What this thread is really missing is a simulation. I can't promise its the enterprise (as the performance constraints are crippling), but here's a bunch of cubes instead:
It seems that what you see is the object flattened on the shadow in front of you, and it remains flattened. Apparently past me didn't implement redshift on objects, but its likely extremely redshifted
Edit:
Here's a second better clip, showing this more clearly
I guess we see a bit of light coming from the top and sides that would normally be hidden, but that ray tracing is making the renderer very sad because sometimes you sample from the back of the polygon that doesn't have texture?
Or something like that, there's definitely something funky going on with the view of the top surface that's unrelated to GR.
Its hard to explain exactly what's accurate here when you're looking at the precise shapes - part of the problem is that this rendering is extraordinarily complex to do in real time, and it relies on the size of the object (tris) being 'small' relative to the curvature of spacetime
There's actually no texturing here at all though, if you see a face disappear its because general relativity is hard. The extra disconnected pieces are something to entirely ignore as well, and occur because tris become nearly parallel with rays leading to long intersection lengths (as well as numerical issues), leading to the local flatness assumption breaking down
It gets a little far into the weeds in a very "the map is not the territory" kind of way, but is fun none the less.
I think (it is not addressed to my satisfaction in the video) that it is implied that from inside the black hole, assuming it was formed from a collapsing star, you would see all around you the event horizon: behind you, the unverise at the time you crossed the event horizin, in front of you, the surface of the star at the moment of its collapse. The singularity, as the post covers, exists only in your future, so you could not see it, even though you will always end up there.
Here's a short fiction idea if anyone wants to take it up.
Forward causation in time with predictive ability must imply Downward causation.
This scenario is similar to the scenario in which the time and space axes inside
the black hole swap places. If that is possible within the blackhole, then it must mean that it should be possible everywhere. Similarly to how we first find
the holographic effect to apply to only blackholes but then generalize to the whole universe.
It is argued that Bob sees light from Alice's crossing of the horizon at the same instant Bob himself crosses. Isn't this true of all matter that enters? When Bob enters, he sees everything that ever fell into the black hole "before" him, at all once? Is it blinding? Does it fry and scramble Bob? Or is it so redshifted that Bob survives?
The article is missing on a couple points. The analysis assumes all Alice's light is emitted radially exactly outward from the center of the black hole. In reality, light is emitted in all directions, and anything emitted at even slightly different angles would get sucked into the black hole. But, Bob might still see it, because he can catch up with it. So when the article says Bob sees Alice cross the horizon when Bob crosses the horizon, it really means that Bob won't see any photons they emitted from inside the BH until Bob crosses into the BH. But similarly, one second prior Bob will be encountering photons Alice emitted roughly 0.99 seconds prior to crossing the horizon, and so on, because those are all getting redshifted too.
It's similar to if you and the car in front of you are accelerating at the same rate, they have a small head start, and they've got someone throwing fastballs at you at 100mph. When you get to the point where you are going 100mph relative to the ground, you'll hit the ball that was thrown from exactly that spot relative to the ground. So, sure, crossing the 100mph barrier implies something interesting mathematically, but it's not something that the observer would particularly notice. The math for GR isn't exactly the same (baseballs won't redshift), and in particular there isn't even a piece of dirt to compare the photon's motion to, as the horizon is just a mathematically-defined "place", but to a first-order approximation, it's the same thing going on with photons and spacetime distortion. It's a continuous function.
There's a bit more to it than can be explained in a comment, but the main thing to know is that (as far as we know) nothing special happens at the horizon if you're falling in.
why would bob see anything? I understood that the event horizon is a threshold, not a shell that you cross and suddenly can see inside.
to see something photons have to bounce on something and reach our eyes, we stop seeing stuff inside the horizon because those photons don't bounce back and they are pulled into the singularity.
my logic said that if light can't escape the horizon, then it can't escape alice to reach bob, even if he's inside the horizon, photons can't suddenly go backwards from alice until bob, they are being pulled further inside.
