled object), sent into a black hole, reveal anything new about black holes, with the connected entangled partner outside the black hole? Can entanglement escape the singularity and communicate with its partner? I've heard the singularity is a rip in...
  9. Z

    I Radiatively efficient accretion onto BHs

    It is said that accretion disk around compact objects like black hole can convert up to 40% of the mass of an infalling material into energy. This means, to my understanding, that if we throw 1kg of matter onto a BH through an accretion disk, during the accretion 0.4kg worth of energy in the...
  10. R

    B How do Black Holes Grow? A Far-Away Observer's Perspective

    Black holes are everywhere in astrophysics. There are numerous discussion about how black holes look like, what happens to gas falling into black holes, how light bends around black holes, whether there is loss of information when mass or energy falls in, etc. There is thought to be a black hole...
  11. Halc

    I Do black holes destroy quantum states?

    This pop article popped up (isn't that what they do, by definition?) on my google news page. https://www.sciencenews.org/article/black-hole-paradoxes-quantum-states It claims that a thought experiment shows that doing a double-slit experiment near a black hole event horizon can reveal...
  12. Aerodyn

    I Escape from Two Black Holes? Solve the Mystery!

    Hello everyone, Imagine the following situation: You are inside the events horizon of a static black hole (called BH1) Now, due to the bended space-time, your future points to be inside the event horizon until eternity (You will never be outside) Now, a second static black hole (BH2)...
  13. C

    B Black Holes: How Do They Move?

    The event horizon is an area where the curvature of space is so great that no particles from within the black hole may travel past that point. For analogy, it is as if a highway ramp curved all the way back to its point of origin, an automobile moving forward would just end up going back where...
  14. haushofer

    I Formation of jets with accretion disks at black holes

    Dear all, I'm going to give a course about black holes at an astrophysics association. The public will consist mainly of lay persons, perhaps wit a little bit of physics background. My background in General Relativity is good, but my background in astrophysics at bit less. My question is if you...
  15. S

    I Kerr & Nordström Black Hole Evaporation: Q&A

    When a Kerr black hole evaporates, what will the Kerr parameter do? Stay constant at initial value? Approach zero? Approach unity? Approach a target value somewhere between zero and unity? Also, Nordström black holes in practice (with matter around) would have a strong tendency to attract...
  16. H

    I Black holes to explode due to evaporation?

    according to Hawking a black hole slowly evaporates and shrinks (when not feeded from outside with matter falling in). as I understood there comes a moment when it explodes and the singularity disappears. Is this true and if so, how is this triggered? and is there something such as a critical...
  17. B

    B The twin paradox and black holes

    Does the twin paradox hold around a black hole (or maybe less extreme gravitational fields)? In a gravitational fields like that of the Earth it seems to apply. If two particles fall together, with synchronized clocks, and one of them rests on a platform for a while, after which it accelerates...
  18. Bandersnatch

    A Black hole mass coupled to expansion -- astrophysical source of dark energy?

    Observational evidence for cosmological coupling of black holes and its implications for an astrophysical source of dark energy Comments?
  19. Feynstein100

    I Time dilatopause for black holes

    I was watching a video on GR by mathematician Matt Parker (he of the Parker Square) where he corrected some general misconceptions about GR. Most of the video wasn't anything special but there was one concept that stood out to me: the time-dilato-pause. The idea behind it is quite simple: for...
  20. V

    A Black holes: merged remnants have similar spin - why?

    Hello everyone, I hope I'm asking in the correct section, if not please point me. I read a list of gravitational wave detection. I focused on black hole - black hole events and I noticed the resulting black hole spin is very similar about a=0.7. I didn't find any explanation for this. List of...
  21. DaveC426913

    B Spaghettification & Black Holes: Classical & Einsteinian

    (Classical model) The radial "stretching" is caused by differential gravity (tides), but what is the lateral squashing caused by? Is it because the "force" of gravity is not parallel, but instead comes from a point, forming an acute angle? (Einsteinian model) I guess it's pretty trivial to...
  22. W

    B Question regarding supermassive black holes in the early Universe

    I'm really curious about this, but I want to know how wrong I am. I've seen in a lot of content recently about how observations of early supermassive blackholes are observed to be more massive than they should be. If I understand it correctly it has to do with the maximum rate a super massive...
  23. B

    A Do Astrophysical Black Holes Contain CTCs?

