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Q:
I have always wondered if a black hole can move through space? And, if they trap everything around them how can we be sure they don't move, if we can't see it?
A:
Black holes can indeed move through
space. The really massive black holes at the centers of galaxies will stay
there unless something catastrophic happens, like a direct collision
between two galaxies.
The much smaller black holes formed from the explosions of stars can move
fairly quickly because they receive a kick from the explosion. See, for
example:
http://hubblesite.org/newscenter/newsdesk/archive/releases/2002/30/text/
Astronomers know about the rapid motion of this black hole because it has a
nearby companion star that's slowly being pulled apart by the gravity of
the black hole. Rapidly moving black holes that are flying solo will be
essentially impossible to detect, since space is very big and they will
encounter other objects only very rarely. Only the most massive stars
produce black holes, and such stars are rare, so there's no need to worry
about a rogue black hole heading towards the solar system.
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