Editing Black hole (section)

=== Singularity ===
{{Main|Gravitational singularity}}

At the centre of the unrealistically simple Schwarzschild model of a black hole is a [[gravitational singularity]]  a region where the spacetime curvature becomes infinite.<ref>{{harvnb|Carroll|2004|p=205}}</ref> For a non-rotating black hole, this region takes the shape of a single point; for a rotating black hole it is smeared out to form a [[ring singularity]] that lies in the plane of rotation.<ref>{{harvnb|Carroll|2004|pp=264–265}}</ref> In both cases, the singular region has zero volume. It can also be shown that the singular region contains all the mass of the black hole solution.<ref>{{harvnb|Carroll|2004|p=252}}</ref> The singular region can thus be thought of as having infinite [[mass density|density]].<ref>{{cite news |title=Sizes of Black Holes? How Big is a Black Hole? |url=https://www.skyandtelescope.com/astronomy-resources/how-big-is-a-black-hole/ |access-date=9 October 2018 |work=[[Sky & Telescope]] |date=22 July 2014 |archive-date=3 April 2019 |archive-url=https://web.archive.org/web/20190403035741/https://www.skyandtelescope.com/astronomy-resources/how-big-is-a-black-hole/ |url-status=live }}</ref>


Even though the Schwarzschild model is not valid at the singularity,<ref>{{harvnb|Carroll|2004|p=205}}</ref> observers falling into a Schwarzschild black hole (i.e., non-rotating and not charged) cannot avoid being carried into the singularity once they cross the event horizon. They can prolong the experience by accelerating away to slow their descent, but only up to a limit.<ref>{{Cite journal |last1=Lewis |first1=G. F. |last2=Kwan |first2=J. |title=No Way Back: Maximizing Survival Time Below the Schwarzschild Event Horizon |journal=Publications of the Astronomical Society of Australia |volume=24 |issue=2 |pages=46–52 |date=2007 |doi=10.1071/AS07012 |arxiv=0705.1029 |bibcode=2007PASA...24...46L |s2cid=17261076}}</ref> When they reach the singularity, they are crushed to infinite density and their mass is added to the total of the black hole. Before that happens, they will have been torn apart by the growing [[tidal force]]s in a process sometimes referred to as [[spaghettification]] or the "noodle effect".<ref>{{harvnb|Wheeler|2007|p=182}}</ref>

In the case of a charged (Reissner–Nordström) or rotating (Kerr) black hole, it is possible to avoid the singularity. Extending these solutions as far as possible reveals the hypothetical possibility of exiting the black hole into a different spacetime with the black hole acting as a [[wormhole]].<ref>{{harvnb|Carroll|2004|pp=257–259 and 265–266}}</ref> The possibility of travelling to another universe is, however, only theoretical since any perturbation would destroy this possibility.<ref>{{Cite journal |title=Black holes: the inside story |first1=S. |last1=Droz |first2=W. |last2=Israel |first3=S. M. |last3=Morsink |journal=Physics World |volume=9 |issue=1 |pages=34–37 |date=1996|bibcode=1996PhyW....9...34D |doi=10.1088/2058-7058/9/1/26}}</ref> It also appears to be possible to follow [[closed timelike curve]]s (returning to one's own past) around the Kerr singularity, which leads to problems with [[causality (physics)|causality]] like the [[grandfather paradox]].<ref>{{harvnb|Carroll|2004|p=266}}</ref> It is expected that none of these peculiar effects would survive in a proper quantum treatment of rotating and charged black holes.<ref>{{cite journal |last1=Poisson |first1=E. |last2=Israel |first2=W. |title=Internal structure of black holes |journal=Physical Review D |volume=41 |issue=6 |pages=1796–1809 |date=1990 |doi=10.1103/PhysRevD.41.1796 |pmid=10012548 |bibcode=1990PhRvD..41.1796P}}</ref>

The appearance of singularities in general relativity signals the breakdown of the theory.<ref>{{harvnb|Wald|1984|p=212}}</ref> This breakdown occurs where [[quantum effects]] should describe these actions, due to the extremely high density and therefore particle interactions. To date, it has not been possible to combine quantum and gravitational effects into a single theory, although there exist attempts to formulate such a theory of [[quantum gravity]]. It is generally expected that such a theory will not feature singularities.<ref>{{cite web |url=http://www.damtp.cam.ac.uk/user/gr/public/bh_hawk.html |title=Black Holes and Quantum Gravity |website=Cambridge Relativity and Cosmology |last=Hamade |first=R. |date=1996 |publisher=University of Cambridge |access-date=26 March 2009 |archive-url=https://web.archive.org/web/20090407043337/http://www.damtp.cam.ac.uk/user/gr/public/bh_hawk.html |archive-date=7 April 2009 }}</ref><ref>{{cite web |url=http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980420b.html |title=Ask an Astrophysicist: Quantum Gravity and Black Holes |last=Palmer |first=D. |publisher=NASA |access-date=26 March 2009 |archive-url=https://web.archive.org/web/20090328064842/http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980420b.html |archive-date=28 March 2009 }}</ref>