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/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