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Black Holes

Date: 11/12/97 at 23:18:05
From: Parminder Kang
Subject: Black Holes

I have always been somewhat dumbfounded by the conclusions one can 
draw from the existence of black holes. This problem has been 
bothering me. Imagine an observer outside a black hole watching 
someone wearing a clock moving into the black hole. I know that the 
outside observer will see the clock moving slower and slower as the 
person approaches the black hole. Of course, after the person passes 
the event horizon, he will lose contact with the rest of the universe. 
I know that the person will still be able to see me as he is entering 
the black hole. 

My question is, how will the person in the black hole see the 
observer? Since the person inside the black hole perceives time to 
be slowing down, he must see my time speed up. Consequently, he will 
see me age very fast. Does this also imply that he is actually seeing 
into the future?

Date: 11/13/97 at 11:29:26
From: Doctor Mitteldorf
Subject: Re: Black Holes

Dear Parminder,

He's seeing the future the same way that a person who travels very 
close to the speed of light and then returns to earth sees the future:  
by slowing down his internal clock and living a long time.

From the point of view of a person falling into a black hole, it takes
a short time. But from the point of view of someone outside, it takes 
an infinitely long time for him to fall in. Yes - he sees the entire 
future history of the universe on the way in. But another way of 
saying this is that he won't make it into the black hole during the 
(infinite) lifetime of this, our universe.

In fact, it takes an infinite amount of time for matter to collapse to
form a black hole, too. All the astronomical objects that are loosely
called "black holes" are really in an irreversible, dynamic process of
collapsing, and are asymptotically close to their event horizon, but 
not through it. Technically, you might reserve the word "black hole" 
for matter that is inside its event horizon; for that to be the case, 
the black hole must have existed since the beginning of (our) time.

-Doctor Mitteldorf,  The Math Forum
Associated Topics:
College Physics
High School Physics/Chemistry

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