Editing Black hole (section)

=== Event horizon ===
{{Main|Event horizon}}
{{multiple image
| align             = right
| direction         = vertical
| width             = 300
| image1            = BH-no-escape-1.svg
| caption1          = Far away from the black hole, a particle can move in any direction, as illustrated by the set of arrows. It is restricted only by the speed of light.
| image2            = BH-no-escape-2.svg
| caption2          = Closer to the black hole, spacetime starts to deform. There are more paths going towards the black hole than paths moving away.{{refn|The set of possible paths, or more accurately the future [[light cone]] containing all possible [[world line]]s (in this diagram the light cone is represented by the V-shaped region bounded by arrows representing light ray world lines), is tilted in this way in [[Eddington–Finkelstein coordinates]] (the diagram is a "cartoon" version of an Eddington–Finkelstein coordinate diagram), but in other coordinates the light cones are not tilted in this way, for example in [[Schwarzschild coordinates]] they narrow without tilting as one approaches the event horizon, and in [[Kruskal–Szekeres coordinates]] the light cones do not change shape or orientation at all.<ref>{{harvnb|Misner|Thorne|Wheeler|1973|p=848}}</ref>|group="Note"}}
| image3            = BH-no-escape-3.svg
| caption3          = Inside of the event horizon, all paths bring the particle closer to the centre of the black hole. It is no longer possible for the particle to escape.
}}

The defining feature of a black hole is the appearance of an event horizon—a boundary in [[spacetime]] through which matter and light can pass only inward towards the mass of the black hole. Nothing, not even light, can escape from inside the event horizon.<ref>{{cite book |title=The New Physics |edition=illustrated |first1=Paul |last1=Davies |publisher=Cambridge University Press |year=1992 |isbn=978-0-521-43831-5 |page=26 |url=https://books.google.com/books?id=akb2FpZSGnMC |access-date=25 September 2020 |archive-date=17 August 2021 |archive-url=https://web.archive.org/web/20210817161727/https://books.google.com/books?id=akb2FpZSGnMC |url-status=live }} [https://books.google.com/books?id=akb2FpZSGnMC&pg=PA26 Extract of page 26] {{Webarchive|url=https://web.archive.org/web/20210815222341/https://books.google.com/books?id=akb2FpZSGnMC&pg=PA26 |date=15 August 2021 }}</ref><ref>{{cite book |title=A Student's Guide to the Mathematics of Astronomy |edition=illustrated |first1=Daniel |last1=Fleisch |first2=Julia |last2=Kregenow |publisher=Cambridge University Press |year=2013 |isbn=978-1-107-03494-5 |page=168 |url=https://books.google.com/books?id=x4gaBQAAQBAJ |access-date=25 September 2020 |archive-date=17 August 2021 |archive-url=https://web.archive.org/web/20210817045139/https://books.google.com/books?id=x4gaBQAAQBAJ |url-status=live }} [https://books.google.com/books?id=x4gaBQAAQBAJ&pg=PA168 Extract of page 168] {{Webarchive|url=https://web.archive.org/web/20210817113029/https://books.google.be/books?id=x4gaBQAAQBAJ&pg=PA168 |date=17 August 2021 }}</ref> The event horizon is referred to as such because if an event occurs within the boundary, information from that event cannot reach an outside observer, making it impossible to determine whether such an event occurred.<ref>{{harvnb|Wheeler|2007|p=179}}</ref>

As predicted by general relativity, the presence of a mass deforms spacetime in such a way that the paths taken by particles bend towards the mass.<ref>{{harvnb|Carroll|2004|loc=Ch. 5.4 and 7.3}}</ref> At the event horizon of a black hole, this deformation becomes so strong that there are no paths that<!--to avoid false positives in search for 'that led/lead' typo--> lead away from the black hole.<ref>{{cite web |title=Singularities and Black Holes > Lightcones and Causal Structure |url=https://plato.stanford.edu/entries/spacetime-singularities/lightcone.html |website=plato.stanford.edu |publisher=[[Stanford Encyclopedia of Philosophy]] |access-date=11 March 2018 |archive-date=17 May 2019 |archive-url=https://web.archive.org/web/20190517122738/https://plato.stanford.edu/entries/spacetime-singularities/lightcone.html |url-status=live }}</ref>

