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White holes are speculative in nature. Black holes are known to exist (supermassive black holes in galactic nuclei and stellar-mass black holes in binaries).

My question: if white holes are expected to be very luminous objects, how do we know theoretically that quasars are not actually powered by white holes?

And observationally, is this possibility definitively ruled out for all (or any) quasars and/or are there constraints for the possibility?

This question seems related, but the questions asked there are different than here, I think.

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    $\begingroup$ Well there is the one that has been imaged - M87. $\endgroup$
    – ProfRob
    Commented Jun 8, 2021 at 14:23
  • $\begingroup$ I think the shadow of the event horizon of M87 was imaged. I do not know if that rules out the possibility of it being a white hole? $\endgroup$ Commented Jun 8, 2021 at 14:31
  • $\begingroup$ if white holes exist, then they are expected to be very bright” - Not true. For example, a Schwarzschild white hole is a vacuum solution. Therefore it emits no radiation. Its external metric is the same, so no experiment can tell a black hole from a non-radiating white hole. $\endgroup$
    – safesphere
    Commented Jun 8, 2021 at 16:05
  • $\begingroup$ In principle you are correct. But I'm specifically asking about white holes in the context of gaseous environments such as AGN - not the vacuum. Besides, we infer the existence of black holes in quasars from their accretion. I've edited it to be more clear. If I have a misunderstanding about white holes please point it out. $\endgroup$ Commented Jun 8, 2021 at 16:08
  • $\begingroup$ If I have a misunderstanding about white holes” - Black and white holes are vacuum solutions that don’t radiate. If you throw a gas to a black hole, you get an accretion disk and jets that do radiate. The metric outside the horizon is the same for a black and white hole. You cannot tell them apart. So if you throw a gas to a white hole, you get the same radiating accretion disk and jets. It is physically impossible to tell a white hole from a black hole. They are the same for external observers. $\endgroup$
    – safesphere
    Commented Jun 12, 2021 at 20:18

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White holes cannot exist in a universe with a finite age. As it happens black holes cannot exist either, though we expect there will be objects that are so similar to black holes that they cannot be distinguished from a true black hole.

So the simplest answer to your question is that quasars cannot be white holes since white holes don't exist. However there are other objections as well. The emission from a white hole is fundamentally unpredictable so we have no way of knowing what would come out of them. By contrast the emission from quasars is well predicted by models that describe them as supermassive black holes in galactic nuclei. While we cannot prove quasars are not white holes Occam's razor suggests this is unlikely.

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    $\begingroup$ Why do you say black holes cannot exist? Sure, they can evaporate, so the "nothing can get out" description isn't valid (which was Stephen Hawking's point), but most quantum-gravity theorists still call them black holes. Anything that falls into them comes out thoroughly scrambled, and they still need a name, so why not "black hole"? (But +1 anyway, because the answer's content is good regardless of my minor quibble about the side-issue.) $\endgroup$ Commented Jun 9, 2021 at 0:57
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    $\begingroup$ @ChiralAnomaly it's really just terminology. It would be more precise to say that event horizons don't exist because they take infinite (coordinate) time to form, so if you define a black hole as an object that has an event horizon then black holes can't exist. That doesn't stop everyone (me included) from using the expression black hole to describe objects that don't currently have a horizon but will form a horizon in finite proper time. $\endgroup$ Commented Jun 9, 2021 at 4:17
  • $\begingroup$ Isn't all of that based on speculative semi-classical approximations? Whereas white holes arise principally from "classical" general relativity, which is really the aim of my question in terms of theory. While we cannot prove quasars are not white holes Occam's razor suggests this is unlikely. I understand this as a philosophical answer. We do not know a priori that white holes do not exist, so I don't find that answer satisfactory, especially since absence of evidence is not generally evidence of absence (only in certain, well demonstrated cases it that so!). $\endgroup$ Commented Jun 9, 2021 at 13:06
  • $\begingroup$ The emission from a white hole is fundamentally unpredictable so we have no way of knowing what would come out of them. I think this gets closer to what I'm wondering. Is there any kind of model that could be used to try to correlate photons leaving a quasar with each other? That way they'd show if they're completely random or if they're correlated by being emitted from a common source, e.g. emission from an accretion disk? Also, is it impossible for an accretion disk to form around a white hole? This is more of what I was wondering, rather than even more controversial Hawking stuff :D $\endgroup$ Commented Jun 9, 2021 at 13:12
  • $\begingroup$ Sorry I did not make that clear! I can certainly edit my question to make it more clear if you'd prefer! $\endgroup$ Commented Jun 9, 2021 at 13:12

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