1
$\begingroup$

This question already has an answer here:

According to general relativity, I had understood that time appears to slow down when looking into high gravitational fields from afar, so that as a black hole forms, the light from a collapsing star appears to get redder and dimmer, and redder and dimmer.

But in that case, does the light ever switch off completely with the formation of a true black hole and event horizon, or does it just keep getting redder and dimmer forever, so that we can no longer detect it?

It seems strange to me that something that gets redder and dimmer, ever goes truly black as it approaches an asymptote, and like any asymptote, never actually reaches its final destination of total blackness. So does a black hole ever really form while we are looking at it from outside? Or does it only form if and when we actually fall into it?

When does the event horizon actually form, to an outside observer?

$\endgroup$

marked as duplicate by Alfred Centauri, CuriousOne, Community Apr 28 '16 at 0:01

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.