Expert: James Gort - 9/2/2007

Question
When a large star collapses to a black hole why doesn't the collapse velocity approach the speed of light?  As far as I can tell there is no force that can counter act the collapse of a very massive star therefor it should accelerate to the speed of light.  And why doesn't time slow down and nearly stop for the star as it collapses and why doesn't the star's mass increase and approach infinite mass?  Also shouldn't any matter that's sucked into the black hole be accelerated to the speed of light with the same consequences occurring to the time and mass of the in-falling object?


Answer
Hi Paul,

I forgot to mention on the "rotation" question, all angular momentum is measured at the event horizon (which drags space-time along with it).  What happens to the angular momentum (or anything else) within the event horizon is anybody's guess.  It's thought that both relativity and quantum mechanics break down, so the laws of physics are uncertain in that region.

At the event horizon, you need an infinite force just to remain stationary, so you are correct that nothing can stop the acceleration at the EH or within the black hole.  So matter accelerates, but not necessarily to the speed of light.  IF relativity holds, the mass of fast moving matter increases, so some of the gravitational energy goes to increasing the mass of incoming matter, which makes it harder to accelerate.  Current theory says the matter will "free fall" as if all the mass was concentrated at the singularity, and may even approach the speed of light (if the black hole is large enough).  But (and we don't know this for sure!) matter probably never reaches the speed of light.

At the event horizon, time appears to stop AS OBSERVED BY A DISTANT OBSERVER.  For matter at the event horizon (or within it), time appears normal.  That's one of the consequences of relativity!

The star's mass doesn't become infinite because there's only so much matter and energy there.  As matter accelerates, its mass increases, but at the expense of gravitational energy.  The total (mass plus energy) must be constant.  That limits the speed the matter can obtain!  So in spite of the matter being "infinitely dense" at the singularity, it only exerts a standard gravitational force on incoming matter (although the matter is in free fall).  One caveat - I mentioned before that the laws of physics are uncertain within the event horizon.  We're sure of a few things (like the total mass plus energy can't increase), but the mechanics of what goes on inside the event horizon is uncertain and will remain hidden from view.

Hope that helps.

Prof. James Gort