- #1

haushofer

Science Advisor

- 2,665

- 1,124

- TL;DR Summary
- How is black hole formation observed by a distant observer?

Dear all,

For a new book I'm writing I'm investigating some common misconceptions in physics. And of course, that means confronting myself with my own confusion. One thing I've never got clear in my head, and which I find hard to answer using google and my textbooks on GR, is the following: how exactly is black hole formation observed by an observer sitting at a distance? If we take a solution of, say, infalling matter or light, crushing itself into a black hole, how much coordinate time would an observer at a distance measure for the matter or light to form an event horizon? I'm sure this can be answered technically by looking at Penrose diagrams, but I've never felt completely comfortable with those. My intuitive answer would be that it would take a huge amount of coordinate time (an infinite amount?) according to our observer for the matter to collapse into a volume smaller than its Schwarzschild radius such that the event horizon actually forms. Am I right? And if so, can someone give some details? What would this mean concretely for the black holes we observe, especially the famous picture by the Event Horizon Telescope? In the end I want to translate this to something concrete in my book, like this picture.

As a follow up one could also ask the same question about black hole evaporation of course, but let's first focus on the black hole formation process. Any insights are more than welcome.

For a new book I'm writing I'm investigating some common misconceptions in physics. And of course, that means confronting myself with my own confusion. One thing I've never got clear in my head, and which I find hard to answer using google and my textbooks on GR, is the following: how exactly is black hole formation observed by an observer sitting at a distance? If we take a solution of, say, infalling matter or light, crushing itself into a black hole, how much coordinate time would an observer at a distance measure for the matter or light to form an event horizon? I'm sure this can be answered technically by looking at Penrose diagrams, but I've never felt completely comfortable with those. My intuitive answer would be that it would take a huge amount of coordinate time (an infinite amount?) according to our observer for the matter to collapse into a volume smaller than its Schwarzschild radius such that the event horizon actually forms. Am I right? And if so, can someone give some details? What would this mean concretely for the black holes we observe, especially the famous picture by the Event Horizon Telescope? In the end I want to translate this to something concrete in my book, like this picture.

As a follow up one could also ask the same question about black hole evaporation of course, but let's first focus on the black hole formation process. Any insights are more than welcome.