A $205
million upgrade will allow a laser-wielding observatory to monitor tens of
thousands of galaxies for mysterious gravitational waves.
Leading
investigators are confident that the Advanced LIGO (Laser
Interferometer Gravitational-wave Observatories) Project will be able for
the first time to detect gravitational waves from neutron stars and black holes,
as predicted by Einstein's theory of general relativity.
"With
the limited LIGO range at time, it wasn't guaranteed detection," said Albert Lazzarini, deputy director of LIGO at the
California Institute of Technology. "With Advanced LIGO, it'd be very
surprising from a relativity perspective if we didn't observe anything."
Gravitational
waves are ripples thought to occur in the fabric of space-time that result from
interstellar
collisions, explosions, or the dramatic movement of large and extremely
dense objects such as neutron stars. Those ripples can then pass through the
space-time that Earth occupies, causing a slight distortion which Advanced LIGO
is meant to pick up on.
How it
works
LIGO tries
to detect gravitational
waves using highly precise lasers to measure the time it takes light to travel
between mirrors. Two sets of facing mirrors sit at a 90 degree angle, forming
something like an "L" shape that meets at a corner. A laser beam is shot
through an "L" shaped splitter at the corner, which splits the beam
into two beams that strike each set of mirrors.
The laser
interferometer measures how long the laser light bounces back and forth between
the mirrors
on the "L" legs before returning to a light detector at the
"L" corner. They should theoretically return to the light detector at
the same time because the mirror legs are identical distances – unless a
passing gravitational wave distorts the local space-time fabric and changes the
distance.
But the
observatory, operational since 2002, has yet to detect the elusive,
still-theoretical waves.
Scientists foresaw that advances in laser technology and
mirrors would allow for even greater sensitivity when LIGO was first proposed,
and so the Advanced LIGO Project became a natural upgrade for the observatory.
The National Science Foundation recently approved the proposal to upgrade LIGO
over the next seven years, starting with $32.75 million in 2008.
"The
first several hours of observation with new instruments will equal almost the
first year of observation with LIGO's current instruments," Lazzarini
said. "We can probe something like several hundred galaxies out to the
Virgo cluster [59 million light-years away] with LIGO, but increase that by a
factor of one thousand and you go to the cosmological regime of measuring many
tens of thousands of galaxies."
That
thousand-fold increase in coverage comes from boosting LIGO's sensitivity 10 times
over.
Larger mirrors made of better materials will reduce the
background "noise" from the random motion of atoms at room
temperature, and the laser power is being pumped from 10 watts to 180 watts.
Advanced LIGO will also be better cushioned from any terrestrial vibrations
coming through the ground, thanks to an active servo-controlled system that
replaces an older, passive spring system.
"We
achieved several milestones with the initial LIGO sensitivity," Lazzarini
noted, pointing out that the two main LIGO facilities at Hanford, Washington and Livingston, Louisiana had just finished a two-year run to test the design's
sensitivity. LIGO requires at least two widely separated detectors working
simultaneously to rule out false signals and confirm when gravitational waves
might pass through the Earth.
More
planned
Advanced
LIGO may eventually become part of a greater global network of gravitational
wave detectors, thanks to strong international collaboration.
German and British contributors are providing the laser and
mirror suspension systems respectively for the upgraded observatory, and
Advanced LIGO has grown its cooperation with Europe's Virgo detector located near Pisa, Italy. Japan has also begun working towards
building a gravitational wave detector.
"The
gravitational wave community supports very strongly indeed the upgrade to LIGO
– this upgrade was in fact planned from the very start of the LIGO project and
has always been an integral part of the planned evolution of the detector
performance," said Jim Hough, University of Glasgow physicist.
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