translation.en: And it's actually quite a useful concept, because it tells us not only when a black hole will form, but it also gives us the key elements for the proof of a black hole.
translation.zh: 它实际上是一个相当有用的概念， 因为它不仅告诉我们 黑洞何时形成， 同时还带给我们黑洞存在证据的关键要素。