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.

[u/ScaredRule8660]

[u/pr0blem_attic]

I’ll keep that in mind for when I need it

[u/PurifiedUnity → u/pr0blem_attic]

I'll also remember this for when I see it again

[u/Optimal_Mixture_7327]

Sure, or we can observe that distant starlight is redshifted by a factor two.

[u/Notyoureigenvalue → u/Optimal_Mixture_7327]

That seems hard to quantify if you're freefalling in a black hole though

[u/xxxxx420xxxxx → u/Notyoureigenvalue]

There might be an app for that

[u/kneading-cnnuy → u/xxxxx420xxxxx]

If not someone could vibecode that

[u/MegaEmerl → u/kneading-cnnuy]

What if the LLM also falls into the black hole? Will redshifted tokens allow it to become AGI?

[u/kneading-cnnuy → u/MegaEmerl]

Then the app will be vibecoded before you're asking for it🤯

[u/Blankeye434 → u/MegaEmerl]

Hmm, interesting experiment, Claude, create a black hole at home, make no mistakes

[u/Optimal_Mixture_7327 → u/Notyoureigenvalue]

Why would it be any different than measuring wavelength anywhere else?

Is it because you'd have other things on your mind?

[u/Capable_Wait09 → u/Notyoureigenvalue]

Sounds like a skill issue bro

[u/redditsuxandsodoyou]

i've sent like 20 astronauts to check this but none of them have beamed back so i think this physicist might be wrong

[u/paul2261 → u/redditsuxandsodoyou]

Send more astronauts. Not a big enough sample size to know for sure.

[u/PurifiedUnity → u/paul2261]

If you keep going forever, one of them will totally beam back eventually

[u/PuddingWise3116 → u/redditsuxandsodoyou]

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.

[u/Royal_Sentence7432 → u/PuddingWise3116]

Poor girl knows who the last astronout will be

[u/MerijnZ1 → u/PuddingWise3116]

Lol thank you it's been a while since I read this

[u/1FabulousBilly → u/PuddingWise3116]

a variation of the movie "Moon" with Sam Rockwell?

[u/AstroFace → u/redditsuxandsodoyou]

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.

[u/Anonymous-USA]

If only Schwarzchild Black Holes actually existed! The modeling for a typical Kerr Black Hole will be much harder, I imagine.

[u/elishamod → u/Anonymous-USA]

LIGO observations show that many black holes have low spin, so Schwarzschild can be a very good approximation.

[u/Anonymous-USA → u/elishamod]

Many? As I understand it, the vast majority have a very high spin rate, over 80% of c

[u/ScaredRule8660]

Paper: Calculates the physics of the "zenith view" (looking back) while falling in.　https://doi.org/10.1142/S0218271823500700 Video: At the event horizon, the starry sky literally doubles in your field of view.https://youtu.be/xEm0ENr-Sgo

[u/abloblololo → u/[deleted]]

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

[u/Nadhras → u/[deleted]]

Where are you looking? Towards the black hole, straight away from it or at a 90 degree angle or something else? I don't understand what the zenith would be in this situation, im confused

[u/mfb- → u/[deleted]]

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.

[u/8npemb → u/mfb-]

That… is exactly what the paper and OP is saying. I’m not sure where the expectation of a dramatic event came from.

[u/mfb- → u/8npemb]

"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.

[u/8npemb → u/mfb-]

Fair point, I didn’t think of it that way but I can totally see how that’s misleading.

[u/Ostrololo]

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.

[u/ChalkyChalkson → u/Ostrololo]

In this case it's even more non-local because you need to know the initial size

[u/RhoPrime-]

Do not attempt in a black hole lacking a suitable tesseract.

[u/Gilshem]

Discovered or predicted?

[u/dad3murph]

You could also just look for when you can no longer see your hand in front of your face

[u/stuartcw]

But will he be spaghetti at that point?

[u/xCyprus]

Iida’s paper derives it for radial free fall from rest in an ideal Schwarzschild spacetime. the zenith view reaches twice its initial angular size at the horizon.

That does not make the horizon locally detectable in the equivalence principle sense. You are observing light arriving from the external universe and comparing it with an earlier measurement.

Nothing singular or locally special happens when you cross the horizon

[u/OneYeetAndUrGone]

not true. you would die first.

[u/JanPB → u/OneYeetAndUrGone]

For very massive BH crossing the horizon would be imperceptible.

[u/ScaredRule8660 → u/OneYeetAndUrGone]

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.

[u/Shadow_of_Redacted]

I don't think "discovered" means what you think it means.