Anyone who visits a cave gets the total-darkness treatment. You turn off your lights and sit for a few minutes. The darkness is so absolute, it feels like a fine powder or gas that seems to fill the space even though your ears and hands tell you the place is empty. After a while you start to hallucinate. You're as far away as you could imagine from the rest of the universe.
So why do physicists go underground to find signals from the rest of the universe? It's that same isolation. Physicists today watch for extremely subtle signs as they try to figure out what's wrong with the universe.
The Cosmic Deficit
The problem is that we can't find enough mass in the universe. The stars, and the clouds of gas and dust between them, can't provide enough gravity to hold galaxies together. Rather than throw out the physics we trust, we're looking for dark matter, lots of it. The vast majority of the universe, in fact, must be dark matter.
The main candidates for dark matter are MACHOs, WIMPs, and neutrinos.
MACHOs, or MAssive Compact Halo Objects, would be planet-size chunks of ordinary matter, forming a cloud around the disks of stars that make up galaxies. We look for them with telescopes, in the MACHO Project.
Finding WIMPs
WIMPs—Weakly Interacting Massive Particles—and neutrinos are what drive physicists underground. Neutrinos are well-known particles with zero mass, like the photons that make up light, but unlike light they can pierce the Earth itself without hitting anything. But neutrinos might actually have a very small mass, which would go a long way to fix the universe.
WIMPs are hypothetical particles, heavier than neutrons, that formed in the Big Bang and have drifted around the universe ever since. They could only be detected directly when a WIMP collides head-on with an atomic nucleus. This wouldn't happen often. Two blindfolded sailors in the whole Pacific Ocean are likelier to bump into each other. But with 10 trillion WIMPs drifting through every kilogram of matter each second, there would be about one hit per day.
WIMP bumps can be detected with sensitive enough equipment—but lots of other things bump nuclei too, such as cosmic rays and natural radioactivity, and they outnumber WIMP collisions a million to one. This noise problem is similar for neutrinos. Hence the need for a very clean and sheltered place.
Old played-out mines are just the thing. They have shafts with power and elevators in place, deep chambers that are safe for human occupation, and equipment for moving large objects. And 1000 meters down, almost nothing in the outer universe can reach sensitive detectors besides neutrinos and WIMPs (if they exist).
The Whisper of Neutrinos—and Geoneutrinos
The Japanese neutrino observatory Super-Kamiokande reminds me of that lights-out cave experience. Super-K is a 40-meter chamber a kilometer below the ground, filled with ultrapure water and surrounded by 11,000 sensitive electronic eyes pointing into the water. It's been sitting for years in total darkness, waiting for dim blue flashes from neutrino collisions in the water. It sees about five per hour, none visible to the human eye.
A sibling observatory, KamLAND, has detected "geoneutrinos" that originated within the Earth. This is the first direct method of assessing the planet's radioactive heat budget. It also launches a new science: neutrino geophysics. More geoneutrinos were confirmed in 2010 by another lab.
A Cabal of Underground Labs
There are other underground labs on the Web, too.
- Gran Sasso National Laboratory is a set of rooms carved near a roadway tunnel under the Apennine mountains of Italy. Many different experiments are being conducted there. One of them is showing results that fit the lightest predicted WIMP, a particle called the neutralino.
- The Soudan Underground Laboratory is in Minnesota, in a decommissioned iron mine that also is a state park (and one of Minnesota's geological attractions). This lab as well as Gran Sasso and Super-K are all preparing experiments in which other physics labs aim beams of neutrinos toward the detectors hundreds of kilometers through the Earth itself.
- Sudbury Neutrino Observatory, in Canada, is 2000 meters deep in a cobalt mine and an impressive piece of engineering, as told in detail on its Web site. It has a Super-K type of experiment using heavy water.
- The Boulby Mine in Yorkshire, England, produces potash and houses experiments for a European WIMP-hunting team. The ZEPLIN liquid-xenon detector is my favorite for exotic materials and beautiful apparatus.
- The American physics community is reusing the former Homestake Mine in South Dakota for its new Sanford Underground Science and Engineering Laboratory (SUSEL). It was dedicated in June 2009 and is readying a liquid-xenon experiment.
PS: Amazing in a different way is the IceCube neutrino observatory, built in the ice cap at the South Pole. Deep holes were drilled using hot water, then detectors were placed in them and frozen into place. They look downward, through the Earth, for neutrinos coming from the universe on the other side.