    I've been seeing popularizations recently that talk as though it's widely accepted that astrophysical black holes contain CTCs. Example: https://www.abc.net.au/news/2023-01-04/are-black-holes-time-machines-yes-but-there-s-a-catch/101822002 Is this accurate? Eternal black hole solutions contain...
  24. E

    B Calculate Red Shift w/ Shwarzschild Metric: A Hypothetical Black Hole Any Size?

    You can unconventionally use the Shwarzschild metric to calculate red shift at given stellar distances with some accuracy right out to the edge of the universe. Take the mass of the known universe ~10^55kg. Try it! A bit ironic. I understand this is not how to use the Shwarzchild; however...
  25. K

    Exploring "The Mathematical Theory of Black Holes" by S. Chandrasekhar

    In page 67 of book "The mathematical theory of black holes" by S. Chandrasekhar in chapter 2 "Space-Time of sufficient generality" there is a theorem that metric of a 2-dimensional space $$ds^2 = g_{11} (dx^1)^2 + 2g_{12} dx^1 dx^2 + g_{22} (dx^2)^2$$ can be brought to a diagonal form. I would...
  26. slogals

    Two black holes moving in a circular orbit around a point

    I tried solving it and i was able to do a) and b) here is what i did on c), but its not correct according to the solution
  27. F

    B Does a contracting universe destroy its black holes?

    I found related questions being debated on the web so i'm not sure wether the question is closed. The following simple reasoning seems to imply that indeed the contracting universe is able to destroy its blackholes but what's wrong with it ?: The black hole solution is usually computed outside...
  28. Melbourne Guy

    Writing: Input Wanted Black holes as weapons - easy way to calculate the evaporation blast

    I've decided my latest novel uses black hole bombs, which aliens create by a mechanism unknown, and which are small enough to quickly evaporate to blast ships in the vicinity. Knowing it is sci-fi and doesn't have to be perfect, about how fast do BHs evaporate? Alternatively, if I need them to...
  29. U

    A Question about dilaton monopole interaction derivation

    I am trying to understand how one derives the dilaton monopole interaction. In "Black holes and membranes in higher-dimensional theories with dilaton fields", Gibbons and Maeda mentioned that one could obtain the dilaton monopole interaction as such: where the action is given by However, I...
  30. Bob Walance

    B Could the source of dark energy be our parent black hole feeding?

    About a year ago, I heard Leonard Susskind discussing how entangled black holes could create spacetime. As I was listening to Prof. Susskind describe the mechanisms for creating entangled black holes, and how these black holes might create their own spacetime, it occurred to me that if we were...
  31. K

    I What Happens Inside BH Event Horizon?

    I have read about the spaghettification of objects due to tidal forces as they get close to the singularity. Gravity at your feet is stronger than at your head, so you get stretched and pulled apart. In this case, the singularity is a point in space. But I also read about the time coordinate...
  32. S

    Normal vector of an embedding surface

    I will only care about the ##t## and ##x## coordinates so that ##(t, z, x, x_i) \rightarrow (t,x)##. The normal vector is given by, ##n^\mu = g^{\mu\nu} \partial_\nu S ## How do I calculate ##n^\mu## in terms of ##U## given that the surface is written in terms of ##t## and ##x##? Also, after...
  33. H

    I Effects of Supermassive Black Hole Merger on Nearby Bodies

    What would be the effects of a merger of supermassive black holes? Such releases by far the greatest amount of energy of any event in our Universe as gravitational waves. I suppose that a great deal of EM radiation is released, but I'm more interested in the gravitational waves. The...
  34. mef

    B How Can We Observe Black Holes Growing?

    IT IS WIDELY KNOWN that from the point of view of a distant, BUT FROM a FINITE distance, the time of falling into a black hole is INFINITE. Then how does the BH absorb the surrounding matter? It turns out that for any observer located at a finite distance R (the coordinate of the Schwarzschild...
  35. K

    I Higgs inflation and primordial black holes

    I've found highly cited papers that explains inflation with the Higgs as the inflaton and primordial black hole as the dark matter, for examples, These papers solve the inflation with the Higgs and Dark matter with PBH without introducing new particles. Primordial Black Hole production in...
  36. A. Neumaier

    A question about black holes

    Summary:: The most basic issue queried Why do we know so little about black holes?
  37. O

    I Information loss in black hole, why is it a problem?

