What does "the black hole formation is a robust prediction of the general theory of relativity" mean in layman terms?
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$\begingroup$ I've deleted a number of comments. Please focus comments on a question on improving that question- answers should be posted as answers and unrelated comments should go on another post or to chat. $\endgroup$– Chris ♦Commented Jul 16, 2022 at 9:47
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The existence of black holes, or more precisely of an event horizon was discovered by Karl Schwarzschild in 1915 when he discovered the solution to Einstein's equations that we now call the Schwarzschild metric. However at that time it was unclear whether this geometry could be formed by a collapsing star. For example the Schwarzschild metric is spherically symmetric, and it was thought possible that it could only form if the original star was also spherically symmetric. Since no real star is that symmetric maybe a black hole could never be formed.
However in the 1960s Roger Penrose proved a number of theorems now known as the Penrose-Hawking singularity theorems that showed black holes were inevitable subject only to a few apparently reasonable assumptions. The theorems show that any sufficiently massive collapsing object must form a black hole regardless of how asymmetric it is.
This is what we mean when we say black holes are a robust prediction of general relativity. If Einstein's equations are correct then black holes cannot help but form. Given that there is a lot of experimental evidence supporting Einstein's equations, the only way out of this is if GR is an approximation that breaks down in the extreme conditions required to form a black hole. There have been many suggests that quantum effects will modify GR and possibly stop black holes from forming, but so far none of these have been convincing enough to be generally accepted.
answered Jul 14, 2022 at 5:29
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$\begingroup$ "The existence of black holes, or more precisely of an event horizon was discovered by Karl Schwarzschild in 1915..." It took a while (several decennia) for people to realize that the singularity in Schwarzschild's solution was, in fact, an event horizon. $\endgroup$– TimRiasCommented Jul 14, 2022 at 14:56
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$\begingroup$ Some comments removed. Please consider posting an alternative answer instead. $\endgroup$– rob ♦Commented Jul 15, 2022 at 17:20
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$\begingroup$ @rob, I see my comments and answers as my intellectual property. If you as moderator find them improper, or unwanted, just tell me that: I will delete them by my own. If you do not want to respond here (it is truly not the right place for it), please feel free to use my email ([email protected]). $\endgroup$– JanGCommented Jul 16, 2022 at 7:59
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$\begingroup$ @JanGogolin, if you would like to discuss site policy on comments, answers, and intellectual property, please post on Physics Meta. $\endgroup$– rob ♦Commented Jul 16, 2022 at 10:17
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$\begingroup$ Actually Penrose proved that under the reasonable assumptions if a trapped surface forms during gravitational collapse then the space-time will be singular. An event horizon is expected and conjectured but i don't think that it is proven (weak cosmic censorship). $\endgroup$– MBNCommented Jul 19, 2022 at 11:45
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If a prediction is robust, one interpretation is that it means it does not require many assumptions. In the case of black holes, they are a simple solution of the equations of general relativity and do not require many assumptions to predict. It would be hard to invent a Universe where black holes do not exist. And we have much observational evidence that black holes indeed exist.
