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Comment by davedx | Hacker News Reader

This is an awesome simulation but what I don't understand is why you wouldn't see any light from the singularity once you're inside the event horizon. Why is it completely black in there? Surely that's not correct?

I get that we have no experimental evidence of what it looks like inside but surely we could simulate the photonic environment?

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The event horizont isnt a line where things change, its just a line where the speed of infalling space exceeds speed of light. If you held flashlight pointed at your face going in first, once it crosses event horizont its going away from you faster than speed of light and you cannot see it. This holds true even after you cross event horizont too: Yes, you are moving faster than before, but the flashlight is still moving away from you faster than light.

Light doesn't suddenly become free to travel anywhere inside the event horizon. At least not with our current understanding of general relativity. So once you fall behind the event horizon, you could see some light that is technically inside the horizon, but only light that fell in behind you. There's still no way to see the singularity (or anything closer to it than you), at least for static black holes.

Really? Do you have a proof for this? The maths doesn't make sense to me, you're inside the boundary where gravity causes light to fall back towards the singularity

Yes light falls towards the singularity, as opposed to, say, falling into your eyes, right?

So look back and I suppose you'd see a massively phase shifted pinpoint of the universe in the line going from singularity through your eye and out of the event horizon? It wouldn't be straight out, but more like the "deorbit"

No I mean... light is emitted from the singularity (because it's an incredibly super dense piece of something star-like). It shines outwards, losing momentum, and at the event horizon it bends back around and falls back down into the singularity.

So if I'm inside the event horizon, the light that's emitted from the singularity should hit my retina, right? What am I missing?

No. The event horizon is just an imaginary point where the escape velocity equals the speed of light. As you get closer to the singularity, the escape velocity increases further. So you could imagine every shell of a sphere inside the horizon as another horizon. If you sit somewhere between the event horizon and the singularity (technically impossible since not even going at the speed of light could prevent you from falling down further), only lightrays that were emitted outside of your "horizon" can reach you.

I am not a physicist but as I understand it everything inside the event horizon goes towards the singularity. No light can come from it for you to see the singularity

Nobody is replying with any concrete proofs or references for this, so I will remain sceptical about it :)

Then you are skeptical about the concept of event horizon. The proof you are looking for is general relatively and all the research validating it

There's different answers here for different types of black holes. Effectively, significant enough charge or rotational speeds can create a second horizon where inside the black hole, allowing for stable orbits inside the event horizon. The issue is there's no real explanation we have that would explain why a black hole would have accumulated enough charge to create a large enough second horizon for there to be stable orbit for us to sit around in and look at stuff.

It's more complicated with rotating black holes. https://arxiv.org/abs/1103.6140 and some other papers contend it's potentially possible, but I'm not sure if black holes large enough to support this sort of thing can rotate fast enough.

Neither would let you see the singularity, though, because the singularity isn't really a physical thing, and even if it were, there's not really any reason to believe it would emit or reflect light.

But all of this is likely nonsense when it comes to practical reality, though - the very fact the math leads us to a singularity means we're almost certainly missing something, and spacetime is really weird past Cauchy horizons - trying to extend the math to figure out what is going on behind them is probably futile.

I would guess in the real world there's not really anything "interesting" here from a 'seeing stuff while alive in the black hole' perspective, but in theory there are some solutions where you could see whatever else is inside the black hole with you.

> light from the singularity

The singularity is not a place. It's an instant of time--the last one you experience if you fall into the hole.

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