Squish

I might misunderstand what you're saying, but I think it's that as a large mass approaches a black hole that the black hole's event horizon gradually expands toward that mass. Do I have that right?If so, then this seems wrong. Consider the case where the large mass is actually a very, very large mass, i.e., another black hole of similar size. As the two black holes approach each other their mutual event horizons closest to the other will retreat toward their mutual centers. This is because the point midway between the black holes is a point of zero gravity (they cancel each other out). When their event horizons eventually "meet" they won't actually be at the midpoint anymore, having retreated some distance away.If that's correct then when a large mass, but not so large as to be a black hole, approaches a black then the black hole's event horizon will retreat away. If the large mass is considerably smaller than the black hole then the retreat will be negligible, but if it's a truly large mass such as a star then it should be measurable.Perhaps you were thinking that the gravity of the large mass would exert an attractive force on the event horizon. It can't do that because the event horizon isn't an actual object upon which gravity can exert a force. It's conceptual. The concept is very real, and there is matter at the event horizon (or at least so our current theoretical understanding posits), but as an actual thing consisting of matter it is not real.Interestingly, this case is where the "bowling ball on a trampoline" analogy breaks down. As two bowling balls on a trampoline approach each other the depression at the midpoint between them becomes greater rather than lesser.--Percy

Aren't tidal forces near a black hole so great that long before you reached the event horizon you would already be spaghettified? Consider the example of a fairly typical black hole of 10 solar masses, or 2*10³¹ kg. The radius of black holes follows a simple formula of R=3M, where M is the multiple of solar masses and R is in kilometers, yielding a radius for this black hole of 30 km. What would the difference in gravity be for an object 31 km distant from the black hole's center versus 31.002 km distant. .002 km is roughly the height of a man, so the question being asked is what is the difference in gravity between a man's head and feet a kilometer distant from the black hole's event horizon?Just doing the very simple, the ratio in gravity between the two distances only 2 meters apart is 1.00013, so close to 1 as to seem negligible.But it's not negligible because given the huge gravitational force .00013 of it must be enormous. The gravitational force a kilometer out from the event horizon is 6.67*10¹⁴ newtons, so .00013 of that is 8.67*10¹⁰ newtons, or translating it into something more familiar, something in the neighborhood of 20 billion pounds.A difference in gravitational pull of 20 billion pounds between head and toes would be far more than enough to spaghettify a person, so long before you ever reached the event horizon you would already be spaghettified.I don't have the background to take relativistic effects into account.--Percy

This is off-topic, but as Mike's doing some more investigating I thought I'd introduce a puzzler.Remember that old puzzler about the innkeeper who charges three men $30 for their room, $10 each, but later realizes he should only have charged them $25. He gives the bellhop $5 to bring to the men's room, but the bellhop realizes $5 doesn't divide neatly by 3 and pockets $2, giving $1 to each man. So each man paid $9 and the bellhop kept $2 for himself for a total of $29. What happened to the other dollar?This puzzle misleads its victims by structuring the question improperly, and I think I have the relativistic equivalent of this puzzlez.A kilometer long alien spacecraft is traveling perpendicular to us at .97c and so is perceived by us as only a quarter kilometer long (approximately). It passes behind a moon a half kilometer in diameter. When the spacecraft is centered behind the moon it will be completely hidden from us, but observers in the spacecraft's front and rear can clearly see us at the same time. They take pictures of our clocks. When we meet later and compare notes we discover that at the same time they could see us clearly from both ends of their spacecraft they were completely invisible to us. How is this possible?--Percy

Oh, of course. So if instead of a typical black hole I had used the one at the center of our galaxy I would have gotten a different answer. That possibility hadn't occurred to me. Neat.--Percy