1 DECEMBER 2017 • VOL 358 ISSUE 6367 1117 SCIENCE sciencemag.org
Relativity survives drop test
Satellite tightens measurements of equivalence principle
Akey tenet of Albert Einstein’s general theory of relativity has passed yet another test with flying colors—and for the first time in space. A French satellite experiment has shown that objects with different masses fall at
exactly the same rate under gravity, just as
relativity dictates. The result is the most
precise confirmation yet of the equivalence
principle, first tested more than 400 years
ago by Galileo Galilei. “The mission appears to have performed fantastically,” says
Clifford Will, a theoretical physicist at the
University of Florida in Gainesville.
Physicists scrutinize the equivalence principle because any violation could point to
new forces of nature that might
resolve a long-standing impasse
between general relativity and
quantum theory. The satellite,
called MICROSCOPE, found no
discrepancy in the acceleration
of two small test masses to about
one part in 100 trillion (1014).
That’s more than 10 times better
than the most sensitive ground-based experiments, which look
for disparities in the response of
weights to Earth’s spin.
The €200 million MICROSCOPE, launched by France’s
space agency CNES in April
2016, benefits from avoiding terrestrial vibrations. It relies on a
pair of concentric cylindrical
shells a few centimeters long.
The outer cylinder is made of a
titanium and aluminum alloy, whereas the
inner one is composed of much denser platinum and rhodium. As the spacecraft orbits
Earth, the cylinders are in continuous free
fall. Electrodes monitor their position and
keep them centered by applying tiny voltages
to nudge them electrostatically whenever
they stray. As the satellite traces out a
1.5-hour-long orbit, a characteristic rise and
fall in the difference between the two applied voltages would indicate that one of
the cylinders is falling slightly faster than
the other—and signal a violation of the
After more than 1500 orbits by the sat-
ellite, the MICROSCOPE team—with re-
searchers from France, Germany, the
Netherlands, and the United Kingdom—
found no such signal, they report in an
accepted paper at Physical Review Letters.
With another 900 research orbits before the
mission ends next year, the team may reach
its goal of confirming the equivalence principle to one part in a quadrillion (1015).
Will says that so far, the measurements
don’t rule out any specific alternatives to
relativity that predict a violation of equivalence. Nevertheless, he argues it is important to keep raising sensitivities in case new
physics lurks. “Until we get there we don’t
know,” he says.
A proposed Italian satellite, aptly named
Galileo Galilei, would test equivalence to
a precision of one part in 1017, partly by
spinning rapidly and isolating any signal
from more slowly varying systematic effects. Researchers at Stanford University
in Palo Alto, California, have proposed
a satellite that aims to reach one part in
1018 using noise-reducing cryogenics. Still
other researchers hope to use Bose-Einstein
condensates—clouds of cold atoms that behave as a single quantum wave—to reach
tight limits (Science, 8 September, p. 986).
Anna Nobili, a physicist at the University
of Pisa in Italy and Galileo Galilei principal
investigator, admits that finding the money
for another space mission will not be easy.
But the latest result “demonstrates that
these tests are easy in space,” she says. j
Edwin Cartlidge is a journalist in Rome.
By Edwin Cartlidge
work of poachers; in both Slovenia and
Croatia, traps set for legitimate scientific
reasons are required to carry a label with
the institution’s name and a license number.
From clues such as the trap locations,
Delić believes the poachers are primarily
looking for rare predatory ground beetles of
the Trechinae subfamily, apparently interesting to collectors because they are hard
to find. One “capital trophy,” Delićsays, is
Aphaenopidius treulandi, one of the largest cave beetles in the world, which occurs
in fewer than 10 caves in the Karavanke
Alps in Slovenia; traps have been found in
all of them, Delić says. Specimens likely
fetch hundreds of euros.
Last year, Delić’s team reported its
findings to national nature protection
authorities, who then alerted Slovenian
police. With the help of biologists and
speleologists, police have so far checked
10 caves and found traps in three of them,
says Uršula Belaj, a senior inspector at the
Criminal Police Directorate in Ljubljana.
Confiscated traps are searched for fingerprints and DNA traces, then sent to the
SubBio lab for analyses of their content.
Nabbing poachers—who face up to
3 years in prison under Slovenian law—has
proved to be difficult, in part because the
traps are often left alone for months. So
far, no arrests have been made, but the investigation is ongoing. “We don’t know yet
exactly where and at what price these cave
insects have been sold, but we are aware
that there are specialized fairs and online
forums where people trade these animals,”
Belaj says. In Croatia, HBSD reported the
illegal traps to government authorities in
late 2015; so far, this has resulted only in
stricter regulations for scientific traps.
Culver says cave poaching appears to be
unique to the region. “There is no equivalent set of collectors outside of Europe,
and I have not heard that this is a problem
in the Pyrenees,” he says. (In the United
States, there have been arrests for killing endangered bats and for vandalizing
Time may be running out for the Balkans’ troglobionts. “Pitfall traps are not
selective,” Zagmajster says. “They attract
an extremely large number of specimens
from different taxonomic groups,” especially when left out for so long; some have
been found with more than a thousand
dead animals. Given that many species
here have such a limited distribution, says
Zagmajster—some have been found in just
a single cave—poaching might eradicate
species before they’re even discovered. j
Vedrana Simičević is a science journalist
based in Rijeka, Croatia.
MICROSCOPE has shown that gravity tugs objects at the same rate.
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