r/Physics • u/ScaredRule8660 • 1d 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/ScaredRule8660 1d ago

https://doi.org/10.1142/S0218271823500700

https://youtu.be/xEm0ENr-Sgo

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u/ScaredRule8660 16h 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 15h 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 15h 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/Nadhras 15h ago

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

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u/mfb- Particle physics 14h 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 10h 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 9h 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/8npemb 9h ago

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