Even though black hole solutions were known for a while, experts were not convinced they were real. They had good reason. Only for some very special cases could it be proven that the collapse of a star would form a black hole.

Before Penrose, the chief tool of general relativity was differential equations. They gave detailed predictions, but only for idealized situations. From solving differential equations, it had been known that if a perfectly spherical star collapsed, it would form a black hole.