“I think Earth’s worst problems are caused by the power shortage,” he said. “That affects everything else. Why doesn’t Earth use the kernels for power, the way that the USF does?”

“Too afraid of an accident,” replied McAndrew. His irritation evaporated immediately at the mention of his specialty. “If the shields ever failed, you would have a Kerr-Newman black hole sitting there, pumping out a thousand megawatts — mostly as high-energy radiation and fast particles. Worse than that, it would pull in free charge and become electrically neutral. As soon as that happened, there’d be no way to hold it electromagnetically. It would sink down and orbit inside the Earth. We couldn’t afford to have that happen.”

“But couldn’t we use smaller kernels on Earth?” asked Yifter. “They would be less dangerous.”

McAndrew shook his head. “It doesn’t work that way. The smaller the black hole, the higher the effective temperature and the faster it radiates. You’d be better off with a much more massive black hole. But then you’ve got the problem of supporting it against Earth’s gravity. Even with the best electromagnetic control, anything that massive would sink down into the Earth.”

“I suppose it wouldn’t help to use a nonrotating, uncharged hole, either,” said Yifter. “That might be easier to work with.”

“A Schwarzschild hole?” McAndrew looked at him in disgust. “Now, Mr. Yifter, you know better than that.” He grew eloquent. “A Schwarzschild hole gives you no control at all. You can’t get a hold of it electromagnetically. It just sits there, spewing out energy all over the spectrum, and there’s nothing you can do to change it — unless you want to charge it and spin it up, and make it into a kernel. With the kernels, now, you have control.”