    If an observer never falls into a black hole, something it observes that does fall in takes an infinite time to reach the event horizon. If an observer falls into a black hole along with an object, it will not lose information on the falling object, but will lose that from the outside of the...
  38. Godot_

    I Does Dark Matter Fall into Black Holes?

    Does dark matter(DM) fall into black holes(BHs)? IMHO it should... ...as it does interact with normal matter gravitationally. Once it's done so, does it add to the BH's mass? Again, IMHO it should. AFAIK it does have mass... Would that "quasi-convert" it to normal matter? ...after all, this...
  39. U

    Finding Event Horizon & Ergosphere: Derivations & Formulas

    Homework Statement:: See below. Relevant Equations:: See below. I am trying to calculate the event horizon and ergosphere of the Kerr metric. However, I could not seem to find a proper derivation or formula to calculate the event horizon and ergosphere. Could someone point me to the...
  40. S

    I Calculate Ricci Scalar & Cosm. Const of AdS-Schwarzschild Metric in d-Dimensions

    I know some basic GR and encountered the Schwarzschild metric as well as the Riemann tensor. It is known that for maximally symmetric spaces there is a corresponding Riemann tensor and thus Ricci scalar. Question. How do you calculate the Ricci scalar ##R## and cosmological constant ##\Lambda##...
  41. Joe Bond

    I Can Placing Earth Between 3 Black Holes Slow Down Time?

    I am just wondering if placing the Earth equidistant from 3 black holes that are spinnng would slow down time on the Earth, due to the time dilation effect. Would that give us more time to live?
  42. E

    B Black Holes: Can Anything Reduce Their Size?

    I'm pretty sure I know the responses I'm going to get, as this question is pretty silly, but I'll ask anyway. Is there anything - a process, or a material, that would reduce the size of a black hole and/or evaporate it away quickly, or is that just hocus pocus thinking?
  43. A

    A Merger of Extremal BHs Causes Naked Singularity?

    Let's say you have two extremal black holes containing the maximum amount of possible charge. Now let's say they're orbiting each other such that they will eventually merge. As the black holes merge they are producing gravitational waves. Once the merger is complete the new black hole mass will...
  44. T

    I Is the sign of the Einstein equation right in Chandrasekhar's Black holes?

    In Chandrasekhar's book, The Mathematical Theory of Black Holes. The sign of Einstein equations is minus "-" , Eq. (1-236). However, the sign of Riemann and Ricci tensor are the same as MTW's book. The sign of Einstein equations in MTW's book are "+"! Is there a error?
  45. A

    I Merging black holes: gravitational waves and information

    A popular theory is that black holes gradually leak information because of black hole evaporation due to hawking radiation. When black holes merge, a significant portion of their mass is converted into gravitational waves. If it's true that black holes leak information due to hawking radiation...
  46. DaveC426913

    B Stars massive enough to evolve into black holes

    I keep seeing this figure of 3-4 Solar masses is all that's required for a star to end its life by collapsing into a black hole. Since the sun is a pretty average star, that would suggest that most of the stars in the universe (like >50%) will end their lives as a black hole. The implication...
  47. E

    I Black holes and relation to creation

    Are there any papers or articles that reference black holes being the creations of the big bang or being considered in creating universes in alternate dimensions? Thanks for the help.
  48. P

    I Could dark matter consist of primordial black holes?

    Could dark matter consist of black holes formed shortly after the big bang? They would form the perfect development seed. If they all have Sun-like masses then they are not detectable from here (they are just 3 kilometers wide!). They have virtually no collisions with stars and could form a...
  49. I

    B The superposition of a photon

    When a photon travels through space it is spreading out like a fan while in its superposition (except that it is spreading also vertically in addition to horizontally). So, what happens if for instance the right outer edge of the photon's superposition is captured by the gravity of a black hole...
  50. Antman0115

    B Black Holes & Stars: Quantum Entanglement Possibility?

    Would it possible that black holes share a quantum entanglement with stars(such as white dwarfs), and the reason we observe the tunnel closing is the star dies out/explodes? I understand that there are different types of black holes and stars, varying in size and properties of mass/spacial...