Appearance of a Kerr black hole from above

In summary, the Kerr black hole appears to be the same as a Schwarzschild black hole, but there are some small differences in the appearance of the two.
  • #1
Creedence
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TL;DR Summary
How does it look like?
How does a Kerr black hole look like from above? Is it the same as a Schwarzschild one?
I could not find any convincing visualization for this question.
By above I mean from a point r>0 and ϑ=0.
 
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  • #2
What is the metric for a Kerr black hole and what do you get when you substitute 0 in?
 
  • #3
Dale said:
What is the metric for a Kerr black hole and what do you get when you substitute 0 in?
Where to substitute?
 
  • #4
Creedence said:
How does a Kerr black hole look like from above? Is it the same as a Schwarzschild one?
I could not find any convincing visualization for this question.
By above I mean from a point r>0 and ϑ=0.
Are you asking for a reasonably technically accurate illustration? That’s what I’m inferring from the word “visualization”.

Of course the black hole itself doesn’t look like much of anything because it neither emits nor reflects light. All the interesting visual effects come from the matter nearby: accretion disks, radiation from infalling matter, jets, ...
 
  • #5
One could imagine a large piece of paper with a polar coordinate system drawn on it behind the black hole and ask how the pattern changes as the angular momentum increases, I suppose. Presumably you should hold the irreducible mass constant while doing it, and come up with some way of defining how far away you are in a reasonably consistent manner.
 
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  • #6
Creedence said:
Where to substitute?
You said ϑ=0, so substitute 0 into ϑ. It is your scenario, so I don’t know why you asked me that.
 
  • #7
Creedence said:
Summary: How does it look like?

How does a Kerr black hole look like from above? Is it the same as a Schwarzschild one?
I could not find any convincing visualization for this question.
By above I mean from a point r>0 and ϑ=0.
This is relevant:
https://iopscience.iop.org/article/10.1088/0264-9381/32/6/065001
But I don't know if they ever made any visualization looking perpendicular to the plane of rotation.
 
  • #8
A.T. said:
This is relevant:
https://iopscience.iop.org/article/10.1088/0264-9381/32/6/065001
But I don't know if they ever made any visualization looking perpendicular to the plane of rotation.
Yes, this is what bugging me. I've created a simple raytracer some years ago and generated some videos, for example: .
A few weeks ago I started to refactor it and fix its bugs (mostly numerical in the nonlinear computations). For some reasons the black hole is in a fixed position and the rotation axis points to the viewer. The output totally looks like a nonspinning BH. I can't find anything to validate the output.

Edit: The other reason for this question is that the raytracer's old version knew only the Schwarzschild solution. Now I try to add the Kerr solution.
 
  • #9
Creedence said:
The output totally looks like a nonspinning BH.
What differences did you expect from that view angle?
 
  • #10
A.T. said:

What differences did you expect from that view angle?
I rendered images of the BH using a square grid background. From that viewpoint the grid may get a vortex-like distortion in the case of a>0. Because of gtf ≠ 0.

Or with another not so precise wording: the lightrays come from infinity and "parallel" with the rotational axis of the BH get an eφ speed component. So the image gets a twist in the near of the BH shadow.
 
  • #11
Creedence said:
I rendered images of the BH using a square grid background.
Try a background that is not scrolling and has better contrast (black lines on white). And increase the spin rate and/or the resolution, in case the effect is still too weak to see.
 
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  • #12
I would think a better background would be the polar coordinate system I suggested above. I would expect that evenly spaced rings would not appear so, and radial lines would spiral somewhat. One could easily cheat on the rendering in this case by only analysing a single radial line and noting the symmetry. Or maybe a couple of radial lines, to check the symmetry.
 
  • #13
Ibix said:
One could easily cheat on the rendering in this case by only analysing a single radial line and noting the symmetry. Or maybe a couple of radial lines, to check the symmetry.
@Creedence As Ibix suggests: Before rendering full size videos, one should do some quick tests, to find out what parameters would produce a visible effect. And to verify that the code actually produces that effect at all.
 
  • #14
The video I posted is OK. It's generated by my old program version. The actual one generates images with grid background.

Ibix: radial lines is a good idea, thanks.
 
  • #15
Ibix said:
I would think a better background would be the polar coordinate system I suggested above. I would expect that evenly spaced rings would not appear so, and radial lines would spiral somewhat. One could easily cheat on the rendering in this case by only analysing a single radial line and noting the symmetry. Or maybe a couple of radial lines, to check the symmetry.
The effect is indeed really small, but I could catch it:

 
  • #16
Creedence said:
The effect is indeed really small, but I could catch it:

Interesting. Is the perceived rotation in two opposite directions just a pattern aliasing effect? Maybe a polar coordinate chart as suggested by @Ibix would indeed be better.
 
  • #17
A.T. said:
Interesting. Is the perceived rotation in two opposite directions just a pattern aliasing effect? Maybe a polar coordinate chart as suggested by @Ibix would indeed be better.
I think it's anti-aliasing, but I'm not sure. I'll try the polar chart.
 
  • #20
A.T. said:
Interesting. Is the perceived rotation in two opposite directions just a pattern aliasing effect? Maybe a polar coordinate chart as suggested by @Ibix would indeed be better.
Rendered a video using a radial pattern in the background:

It is "little" buggy (the ring inside should not be there I think), but the main effect can be seen on it. The spin parameter goes from 0.0 to 2.0 .
 
  • #21
Creedence said:
It is "little" buggy (the ring inside should not be there I think),
You could check the paths taken by the rays that generate those non-black pixels in that ring, and the black pixels that separate it from the rest.
 

1. What is a Kerr black hole?

A Kerr black hole is a type of black hole that is characterized by its angular momentum, or spin. It was first described by physicist Roy Kerr in 1963 and is a solution to Einstein's field equations of general relativity.

2. How can we observe the appearance of a Kerr black hole from above?

The appearance of a Kerr black hole from above can be observed through computer simulations and theoretical models, as it is not possible to directly observe a black hole due to its strong gravitational pull.

3. What makes the appearance of a Kerr black hole unique?

The appearance of a Kerr black hole is unique due to its spinning motion, which causes the event horizon (the point of no return) to be oblate rather than spherical. This results in an asymmetrical shape and a distortion of light around the black hole.

4. Can the appearance of a Kerr black hole change over time?

Yes, the appearance of a Kerr black hole can change over time due to its spin and the effects of matter falling into it. As the black hole gains more mass and angular momentum, its appearance may become more distorted.

5. How does the appearance of a Kerr black hole differ from other types of black holes?

The appearance of a Kerr black hole differs from other types of black holes, such as Schwarzschild and Reissner-Nordström black holes, due to its spin and resulting asymmetrical shape. This also affects the behavior of matter and light around the black hole, making it distinct from other types of black holes.

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