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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 Jun 18 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 Jun 18 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 Jun 18 at 7:44
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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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