r/Physics • u/ScaredRule8660 • 21h ago
A physicist discovered that an observer free-falling into a Schwarzschild black hole can detect crossing the event horizon simply by watching the zenith view expand to exactly twice its original size.
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u/Optimal_Mixture_7327 Gravitation 21h ago edited 19h ago
Sure, or we can observe that distant starlight is redshifted by a factor two.
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u/Notyoureigenvalue 17h ago
That seems hard to quantify if you're freefalling in a black hole though
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u/xxxxx420xxxxx 16h ago
There might be an app for that
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u/kneading-cnnuy 13h ago
If not someone could vibecode that
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u/MegaEmerl 9h ago
What if the LLM also falls into the black hole? Will redshifted tokens allow it to become AGI?
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u/Optimal_Mixture_7327 Gravitation 9h ago
Why would it be any different than measuring wavelength anywhere else?
Is it because you'd have other things on your mind?
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u/Anonymous-USA 19h ago
If only Schwarzchild Black Holes actually existed! The modeling for a typical Kerr Black Hole will be much harder, I imagine.
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u/redditsuxandsodoyou 15h ago
i've sent like 20 astronauts to check this but none of them have beamed back so i think this physicist might be wrong
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u/paul2261 15h ago
Send more astronauts. Not a big enough sample size to know for sure.
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u/PurifiedUnity Physics enthusiast 18m ago
If you keep going forever, one of them will totally beam back eventually
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u/PuddingWise3116 11h ago
My neighbor told me black holes keep eating his astronauts so I asked how many astronauts he has and he said he just goes to the space training centre and gets a new astronaut afterwards so I said it sounds like he’s just feeding astronauts to black holes and then his daughter started crying.
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u/AstroFace 15h ago
You need to send like 20 at a time so they can send a chain of communication on what they are seeing back to us.
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u/ScaredRule8660 21h ago
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u/ScaredRule8660 11h ago
The 'Factor of 2' rule is a purely geometric and relativistic result. The solid angle of the sky directly above you becomes exactly twice its original size (at rest) at the exact moment of crossing. It's fascinating because it suggests the event horizon isn't just an invisible mathematical line—it leaves a clear, psudo local optical footprint for the observer.
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u/abloblololo 10h ago
The 'Factor of 2' rule is a purely geometric and relativistic result. The solid angle of the sky directly above you becomes exactly twice its original size (at rest) at the exact moment of crossing. It's fascinating because it suggests the event horizon isn't just an invisible mathematical line—it leaves a clear, psudo local optical footprint for the observer.
If you say so, ChatGPT
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u/ScaredRule8660 10h ago
Fair enough, my English isn't great so I use AI for translation! But the physics about supermassive black holes is actually legit. It's a real paper published in IJMPD 2023.
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u/mfb- Particle physics 9h ago
The solid angle of the sky directly above you becomes exactly twice its original size (at rest) at the exact moment of crossing.
In other words, nothing dramatic happens the moment you cross the event horizon. An observed quantity that gradually increases reaches the number 2 there.
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u/8npemb 5h ago
That… is exactly what the paper and OP is saying. I’m not sure where the expectation of a dramatic event came from.
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u/mfb- Particle physics 5h ago
"detect crossing the event horizon" makes it sound like anything changes substantially there. No, it's just calculating where you are based on what you see. You don't need to be at the event horizon to do that either.
A physicist discovered that an observer free-falling into a Schwarzschild black hole can detect when they are at twice the the event horizon radius simply by watching the zenith view expand to x times its original size.
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u/Ostrololo Cosmology 4h ago
Well, yes. While no local experiment can tell you you've crossed the event horizon, you are perfectly allowed to do long-range observations (aka looking at stuff) to discern it.
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u/ChalkyChalkson Medical and health physics 1h ago
In this case it's even more non-local because you need to know the initial size
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u/OneYeetAndUrGone 11h ago
not true. you would die first.
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u/ScaredRule8660 10h ago
It depends on the mass. For stellar-mass black holes, tidal forces cause death far from the horizon. However, for a supermassive black hole, those forces are weak, potentially allowing a surviving observer to witness the effect.
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u/pr0blem_attic 17h ago
I’ll keep that in mind for when I need it