Expert: Jayendra Upadhye - 9/11/2008

Question
QUESTION: i never quite understood what happens to a black hole if its radius exceeds the Schwarzchild radius.How is the angular momentum of the black hole affected?

ANSWER: Hi Shruthi,
No one asked me this one before!

Now let me get this right..
You are implying there are black holes with radius exceeding their Schwarzchild radius, and that that affects their angular momentum.

To my knowledge, for all black holes, the event horizon is of a greater radius than the compact body itself.

For non-rotating black holes (zero angular momentum), that is a singularity which is dimensionless and no "radius" implies zero angular momentum.

For rotating black holes, the central compact object is still "within the event horizon" which has radius equal to the Schwarzchild radius. But which has zero thickness and some small finite radius..ie it is a ring or annulus, with some angular momentum.

As more mass is accreted by the hole, the event horizon expands outward and the radius of the annulus too inceases, (but not for a non-rotating one, which remains a singularity at all times.

I dont know what makes you think a hole will ever "over shoot" its Schwarzchild radius.

Please elaborate, and we will see.

Link:- http://en.wikipedia.org/wiki/Black_hole

regards
Jayen



---------- FOLLOW-UP ----------

QUESTION: If we consider a supermassive black hole, won't it accrete enough matter to exceed the S.radius?

Answer
Hi,
To my knowledge, ALL black holes ALWAYS never transcend their swartzchild radius. Because the moment that happens (though it is a contradiction to say that), the object in question will cease to be a black hole.

Because, Black holes by definition of their structure are double entities.
1 - The central Singularity which is a point entity (non-rotating).
   Or at best a planar ring. (Rotating black holes).
2 - The event horizon marks the swarzchild radius in a 3 dimensional sphere. It is always bigger than the hole itself.

A super massive black hole is still a black hole! If it is non rotating, it would still be a singularity, a dimensionless point!
If it were rotating, the "supermassive" gravity would still keep the ring within bounds of the swarzchild radius.

Hope that suffcies.
regards
Jayen