Can one create a black hole when already inside the black hole ?
You mean a blacker hole? :)
Your question is really hypothetical. I think that may not possible.
As far as we know a black hole exists in our 3+1 dimensional space but it itself is considered to be in a state known as singularity so its size is infinitly small and does not consume more that one dimension but its strong gravity bends the space around it so much that it pulls virtually everything into it eve the light which makes a dark region around it and creates an optical illusion that the black hole has large sizes. The gravity of the black hole is dependent on the mass of the star once it was so, the more massive the black hole is the greater will be the size of its event horizon. The event horizon is the region beyond which nothing (even light) can escape the pull of the black hole.
since the size of orginal black hole consider as infinetly small we can't able to create a another black hole inside it.And there also another important thing is! if anything goes into black hole it doesnot exist. the things may converted into neutrons.therefore no blackhole created inside a blackhole
The gravity of a black hole inside its event horizon is too strong [remember the part about not even light can escape] to permit gravitational collapse of mass to occur.
The singularity is of zero size. The black hole itself (volume within the event horizon) has a nonzero size. More massive black holes are indeed larger than less massive black holes. It's no optical illusion.
I don't think it's accurate to say that they don't exist anymore. We just don't really know what happens to them. Conversion to other particles is certainly a possibility though.
It depends on the definition of "black hole".
If we say "everything behind the event horizon as seen from infinitely far away" then we cannot by definition, because an object can only be behind an event horizon or not.
If we say "everything behind a local event horizon relative to some other position" (in some mathematical sound way), then it is possible. Locally, things always look like you are in free space, so you can create your own local black hole while you are in a larger black hole.
Intriguing thought. If a star was in the process of collapsing to start a supernova as it passed the event horizon of a large black hole, what would cause it to cease being able to collapse? Tidal forces pulling it apart? If the large black hole is really large, tidal forces would be fairly small. So I'm not sure about that.
I'd hate to the math for a gravitational collapse inside the event horizon of a black hole. Relativistic corrections would be pretty nightmarish.
The OP question is very interesting one. I think russ_watters is right. What can't prevent the star to become supernova and turned into black hole? For a larger black hole, the event horizon is very far right. So in between before the singularity, or before the final stage star becomes spaghettization, it can collpase into a black hole itself.
All paths inside the event horizon lead towards the singularity. So a particle trying to gravitationally collapse still must move towards the singularity, not towards the center of mass of the region trying to collapse.
Those two options don't exclude each other.
I have difficulty visualizing how a collapse would proceed from such a state.
All material would move inwards (for some suitable definition of "inwards"), but the material further out moves inwards faster. This is easier to model with weak fields (imagine two asteroids close together quickly falling towards the sun), but I don't see why this should be impossible with strong fields.
We had a thread on this some time ago:
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