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So AFAIK the objects that have been confirmed to be black holes by direct observation of the event horizon("black hole shadow"), like M87 for instance, also show observable evidence of frame dragging (the characteristic of a black hole with a Kerr metric). Now the thing about Schwarzschild black holes is that a point singularity of infinitesimal size and infinite density is a direct violating of the uncertainty principle. Now Cygnus X-1 is often listed as a Schwarzschild black hole, but is there any observational evidence that is has both an event horizon and lack of frame dragging?

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    $\begingroup$ No black holes are Schwarzschild black holes. Everything spins to some extent. $\endgroup$
    – ProfRob
    May 30 '20 at 22:31
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    $\begingroup$ See the graph at the end of this answer astronomy.stackexchange.com/a/20292/16685 which shows the spin of 19 supermassive black holes. I expect that stellar black holes are also have high spin parameters. $\endgroup$
    – PM 2Ring
    May 30 '20 at 22:53
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    $\begingroup$ Also note that, strictly speaking, a pure Schwarzschild BH is eternal, and it's in a universe that doesn't contain anything else. But the Schwarzschild solution is still a useful approximation. $\endgroup$
    – PM 2Ring
    May 30 '20 at 22:57
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    $\begingroup$ @StephenG Fair point, which is why I just said I expect them to have high spin, but I can't find much in the way of solid observational data about stellar mass BH spin. I assume they'd inherit a lot of angular momentum from their progenitor star. OTOH, supernova explosions tend to be quite asymmetrical, so I suppose that affects the remnant's angular momentum as well as its linear momentum. $\endgroup$
    – PM 2Ring
    May 31 '20 at 0:44
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    $\begingroup$ The Cyg X-1 object is an accreting black hole in a binary system. It cannot have zero spin. $\endgroup$
    – ProfRob
    May 31 '20 at 8:29
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All the available evidence suggests that the known compact objects in binary systems (high-mass and low-mass versions) have high spin parameters in general (e.g. Nielsen 2016. They have ample opportunity to spin up during their progenitor phase, through tidal locking, the supernova explosion and then by accreting material from a companion afterwards (which is how they are detected and how their spins are measured). In particular, Cygnus X-1 is thought to be a "near-extremal" (Nielsen 2016) example of a black hole with an "extreme spin" of $a>0.95$ (Gou et al. 2011).

The available evidence from merging black hole binaries is that the spins inferred from gravitational wave signatures are not extremal but low and poorly constrained, but could be consistent with zero in some cases (e.g. Tiwari et al. 2018).

Thus the idea of a non-spinning Schwarzschild black hole is an idealisation that seems unlikely to be exactly met in nature, though it might serve as an approximation for some rare, low-spinning high-mass black hole binary systems. Of course we know very little about isolated black holes, although we do know that newly born neutron stars spin rapidly...

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    $\begingroup$ @safesphere All neutron stars are born in supernovae as far as we know. $\endgroup$
    – ProfRob
    May 31 '20 at 21:18
  • $\begingroup$ A spin of 0.95 would not be “near extremal”. Typical near extremal behavior does not set in until well above 0.99 or even 0.999. $\endgroup$
    – mmeent
    May 31 '20 at 23:36
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    $\begingroup$ @safesphere The Crab pulsar. $\endgroup$
    – ProfRob
    Jun 1 '20 at 6:37
  • $\begingroup$ @RobJeffries Thanks for an actual answer and not just a comment. I did wonder about this since a point singularity would pose a huge problem for physics as we understand it. $\endgroup$
    – Mr X
    Jun 3 '20 at 3:16

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