B How Do Black Hole Animations Capture Optical Effects?

[anorlunda]

Mind bending. It makes our attempts to describe such things using words on PF seem so futile.

https://apod.nasa.gov/apod/ap210416.html

 

[etotheipi]

Very cool. Did they also make their equations available somewhere?

 

[berkeman]

Also, I looked but could not find if the speed of the animation is real-time, or sped up. Did anybody see if they mentioned that?

 

[anorlunda]

Also, I looked but could not find if the speed of the animation is real-time, or sped up. Did anybody see if they mentioned that?

My impression is that there is no attempt to show time or actual orbits. The objects in the animation never really merge or spiral inward. The purpose was to show optical effects.

 

[Vanadium 50]

It is almost certainly solved numerically and it is certainly sped up. These black holes are severla light minutes across.

 

[Ibix]

My impression is that there is no attempt to show time or actual orbits. The objects in the animation never really merge or spiral inward. The purpose was to show optical effects.

The holes aren't that close. The center to center distance is something like ten times the diameter of the inner bright ring around the larger hole. That ring is at the photon sphere, so the holes are something like thirty times the Schwarzschild radius of the larger hole distant from one another (I know they're Kerr holes, but back of the envelope). I don't think approximating the orbits as Newtonian is completely crazy at this level of hand waving, and that gives an orbital period of several hours. So definitely sped up.

I'm not sure how much time solving Einstein's Field Equations numerically would require in this case. The blurb reckons they used 2% of a 129,000 processor cluster for a day, or a little over 60,000 hours of computer time. That seems a little bit steep to me for just ray tracing (even nice high-res ray tracing in curved spacetime), so I lean towards it being a proper simulation (although that's, at best, only a partially educated guess).