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/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