1098 6 MARCH 2015 • VOL 347 ISSUE 6226 sciencemag.org SCIENCE
like an additional form of gravity but pulls
on different types of particles with different strengths. So, two objects with the
same internal energy and inertial mass
may have different gravitational masses,
violating the equivalence principle. The ratio of a nucleus’s inertial and gravitational
masses could depend on the tally of protons and neutrons in it or the difference
in the numbers of protons and neutrons,
PHYSICISTS HAVE ALREADY TESTED the
equivalence principle to exquisite precision.
The best test comes from Eric Adelberger, a
physicist at the University of Washington, Seattle, and colleagues in the Eöt-Wash Group.
“They’re the gold standard right now,” Will
says. Eöt-Wash researchers don’t drop things,
but instead follow an approach pioneered
in the 1800s by Hungarian physicist Loránd
Eötvös, after whom the group is named.
Eötvös used a small dumbbell of weights
of different materials suspended horizontally
from a thin fiber. Gravity pulls each weight
toward the center of Earth. But
Earth also spins, so the inertia of
the weights creates a tiny centrifugal force that flings them away from
the planet’s axis. The sum of the two
forces, which align only at the equator, defines the direction “down”
for each weight. If the equivalence
principle holds, then the centrifugal force on each weight is locked
into proportion to the gravitational
one, so down is the same for both
weights. Then, the dumbbell will
rest pointing in any direction.
But if inertial and gravitational
mass are different, then the fling-
ing will affect the weights dif-
ferently and the net force on each one
will point in a slightly different direction.
“If the equivalence principle is violated,
then every material has its own down,”
Adelberger says. That difference would
cause the dumbbell to twist toward a particular orientation. In 1889, Eötvös saw no
such sign and confirmed the equivalence
principle to one part in 20 million.
For 25 years, Eöt-Wash researchers have
refined this test. Their latest rig consists
not of a dumbbell but of a nearly cylindri-
cal shell studded on either side with weights
of different materials. Instead of looking for
a static twist, they slowly rotate the entire
rig and look for a periodic twisting of the
cylinder. Using beryllium and titanium, they
found gravitational and inertial mass equal
to one part in 10 trillion, as they reported in
Physical Review Letters in 2008. That’s not
quite precise enough to test string theory
predictions. “In principle, we could get an-
other order of magnitude,” Adelberger says.
“There are difficulties, but that’s the goal.”
Now the MicroSCOPE team aims to
probe the equivalence principle to one part
in a quadrillion. “When you have the ability
to do such a test, you have to do it,” says
project leader Touboul. MicroSCOPE will
carry aloft two cylindrical shells, one the
size of a toilet paper roll and made of tita-
nium and a smaller one inside it made of
platinum-rhodium. If the equivalence prin-
ciple holds, both will glide on precisely the
same orbit. If not, one should slip Earth-
ward relative to the other.
In practice, MicroSCOPE researchers will
apply electrostatic forces to counteract any
motion and use the force as their signal. As
an extra test, they’ll periodically flip the satellite so that if one cylinder does tend to circle closer to Earth, researchers will have to
switch the direction of the force at the same
time. MicroSCOPE will also carry a second
pair of cylinders, both made of platinum-rhodium, to act as a control.
For such an ambitious experiment, MicroSCOPE is relatively cheap. The instrument
cost about €20 million, Touboul says, and
the entire mission less than €200 million.
Nevertheless, Damour says that
according to some models MicroSCOPE has a shot at seeing a violation of the equivalence principle.
“There is no sharp prediction,” he
says, “but there are models that
say MicroSCOPE should see a
If that sensitivity isn’t enough,
Mark Kasevich, a physicist at Stanford University in Palo Alto, California, thinks he can do 100 times better still. He is working on an atomic
version of Galileo’s drop test that
will compare two different atoms:
rubidium-87, which has 37 protons
and 50 neutrons, and rubidium-85, IMAG
Three ways to test the equivalence principle
To tell whether inertial and gravitational mass are the same, scientists can check whether objects made of different materials fall at different rates,
orbit at different distances above Earth, or cause a twist in a torsional oscillator. The twist would come about if the net force produced by gravity’s
pull toward the center of Earth and the centrifugal force produced by Earth’s rotation pointed in a different direction for each weight.
The MicroSCOPE satellite is scheduled for launch in April 2016.
SPECIAL SECTION GENERAL RELATIVITY