2
$\begingroup$

All stars rotate. And the more they contract the faster the rotation, so is there such a thing as a non-rotating black hole? And as gravity is less at the equator of a rotating star, assuming that they are still stars after collapsing to become black holes, at a very high rate of rotation could the escape velocity at the equator be less than $c$ while away from the equator it exceeds $c$? The fact that they still rotate and have a gravitational field after becoming black holes suggests that they are still stars even though we can’t see them, and gravity should still be slightly less at the equator of the spherical event horizon.

$\endgroup$
  • 1
    $\begingroup$ One question per question, please! A rotating star cannot have an escape velocity of c. $\endgroup$ – PM 2Ring May 27 at 10:40
  • $\begingroup$ In some basic sense, you are right. The horizon is farther away from the center of a Kerr black hole on the equatorial plane that is at the poles. See: en.wikipedia.org/wiki/Kerr_metric#/media/File:Kerr-surfaces.png Also, all astrophysical black holes are presumably Kerr black holes, yes. $\endgroup$ – Dvij Mankad May 27 at 10:49
  • $\begingroup$ What if it collapses to become a black hole? We don't know what goes on inside the event horizon,though some people pretend that they do. Its worth remembering a quote from Bertrand Russell,who knew a thing or too about mathematics himself: "Mathematics is the subject in which we never know what we are talking about,nor whether what we are saying is true." Obviously he was speaking tongue in cheek,but he was only half joking. I think he had quantum mechanics in mind when he said it. $\endgroup$ – Michael Walsby May 27 at 10:55
  • 3
    $\begingroup$ @Michael Walsby "don't know what goes on inside the event horizon" we don't, but we have no reason to believe our theory fails right after the event horison. The equivalence principle of GR tells you, that unless the tidal forces are really huge, there is no reason to believe the theory breaks. And for supermassive black holes the tidal forces right after the event horizon can be weaker than on Earth. $\endgroup$ – Umaxo May 27 at 16:15
  • 1
    $\begingroup$ @MichaelWalsby You shouldn't treat black holes as stars because they are not. For starters, stars are supposed to be made of matter. A black hole is just a region of spacetime and has no matter whatsoever. $\endgroup$ – Dvij Mankad Jun 16 at 14:59
1
$\begingroup$

Yes, the expectation is that all (astrophysical) black holes have at least some rotation. Strictly speaking the chance of one having exactly zero rotation is zero. However, the rotation can be so slow that the non-rotating solution is a good approximation.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.