In the simple explanation that a black hole appears when a big star collapses under missing internal pressure and huge gravity, I can't see any need to invoke relativity. Is this correct?
Answer
By a coincidence, the radius of a "Newtonian black hole" is the same as the radius of the Schwarzschild black hole in general relativity. We demand the escape velocity v to be the speed of light c, so the potential energy GMm/R=mc2/2, i.e. R=2GMc2
The agreement, especially when it comes to the numerical factor of 2, is a coincidence. But one must appreciate that these are totally different theories. In particular, there's nothing special about the speed c in the Newtonian (nonrelativistic) gravity. To be specific, objects are always allowed to move faster than c which means that they may always escape the would-be black hole. There are no real black holes (object from which nothing can escape) in Newton's gravity.
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