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The Question

(Submitted June 18, 1997)

If time comes to a stand still in a black hole, where does time began again out side the black hole?

The Answer

It is a bit misleading to say that 'time stands still' inside a black hole. Actually, if you could survive a trip into a black hole (which you couldn't) you would not be aware of any slowing down of any clock you carried as you fell in. However, if you could compare the speed of your clock with that of a reference clock kept far away, then then the clock falling into the black hole would appear to slow down relative to the clock far from the hole. The place where the falling in clock would appear to 'stop' is the spherical surface called the event horizon. This is also regarded as the boundary of the black hole, since nothing crossing this surface can escape. For more information, look at the entries for event horizon on our web page:

http://imagine.gsfc.nasa.gov/docs/dictionary.html

I hope this helps.

Tim Kallman
for the Ask an Astrophysicist Team

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