Almost there. As in this drawing, the Alpha Magnetic Spectrometer will perch on the space station.
in 1994, he conceived of it as a simpler
instrument costing tens of millions of dollars that would hunt an exotic prey: atomic
nuclei made of antimatter, which would have
charges opposite those of ordinary nuclei
(Science, 12 January 1996, p. 142). Apart
from antiprotons, which are made in abundance by the collision of protons in space, our
corner of the universe contains essentially all
matter and no antimatter. However, the laws
of physics do not favor one over the other.
So some physicists have speculated that the
universe may harbor vast regions containing
stars and galaxies of pure antimatter. If so, a
few antihelium or anticarbon nuclei might
find their way to our galaxy as cosmic rays.
However, analyses from the late 1990s
put the kibosh on that scenario, cosmic ray
experts say. If such “domains” of antimatter did exist, then scientists would see a haze
of gamma rays from particle annihilations
at the borders with domains of matter, says
Gregory Tarlé of the University of Michigan,
Ann Arbor. Even if antimatter galaxies did
exist, he says, calculations show that there’s
no chance nuclei from them could reach our
galaxy. “The major justification for doing
AMS has evaporated,” Tarlé says. Ting
argues that the question remains open and
should be settled experimentally.
AMS, which has evolved into a far
more comprehensive detector, has a better chance of discovering evidence of dark
matter, many physicists say. Some theories
predict that when dark matter particles collide, they ought to annihilate one another
to produce electron-positron pairs, leading
to a telltale excess of positrons. Hints of an
excess were spotted in the mid-1990s by the
balloon-borne High-Energy Antimatter Telescope (HEAT). In 2008, physicists with the
Italian experiment Payload for Antimatter
Matter Exploration and Light-Nuclei Astrophysics, which sits aboard a Russian satellite,
reported further evidence of such an excess.
Confirming that signal would be AMS’s best
shot at glory, says Stéphane Coutu, a cosmic-ray physicist at Pennsylvania State University, University Park.
CREDIT: NASA/GLENN BENSON
However, AMS would not be able to
prove that the positrons did not come from
a more mundane source, such as a nearby
pulsar, Tarlé says. “Given that it’s not a con-
clusive experiment, the billions of dollars in
expenditures—and the risk of lives in
extending the shuttle program—is just not
worth it,” he says. Tarlé argues that similar
measurements could have been made by fly-
ing HEAT, which flew for 1 day at a time, on
a monthlong mission around the South Pole.
In 2003, NASA turned down a proposal to
do that, says Coutu, who worked with Tarlé
Undeniable. By sheer determination, Samuel Ting kept the project alive, all agree.
say. “They are not going to go into terra
incognita there,” Müller says. Moreover,
some researchers worry that AMS may not
be able to make measurements up to 1 TeV.
That’s because a year ago Ting and colleagues swapped in the permanent magnet, used during a test run on the shuttle in
1998, to replace a superconducting coil with
a field five times as strong (Science, 30 April
2010, p. 561). That move was made because
the superconducting magnet requires liquid
helium to keep it extremely cold, and tests
showed that it would boil away its 2500-liter
supply faster than planned. AMS researchers
say the reconfigured device will have nearly
the same precision, but others are skeptical.
The scientific controversy surrounding
AMS intertwines with the personal con-
troversy surrounding Ting, who shared the
1976 Nobel Prize in physics for the discov-
ery of a particle called the charm quark. All
agree that Ting kept AMS alive when NASA
proposed balloon missions such as the polar
flight of HEAT in part because AMS was
going to do similar work.