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There are a lot of questions about the m87 image on this site, non of them actually answer my question.

I have read this question:

Does an expanding event horizon "swallow" nearby objects?

For the distant observer no event horizon ever forms and the the infalling observer takes an infinite time to pass the point r=rs where the horizon would form given infinite time.

And here is the real image of an actual black hole:

enter image description here

https://www.nasa.gov/mission_pages/chandra/news/black-hole-image-makes-history

Based on John Rennie's answer, the event horizon never forms to an outside observer, just like us here on Earth. So if the event horizon never forms, then that black area in the middle is the effect of something else.

Now if the event horizon never forms for us outside observers, then what is that we see in the middle area, the black part? If that is not the event horizon, then it could be maybe an effect of GR lensing or EM shielding, or any other effect?

Question:

  1. If an event horizon never forms for an outside observer, then what do (or don't) we see on this real image of an actual black hole?
Árpád Szendrei
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  • That's not exactly the event horizon, it's generally called the black hole shadow. See https://astronomy.stackexchange.com/q/30317/16685 & https://astronomy.stackexchange.com/q/33387/16685 – PM 2Ring Oct 19 '20 at 23:58

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If you simulated this image (or a sharper version of it) in a 4-D ray tracer, the past-directed lightlike geodesics corresponding to those pixels would pass through the black hole's accretion disk (which is semitransparent), and ultimately hit some opaque matter that fell into the hole long ago. The light from that matter is ludicrously redshifted. The redshift doubles in a time comparable to the light-crossing time of the hole, which for the M87 hole is around 1 day, so after a year it's roughly 1 googol. Therefore, unless the hole ate a star quite recently, you can't actually see whatever the ray hits, and it'll appear perfectly black (plus some orange for the glowing semitransparent matter that the ray also passed through).

If you extend the ray past that opaque object, it'll probably hit another one that's even more redshifted. If you keep going, it will eventually head off to infinity after (i.e., before) passing through the earliest matter that collapsed to form the hole. It never crosses the event horizon.

benrg
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