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If I were looking at a black hole with an accretion disk, I presume it would appear something like this: obtained from astronomycafe.net

I.e., the black hole is as black as black can be, and the disk on the far side of the hole is visible to us, unlike e.g. Saturn's rings.

What would a white hole look like? By white hole, I refer to a region of spacetime into which nothing may enter. If it were surrounded by a disk like the black hole above, what would that disk look like to us?

Qmechanic
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Diffycue
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    White holes are a wild theoretical idea and I don't think they're taken seriously in mainstream physics. However the closest thing to what you want is this video. – StephenG - Help Ukraine Jun 09 '17 at 05:16
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    Whether or not white holes exist is irrelevant; I just want to know what one would look like if one did exist. The idea is no more "wild" than black holes, except that one ought to exist while the other ought not if we expect the second law of thermodynamics to hold. For an example of a mainstream physicist discussing white holes briefly, consider Bob Wald's article on the history of QFT in curved spacetime. – Diffycue Jun 09 '17 at 07:50
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    Actually this is not a simple question. Since a white hole -- roughly speaking -- is something ejecting everything in its neighborhood. You should see the singularity, and I don't know how a singularity would look like. – Oscar Jun 09 '17 at 10:14
  • @Oscar, Indeed, I don't have a good idea of what the singularity would look like either. Discounting the singularity, could we maybe imagine something like a neutron star (i.e. can see its reverse side) with a perfectly reflective surface? Is the warped image of the accretion disk going to be the same? – Diffycue Jun 09 '17 at 11:30
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    The point is that in a white hole there is no accretion disk. – Oscar Jun 10 '17 at 17:49
  • @Oscar I'm not concerned with whether there is or is not such a disk or whether a white hole does or does not exist, just what it would look like if one did exist. As I understand it this is a question that has a definitively computable answer with raytracing techniques but I was hoping someone with good facility in differential geometry could answer it without going through all the computational work. – Diffycue Jun 11 '17 at 19:46
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    I got your point. And my second comment was about your question " Is the warped image of the accretion disk going to be the same?" No, since in a white hole no accretion disk is present. there nothing special at the horizon in a white hole, even from a visual point of view. The only thing you should see in a white hole is the naked singularity. nothing else special. – Oscar Jun 12 '17 at 23:56

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