This question already has an answer here:

Black holes are said to be composed of two parts:

  • A singularity at the center, where all the mass of the black hole is condensed in a point of zero volume.
  • A black and empty ball delimited by the event horizon.

Also, relatively to an observer outside the black hole, the time for a particle inside the black hole (inside the event horizon) is stopped/frozen.

Suppose we had a particle very similar to photons but which wasn't affected by the gravitational force. Using this particle as our light we could see inside the black hole.

What would we see? We would see that particles who fall in the black hole become slower and slower because the more we come close to the black hole and the more time slows down. And once particles reach the event horizon, they stop completely, as time freezes.

So my question is: How can particles reach the singularity if time stops at the event horizon?

Scientists always say that, when a star collapses into a black hole, all the matter will be compressed in a singularity... But how can this singularity form if time stops at the event horizon?


marked as duplicate by John Rennie general-relativity Mar 28 '15 at 11:05

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ I've suggested what seems to me an obvious duplicate, and there are many other related posts on this site (though getting the right search terms to find them isn't trivial!). If you aren't happy that my suggested duplicate answers your question reply to this comment and I'll withdraw my clse vote. $\endgroup$ – John Rennie Mar 28 '15 at 11:07
  • $\begingroup$ @John Rennie: Thanks. I've read the question you suggested as well as two others. So it seems that the answer is: nobody knows ? In this question physics.stackexchange.com/questions/21319/… the best answer says that nothing ever passes through the event horizon so nothing every reach the singularity ; while in this answer physics.stackexchange.com/questions/82678/… the best answer says that matter does reach the singularity. $\endgroup$ – Quantum Force Mar 28 '15 at 12:38
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    $\begingroup$ @QuantumForce: the key point is that in GR different observers can and do observe different things. That doesn't mean one is wrong and the other right - they are both right. For a Schwarzschild observer nothing can ever pass through the event horizon. For a free falling observer hitting the singularity takes a finite (usually very short!) time. The nothing can hit the singularity is conditionally true, and likewise conditionally false. $\endgroup$ – John Rennie Mar 28 '15 at 12:53
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    $\begingroup$ @John Rennie: So this means that all the black holes born after the Big Bang has happened (from the collapse of stars for example) do no have any singularities, correct? $\endgroup$ – Quantum Force Mar 28 '15 at 13:04
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    $\begingroup$ @JohnRennie: I think we need some fresh answers to this question. IMHO the question arises continually on various web sites because the standard answer contains severe issues. $\endgroup$ – John Duffield Mar 28 '15 at 14:10