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Comment by mabbo | Hacker News Reader

Fun question: can spinning this fast allow for larger neutron stars?

The neutron star keeps growing, and eventually may have too much mass, causing it to collapse into a black hole. The forces pulling inward are just too much.

But if it's spinning fast enough, then centripetal forces would be pulling outward as well. This one is completing a full rotation in 1.41 ms- that's a lot of rotational speed.

Or is there some intrinsic property of black hole formation that does not care about this?

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Rotation does indeed allow for heavier neutron stars, the outward force helps counteracting the inward pull of gravity that would otherwise overcome the forces that keep neutrons not turning into black holes.

edit:

There is a relativistic limit, as faster rotation eventually is not possible without adding ludicrous amounts of energy that would make the neutron star too heavy.

The non-rotating limit is about 2.1 solar masses (2.1 to 2.9), the rotating limit is about 20% higher (2.5 - 3.6 solar masses)

> There is a relativistic limit, as faster rotation eventually is not possible without adding ludicrous amounts of energy that would make the neutron star too heavy.

That is exactly the kind of thing I'm here for. Spinning so fast that the energy adds more mass than the spinning can counteract. Delightful.

There is an upper limit to how fast a black hole can spin. I'm not sure if that's the same limit you'd get if you tried spinning a material object as fast as possible while increasing its mass until it became a black hole.

Black holes don't have a limit on their rotation rate as far as current physics go, since black holes do not have a traditional surface. If the spin exceeds the speed of light, that simply makes the event horizon go away and leaves you with a naked singularity.

Can one theoretically increase the spin of a black hole by adding mass at the right angle? IE: give it more and more angular momentum until you achieve the naked singularity?

Perhaps that is the final experiment of Kardashev level whatever civilization.

You absolutely could keep adding momenting in just the right way. Though I will add, as spin goes up, so does frame dragging, which makes it actually difficult to hit a black hole as space begins spinning with it.

It makes for a very nice sci-fi story setting though. A very far flung future in which some very post-humans are attempting to break the universe in this way.

From what I've read, making the event horizon go away is a limit of current physics, in part because current physics can't cope with singularities.

The event horizon going away is entirely valid, the naked singularity is not. So the limit is rather in that we can't explain what would come out of it, rather that the event horizon can't go away.

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