I have heard it explained that a clock will not work in a black hole,
and thus time "stops" in a black hole.  However, a clock is a man-made
machine used to measure the intangible thing we call time, not the
intangible concept of "time" itself.  So, when astronomy theorists
speak of time stopping in a black hole, what do they mean?

Hi there!

A clock is man-made, of course, but it is designed to measure an
actual phenomenon of our physical universe. Time is actually a
component of space/time, which is defined as:

"A geometry that includes the three dimensions and a fourth dimension
of time. In Newtonian physics, space and time are considered as
separate entities and whether or not events are simultaneous is a
matter that is regarded as obvious to any competent observer. In
Einstein's concept of the physical universe, based on a system of
geometry devised by H. Minkowski, space and time are regarded as
entwined, so that two observers in relative motion could disagree
regarding the simultaneity of distant events. In Minkowski's geometry,
an event is identified by a world point in a four-dimensional
continuum."

(http://physics.about.com/library/dict/bldefspacetime.htm)

Now, that's relatively dense stuff. Here's what it means -- at least
as far as Einstein and Minkowski are concerned. Two people who are in
the same relative frame of reference (moving at about the same speed
in the same direction) will experience time the same way. So, if you
and your best friend got on a space ship that started travelling at
almost the speed of light, the two of you would experience time at
exactly the same rate. And, assuming your watches were synchronized,
they would confirm your suspicions.

However, when two observers are in different frames of reference,
suddenly time might seem very different. You see, the faster a person
travels through space/time, relative to another observer, the more
time seems to slow down. If you and your friend go travelling at
nearly the speed of light in your spaceship, it might seem to the two
of you that only one year passes between taking off and returning.
However, because of your great speed relative to Earth, the people
back on Earth will experience time more quickly than you do, so that
decades might have passed before you arrived. In physics, this is
called Time Dilation, and it's defined this way:

"The principle, predicted by Einstein's special theory of relativity,
that intervals of time are not absolute but are relative to the motion
of the observers. If two identical clocks are synchronised and placed
side by side in an inertial frame of reference they will read the same
time for as long as they both remain side by side. However, if one of
the clocks has a velocity relative to the other, the travelling clock
will show, to that observer, that less time has elapsed than the
stationary clock. The principle has been verified in a number of ways;
for example, by comparing the lifetimes of fast muons, which increase
with the speed of particles to an extent predicted by this factor."

(http://physics.about.com/library/dict/bldeftimedilation.htm)

Now, time dilation takes place at high speed, but it also takes place
in regions of intensely high gravitational pull. This is because
gravity is 'created' by the mass of an object literally causing
space/time near it to curve. This curvature occurs with all mass --
you create a very small gravitational pull by your very existence and
so do I. Because time and space are entangled together, frames of
reference are changed by high gravity the same way they're changed by
high speed. Now, in most normal gravitational fields, you'll never
notice the difference. No one on the Moon seems to be in a different
rate of time than someone on Earth, for example, even though the Moon
only has one sixth the gravity of Earth. But, when you get close to a
black hole, the gravity changes become vastly different. As one
approaches the singularity -- the 'hole' itself -- the shift in
gravity becomes so tremendous that time slows down intensely, to the
point that it seems to stop entirely, from the point of view of a
hypothetical observer outside the black hole's event horizon.
Literally billions of years could pass outside before the ship you
were in went forward even an inch towards the actual singularity.

However, as far as you're concerned inside the space ship, time will
still work normally, and your clock will still tick once a second.
After all, you and the clock will share a frame of reference, so as
far as you're concerned, time will still be working normally. However,
looking back out at the rest of the universe you would see their time
speed up incredibly or even infinitely fast.

Of course, the gravity would tear you and your ship apart long before
any question of you stopping in time would come up, but that's neither
here nor there.

ADDITIONAL INFORMATION can be found at

http://casa.colorado.edu/~ajsh/singularity.html

I hope this helps!

gsuch,

i am no physicist but here's a layman's shot at trying to explain
"high" physics.  its has to do with a term called "relativity".

but let's start with the basics:
There are 2 kinds of time: The time that we normally associate with is
what is sometimes called an absolute measurement, i.e. without us
doing anything, absolute time would still pass us by (hence the saying
time [and tide] awaits for no man).  The 2nd kind of time is
dimensional and is a relative form of measurement: this is obvious
even in a normal circumstance i.e. for example when we cross the
International Dateline - we either "gain" or "loose" a day or 2 depend
on the direction we were traveling. But remember this loss or gain is
actually relative to the place you are arriving, the site that you
have left would still be running its "normal" time.

So when the scientists mentioned about time being stopped - they are
in reality referring to a relative measurement of time with respect to
the body being subject to the black hole scenario as compared to
another similar body not subjected to the same black hole scenario.
i.e. a comparative measure.

experts please shoot me if i am wrong. 8-)

unstable, I think you just about nailed it, although I'm no expert
either!

A good layman's guide to cosmology and relativity is Stephen Hawking's
"A Brief History of Time." Excellent reading.

Time doesn't stop for the person entering the black hole, it only
appears that way to an outside observer. If you were in a safe
position outside of the black hole and were able to watch that same
clock enter it, you would see the hands slow to a crawl and eventually
stop just as the clock reaches the event horizon. The clock would
still keep on ticking away (at least until it's torn apart by
gravity), it's just that the light you see can't escape the gravity of
the black hole.

It is my understanding that the space craft occupants get no internal
clue that they are traveling close to light speed and/or inside a
black hole. That is because their speed relative to the ship is close
to zero. The only exception; in a small black hole they will
experience a destructive gravity gradient (some times called tide)
They will be unable to communicate with back home and colliding with
even a tiny partical may be a disaster. It will take forever to leave
the black hole, but nothing happens inside the ship other than the
tide effect.  Neil