A single photon can't be seen multiple times, right? So, if photon A goes into Alica's retina, then Bob can't see photon A. If a big, opaque object passes through the event horizon right in front of you, it would absorb or scatter the photons in its path, and you would not see them.
I'm going to guess. From Bob's perspective, Alice's ship would still be able to block light. So he wouldn't be able to see what was ahead of him through the back of Alice's ship; Alice's ship would occlude his view.
Fun! I think there's an interesting hidden puzzle in the first sentence:
> 101 starship captains, bored with life in the Federation, decide to arrange their starships in a line, equally spaced, and let them fall straight into an enormous spherically symmetrical black hole—one right after the other.
Does this problem have a globally consistent solution? In the curved spacetime around the blackhole, can everyone agree on what equal spacing means?
No, they can't. One could very well come up with some other coordinate system whose time & space coordinates are (non-linear) combinations of the usual Schwarzschild time & space coordinates. In that coordinate system, spatial slices of equal time would be different, so spatial distances would be measured differently.
TL;DR Just pretend the author wrote "[…] equally spaced with respect to some observer's frame of reference" (e.g. a stationary observer at infinity that uses the usual Schwarzschild coordinates).
> However, when any of the starships behind him crosses the horizon, the captain of that starship will see Bob in front of them, also crossing the horizon!
This is a tricky point. The 51-st captain will see the ships in front of him getting slower and slower, the distance between them growing less and less. At the horizon, they'll be separated by zero distance.
But how can that be? Imagine that you're looking at a line of cars in front of you, while sitting in a bus. You can see far ahead, and you can project the distances between cars onto your windshield. Now you go out of the bus and look at the cars ahead of you again from your regular height. Now you get down on your knees, and look again. And then lie down on the road and look ahead again.
That's exactly how it would feel for that captain, any lateral distances between ships will keep getting smaller and smaller, until they completely disappear at the horizon. All the ships in front of you will be squished into a line, and the ship in front of you would block the view.
If I got BH theory right, each particle of the ship entering the event horizon will almost fully stop, while the next particles entering will still slightly move, compressing the whole ship into a thin shell or crust that, due to atomic mechanics, won't function as normal matter, so the crew won't know what happened to them. The entering ship will redshift until it dissapears. Their particles will "spacetime-travel" far far away into the future until the compressed crust bounces back to space as radiation. Did I get it right?
Ignoring the "atomic mechanics" and "spacetime travel" parts, what you're describing is roughly what might be seen by an external observer, far from the black hole. From the ship's point of view, passing the event horizon happens (and in the case of a large-enough black hole, could be quite uneventful). This puzzle is all about reconciling those two points of view, and exploring intermediate points of view. If you're trying to describe things from a single authoritative point of view, of course, there is no such thing (although here's the disclaimer: I am also not an astrophysicist)
Granted. Thinking twice, BHs move in space really fast, for example when they orbit other BHs. Meaning their mass is not frozen in time, otherwise these will elongate or rip apart. So the time dilation wouldn't be have such big effect, and atoms might not be compacted too much?
Let's see if I can state this properly. The atoms of the ship will pass right through the event horizon like nothing. To see the ship, though, photons have to travel from the ship to your eyes. As the ship goes deeper into the black hole's gravity, the photons will "appear to" be getting slowed down by the black hole's gravity well, each photon more than the last. So, an outside observer would have to wait longer and longer to get the next photon. In fact, he'd have to wait an infinitely long time to get all of them. It's like an optical illusion, except that a real physicist would say it's not an optical illusion; it's time dilation, etc.
