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.

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  1. T

    I Evaporating black hole: First principles consideration

    Here is first principles consideration: Since it is a black hole there is an event horizon where timelike worldlines can enter but not exit, this is what defines a black hole instead of a white hole. When evaporation is finished there is no more event horizon, this is what defines the...
  2. M

    B Falling into a black hole

    If, from an outsider's perspective it takes an infinite amount of time to cross the event horizon, but a finite amount of time for the black hole to evaporate, how does that look like from the perspective of the falling observer. For that matter, how does it look like from the perspective of the...
  3. sbrothy

    I Unruh Radiation and Black Holes

    The level may be higher that Intermediate but I use that in the hope that I'll be able to understand the answer. I moved it here as it didn't really belong in the other thread. Feel free to move it further. I admit I haven't yet read the paper in it's entirety but I hope you'll bear with me. I...
  4. A

    I Light curving around vortex simulating a black hole

    So I just had a question. In a Netflix documentary, they showed these people trying to simulate the conditions of a black hole by having a vortex in a pool of water. they said that light cannot pass through the hole and the physics around is essentially adjacent to the event horizon and what...
  5. Grinkle

    B Detection of red-shifted photons near an event horizon

    I realize that something I thought I understood about a beacon approaching a black hole I am unsure of. My hypothetical - I am at a safe distance from as simple a black hole as it makes sense to discuss. I launch a beacon at it, and I calculate that in one hour of my own proper time the...
  6. ergospherical

    NASA Black Hole Visualisation

    https://svs.gsfc.nasa.gov/14576 (NASA's Goddard Space Flight Center/J. Schnittman and B. Powell) Cool!
  7. S

    I Questions about Hawking radiation and extremal black holes...?

    I'm studying if there is some way to avoid black hole evaporation, even if it requires a very special set up of conditions... Theoretically, extremal black holes (both for rotating Kerr and Reissner-Nordström ones) would avoid evaporation as they would not emit Hawking radiation. Since...
  8. E

    4 foot diameter black hole comes to rest in Kuiper belt - then what?

    I am a writer completing a science fiction novel involving a four foot diameter black hole (with approx 1.5 times the mass of Saturn) At one point in the story, this small black hole has sling shot past the sun, is headed outward toward the Kuiper belt & Oort Cloud. As it moves, the sun's...
  9. S

    B What is the actual shape of black holes?

    When looking at depictions of black holes, we often get illustrations of a two-dimensional space-time where everything spirals into a "hole". The hole then leads to a singularity usually depicted as a funnel shape. Then there are images of black holes shown as spherical with glowing rings...
  10. S

    I Can there be slowly-falling accretion disks in black holes?

    Black holes accrete mass around them and it falls gradually up to the even horizon where mass is trapped by the black hole forever. However, the rate of mass falling from the accretion disk to the black hole ranges from being very fast to very long-lived, depending on various conditions...
  11. E

    A four foot diameter black hole destroys our Sun

    Homework Statement: I am a writer completing a science fiction novel involving a four foot diameter black hole (with approx 1.5 times the mass of Saturn) In my novel, the black hole is being drawn toward our much more massive sun. I assume the black hole would begin to consume plasma/energy...
  12. S

    I Does Kerr's argument against singularities apply to all black holes?

    Roy Kerr has recently written a preprint (https://arxiv.org/abs/2312.00841) in which he strongly argues against the possible existence of singularities inside Black Holes. I've read that his arguments are really powerful and that he is most likely right. But, does it mean that Kerr has...
  13. K

    I Is the Black Hole Information Paradox Truly Resolved?

    from what I understand it is believed that information is preserved but we are still working out how exactly, is this the case?
  14. Omega0

    B Why Does Black Hole Entropy and Information Loss Matter?

    We know that there is no law of conservation for the entropy. It is quite the contrary: If we have a closed system without exchange of heat the entropy cannot get less. It will reach the max. If we have not a closed system but a stream of entropy only into a system, the entropy will increase...
  15. K

    B Question about Hawking Radiation and Iron Stars

    So, I was thinking, the most massive black holes are expected to evaporates in roughly a googol year. Fine. But I was reading that if protons are stable, every planet and non black hole star remains are basically expected to turn into iron star from quantum tunneling after mind numbing time on...
  16. P

    B Is crossing a black hole's event horizon possible?

    As I understand things if you're hanging out in your space suit some distance away from a black holes event horizon and your buddy decides to dive on in you will never see him cross the event horizon. You'll see him approach the event horizon but never cross it. It would seem the time needed to...
  17. S

    B Black hole electromagnetic spectrum

    Of course neither a single Schwarzschild nor a single Kerr black hole, nor a pair of these has an emission spectrum, other than the Hawking one. (Nordström and Newman holes must have it while in binaries, but they are not common). But not having emission spectrum does not rule out having...
  18. M

    B How would physics change if Planet 9 were a primordial black hole?

