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I have the following assumption based on limited knowledge. A black hole appears dark to us because any light that would be emitted from it, and any light that passes nearby, is caught by the gravity well and either captured or bent from its original course.

If this is true, from any point within the gravity well or towards the center of a black hole, looking outward there would be immense amounts of light and radiation coming in.

Edit for specificity: The idea is not necessarily that a human or object would be looking out while at a point, but theoretically a fixed point that has light and matter passing by. Of perhaps if we somehow invented something capable of not exceeding but matching the gravitational force and could stay still. Or a sensor reading at a certain point, pointing a camera or infinite lumen sensor outward.

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  • $\begingroup$ @safesphere you provided an overall calculation, which I believe can be summarized to "yes looking outward from a fixed point would mean intense light passing by" - is that correct? $\endgroup$
    – FaultyJuggler
    Nov 9, 2019 at 19:02
  • $\begingroup$ @safesphere I would say both answers are correct and I assume I can't mark both? Ideally them combined is the answer as you nicely provided the direct calculations but didn't give an executive summary, that was my only hesitation to mark either as the answer. $\endgroup$
    – FaultyJuggler
    Nov 13, 2019 at 18:20

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Anything that crosses the event horizon (including light) will swiftly proceed towards the "central" singularity (at least until it crosses the inner horizon; we will get back to that). In particular, it is impossible for an observer to hover inside the black hole. Hence any observer will be falling further down the black hole. However, it is possible for light to catch up to the falling observer. The amount of light observed in a particular time interval is however always finite, for any timelike observer.

At least this is the picture for non-spinning/non-charged black holes. Generic black hole solutions with spin or charge will feature two horizons, an inner and outer horizon. Inward motion is only necessary in the region between the two horizons. One definition of the inner horizon is that it is the boundary of the causal future of the region outside the black hole. For an observer falling into the black hole this means that as he is about to cross the inner-horizon (and he must do see in a finite amount of time), he will witness the all the light signals that entered the black hole after him for the entire future of the universe. Because the signals are compressed into a finite amount of time, both their intensity and blueshift will diverge before crossing the inner-horizon.

One consequence of this is that the naive picture of the black hole as a vacuum solution breaksdown at the inner-horizon, and we can therefore no longer trust this solution. It is expected that these divergent perturbations lead to the formation of a spacelike or lightlike singularity at the location of the inner horizon. This expectation is sometimes known as (a version of) the strong cosmic censorship conjecture. Its exact status (and the nature of the singularity at the inner horizon) is subject of ongoing scientific debate.

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  • $\begingroup$ So in theory are black holes actually stars with gravity so great due to mass that even light cannot escape? This is why my thought was looking out from in outside the event horizon) a fixed point would be awash with light, like water passing by in a riptide $\endgroup$
    – FaultyJuggler
    Nov 9, 2019 at 19:05
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The light is not exactly bent from it's original course, the blackhole curves spacetime and the light follows that curvature. The thing is that the time component of "spacetime", as well, is bent near a black hole, so that as the light approaches the event horizon, the time it takes (from an outside perspective) for the light to get closer and closer to the event horizon gets longer and longer such that eventually, for the light to proceed an inch closer to the horizon, in the necessary time elapsed the universe would have died. So there is, seemingly, a paradox here, in the "time" it takes you to "perceive" anything at the event horizon (assuming you somehow can survive the gravitational tides) the universe would have ended. Of course your experience is separate and equal from an outside perspective and time would feel normal to you, but the question is, if the universe in which you are contained is destroyed in a different perspective, what happens to you from your own perspective? There really is no currently conceivable way to know the answer to your question.

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  • $\begingroup$ @definitelynotbs again "There really is no currently conceivable way to know the answer to your question" this is not true. All you need to do is follow the path of all the photons going inside the black hole which catch up to the falling observer. This is nice computation that requires no knowledge of some distant outside observers (who is irrelevant for the whole situation) time. The fact that outside observer will never see this happen does not matter for the question at all. When i say it that way - the physics of GR is local, you don't need to bring outsiders to your analysis. $\endgroup$
    – Umaxo
    Nov 6, 2019 at 11:35
  • $\begingroup$ @Umaxo I do not think that you understand what you have read. The universe will end before the photons catch up. Relativity does not mean that it will end from one persons perspective and not the other, it means that the "speed" at which time unfolds seems different. Perceptions are a function of time and will feel the same, but what is split seconds to the blackhole observer is billions of years to (most of) the rest of the universe. I am not bringing outsiders into the analysis, I am merely taking into consideration the system (the universe) in which the relevant observer exists. $\endgroup$
    – definitelynotbs
    Nov 7, 2019 at 3:17
  • $\begingroup$ @definitelynotbs How will the universe end? Are you assuming some cosmological model? Anyway this is not true "The universe will end before the photons catch up". This is kind of a claim that should be checked up by calculations and those will show you there is no problem, unless you consider some crazy comsology. In real universe though, there is no problem....Have you heard of penrose diagrams? If not, check them out. $\endgroup$
    – Umaxo
    Nov 7, 2019 at 5:24
  • $\begingroup$ @Umaxo ok, go do some calculations and get back to me when you have some idea of what you're talking about. Hate to inform you, but the "real universe" isn't a diagram. $\endgroup$
    – definitelynotbs
    Nov 7, 2019 at 8:00

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