the mirrors. To smooth out such noise, re-
searchers crank up the amount of light and
deploy massive mirrors. At frequencies of
tens of hertz and lower, seismic vibrations
dominate, so researchers dangle the mir-
rors from elaborate suspension systems
and actively counteract that motion. Still,
a large earthquake anywhere in the world
or even the surf pounding the distant coast
can knock the interferometer off line.
To boost the Hanford and Livingston detectors’ sensitivity 10-fold, to a ten-billionth
of a nanometer, physicists have completely
rebuilt the devices. Each of the original 22-
kilogram mirrors hung like a pendulum from
a single steel fiber; the new 40-kilogram
mirrors hang on silica fibers at the end of
a four-pendulum chain. Instead of LIGO’s
original 10 kilowatts of light power, researchers aim to circulate 750 kilowatts.
They will collect 100,000 channels of data to
monitor the interferometer. Comparing the
new and old LIGO is “like comparing a car
to a wheel,” says Frederick Raab, a Caltech
physicist who leads the Hanford site.
The new Livingston machine has already doubled Initial LIGO’s sensitivity. “In
6 months they’ve made equivalent progress
to what Initial LIGO made in 3 or 4 years,”
says Raab, who adds that the Hanford site is
about 6 months behind. But Valery Frolov, a
Caltech physicist in charge of commissioning
the Livingston detector, cautions that machine isn’t running anywhere close to specs.
The seismic isolation was supposed to be
better, he says, and researchers haven’t been
able to keep the interferometer “locked” and
running for long periods. As for reaching design sensitivity, “I don’t know whether it will
take 1 year or whether it will take 5 years like
Initial LIGO did,” he warns.
Still, LIGO researchers plan to make a
first observing run this year and hope to
reach design sensitivity next year. “We will
have detections that we will be able to stand
up and defend, if not in 2016, then in 2017
or 2018,” says Gabriela González, a physicist
at LSU and spokesperson for the more than
900-member LIGO Science Collaboration.
That forecast is based on the statistics of
the stars. LIGO’s prime target is the waves
generated by a pair of neutron stars—the
cores of exploded stars that weigh more
than the sun but measure tens of kilometers
across—whirling into each other in a death
spiral lasting several minutes. Initial LIGO
could sense such a pair up to 50 million
light-years way. Given the rarity of neutron-
star pairs, that search volume was too small
to guarantee seeing one. Advanced LIGO
should see 10 times as far and probe 1000
times as much space, enough to contain
about 10 sources per year, González says.
However, Clifford Will, a theorist at the
University of Florida in Gainesville, notes
that the number of sources is the most un-
certain part of the experiment. “If it’s less
than one per year, that’s not going to be too
good,” he says.
The hunt will be global. As well as com-
bining data from the two LIGO detectors,
researchers will share data with their peers
working on the VIRGO detector, an inter-
ferometer with 3-kilometer arms near Pisa,
Italy, that is undergoing upgrades, and on
GEO600, one with 600-meter arms near
The ultimate motion sensor
In a LIGO interferometer, light waves leaking out of the two storage arms ordinarily interfere to send light back to the laser. By stretching the two arms by different amounts, a
gravitational wave would alter the interference and send light toward a photodetector.
General relativity hit the ground running—and thrilled Albert Einstein—by explaining a decades- old puzzle regarding Mercury’s orbit. According to Newtonian gravity, an
isolated planet would follow exactly the
same elliptical path on each orbit around
its star; only the influence of neighboring planets would cause the ellipse to
gradually shift, or precess, about the sun.
In 1859, however, astronomer Urbain Le
Verrier pointed out that Mercury was
precessing slightly more than purely
Newtonian gravity predicted.
Scientists proposed a slew of
possible but unlikely explanations.
Some insisted there must be a new
planet between Mercury and the sun,
which became known as Vulcan. Others
proposed bands of dust near the sun,
or an unseen moon around Mercury.
Some tried to solve the issue by tweaking Newton’s gravity. But none of these
explanations withstood scrutiny.
In November 1915, Einstein finally
had the solution: His new theory fully
explained Mercury’s extra precession.
Einstein later said that the thrill of this
discovery had given him heart
palpitations. “For a few days, I was
beside myself with joyous excitement,”
The result immediately boosted the
theory’s credibility. Mathematician David
Hilbert wrote to congratulate Einstein and
praised him on the speed of his calcula-
tions, which Einstein had performed
in only a week. What Einstein didn’t let
on was that his speed was the result of
practice: He had done the calculations
once before with an incorrect version of
his theory. ■ –E.C.
Mercury delivers good news about a newborn theory