Editing Gravitational singularity (section)

{{short description|Condition in which spacetime itself breaks down}}

[[File:Black_hole_lensing_web.gif|thumb|upright=1.2|Animated simulation of [[Gravitational lens|gravitational lensing]] caused by a [[Schwarzschild metric|Schwarzschild black hole]] passing in a line-of-sight planar to a background galaxy. Around and at the time of exact alignment ([[syzygy (astronomy)|syzygy]]) extreme lensing of the light is observed.]]

{{General relativity sidebar |phenomena}}

A '''gravitational singularity''', '''spacetime singularity''', or simply '''singularity''', is a theoretical condition in which [[gravitational field|gravity]] is predicted to be so intense that [[spacetime]] itself would break down catastrophically. As such, a singularity is by definition no longer part of the regular spacetime and cannot be determined by "where" or "when”. Gravitational singularities exist at a junction between [[general relativity]] and [[quantum mechanics]]; therefore, the properties of the singularity cannot be described without an established theory of [[quantum gravity]]. Trying to find a complete and precise definition of singularities in the theory of general relativity, the current best theory of gravity, remains a difficult problem.<ref>{{harvnb|Earman|1995|loc=Section 2.2 ''What is a singularity?''|pp=28–31}}</ref><ref name="curiel" /> A singularity in general relativity can be defined by the [[Curvature invariant (general relativity)|scalar invariant]] [[Curvature of Riemannian manifolds|curvature]] becoming [[Infinity|infinite]]<ref>{{cite web|url=http://www.physicsoftheuniverse.com/topics_blackholes_singularities.html|title=Singularities |website=Physics of the Universe }}</ref> or, better, by a [[Geodesics in general relativity|geodesic]] being [[Geodesic manifold#Non-examples|incomplete]].<ref>{{Cite journal |last=Uggla |first=Claes |year=2006 |title=Spacetime Singularities |url=http://www.einstein-online.info/spotlights/singularities |url-status=dead |journal=[[Einstein Online]] |publisher=[[Max Planck Institute for Gravitational Physics]] |volume=2 |archive-url=https://web.archive.org/web/20170124030605/http://www.einstein-online.info/spotlights/singularities |archive-date=2017-01-24 |access-date=2015-10-20 |number=1002}}</ref>

General relativity predicts that any object collapsing beyond its [[Schwarzschild radius]] would form a black hole, inside which a singularity will form.<ref name=curiel>{{cite encyclopedia|url=http://plato.stanford.edu/entries/spacetime-singularities/|title=Singularities and Black Holes|last=Curiel|first=Erik|publisher=Metaphysics Research Lab, Stanford University |name-list-style=amp|encyclopedia=Stanford Encyclopedia of Philosophy|date=2021 |access-date=1 October 2021}}</ref> A black hole singularity is, however, covered by an [[event horizon]], so it is never in the [[causal past]] of any outside observer, and at no time can it be objectively said to have formed.<ref>{{cite journal
 | last1 = Narlikar
 | first1 = J. V.
 | author-link1 = Jayant Narlikar
 | last2 = Padmanabhan
 | first2 = Th.
 | author-link2 = Thanu Padmanabhan
 | date = June 1988
 | title = The Schwarzschild solution: Some conceptual difficulties
 | journal = [[Foundations of Physics|Found Phys]]
 | volume = 18
 | publisher = [[Springer Nature]]
 | pages = 659–668
 | doi = 10.1007/BF00734568
}}</ref> General relativity also predicts that the initial state of the [[universe]], at the beginning of the [[Big Bang]], was a singularity of infinite density and temperature.<ref>{{harvnb|Wald|1984|p=99}}.</ref>{{Obsolete source|reason=this source is forty years old - a lot has changed since then.|date=October 2024}} However, [[classical field theory|classical]] gravitational theories are not expected to be accurate under these conditions, and a quantum description is likely needed.<ref>{{cite web |last=Hawking |first=Stephen |title=The Beginning of Time |url=http://www.hawking.org.uk/the-beginning-of-time.html |work=Stephen Hawking: The Official Website |publisher=[[Cambridge University]] |access-date=26 December 2012 |archive-date=6 October 2014 |archive-url=https://web.archive.org/web/20141006200729/http://www.hawking.org.uk/the-beginning-of-time.html |url-status=dead }}</ref> For example, quantum mechanics does not permit particles to inhabit a space smaller than their [[Compton wavelength]]s.<ref>{{cite book |last=Zebrowski |first=Ernest |url=https://books.google.com/books?id=2twRfiUwkxYC |title=A History of the Circle: Mathematical Reasoning and the Physical Universe |date=2000 |publisher=[[Rutgers University Press]] |isbn=978-0813528984 |location=Piscataway New Jersey |page=180}}</ref>