Cosmic Power Producers

Never say that cosmologists are impractical. Two have invented a scheme for making the most efficient engine yet envisioned--a black hole engine.

Black holes, which are collapsed stars, qualify as engines because they convert matter into energy: Matter speeds up and radiates high-energy light as gravity causes it to plunge from a spinning disk into the hole. In 1974, physicist Kip Thorne of the California Institute of Technology showed that about 31% of the infalling mass is converted to usable energy. The rest is lost, as it goes into increasing the size of the hole.

For years, physicists were unable to figure how black holes could burn energy more efficiently than Thorne's calculations suggested. Now Princeton physicist Bohdan Paczynski and student Li-Xin Li have come up with a better model: A black hole that is, in effect, a two-cycle engine.

First, says Paczynski, you let matter fall into the black hole as Thorne did in his calculations. But then, as the infalling matter causes the black hole to spin faster, you stop feeding in the matter. The hole's magnetic field then increases the speed of the whirling disk, extracting another dollop of energy. "By magnetic coupling [of the hole] to the nearby disk, energy extraction from the spinning black hole can be made even more efficient," says Paczynski, who notes that the efficiency is now a whopping 43%.

The duo have posted a paper describing their improved black hole at Los Alamos National Laboratory's preprint server. But don't expect to get a car running on more miles per matter anytime soon. "There are no specific applications," says Paczynski. "It's just a mental exercise."