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Comment by ridgeguy | Hacker News Reader

The referenced article suggested there's an upper limit to the spin rate of a black hole. From the penultimate paragraph: "The larger black hole in this binary, which was about 40 times more massive than the Sun, was spinning almost as fast as physically possible."

I don't get what limits black hole spin rate. As I understand it, The hole isn't a material surface, nor is there mass within, except for what's caused the singularity. Which might or might not be larger than a point.

Can anybody clue me in as to whether there's an upper spin rate limit, and if so, why? Thanks.

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This PBS Space Time video does a good job of explaining it https://youtu.be/1Z5fnwUmTSY. tl;dr the inner and outer event horizons of spinning black hole cancel out leaving a naked (ring) singularity which is believed to be prevented by the “Cosmic Censorship Hypothesis”. In the math it causes the event horizon radius to become complex which is believed to be “unphysical”. However maybe not, many “unphysical” mathematical objects have later turned out to be real such as black holes themselves (Einstein thought they were just a mathematical artefact) and anti matter (a result of negative roots of the Dirac Equation). Complex numbers are also an intrinsic part of the way Quantum Mechanics works, so perhaps there are naked singularities.

Correct me if I’m wrong, but the Schrödinger equation isn’t the only mathematical treatment of the stochastic nature of quantum phenomena.

I thought matrix mechanics was with real numbers and was mathematically equivalent. I also thought there were different statistical systems that included “negative” probabilities that can be used to describe wave packets without needing complex numbers.

I do agree that sometimes the maths leads you to a truth in reality. Though sometimes it doesn’t.

I’m not too familiar with QFT, does that require complex numbers?

What do you mean by “require”? Complex numbers can be represented as a 2x2 matrix of real numbers (a scaled rotation around the origin), so transitively, that’s true for any equation involving complex numbers.

I don’t remember exactly since it’s been a while since I read it, but in Scott Aaronson’s Book “Quantum Computing Since Democritus” which is about how physics informs what’s computable. He talks about how the behaviour of quantum mechanics follows fairly straightforwardly from considering how probability works with complex numbers / bloch spheres. You can’t properly reproduce Bell’s Inequality without them I think?

Thanks for this. The subject is way out of my wheelhouse, but I enjoy clinging to the coattails of explorers of worlds I don't understand.

> The hole isn't a material surface, nor is there mass within

Are you saying a black hole is considered to have no mass at all? That it is literally considered a hole?

Where would matter "go" when it falls in? Would it be emitted in its entirety (and no loss) as Hawking radiation?

I thought the enormous gravity defines it to be a black hole where not even light can escape, but there is still "matter".

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