On 20 February, dignitaries will descend on Virgo, Europe’s premier gravita- tional wave detector near Pisa, Italy, for a dedication ceremony to celebrate a 5-year, €24 million upgrade. But the pomp will belie nagging problems
that are likely to keep Virgo from joining its
U.S. counterpart, the Laser Interferometer
Gravitational-Wave Observatory (LIGO), in a
hunt for gravitational wave sources that was
meant to start next month. What has hobbled the 3-kilometer-long observatory: glass
threads just 0.4 millimeters thick, which
have proved unexpectedly fragile. The delay,
which could last a year, is “very frustrating
for everyone,” says LIGO team member Bruce
Allen, director of the Max Planck Institute for
Gravitational Physics in Hannover, Germany.
A year ago, LIGO confirmed a prediction
made by Albert Einstein a century earlier:
that violent cosmic events, like the merger
of two black holes, would wrench the fabric of spacetime and emit ripples (Science,
12 February 2016, p. 645). But LIGO, with
two instruments in Livingston, Louisiana,
and Hanford, Washington, cannot pinpoint
the sources of the waves, which would let
astronomers train other telescopes on
them. Triangulating on the sources requires
a third detector: Virgo.
The detectors all rely on optical devices
called interferometers: two straight arms,
several kilometers long, positioned at right
angles. Inside each arm a laser beam bounces
back and forth between mirrors at each end
of a vacuum tube, resonating like sound in
an organ pipe. The laser light is combined
where the two arms meet so that the peak
from one laser wave meets the trough of the
other and they cancel each other out. But
if something, such as a gravitational wave,
stretches space and changes the length of the
two arms by different amounts, the waves
will no longer match up
and the cancellation will
be incomplete. Some light
will pass through an exit
known as the dark port
and into a detector.
The tiniest vibrations—
earth tremors, the rumble
of trains, even surf crash-
ing on distant beaches—
can swamp the signal
of gravitational waves.
So engineers must pain-
stakingly isolate the de-
tectors from noise. At
Virgo, for example, the
mirrors are suspended
at the end of a chain of
seven pendulums. For the
upgrade, steel wires connecting the mirror
to the weight above it were replaced with
pure glass fibers to reduce thermal and me-
chanical noise.
But a year ago, the glass threads began
shattering, sometimes days or weeks after
the 40-kilogram mirrors were suspended
from them. After months of investigation,
the team found the culprit: microscopic
particles of debris from the pumps of the
upgraded vacuum system. When these
particles settled on the glass fibers they
created microcracks, which widened over
days and weeks until the
fibers failed. “The fibers
are very robust until
something touches their
surface,” says Giovanni
Losurdo, Advanced Virgo
project leader at Italy’s
National Institute for
Nuclear Physics in Pisa.
During the investiga-
tion, the team tempo-
rarily replaced the glass
fibers with steel wires—
as in the original Virgo—
and pressed ahead. But
other problems com-
pounded the delays. An
examination of small
steel triangles that act
17 FEBRUARY 2017 • VOL 355 ISSUE 6326 673 SCIENCE
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Glass fibers used to suspend
40-kilogram mirrors have shattered.
By Daniel Clery
ASTROPHYSICS
European gravitational wave detector falters
Fragile glass fibers hurt Virgo’s chances to observe in 2017 with U.S. counterpart
IN DEPTH
Virgo stretches its
3-kilometer arms
across the Tuscan
plain near Pisa, Italy.
