Schwarzschild’s first calculations focused on the curvature of spacetime outside a spherical, nonspinning star. A few weeks later, he sent Einstein another paper on what it would be like inside such a star. In both cases, something unusual seemed possible, indeed inevitable. If all the mass of a star (or any object) was compressed into a tiny enough space—defined by what became known as the Schwarzschild radius—then all of the calculations seemed to break down. At the center, spacetime would infinitely curve in on itself. For our sun, that would happen if all of its mass were compressed into a
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