On an early summer morning in orthern Minnesota, a crew of about a dozen waits by the top of mine shaft No. 8. Donning
hard hats, the engineers and physicists
pile into a creaky, double-decker elevator cage. It is pitch black for most of the
three-minute descent. Ears pop, the cage
floor vibrates and a giant motor dating
from 1925 thunders overhead.
When the cage door slides open, the
team is 713 meters below the surface.
Directly ahead lies a maze of tunnels — an
abandoned mine where laborers once
extracted iron ore of uncommon purity.
But the scientific crew takes a U-turn
into a huge and unexpectedly spacious
two-room cavern known as the Soudan
The workers have journeyed deep into
the Earth to plumb the darkest depths
of the cosmos, hunting for the missing
material believed to account for 83 percent of the universe’s mass.
That material, known as dark matter,
must exist, astronomers say, because
the cosmic allotment of ordinary, visible matter doesn’t provide enough
gravitational glue to hold galaxies
together. Although the missing material shouldn’t be any more prevalent
in the underworld than above ground,
dark matter hunters have good reason
to frequent Soudan and other subterranean lairs. Because dark matter particles
would interact so weakly, experiments
designed to detect the dark stuff could
easily be overwhelmed by the cacophony
of other particles. So scientists at Soudan
and elsewhere use Earth’s crust to filter
out cosmic rays — charged particles from
space that bombard Earth’s atmosphere.
Physicists have been directly searching for dark matter for more than two
decades. But until recently, only one
experiment, beneath a mountain in central Italy, had consistently reported evidence of the invisible particles. Now two
more experiments have found similar
hints. When taken together, the findings
suggest that the most popular models for
dark matter may not be correct —the
particles pegged have a lower mass than
many physicists had proposed.
“Any discovery of dark matter would
be a major revolution,” says theorist
Neal Weiner of New York University.
“But if these results are right, I think it’s
even more exciting than that.”
If the low-mass measurements are
confirmed, a second revolution is in the
making: In addition to dark matter, a new
force may be needed to explain the work-
ings of the universe. Favorite particle
physics theories may require revision or
may even have to be discarded.
But not so fast, some scientists say.
Other recent work questions whether
researchers have actually spotted low-mass dark matter particles. And with so
much at stake, including the likelihood
of a Nobel Prize for whoever discovers dark matter first, rival teams have
resorted to name-calling. One team has
twice publicly ridiculed the results of
a second, while the team whose analysis has come under fire has likened the
attacks to the Spanish Inquisition.
It’s an exciting but confusing time,
Catching some WIMPs
Physicists have long had a leading model
for dark matter. They believe that it consists of a proposed particle left over from
the Big Bang called the WIMP, for weakly
interacting massive particle. WIMPs
sense only gravity and the weak force,
the interaction that governs radioactive
decay. Particle physicists like WIMPs
Into the darkness the visible extent
of the Milky way galaxy (center), which has a
diameter of about 100,000 light-years, is
dwarfed by a surrounding halo of dark matter.