Black Hole Puzzle
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Black Hole Puzzle
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Re: Black Hole Puzzle
#2This is a relevant-ish short story by Greg Egan on humans jumping into a black hole: https://www.gregegan.net/PLANCK/Complete/Planck.html
Re: Black Hole Puzzle
#3It 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?
Re: Black Hole Puzzle
#4There's a good episode of PBS Spacetime about Penrose diagrams: https://www.youtube.com/watch?v=T4oYvSH6jJ8
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.
Re: Black Hole Puzzle
#5It 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?
this is my uninformed guess.
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.
Re: Black Hole Puzzle
#6It 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?
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.
Re: Black Hole Puzzle
#7It 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?
Re: Black Hole Puzzle
#8> 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.
Re: Black Hole Puzzle
#9Fun! 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?
Re: Black Hole Puzzle
#10It 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?
this is my uninformed guess. 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 i…
No, we're talking about images of things, photons emitted by everything that has fallen in before, before they crossed the horizon.