On April 4, the physics department at Columbia University held an unblinding party. For 100.9 days between January 13 and June 8, 2010, a detector 4,500 feet underground at the Laboratori Nazionali del Gran Sasso, in central Italy, had been collecting data. Following the protocol of a “blind” analysis, the data had instantly disappeared into a “box”—a Pandora-esque hard drive—so as not to prejudice the human analysis. Now some of the collaborators on the experiment had gathered in a laboratory at Columbia to watch as a software program lifted the lid on the box and allowed them all a first peek at whatever there was to see.
What they saw on the computer screen were six red dots. Six: a number statistically significant enough to allow them to claim a detection…and for the leader of the team, Elena Aprile, to cop a Nobel Prize.
Cheers! Hugs! Kisses!
Also (straighten tie; smooth skirt), analysis. Over the next few days they discovered that they had to attribute three events to electronic noise. Which still left them with three events. But they had known in advance that no matter how many or how few “detections” they found, they would have to discount two—or, more accurately, 1.8 ± 0.6, the number they had calculated that in a sample of this particular size would be due to radioactive interference. Which left them with one, a statistically insignificant number. On April 13—nine days after the unblinding party had turned into a party party—the collaboration posted their paper online, including this conclusion in the abstract: “no evidence for dark matter.”
Scientists have been searching for dark matter—the mysterious substance that makes up more than four-fifths of the matter in the universe—for more than thirty years now. In the 1970s, astronomers used the gravitational motions of galaxies and clusters of galaxies to infer the existence of matter they couldn’t see. At the same time, particle physicists trying to solve problems in the Standard Model independently came up with hypothetical particles that would possess just the right properties to account for the missing matter.
The problem was that these particles, if they existed, wouldn’t interact with electromagnetism, so they would be invisible in any wavelength of light. They also wouldn’t interact through the strong nuclear force, meaning that they would rarely interact with atomic nuclei.
The key word, though, is “rarely.” The very occasional exception would allow dark-matter detectives to take evidence that would be inaccessible to our senses and transform it into evidence that would be accessible: an emission of heat and an electric charge that would be the unique signature of a dark matter particle coming into contact with the nucleus of a target atom. In the case of the Gran Sasso experiment, the target atoms were liquid xenon; hence the name of the experiment, XENON 100.
By the late 1970s, then, physicists had a method for detecting dark matter. They also had the means—any of the two or three dozen dark matter experiments that would have been up and running somewhere in the world at any given moment.
And so it seemed completely reasonable that:
• in 1980 Vera Rubin, one of the astronomers who had discovered some of the most compelling evidence for the existence of dark matter, predicted the discovery of dark matter within ten years;
• ten years later, in 1990, British astronomer (and future Astronomer Royal) Martin Rees predicted the discovery of dark matter within ten years;
• eleven years later, in his book Our Cosmic Habitat, Rees wrote, “I think there is a good chance of achieving this goal within ten years.”
• five years later, at an American Institute of Physics symposium, Rees doubled down on that prediction: five more years, he vowed. Vera Rubin, who happened to be in the audience, stood up. “I know of earlier predictions,” she said.
Rubin provided this last bullet point over dinner this past January, a couple of weeks after the calendar had flipped to 2011. “And now it’s been five years,” she said, laughing.
Of course, 2011 is far from over. What if XENON100 had found six solid events? And any day now the Large Hadron Collider might spit a dark matter particle into the universe. Hope springs eternal.