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The way I understand it, black holes have extreme gravity, and time moves more slowly in high gravity.

For outside observers, it would seem that time would stop at the event horizon.

Does this mean that light which was traveling towards the star at the time of the gravitational collapse would possibly get "stuck" near the event horizon, for possibly billions of years?

If that is the case, would it then be possible to examine such light (if it doesn't spiral into the black hole) to examine the early universe?

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  • $\begingroup$ Yes, time stops at the event horizon for a distant observer, but not if you're falling towards the BH. See physics.stackexchange.com/q/79054/123208 & physics.stackexchange.com/q/21319/123208 $\endgroup$
    – PM 2Ring
    Jul 19, 2022 at 9:33
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    $\begingroup$ But how would you examine that light? It's heading towards the event horizon (EH), so to see it you'd need to somehow get between it & the EH. $\endgroup$
    – PM 2Ring
    Jul 19, 2022 at 9:38
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    $\begingroup$ Presume then that the light was not traveling straight towards it. Instead, it could travel close to the event horizon (possibly get captured in an unstable orbit), where it eventually escapes. $\endgroup$
    – Zobody
    Jul 19, 2022 at 9:45
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    $\begingroup$ Oh, ok. But those orbits are very unstable. And as the BH gains mass, the photons that were in the photon sphere fall into the BH. See physics.stackexchange.com/a/680961/123208 But even if you manage to observe some photons that were orbiting for a while just above the photon sphere, it won't be easy to extract useful info from them. It'd be like trying to read a book that's been through a blender. ;) $\endgroup$
    – PM 2Ring
    Jul 19, 2022 at 9:55

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Infalling objects pass right through the event horizon. They don't freeze there. The vicinity of the event horizon is locally just like any other part of spacetime.

If a light-emitting or light-scattering object falls into the hole while you watch from a distance, you will see it slow down and freeze (and redshift) at the event horizon, but not because it actually does. You don't see the object directly: what you see is light that hits the retina of your eye. Light from near the event horizon takes a long time to escape to your distant location, and light from inside the event horizon never escapes, so you see a frozen image from near the event horizon, even through the object isn't physically frozen there.

That does mean that in a classical universe, in principle, you could take a snapshot of light emitting/scattering objects that fell into a black hole (stars, rocks, but not light since light doesn't emit light) with an infrared camera. In reality, it's impossible because of quantization if nothing else. Each object emits/scatters only finitely many photons before crossing the event horizon, and the time at which the last photon escapes is a fairly small multiple of the light crossing time of the black hole. The light-crossing time is a few microseconds per solar mass, and the last-photon time is probably well under 1 millisecond per solar mass, which even for a supermassive black hole like M87's is on the order of one year, not billions.

As mentioned in comments, it's also possible for light to stick around near a black hole by orbiting it, but the orbits are unstable, so this runs into the same problem as the snapshot-viewing idea: the orbital-decay half life of photons is some small multiple of the light-crossing time, so there's no realistic chance of finding any ancient light there.

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An inner horizon firewall

Objects appear to freeze at the event horizon but actually fall in quickly. Objects dropped in, after a few moments, cannot be intercepted outside the black hole. Even though there are still visible. A similar phenomenon happens with distant galaxies.

But the inner horizon is different. Most black holes rotate and they approximate the Kerr metric. The Kerr metric predicts two one way doors: the event horizon and the inner horizon. In the space between these "doors" all objects and light is forced inward toward the inner horizon.

Objects take forever to get to the inner horizon: no matter how long you wait, you can always reach them just outside the inner horizon. But in the object's reference frame it falls in quickly. It sees the entire future of the outside universe infinitely blue-shifted at the inner horizon.

This infinite wall of "stored ancient light" and particle energy signals the breakdown of the Kerr metric. The carnage happens deep inside the hole and no news of the maelstrom gets outside.

Quantum mechanics likes black hole firewalls (since it allows Hawking radiation to encode an object's information without leaving a copy inside). Most firewall models place the instant-doom at the event horizon, which is considered a "paradox" since general relativity predicts nothing special to happen at the outer horizon. But the inner-horizon firewall makes both relativity and the no-cloning theorem happy.

This is really sad: the Kerr metric has a lot of fun stuff for travelers who pass the inner horizon. But anything past the inner horizon a mathematical mirage, a starry-eyed analytical solution that doesn't exist in reality.

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  • $\begingroup$ Having a firewall at the inner horizon does nothing to make quantum mechanics happy. It makes the situation no better (from a quantum perspective) than the Schwarzschild case. $\endgroup$
    – TimRias
    May 4, 2023 at 16:54
  • $\begingroup$ @TimRias: Why would a firewall at the outer horizon be any better than one at the inner horizon at solving the information paradox? Of course, the separate problem of how quantum gravity works at singularities is still unknown. I edited my question to say "information paradox" to be more clear. $\endgroup$ May 5, 2023 at 9:16

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