When a star collapses, it sort of falls in on itself.” Travers curved his hand into a semicircle and then brought the fingers in. “And sometimes it reaches a kind of point of no return where the gravity pulling in on it is stronger than the nuclear and electric forces, and when it reaches that point, nothing can stop it from collapsing and it becomes a black hole.” He closed his hand into a fist. “And that critical diameter, that point where there’s no turning back, is called the Schwarzschild radius.” Travers paused, waiting for me to say something.

He had come to see me every day for a week, sitting stiffly on one of my chairs in an unaccustomed shirt and tie, and talked to me about black holes and relativity, even though I taught biology at the university before my retirement, not physics. Someone had told him I knew Schwarzschild, of course.

“The Schwarzschild radius?” I said in my quavery, old man’s voice, as if I could not remember ever hearing the phrase before, and Travers looked disgusted. He wanted me to say, “The Schwarzschild radius! Ah, yes, I served with Karl Schwarzschild on the Russian front in World War I!” and tell him all about how he had formulated his theory of black “holes while serving with the artillery, but I had not decided yet what to tell him. “The event horizon,” I said.

“Yeah. It was named after Schwarzschild because he was the one who worked out the theory,” Travers said. He reminded me of Müller with his talk of theories. He was the same age as Müller, with the same shock of stiff yellow hair and the same insatiable curiosity, and perhaps that was why I let him come every day to talk to me, though it was dangerous to let him get so close.

“I have drawn up a theory of the stars,” Müller says while we warm our hands over the Primus stove so that they will get enough feeling in them to be able to hold the liquid barretter without dropping it. “They are not balls of fire, as the scientists say. They are frozen.”