Cauchy horizon:
The Cauchy horizon is the spot where determinism breaks down, where the past no longer determines the future. Physicists, including Penrose, have argued that no observer could ever pass through the Cauchy horizon point because they would be annihilated.
As the argument goes, as an observer approaches the horizon, time slows down, since clocks tick slower in a strong gravitational field. As light, gravitational waves and anything else encountering the black hole fall inevitably toward the Cauchy horizon, an observer also falling inward would eventually see all this energy barreling in at the same time. In effect, all the energy the black hole sees over the lifetime of the universe hits the Cauchy horizon at the same time, blasting into oblivion any observer who gets that far. [“Some black holes erase your past,” Robert Sanders, UC Berkeley News]
I’ve read about black holes all my life, but Cauchy horizon? New to me. Noted in “The hidden pockets of the universe where the future can cause the past,” Leah Crane, NewScientist (30 May 2026, paywall):
You’re falling into a black hole. Somehow, you’ve managed to protect yourself from the spaghettification that’s happening to every object around you as the black hole’s powerful gravity pulls on the near end of each object more than the far end, stretching everything into noodles before shredding it to pieces. Maybe you’ve got some sort of high-tech compression suit holding you together; congratulations on your invention. As you pass the event horizon, the point of no return, all you see is blackness punctuated by streaks of light falling towards the singularity at the heart of the cosmic behemoth. Your impossible suit also protects you from those streaks, which would otherwise be ripping through your molecules at near-light speed.
And then you pass a second, lesser-known horizon, and time and space switch places. This second boundary is called the Cauchy horizon; if they exist within black holes, their insides are the strangest places in the universe.
