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I have already read this question - Orange ring in a black hole image

And also have seen several videos about it (Specificaly one from Veritasium -https://www.youtube.com/watch?v=zUyH3XhpLTo&ab_channel=Veritasium

and one from TED https://www.youtube.com/watch?v=uyMtsyzXWd4&ab_channel=TED)

From those I understood that what we actually see is really the photon orbit, not the Accretion disc.

What I do not quite understand is the reason, why part of the "disc" around the event horizont is brighter and the other one darker.

In the video from TED (time 6:42), they say that it is caused by one part of the gas moving towards us and one away from us. (Doppler's effect probably).

But what gas are they talking about?

If the light comes from photon orbit, there is no gas in there, only the light. The gas should be part of the acretion disc.(Which as mentioned before, we do not see on the image of black hole.)

Could someone clarify this?

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  • $\begingroup$ They're talking about gas ejected from the accretion disk. $\endgroup$
    – PM 2Ring
    Commented Mar 25, 2022 at 17:43
  • $\begingroup$ @PM2Ring So, it is gas that has left accretion disc and entered the photon orbit ? (And afterwards Event Horizon) $\endgroup$
    – John Ronald
    Commented Mar 25, 2022 at 17:52
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    $\begingroup$ Well, only photons can orbit in the photon sphere. But yes, it's gas that's falling towards the event horizon. Some of the photons it emits will temporarily orbit in the photon sphere. $\endgroup$
    – PM 2Ring
    Commented Mar 25, 2022 at 18:01
  • $\begingroup$ That's the answer I was looking for, Thank you! $\endgroup$
    – John Ronald
    Commented Mar 25, 2022 at 18:05
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    $\begingroup$ FWIW, I have some info about photon sphere trajectories here: physics.stackexchange.com/a/680961/123208 although that's for a Schwarzschild BH, not a rotating one, which is rather more complicated. $\endgroup$
    – PM 2Ring
    Commented Mar 25, 2022 at 18:06

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