[HN Gopher] Black Hole Puzzle
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Black Hole Puzzle
Author : besmirch
Score : 24 points
Date : 2024-12-02 20:10 UTC (2 hours ago)
(HTM) web link (johncarlosbaez.wordpress.com)
(TXT) w3m dump (johncarlosbaez.wordpress.com)
| dmitrygr wrote:
| This is a relevant-ish short story by Greg Egan on humans jumping
| into a black hole:
| https://www.gregegan.net/PLANCK/Complete/Planck.html
| cvoss wrote:
| 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?
| kobalsky wrote:
| 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.
| Sharlin wrote:
| No, we're talking about _images_ of things, photons emitted
| by everything that has fallen in before, _before_ they
| crossed the horizon.
| cyberax wrote:
| The objects can emit photons by themselves.
|
| 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.
| mtdewcmu wrote:
| 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.
| insapio wrote:
| https://en.wikipedia.org/wiki/Firewall_(physics)
| cyberax wrote:
| > Isn't this true of all matter that enters?
|
| 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.
| _petronius wrote:
| There'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.
| cyberax wrote:
| > 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.
| itishappy wrote:
| 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?
| jiggawatts wrote:
| Any such blog / article / video that features a Penrose diagram
| is just wrong, because it's using mathematics that doesn't apply
| to the physical universe.
|
| Penrose diagrams draw black holes as if they have existed
| forever, and will last forever -- that's what the "future
| infinity" line _means_.
|
| Obviously black holes form at some finite time, and Stephen
| Hawking showed that they evaporate in a finite time.
|
| This matters. A lot!
|
| Fundamentally, relativity does _not_ allow observers to disagree
| on observations of _what_ , only _when_. If an outside observer
| never observes someone falling into a black hole _then they
| cannot fall in_. It 's just that simple! Again, for the slow
| people at the back of the class: Observers _cannot disagree on
| this_. If there is a paradox in your model, then your model is
| broken, end of story.
|
| There is this weird aspect of modern physics of holding on to the
| almost-mystical "woo" aspects far more than is justified in
| either the mathematics or observations. Quantum Mechanics and
| cosmology are especially rife with popularisations of what
| amounts to science fiction story telling. It's fun to think about
| wormholes, white holes, alternate universe accessible trough
| black holes, etc... I've read these novels, they were fun! Not
| physical though, natch.
|
| Much more realistically: An infalling observer is slowed down
| relative to the outside universe _and the stellar remnant that
| formed the black hole_. Effectively, the hapless infalling victim
| sees the black hole and the universe both "speed up".
|
| The error most people make here is that they assume that the
| black hole is already fully formed in the _infinite past_ and is
| "completed", making it a mathematically perfect sphere in a
| sense.
|
| No!
|
| It _hasn 't finished forming_ because as its gravitational field
| increases its time distortion increases. Its formation is "frozen
| in time" (actually just very very slow), it is _never fully
| formed_.
|
| Infalling observers see this slowdown _speeding back up_ , so
| what they observe is the light of the black hole evaporation
| getting blue-shifted and brighter until right before
| mathematically they would have "fallen past the event horizon"
| what they're actually seeing is akin to a supernova exploding in
| their face. They're blasted into subatomic particles, joining the
| radiation of the black hole evaporation many trillions of years
| into the future... but not _infinitely_ into the future.
|
| This model solves _every_ paradox of black holes, making them
| boring and not sci-fi exciting again, which is why you haven 't
| heard about it! Uncool stories don't get repeated on blogs for
| ad-clicks.
|
| Just remember: there are no infinities in the universe, and
| anyone using one in their theory is almost certainly just making
| stuff up to sound cool.
|
| PS: Penrose is also behind the kook notion that brains are
| quantum _despite all evidence_.
| while_true_ wrote:
| It irks me so many physicists/cosmologists jump from the
| mathematical GR singularity at the center of a BH to "matter
| there has infinite density." That's highly unlikely, it's
| probably quark plasma.
| coolThingsFirst wrote:
| Pure existential dread. There is a force out there that can eat
| our solar system as if though it's a cracker.
|
| This universe is bizarre af, it shouldn't even exist and us too
| but somehow it does.
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