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Comment by ffhhj | Hacker News Reader

In the comparison image M87's blackhole has an almost perfectly circular black spot, while in the simulated image the center is half a circle due to the disk. Is this caused by some missing effect in the calculations, the angle of the disk, or something else?

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We're looking down on M87* from one of its poles. The image from the paper is viewed close to the equator.

I never understood this, do black holes have poles from any distance?

I know if you're too close, everything gets distorted, but I would have thought they look the same from any direction or angle, never managed to grasp the concept.

Maybe there is some 3d sim to explain this?

The stuff falling onto the black hole has an orientation -- it forms an accretion disk at the "equator".

If the black hole is spinning, it has poles. We expect all black holes to be spinning. This is a subtle change in the shadow itself, but...

And if the accretion disk and black hole have misaligned poles, there should be a huge warp in the disk close to the black hole as the black hole frame-drags the accretion disk.

Is the misalignment _necessary_? I always thought the hole got its rotation from the infalling matter.

Over billions of years, the angular momentum of the stuff falling in changes.

But if it's infinitely small(not the event horizon, the black holes singularity), then a rotation or poles are hard to imagine. Is the singularity rotating or the whole event horizon area? It's hard to put this in descriptive words, i hope my point gets across.

Yeah I thought about this and looked into it a little bit. A "point" has zero dimensions so my intuition thought that there's nothing TO spin.

What happens at the singularity in a rotating black hole depends on your quantum theory of gravity. Some theories have the singularity as a "ring" instead of a point. I believe loop quantum gravity has a "planck star" in the center of a black hole.

Macroscopically, rotating black holes can be thought of as rotating the space around the event horizon. So the spacetime curvature doesn't just pull you directly to the center, there's an angular aspect as well.

https://en.wikipedia.org/wiki/Planck_star

https://www.youtube.com/watch?v=UjgGdGzDFiM

I’m only an enthusiast, but my understanding is that it’s the black hole’s rotation that causes poles in 3D. The visible accretion disc, I believe lies in the plane in which the black hole rotates.

If the black hole is not spinning, it should look the same from every angle. A spinning black hole is deformed (like the spinning Earth.)

Anyway, we are not seeing the black hole (because it looks just like a black circle in front of a black background). The interesting part of the image is a disk of material spinning around it. There is a nice 3d cardboard simulation and clear explanation in a video by Veritasium https://www.youtube.com/watch?v=zUyH3XhpLTo

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