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Every star or other massive body in the universe rotates, if only a little. If such a body collapses, its spin, any spin at all, and thus, angular momentum approach infinity as r approaches 0. Angular momentum must be conserved. In order to produce a true singularity or a non-rotating black hole, there must be some process by which ALL angular momentum (energy) is dissipated or converted to something else.

Is there such a process? Otherwise all black holes must rotate, perhaps too fast or too slowly for us to detect, but they must rotate. Am I missing something? I've never heard this discussed before.

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  • $\begingroup$ Related: physics.stackexchange.com/questions/91186/… $\endgroup$
    – PM 2Ring
    Commented Jun 18, 2019 at 5:19
  • $\begingroup$ Why do you think black holes can’t or don’t rotate? The holes observed by LIGO do. $\endgroup$
    – G. Smith
    Commented Jun 18, 2019 at 5:32
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    $\begingroup$ You're not missing anything. You correctly applied the simple idea of angular momentum conservation to deduce that astrophysical black holes must be spinning. Congratulations. $\endgroup$
    – Avantgarde
    Commented Jun 18, 2019 at 7:44
  • $\begingroup$ This addresses the topic of rotation but the other part of my question addressed singularities. With rotation and a finite (and likely measurable) r>0, then true singularities cannot exist either. Yet they're spoken of so matter-of-factly. Even the Kerr ring singularity seems to me like an attempt to cling to this unnecessary construct. Math alone cannot describe everything in the universe, at least not without an equation taking into account ALL variables, some of which we may not yet understand. $\endgroup$
    – Scott Ewart
    Commented Jun 21, 2020 at 17:22

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There are, to our knowledge, no non-rotating black holes (or other massive bodies).

Perhaps you have heard something like the Schwarzschild metric being discussed. The Schwarzschild metric was discovered in 1915 and is a solution to non-rotating bodies. It is a useful approximation for describing slowly rotating astronomical objects, including Earth and the Sun. It is also much simpler to solve for non-rotating bodies than for rotating bodies.

However, the Kerr metric is a generalization of the Schwarzschild metric and describes a solution for a rotating black-hole. This solution was not discovered until 1963. An extension to this solution, the Kerr–Newman metric, was discovered shortly thereafter in 1965.

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