    I watch John Michael Godier on YouTube. He is a futurist and makes fantastic videos where he speculates on many things. Todays video was about primordial black holes and he speculated about Planet 9 not being visible because it may actually be a black hole born in the dawn of the universe. He...
  19. G

    Black hole evaporation mechanism

    [post moved to relativity forum: https://www.physicsforums.com/threads/black-hole-evaporation-mechanism.1059236/]
  20. W

    I If nothing can escape a black hole, it follows that Hawking radiation can't exist?

    Or is Hawking radiation something? Can't be both, however if you choose one theory over another, why do you do so. Those of you who are younger will not remember a World without the information paradox, but when I was younger it did not exist in any way because nothing escaped the event...
  21. Kekkuli

    I Falling into a massive black hole is not necessarily noticeable

    I find it interesting that the more massive the black hole, the weaker the fall acceleration at the distance of the Schwarzschild radius - that's why you wouldn't necessarily notice anything special in the event horizon.
  22. Hill

    I Interpretation of (X,T) coordinates in Kruskal diagram

    These are the points in the book: What is "naturally used"? Does it hold only as the observer crosses the event horizon? How can they "use" them?
  23. Bosko

    I A mirror behind the event horizon

    Thought experiment: What will happen if you place a mirror behind the event horizon and turn on the light in its direction?
  24. I

    I Time Dilation Near a Black Hole: Accelerating Objects Approaching the Event Horizon

    First of all, I wish everyone a Happy New Year. I am interested in your expertise on a special constellation, which I will first briefly describe. If you observe an object that is approaching the event horizon of a black hole, it is said that at some point the distant observer will have the...
  25. Hill

    I Black hole singularity, a time-like surface?

    Below is the description from the book. I thought that hyperbolas in the right quadrant are time-like and hyperbolas in the upper quadrant are space-like. If it were so, the surface ##r=0## would be space-like, but the book says otherwise. -- ?
  26. Demystifier

    A Negative energy in Hawking radiation

    At a descriptive level, negative energy quanta enter a black hole during Hawking radiation. But when one tries to understand it mathematically, it seems that negative "energies" appear in two very different senses, which seem to be totally unrelated to each other. At one level one has Bogoliubov...
  27. M

    B "If a black hole has more mass then the event horizon shrinks" ???

    The title is a direct quote of this video by Dr. Becky Smethurst, an astrophysicist specializing in black hole research. This is a mistake, right? Supermassive black holes, for example, don't have tiny radii, compared to stellar mass BHs. Then there's the equation she presents seconds later...
  28. M

    I From our reference frame, how would a black hole ever form?

    Looking at Kruskal diagrams, it seems to me we should not be able to see evidence of black holes. Assuming our frame is a hyperbola of roughly constant ##r## in such a diagram, as the black hole's constituent mass comes together time slows (from our POV) to the extent that it never crosses the...
  29. haushofer

    A Where exactly is the wormhole in the Kruskal-Szekeres diagram?

    Dear all, recently I was brushing up my knowledge of black holes with (among others) Zee's "Einstein gravity in a Nutshell" and encountered the analytical continuation of the Schwarzschild black hole in the famous Kruskal-Szekeres coordinates (Zee: chapter VII.2). The corresponding diagram can...
  30. T

    I Black hole merger question (effect of mass radiated away via gravitational waves)

    Hi I have two black holes 1 Sun mass each, when they merge a significant percentage of the mass is lost and is radiated away, so the combined black hole remnant is only, say, 1.8M. What is a behavior of the test body on an orbit far away from that binary? I assume that after some time...
  31. Wo Wala Moiz

    I Do any real macroscopic black holes have a singularity?

    Here's my reasoning. The event horizon is the point where the escape velocity becomes greater than the speed of light. This results in the event horizon spacetime boundary having infinite time dilation. So, that must mean that inside the boundary of the event horizon, time dilation must...
  32. haushofer

    I Supermassive black hole, surface gravity and tidal forces

    As the summary says: a light black hole has stronger surface gravity and tidal forces just outside the horizon than a supermassive black hole. So if you want to hoover just outside the horizon of a black hole and care about your well-being it better be supermassive. I understand this perfectly...
  33. D

    I What is the Limiting Mass at the Final Stages of Black Hole Evaporation?

    As Hawking radiation does away with black holes in the eons of time it takes to evaporate, what is the limiting mass at the final stages of the black hole evaporation? Is it Planck mass? or some fractional mass of the initial BH?
  34. J

    I What is the Connection Between Black Holes and the Big Bang?

