101 starship captains, bored with life in the Federation, decide to arrange their starships in a line, equally spaced, and let them fall straight into

Black Hole Puzzle | Azimuth

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2024-12-02 20:30:07

101 starship captains, bored with life in the Federation, decide to arrange their starships in a line, equally spaced, and let them fall straight into an enormous spherically symmetrical black hole—one right after the other. What does the 51st captain see?

A somewhat surprising fact is that the more massive a black hole is, the closer to flat is the spacetime geometry near the event horizon. This means an object freely falling into a larger black hole feels smaller tidal forces near the horizon. For example, we sometimes see stars getting ripped apart by tidal forces before they cross the horizon of large black holes. This happens for black holes lighter than 108 solar masses. But a more massive black hole can swallow a Sun-sized star without ripping it apart before it crosses the horizon! It just falls through the horizon and disappears. The tidal forces increase as the star falls further in, and they must eventually disrupt the star. But because it’s behind the event horizon at that point, light can’t escape, so we never see this.

My puzzle is assuming a large enough black hole that the starships can fall through the horizon without getting stretched and broken.