Re: Is Black Holes like PacMan?

The simple answer is "no, not really". A black hole is not a physical 
construct like the "planet crusher" (and why was the Enterprise so big 
compared to the planet crusher? I mean, if the thing could swallow a planet
then the Enterprise should have been a tiny, tiny speck!)

But the question you ask is provocative. Could a 'black hole' travel 
through space and gobble up matter? Could it be the celestial equivalent
of a voracious PacMan? To answer this, consider what happens to matter as
it approaches a black hole.

First, a black hole is a point - a "singularity" - in space. That is, the
"hole" is a tiny region of space with a tremendous mass. A point of no
return. No one is ever likely to "see" one (Indeed, it would be impossible
to "see" one in our conventional sense of the word as "seeing" would 
involve the detection of light emissions from the black hole but that does 
not occur!) but if they could, it would be a point so small, well, it would 
be a singularity.

But a black hole has a massive effect. Gravity in the region around the
hole is severely affected. Spacetime is curved to the straining point. So,
as one approaches a black hole - from any direction - gravity constantly
increases. The sort of two dimensional, swirling hole in space of the
movies (and STAR TREK, etc) is hollywood. With respect to spacetime around
a black hole, every direction is down with the black hole waiting at the
bottom.

As you approach a black hole, the increase in gravity eventually results in
a spherical region where the gravitational attraction is large enough that
even light can not escape. Outside this region, you could still send a 
radio message that said "help, I am being eaten by a black hole" and 
someone might eventually hear it. Inside this region, you could send the 
message but it, too, would be sucked down into the singularity. This is
the "event horizon" - the point of no return, not even for light.

Of course, anything that can not escape the initial gravitational
attraction of the black hole - regardless of whether or not it is within
the event horizon - would eventually get sucked in. So a black hole does
sweep up all of the matter in the surrounding region because gravity goes
on forever. Black holes do devour their surroundings.

As to do they move about, there is some evidence to suggest that they might
but then we are also moving at tremendous velocities as a consequence of
spinning about earth's axis, travelling around the sun, which is itself
travelling through space and around the axis of the Milky Way, which is
itself travelling through space, .... well, you get the picture. But no,
they would not move to "eat" the surroundings. Gravitational attraction
relies on the mass of both objects. When one of them is of extremely
mass, the other doesn't really stand a chance. Consider the idea that if
you jump up in the air, the Earth is attracted to you just as much as you
are attracted to it - but it is fair to say that only one of you moves
in a noticeable fashion!

The best account I could suggest for reading what coming into contact with
a black hole would be like is in Kip Thorne's "Black Holes and Time Warps". 
The first chapter is a fictional account of what that encounter might be
like for various size black holes. It is written in non-technical terms 
and well worth the read.

Hope this helps. 

[Moderator's Note:  I just want to add that a black hole "gobbling up" the gas 
surrounding it is said to be "accreting" the matter.  As the accretion process 
goes on, the gas gets very hot and emits X-rays; when people say they 
are "observing black holes," they are usually just seeing the X-rays that the 
gas is emitting.]