Q&A: Black Holes

Q:
I read in a book that an average adult astronaut would die if he/she was sucked into a black hole miles before the person was even near the black hole. Is that because the amount of gravity being sucked in by the black hole is crushing the person?

A:
The force of gravity on the side of the astronaut closest to the black hole would be stronger than on the side farthest away, so the astronaut would be stretched in the direction of the black hole. Eventually this force would rotate an astronaut lengthwise so that his/her head or feet would be lined up with the direction to the black hole. By the time the astronaut was 3,000 miles or so away from a black hole containing ten times the mass of the Sun, these forces would pull all the blood to the end closest to the black hole, with fatal effects. As the astronaut got closer, his/her body would be torn apart. So, the gravity in the vicinity of the black hole would pull a person apart before it crushed him/her.

The same effect causes the Earth to be pulled slightly out of round by the Moon's gravity, and causes the ocean tides. For this reason the forces are called gravitational tidal forces.

Reference: Kip Thorne, Black Holes & Time Warps (W.W.Norton, New York, 1994)

For more information see our Field Guide section on Black Holes.