The singularities at the centers of astrophysical black holes mark the breakdown of Einstein’s theory of gravity, general relativity. They represent the only breakdown sites accessible to experimentalists, since the only other known singularity, the big bang, is believed to be invisible due to the vast expansion that occurred afterwards during cosmic inflation.
Every physicist knows these facts but very few discuss black hole singularities, as if the topic is taboo. The reason is simple: to explore the true nature of singularities we need a theory that unifies general relativity with quantum mechanics, and we do not have a unique, well-defined formalism for doing that. Even in the context of specific proposals for a unified model, such as string theory, the nature of black hole singularities is rarely discussed because of its mathematical complexity.
But perhaps the time is ripe now to open up this discussion, given that the 2017 Nobel Prize was awarded to the LIGO team for discovering gravitational waves from collisions of black holes. An observable quantum signal from the embedded singularities could guide us in the search for a unified theory.
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