    *********Disclaimer******** Hi, I want to ask this community about some black hole shinanigans. I've spent some time searching for this topic here because I don't want to be the guy who spams a forum with a question already answered a hundred times over. Since none of the threads I found...
  35. A

    B Is there a lining to our Universe?

    I was wondering if someone could answer my question. I know I may be dumb but I don't really understand why our universe has a limited capacity for mass. If I remember correctly black holes occur when concentration of mass causes a hole in spacetime. I don't really understand I guess how space...
  36. D

    I Inverse Square Law for Black Holes

    When you are calculating the gravitational force between two masses and one of them is a black hole, do you still use the distance to the center of mass as you would in Newtonian gravity to find the force? Or is the distance measured only to the event horizon? Is the inverse square law modified...
  37. Entropix

    I Velocity of a given planet relative to the Galactic Center

    Long time had this question about how fast a planet, dwarf or not or even a rogue asteroid can move relative to the core of the Galaxy, where, if my understanding is correct it has to be a supermassive black hole. Knowing satellites revolves around a planet and this system circles around a star...
  38. E

    Why is it thought that black holes could make us travel to the past?

    I very often see in movies and works of fiction that scientists explain that with a black hole you can travel in time and how the characters use black holes to travel in time, more precisely to the past Do scientists really believe that time travel through black holes is likely? If so, why?
  39. jaketodd

    B Photons on a perpendicular bisector path from a black hole's core

    Not directly from the core, but a trajectory that goes to the event horizon, and gets corrected to a perfect, perpendicular bisector path by the gravity of the core, when it reaches the event horizon. Would they escape the event horizon, since they have to always move at c? On this trajectory...
  40. Antarres

    A Continuation of Coordinates Across Black Hole Horizons

    Studying and tinkering with some solutions, I've come to some realizations and questions regarding the regularization of coordinate singularities, so I'd like to see if my conclusions are good, and I guess I have some questions as well. There are two questions/conclusions, but since they...
  41. .Scott

    B Crossing the Bekenstein Bound at Black Hole Event Horizon

    The Bekenstein Bound places a upper limit on the amount of entropy that a given volume of space may contain. This limit was described by Jacob Bekenstein who tied it quite closely to the Black Hole Event Horizon. Put simply, black holes hold the maximum entropy allowed for their volume. If you...
  42. BadgerBadger92

    Preorder the Hyperspace Watch "Black Hole" Edition

    Just preordered a “Black Hole” version of the watch. It tries to mimic what it would look like to travel through a wormhole, in celebration of Einstein’s theory of relativity.
  43. 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...
  44. T

    I Runaway supermassive black hole?

    There seems evidence for a runaway supermassive black hole. https://phys.org/news/2023-04-strange-streak-young-stars-evidence.html
  45. Kostik

    B Entanglement inside a black hole

    Suppose two electrons are entangled with opposite spins. Electron #1 passes through the event horizon of a black hole, together with Laboratory Assistant #1. Suppose the assistant measures electron #1's spin after they pass through the event horizon (according to #1's [proper] time) and measures...
  46. pinball1970

    B Circumgalactic Black hole imaged by the Hubble Space Telescope

    From NASA: https://www.nasa.gov/feature/goddard/2023/hubble-sees-possible-runaway-black-hole-creating-a-trail-of-stars "We think we're seeing a wake behind the black hole where the gas cools and is able to form stars. So, we're looking at star formation trailing the black hole," said Pieter...
  47. pinball1970

    B AI Enhanced image of M87 black hole

    https://iopscience.iop.org/article/10.3847/2041-8213/acc32d From the abstract, "We present a new reconstruction of the Event Horizon Telescope (EHT) image of the M87 black hole from the 2017 data set. We use PRIMO, a novel dictionary-learning-based algorithm that uses high-fidelity simulations...
  48. .Scott

    I Black Hole Information path described in new article

    I'm posting this in "Quantum Physics" although it is just as much "General Relativity". A paper entitled "Quantum gravitational corrections to particle creation by black holes" was published in Science Direct earlier this month. It appears to provide an event horizon mechanism for holding...
  49. Astronuc

    B Ultramassive Black Hole (UMB), 30 billion solar masses?

    https://www.france24.com/en/live-news/20230329-astronomers-discover-one-of-biggest-black-holes-ever-recorded https://universe.nasa.gov/black-holes/types/ - didn't find this very helpful ESA Euclid mission overview - https://www.esa.int/Science_Exploration/Space_Science/Euclid_overview (to be...
  50. Narasoma

    B Why Don't We Live Inside a Black Hole?

    We know the Big Bang Theory states that our universe was started from a hot-dense point. But should't it became black hole and every matter and radiation pulled to singularity? We would not be her if that is the case.