Expert: Jayendra Upadhye - 4/13/2004

Question
I've read that as a black hole forms, time appears to slow down as the star collapses.  If I understand it correctly, the closer it gets to complete collapse, the slower time goes, so that it takes infinite time (from our point of view) to become a black hole.  So how could there be any black holes at all if an infinite amount of time has obviously not passed?

Answer
Hi Tom,
Technically, the actual hole is a singularity ..a point of no dimensions at all!
But there IS this EVENT HORIZON which envelops a volume of space centered on the hole, and which is proportional to the mass of the black hole in its size.
The boundary of the event horizon starts where speed of light and the escape velocity of the whole become equal.
The "infinite" journey of our falling victim reaches crticality at that point.
Any slowing down / freezing of time etc (for external observers) happens immediately on the "inside" of the event horizon and is hence "unknown" to us.
the singularity is a conjecture as no one could possibly "see" it.
The journey of infalling matter from the horizon to the hole itself will always be a matter of conjecture.
That reminds me of a paradox brought up by Asimov..if the universe itself is a black hole (it has "huge" mass ..), its event horizon would recede into the dim farthest reaches of the known universe! For all we care, we might be residing within a hole you know!
Jayen