Readit News
https://www.youtube.com/watch?v=iTw0pJvTkGw
It seems that what you see is the object flattened on the shadow in front of you, and it remains flattened. Apparently past me didn't implement redshift on objects, but its likely extremely redshifted
Edit:
Here's a second better clip, showing this more clearly
https://www.youtube.com/watch?v=npC6lCwYUN0
Or something like that, there's definitely something funky going on with the view of the top surface that's unrelated to GR.
There's actually no texturing here at all though, if you see a face disappear its because general relativity is hard. The extra disconnected pieces are something to entirely ignore as well, and occur because tris become nearly parallel with rays leading to long intersection lengths (as well as numerical issues), leading to the local flatness assumption breaking down
It gets a little far into the weeds in a very "the map is not the territory" kind of way, but is fun none the less.
I think (it is not addressed to my satisfaction in the video) that it is implied that from inside the black hole, assuming it was formed from a collapsing star, you would see all around you the event horizon: behind you, the unverise at the time you crossed the event horizin, in front of you, the surface of the star at the moment of its collapse. The singularity, as the post covers, exists only in your future, so you could not see it, even though you will always end up there.
Forward causation in time with predictive ability must imply Downward causation.
This scenario is similar to the scenario in which the time and space axes inside the black hole swap places. If that is possible within the blackhole, then it must mean that it should be possible everywhere. Similarly to how we first find the holographic effect to apply to only blackholes but then generalize to the whole universe.
Downward Causation : https://www.preposterousuniverse.com/blog/2011/08/01/downwar...
It's similar to if you and the car in front of you are accelerating at the same rate, they have a small head start, and they've got someone throwing fastballs at you at 100mph. When you get to the point where you are going 100mph relative to the ground, you'll hit the ball that was thrown from exactly that spot relative to the ground. So, sure, crossing the 100mph barrier implies something interesting mathematically, but it's not something that the observer would particularly notice. The math for GR isn't exactly the same (baseballs won't redshift), and in particular there isn't even a piece of dirt to compare the photon's motion to, as the horizon is just a mathematically-defined "place", but to a first-order approximation, it's the same thing going on with photons and spacetime distortion. It's a continuous function.
There's a bit more to it than can be explained in a comment, but the main thing to know is that (as far as we know) nothing special happens at the horizon if you're falling in.
why would bob see anything? I understood that the event horizon is a threshold, not a shell that you cross and suddenly can see inside.
to see something photons have to bounce on something and reach our eyes, we stop seeing stuff inside the horizon because those photons don't bounce back and they are pulled into the singularity.
my logic said that if light can't escape the horizon, then it can't escape alice to reach bob, even if he's inside the horizon, photons can't suddenly go backwards from alice until bob, they are being pulled further inside.
The problem is that (classically) when you cross the event horizon, the photons that you emit at just that moment will _stay_ _in_ _place_ forever.
Not quite. He will see the light emitted by _all_ of the matter that has fallen in before him, but only in an infinitely small area.
> 101 starship captains, bored with life in the Federation, decide to arrange their starships in a line, equally spaced, and let them fall straight into an enormous spherically symmetrical black hole—one right after the other.
Does this problem have a globally consistent solution? In the curved spacetime around the blackhole, can everyone agree on what equal spacing means?
TL;DR Just pretend the author wrote "[…] equally spaced with respect to some observer's frame of reference" (e.g. a stationary observer at infinity that uses the usual Schwarzschild coordinates).
This is a tricky point. The 51-st captain will see the ships in front of him getting slower and slower, the distance between them growing less and less. At the horizon, they'll be separated by zero distance.
But how can that be? Imagine that you're looking at a line of cars in front of you, while sitting in a bus. You can see far ahead, and you can project the distances between cars onto your windshield. Now you go out of the bus and look at the cars ahead of you again from your regular height. Now you get down on your knees, and look again. And then lie down on the road and look ahead again.
That's exactly how it would feel for that captain, any lateral distances between ships will keep getting smaller and smaller, until they completely disappear at the horizon. All the ships in front of you will be squished into a line, and the ship in front of you would block the view.
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