Still, neutrinos poured out of the sun in such vast profusion that collisions did occasionally occur-and heavy water was an ideal target for such collisions. The hydrogen nuclei in heavy water each contain a proton-the normal constituent of a hydrogen nucleus-plus a neutron, as well. And when a neutrino did chance to hit a neutron, the neutron decayed, releasing a proton of its own, an electron, and a flash of light that could be detected by the photomultiplier tubes.

At first, Louise’s dark, arching eyebrows did not rise when she heard the neutrino-detection alarm go ping, the alarm sounded briefly about a dozen times a day, and although it was normally the most exciting thing to happen down here, it still didn’t merit looking up from her copy of Cosmopolitan.

But then the alarm sounded again, and yet again, and then it stayed on, a solid, unending electric bleep like a dying man’s EKG.

Louise got up from her desk and walked over to the detector console. On top of it was a framed picture of Stephen Hawking-not signed, of course. Hawking had visited the Sudbury Neutrino Observatory for its grand opening a few years ago, in 1998. Louise tapped on the alarm’s speaker, in case it was on the fritz, but the keening continued.

Paul Kiriyama, a scrawny grad student, dashed into the control room, arriving from elsewhere in the vast, underground facility. Paul was, Louise knew, usually quite flustered around her, but this time he wasn’t at a loss for words. “What the heck’s going on?” he asked. There was a grid of ninety-eight by ninety-eight LEDs on the detector panel, representing the 9,600 photomultiplier tubes; every one of them was illuminated.

“Maybe someone accidentally turned on the lights in the cavern,” said Louise, sounding dubious even to herself.

The prolonged bleep finally stopped. Paul pressed a couple of buttons, activating five TV monitors slaved to five underwater cameras inside the observatory chamber. Their screens were perfectly black rectangles. “Well, if the lights were on,” he said, “they’re off now. I wonder what-”