In a thought experiment, a distant observer can imagine clocks near a black hole which would appear to tick more slowly than those farther away from the black hole.<ref>{{harvnb|Carroll|2004|p=217}}</ref> This effect, known as [[gravitational time dilation]], would also cause an object falling into a black hole to appear to slow as it approaches the event horizon, taking an infinite amount of time to reach it.<ref>{{harvnb|Carroll|2004|p=218}}</ref> All processes on this object would appear to slow down, from the viewpoint of a fixed outside observer, and any light emitted by the object to appear redder and dimmer, an effect known as [[gravitational redshift]].<ref>{{cite web |url=http://nrumiano.free.fr/Estars/int_bh.html |title=Inside a black hole |website=Knowing the universe and its secrets |access-date=26 March 2009 |archive-url=https://web.archive.org/web/20090423053437/http://nrumiano.free.fr/Estars/int_bh.html |archive-date=23 April 2009 }}</ref> Eventually, the falling object fades away until it can no longer be seen. Typically this process happens very rapidly with an object disappearing from view within less than a second.<ref>{{cite web |title=What happens to you if you fall into a black hole |url=http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/fall_in.html |website=math.ucr.edu |publisher=[[John Baez]] |access-date=11 March 2018 |archive-date=13 February 2019 |archive-url=https://web.archive.org/web/20190213124648/http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/fall_in.html |url-status=live }}</ref>

On the other hand, imaginary, indestructible observers falling into a black hole would not notice any of these effects as they cross the event horizon. Their own clocks appear to them to tick normally, they cross the event horizon after a finite time without noting any singular behaviour.
In [[general relativity]], it is impossible to determine the location of the event horizon from local observations, due to Einstein's [[equivalence principle]].<ref>{{harvnb|Carroll|2004|p=222}}</ref><ref>{{cite news |title=Watch: Three Ways an Astronaut Could Fall Into a Black Hole |url=https://news.nationalgeographic.com/news/2014/01/140130-black-holes-stephen-hawking-work-space-astronomy/ |access-date=13 March 2018 |date=1 February 2014 |archive-date=15 April 2019 |archive-url=https://web.archive.org/web/20190415101947/https://news.nationalgeographic.com/news/2014/01/140130-black-holes-stephen-hawking-work-space-astronomy/ }}</ref>

The [[topology]] of the event horizon of a black hole at equilibrium is always spherical.{{refn|This is true only for four-dimensional spacetimes. In higher dimensions more complicated horizon topologies like a [[Higher-dimensional Einstein gravity#Black hole uniqueness|black ring]] are possible.<ref>{{cite journal |first1=R. |last1=Emparan |first2=H. S. |last2=Reall |title=Black Holes in Higher Dimensions |journal=Living Reviews in Relativity |volume=11 |issue=6 |page=6 |date=2008 |arxiv=0801.3471 |bibcode=2008LRR....11....6E |doi=10.12942/lrr-2008-6 |doi-access=free |pmid=28163607 |pmc=5253845}}</ref><ref>{{Cite book |last1=Obers |first1=N. A. |title=Physics of Black Holes |editor1-last=Papantonopoulos |editor1-first=Eleftherios |volume=769 |pages=211–258 |date=2009 |doi=10.1007/978-3-540-88460-6 |arxiv=0802.0519 |series=Lecture Notes in Physics |location=Berlin |publisher=Springer |isbn=978-3-540-88459-0 |bibcode=2009LNP...769.....P |url=https://cds.cern.ch/record/1164174/files/978-3-540-88460-6_BookTOC.pdf |access-date=27 July 2018 |archive-date=26 July 2018 |archive-url=https://web.archive.org/web/20180726103141/https://cds.cern.ch/record/1164174/files/978-3-540-88460-6_BookTOC.pdf |url-status=live }}</ref>|group="Note"}}<ref>{{harvnb|Hawking|Ellis|1973|loc=Ch. 9.3}}</ref> For non-rotating (static) black holes the geometry of the event horizon is precisely spherical, while for rotating black holes the event horizon is oblate.<ref name= "Smarr1973">{{cite journal|last1= Smarr|first1= L.|title= Surface Geometry of Charged Rotating Black Holes |journal= Physical Review D|volume= 7|issue= 2|year= 1973|pages= 289–295|doi= 10.1103/PhysRevD.7.289|bibcode= 1973PhRvD...7..289S}}</ref><ref name= "Wiltshire2009">{{cite book|author= Visser, M.|editor1= Wiltshire, D.L.|editor2= Visser, M.|editor3= Scott, S.M.|title= The Kerr Spacetime: Rotating Black Holes in General Relativity|url= https://books.google.com/books?id=wymJBq_80Q0C|chapter= The Kerr spacetime: A brief introduction|date= 22 January 2009|publisher= Cambridge University Press|isbn= 978-0-521-88512-6|arxiv= 0706.0622|access-date= 12 January 2020|archive-date= 20 May 2020|archive-url= https://web.archive.org/web/20200520134643/https://books.google.com/books?id=wymJBq_80Q0C|url-status= live}}</ref><ref name= "Delgado2018">{{cite journal|last1= Delgado|first1= J.F. M.|last2= Herdeiro|first2= C.A. R.|last3= Radu|first3= E.|title= Horizon geometry for Kerr black holes with synchronized hair|journal= Physical Review D|volume= 97|issue= 12|page= 124012|year= 2018|doi= 10.1103/PhysRevD.97.124012|bibcode= 2018PhRvD..97l4012D|hdl= 10773/24121|arxiv= 1804.04910|s2cid= 55732213}}</ref>